&EPA
United States
Environmental Protection
Agency
Office of Emergency and
Remedial Response
Washington DC 20460
Superfund
Office of Research and
Development
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-540/2-84-001 Feb. 1984
Slurry Trench
Construction for
Pollution Migration
Control
-------
-------
EPA-5W2-84-001
February 1984
SLURRY TRENCH CONSTRUCTION
FOR
POLLUTION MIGRATION CONTROL
OFFICE OF EMERGENCY AND REMEDIAL RESPONSE
OFFICE OF SOLID WASTE AND EMERGENCY RESPONSE
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
-------
NOTICE
The information in this document has been funded, wholly or
in part, by the United States Environmental Protection
Agency under Contract No. 68-03-3113 to JRB Associates.
It has been subject to the Agency's peer and administra-
tive review and has been approved for publication as an
EPA document.
This handbook is intended to present information on the
application of a technology for the control of specific
problems caused by uncontrolled waste sites. It is not
intended to address every conceivable waste site problem
or all possible applications of this technology. Mention
of trade names or commercial products does not constitute
endorsement or recommendation for use.
-------
FOREWORD
This is one of a series of reports being published to implement CERCLA,
otherwise known as Superfund legislation. These are documents explaining the
hazardous response program and, in particular, the technical requirements for
compliance with the National Contingency Plan (NCP), the analytical and
engineering methods and procedures to be used for compliance, and the back-
ground and documenting data related to these methods and procedures. The
series may include feasibility studies, research reports, manuals, handbooks,
and other reference documents pertinent to Superfund.
This handbook provides in-depth guidance on the use of slurry walls for
the control of subsurface pollutants. It describes how these barriers can be
employed for waste site remediation and presents the theory of their func-
tion and use. It also describes the essential elements of slurry wall feasi-
bility, design, and construction and presents information on site investiga-
tion, associated remedial measures, maintenance and monitoring, and major cost
elements. The handbook provides governmental and industrial technical person-
nel with the means of evaluating essential aspects of the application of this
technique to the clean-up of uncontrolled hazardous waste sites. In conjunc-
tion with other publications in this series, it will assist in meeting the
national goal of a cleaner, safer environment.
111
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ABSTRACT
This report is intended to provide reviewers of remedial action plans
with the necessary background material to evaluate the portions of the plan
dealing with pollution migration control slurry walls.
A discussion of the early development and use of slurry trench
construction techniques, both in Europe and the United States is given to
acquaint the reader with the history of the technique.
An in-depth description of the currently held theories regarding the
functions of bentonite slurries, and the various backfill materials is
presented. Data on filter cake formation, slurry viscosity, thixotropy, as
well as the effects of cement on the functioning of both slurry and backfill
are included. Failure mechanisms are also discussed.
Typical slurry wall configurations are described along with the various
other remedial measures appropriate for use in conjunction with slurry walls.
After the presentation of the fundamentals of slurry wall use, the
procedures for planning a slurry wall installation are given. These include,
site investigation procedures for characterizing the surface and subsurface
conditions, as well as waste and leachate characterization. The factors
considered in slurry wall design are then presented, followed by an outline of
accepted construction practices. The necessary methods to monitor and
maintain a completed slurry wall are also included, along with the factors
that influence costs.
The handbook concludes with a series of evaluation criteria that
correspond to the stages of a slurry wall installation. A Glossary of
commonly used terms is also included.
This report is submitted in fulfillment of Contract No. 68-03-3113 by JRB
Associates under the sponsorship of the U.S. Environmental Protection Agency.
This report covers the period. April 15, 1982, to July 20, 1983, and work was
completed on July 20, 1983.
IV
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CONTENTS
FOREWORD iii
ABSTRACT vi
CONTENTS v
FIGURES xii
TABLES xv
ACKNOWLEDGEMENTS xvii
1. INTRODUCTION 1-1
1.1 Purpose of This Handbook 1~1
1.1.1 Organization and Use 1"1
1.2 Background 1~2
1.2.1 Slurry Trench Construction Techniques 1-2
1.2.2 History of Slurry Trench Construction 1-3
1.2.2.1 Technique Development 1-4
1.2.2.2 Applications 1-4
1.3 Limitations 1~5
1,4 Summary 1-7
2. THEORY OF SLURRY AND BACKFILL FUNCTION 2-1
2.1 Bentonite 2~1
2.1.1 Rationale for Bentonite Use 2-2
2.1.2 Bentonite Properties 2"2
2.1.2.1 Swelling and Hydration 2-2
2.1.2.2 Dispersion 2"3
2.1.2.3 Thixotropy 2-3
2.1.3 Factors Affecting Bentonite Performance .......... 2-3
2.1.3.1 Montmorillonite Content 2-4
a. Montmorillonite Crystal Structure 2-4
b. Theory of Clay Hydration and Swelling. . . . 2-6
c. Theory of Flocculation and Dispersion. . . . 2-10
d. Theory of Gelation and Thixotropy 2-10
2.1.3.2 Relative Sodium and Calcium Concentrations . . . 2-12
2.1.3.3 Bentonite Particle Size 2-13
2.2 Bentonite Slurries . , ..... 2-13
2.2.1 Bentonite Slurry Properties 2"13
2.2.1.1 Viscosity 2~15
2.2.1.2 Gel Strength 2-15
2.2.1.3 Density 2-17
2.2.1.4 Filter Cakes 2-17
a. Filter Cake Formation and Function 2-17
b. Desirable Filter Cake Characteristics. . . . 2-19
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CONTENTS (continued)
2.2.2
2.2.1.5 Resistance to Flocculation
Factors Affecting Bentonite Slurry Performance
2.2.2.1
Filter Cake Performance,
Bentonite Concentration
2.2.2.2
2.2.2.3
2.2.2.4
c.
d.
e.
f.
g-
Gel
Bentonite Quality
Slurry Mixing Methods. . .
Filter Cake Formation Time
Strata Characteristics . .
Hydraulic Gradient ....
Slurry Contamination . . .
Strength
Density
Chemical and Physical Additives
2.3 Soil-Bentonite Walls
2.3.1 SB Wall Properties
2.3.1.1 Low Permeability
2.3.1.2 Resistance to Hydraulic Pressure
and Contaminants
2.3.1.3 Strength and Plasticity
2.3.2 Factors Affecting SB Wall Performance
2.3.2.1 Design Criteria
2.3.2.2 Backfill Composition and Characteristics
a. Native Clay and Bentonite Content. .
b. Water Content
c. Contaminants in Backfill Materials .
2.3.2.3 Backfill Placement Methods
2.3.2.4 Post-Construction Conditions
a. Hydraulic Gradient
b. Presence of Contaminants
2.4 Cement Bentonite Slurries
2.4.1 CB Slurry Properties
2.4.1.1 Differences in Physical Properties . . .
2.4.1.2 Differences in Setting Times
2.4.1.3 Differences in Filter Cake Permeability.
2.5 Cement Bentonite Walls
2.5.1 CB Wall Requirements
2.5.1.1 Strength .* '
2.5.1.2 Durability
2.5.1.3 Cont inuity
2.5.1.4 Set Time
2.5.1.5 Permeability
2.5.2 Factors Affecting CB Wall Performance
2.5.2.1 Slurry Constituents
a. Bentonite Content
b. Water Quality
c. Cement Content
Page
2-20
2-20
2-21
2-21
2-21
2-23
2-23
2-23
2-25
2-25
2-27
2-27
2-28
2-28
2-28
2-28
2-30
2-30
2-30
2-31
2-31
2-31
2-33
2-33
2-35
2-35
2-36
2-36
2-36
2-36
2-37
2-37
2-40
2-40
2-40
2-41
2-41
2-41
2-42
2-42
2-42
2-42
2-42
2-43
2-43
VI
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CONTENTS (continued)
d. Cement Replacements 2-43
e. Chemical Additives 2-46
2.5.2.2 CB Slurry Mixing Methods 2-46
2.6 Summary 2-47
3. SLURRY WALL APPLICATIONS 3-1
3.1 Configuration 3-1
3.1.2 Vertical Configuration 3-1
3.1.2.1 Keyed-In Slurry Walls 3-1
3.1.2.2 Hanging Slurry Walls 3-3
3.1.3 Horizontal Configuration 3-3
3.1.3.1 Circumferential Wall Placement 3-3
3.1.3.2 Upgradient Wall Placement 3-5
3.1.3.3 Downgradient Wall Placement 3-5
3.2 Associated Remedial Measures and Practices 3-12
3.2.1 Groundwater Pumping 3-12
3.2.1.1 Pumping Systems 3-17
a. Well Points 3-17
b. Extraction/Injection Wells 3-23
c. Skimmer Systems 3-24
3.2.1.2 Groundwater Treatment 3-26
3.2.2 Collectors and Drainage Systems 3-26
3.2.3 Surface Sealing . . . . 3-27
3.2.4 Ancillary Measures 3-28
3.2.4.1 Grouting 3-28
3.2.4.2 Sheet Piles l 3-32
3.2.4.3 Synthetic Membrane Liners 3-33
3.3 Summary 3-33
4. SITE INVESTIGATION AND CHARACTERIZATION 4-1
4.1 Physical Constraints 4-1
4.1.1 Topography 4-5
4.1.2 Vegetation Density 4-5
4.1.3 Land Drainage Patterns 4-5
4.1.4 Availability of Water 4-5
4.1.5 Location of Utility Crossings . . . ;.-. 4-5
4.1.6 Proximity to Property Lines . . . . * 4-6
4.1.7 Site Accessibility 4-6
4.1.8 Presence of Other Man-Made Features 4-6
4.2 Subsurface Investigations 4-7
4.2.1 Geology 4-9
4.2.2 Hydrology 4-10
4.2.3 Soils and Overburden 4-12
4.3 Wastes and Leachates 4-13
4.3.1 Effects of Groundwater Contaminants on SB Walls 4-14
4.3.1.1 Effects of Groundwater Contaminants
on Bentonite Slurries 4-14
vii
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CONTENT S (continued)
4.3.1.2 Effects of Groundwater Contaminants
on the Permeability of Cut-Off Walls 4-15
4.3.2 Compatibility Testing 4-18
4.3.2.1 Viscosity Test 4-18
4.3.2.2 Filter-Press Test . 4-19
4.3.2.3 Examination of Bentonite Mineralogy '4-19
4.3.2.4 Permeability 4-19
4.4 Summary 4-22
5. DESIGN AND CONSTRUCTION 5-1
5.1 Design Procedures and Considerations 5-1
5.1.1 Feasibility Determination 5-2
5.1.1.1 Waste Compatibility 5-2
5.1.1.2 Permeability and Hydraulic Gradient 5-2
5.1.1.3 Aquiclude Characteristics 5-3
5.1.1.4 Wall Configuration and Size 5-3
5.1.1.5 Cost and Time Factors. 5-3
5.1.2 Selection of Slurry Wall Type 5-3
5.1.2.1 Permeability and Hydraulic Gradient 5-4
5.1.2.2 Leachate Characteristics 5-4
5.1.2.3 Availability of Backfill Material 5-4
5.1.2.4 Wall Strength 5-5
5.1.2.5 Aquiclude Depth 5-5
5.1.2.6 Site Terrain 5-5
5.1.2.7 Cost 5-6
5.2 Specification Type and Design Components 5-6
5.2.1 Differences in Specification Type 5-6
5.2.2 Components of Design 5-7
5.2.2.1 Scope of Work 5-7
5.2.2.2 Construction Qualifications 5-7
5.2.2.3 Construction Requirements of the Trench
and Wall 5-7
5.2.2.4 Materials 5-8
5.2.2.5 Equipment 5-8
5.2.2.6 Methods. 5-8
5.2.2.7 Quality Control and Documentation 5-9
5.2.2.8 Drawings. 5-9
5.2.2.9 Measurement and Payment 5-10
5.3 Slurry Wall Requirements 5-10
5.3.1 Location 5-10
5.3.2 Depth 5-11
5.3.3 Width and Permeability 5-11
5.3.4 Continuity and Verticality 5-11
5.3.5 Surface Protection 5-12
5.3.6 Materials, Quality Control and Documentation
Requirements 5-13
5.3.6.1 Dry Bentonite 5-13
viii
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CONTENTS (continued)
Page
5.3.6.2 Clay
5.3.6.3 Water
5.3.6.4 Fresh Slurry
5.3.6.5 In-Trench Slurry
5.3.6.6 Backfill Materials
5.3.6.7 Mixed Backfill
a. Slump
b. Density
c. Shear Strength
5.3.7 Equipment
5.3.7.1 Slurry Mixing
5.3.7.2 Trench Excavation
5.3.7.3 Backfill Mixing
5.3.7.4 Backfill Placement
5.3.8 Facilities
5.3.9 Methods
5.3.10 Safety Procedures
5.4 Preconstruction Activities
5.4.1 Slurry Wall Design
5.4.2 Cost Estimates
5.4.3 Bid Package Preparation
5.4.4 Bid Evaluation and Contract Award
5.5 Soil Bentonite Wall Construction
5.5.1 Preconstruction Assessment and Mobilization
5.5.1.1 Plan Layout
5.5.1.2 Equipment Requirements
5.5.1.3 Personnel Requirements
5.5.2 Pre-Excavation Site Preparation
5.5.3 Slurry Preparation and Control
5.5.3.1 Testing Bentonite and Water. . . .
5.5.3.2 Slurry Mixing and Hydration. . . .
5.5.4 Slurry Placement
5.5.5 Trench Excavation
5.5.6 Backfill Preparation
5.5.6.1 Fines Content
5.5.6.2 Slump -:, . . .
5.5.6.3 Wet Density . . .
5.5.7 Backfill Placement
5.5.8 Capping
5.5.9 Clean Up Activities
5.6 Cement Bentonite Wall Construction
5.7 Diaphragm Wall Construction
5.8 Potential Problems During and After Construction .
5.8.1 Unstable Soil
5.8.2 High Water Table
5.8.3 Rock in Excavation
5.8.4 Sudden Slurry Loss
13
15
15
19
19
19
20
-20
22
-22
23
-23
23
-23
25
-25
25
-26
26
-26
-26
-27
-27
-28
-28
-28
30
-31
-31
-31
32
-32
-33
-35
-35
-36
-36
-36
-37
-37
38
-39
41
-41
43
-43
-43
IX
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CONTENTS (continued)
Page
5.8.4.1 Pervious Zones 5-43
5.8.4.2 Pipes and Conduits 5-44
5.8.5 Slurry Flocculation 5-44
5.8.6 Trench Collapse 5-44
5.8.7 Inadequate Backfill Placement 5-45
5.8.7.1 Sediments in Trench Bottom 5-45
5.8.7.2 Slurry Pockets in the Backfill 5-47
5.8.8 Cracking 5-47
5.8.8.1 Conso 1 idat ion 5-48
5.8.8.2 Hydrofracturing 5-48
a. Consolidation » 5-50
b. Piezometers 5-50
.c. High Hydraulic Gradients 5-50
5.8.8.3 Syneres is 5-51
5.8.9 Tunnelling and Piping 5-51
5.8.9.1 Tunnelling 5-51
5.8.9.2 Piping 5-53
5.8.10 Chemical Disruption 5-53
5.9 Summary. 5-54
6. SLURRY WALL MONITORING AND MAINTENANCE 6-1
6.1 Effectiveness Monitoring 6-1
6.1.1 Basal Stability 6-2
6.1.2 Ground Movement 6-2
6.1.3 Groundwater Level and Chemistry 6-4
6.1.3.1 Groundwater Monitoring 6-4
6.1.4 In Situ Permeability Tests 6-5
6.1.5 Surface Water Chemistry 6-6
6.2 Maintenance 6-6
6.3 Wall Restoration 6-8
6.4 Summary 6-12
7. MAJOR COST ELEMENTS 7-1
7.1 Introduction 7-1
7.1.1 Developing Preliminary Cost Estimates 7-2
7.1.1.1 Cost Estimation Example 7-3
a. Feasibility Testing 7-3
b. Temporary Road Construction 7-4
c. Site Clearing and Preparation 7-4
d. Slurry Wall Excavation and Installation. . . 7-4
e. Site Re-Grading and Revegetation 7-8
f. Total Costs 7-8
7.2 Unit Costs 7-8
7.2.1 Feasibility Testing 7-10
7.2.1.1 Geologic and Soils Testing 7-10
7.2.1.2 Hydrologic Testing 7-11
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CONTENT S (continued)
Page
7.2.1.3 Backfill Testing 7-11
7.2.2 Construction Activities 7-14
7.2.2.1 Site Clearing 7-14
7.2.2.2 Excavation 7-15
7.2.2.3 Backfilling 7-15
7.2.2.4 Borrow 7-19
7.2.2.5 Compaction 7-19
7.2.2.6 Grading 7-19
7.2.2.7 Hauling 7-19
7.2.2.8 Mobilization and Demobilization 7-22
7.2.2.9 Site Dewatering 7-22
7.2.3 Completed Wall Costs 7-22
7.2.3.1 Monitoring 7-25
7.2.3.2 Maintenance 7 -25
7.2.4 Materials 7-28
7.2.5 Equipment 7-28
7.3 Summary 7-28
8. EVALUATION PROCEDURES 8-1
8.1 Site Characteristics 8-1
8.1.1 Surface Characteristics 8-3
8.1.2 Subsurface Characteristics 8-4
8.1.3 Waste Characteristics 8-6
8.2 Slurry Wall Applications 8-6
8.2.1 Wall Configuration and Type 8-7
8.2.2 Associated Remedial Measures 8-7
8.3 Construction Techniques and QA/QC Requirements 8-8
8.4 Monitoring and Maintenance 8-9
8.4.1 Monitoring 8-9
8.4.2 Maintenance 8-10
8.5 Costs 8-10
MEASURING UNIT CONVERSION TABLE 9-1
GLOSSARY 10-1
REFERENCES H"1
XI
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LIST OF FIGURES
Number
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
3-1
3-2
3-3
3-4
3-5
Montmorillonite Crystal Lattice, Showing Adsorbed
Cations and Oriented Water Molecules
Viscosity and Weight of Mud in Relation to Percentage
of Bentonites and Native Clays in Fresh Water . . .
Bentonite Particles During Hydration, Gelation,
Flocculation and Dispersion
Relationship Between Filter Cake Permeability and Slurry
Viscosity
Fluid Loss During Filter Cake Formation ,
The Effect of Bentonite Concentration on the Initial
Fluid Loss During Filter Cake Formation
The Effect of Mixing Techniques and Times on Hydration
of a 5% Suspension of Ca-Exchanged Bentonite. . . .
The Effect of Added Sand on Filtration of a 5% Suspension
of a Calcium-Exchanged Bentonite through a Fine
Gravel Bed
Relationship Between Permeability and Quality of Bentonite
Added to SB Backfill
Effect of Plastic and Non-Plastic Fines Content on
Soil-Bentonite Backfill Permeability
Keyed-In Slurry Wall
Hanging Slurry Wall
Plan of Circumferential Wall Placement
Cut-away Cross-section of Circumferential Wall
Placement
Plan of Upgradient Placement with Drain
Page
2-5
2-9
2-11
2-16
2-18
2-22
2-24
2-26
2-32
2-34
3-2
3-4
3-6
3-7
3-8
XI1
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LIST OF FIGURES (continued)
Number
5-4 Schematic
3-6
3-7
3-8
3-9
3-10
3-11
3-12
3-13
3-14
3-15
3-16
3-17
3-18
4-1
4-2
5-1
5-2
5-3
Cut-away Cross-section of Upgradient Placement
with Drain
Plan of Downgradient Placement
Cut-away Cross-section of Downgradient Placement
Shape of a Drawdown "Cone"
Intersection of a Drawdown Cones of Two Adjacent Wells. . .
Schematic of a Well Point Dewatering System
The Effect- of Drawdown in the Absence and Presence of a
Slurry Wall
Well Points Located Behind an Upgradient Slurry Wall,
Cut-Away View
Well Points Located Behind an Upgradient Slurry Wall,
Well Points Located Before an Upgradient Slurry Wall. . . .
Skimmer Systems *
Bottom Key Grouting
Cut-Off Wall-Dike Contact
Rotational Viscometer
Filter Press Test Apparatus .....
Typical Backfill Profile in Trench with . Irregular
Bottom
Typical Slurry Wall Construction Site
Cross-Section of Slurry Trench, Showing Excavation and
Backfilling Operations
P-T__ *-.: nf r^t^nf-inna 1 rasf--in-Place DiaDhraem Wall. . .
3-9
3-10
3-11
3-15
3-16
3-18
3-19
3-20
3-22
3-25
3-30
3-31
4-20
4 71
5-21
5-29
5-34
5-42
X1X1
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LIST OF FIGURES (continued)
Number
- '
5-5 Trench Collapse, Showing Plane of Weakness (a) and
Block Slippage (b)
Page
6-1 Groundwater Pumping to Reduce Hydraulic Head Pressure
on a Slurry Wall
6-2 Wall Breach Due to Localized Chemical Attack ........ 6-11
8-1 Flow Chart of the Evaluation Procedures for a Pollution
Control Slurry Wall ................ 8_2
xiv
-------
LIST OF TABLES
Number ^S6-
2-1 Comparison of Sodium and Calcium-Saturated
Montmorillonites 2~'
2-2 Specified Properties of Bentonite and Cement Bentonite
Slurries 2~14
2-3 Common Slurry Materials and Additives 2-29
2-4 Typical Compositions of Cement Bentonite Slurries 2-38
2-5 Properties of Soil Bentonite and Cement Bentonite
9"*Q
Backfills L Jy
3-1 Summary of Slurry Wall Configurations 3~13
4-1 Types of Physical Constraints and Their Effects on
Slurry Wall Construction 4-2
4-2 Principal Sources of Available Geotechnical Data * 4-8
4-3 Soil Bentonite Permeability Increases Due to Leaching
with Various Pollutants ^-1<
4-4 Increase in the Permeability of Four Brands of Bentonite
Caused by Leaching with Various Pollutants 4-17
5-1 Materials Quality Control Program for SB Walls 5-14
5-2 Comparison of Selected Properties of Clays 5-16
5-3 Common Slurry Properties and Testing Methods 5-18
5-4 Excavation Equipment Used for Slurry Trench
S ^ ft
Construction -> **
5-5 Materials Quality Control Program for Cement/Bentonite
Walls. 5-40
6-1 Potential Problems Related to Slurry Wall Effectiveness
and Possible Associated Monitoring Methods 6-3
xv
-------
LIST OF TABLES (continued)
Page
6-2 Potential Causes for Premature Wall Deterioration and
Associated Maintenance Techniques ............ 6-7
6-3 Possible Restorative Methods for Various Wall Failure
Problems ...................... 6-10
7-1 Estimated Costs for Feasibility Testing - Example Site . . 7-5
7-2 Estimated Costs for Temporary Road Construction -
Example Site _
7-3 Estimated Costs for Site Clearing and Preparation -
Example Site ...................... 7_g
7-4 Estimated Costs for Slurry Wall Installation -
Example Site .................... 7_7
7-5 Estimated Costs for Site Regrading and Revegetating -
Example Site ...................... 7_g
7-6 Estimated Total Costs - Example Site ........... 7-9
7-7 Example Unit Costs for Geologic and Soil Testing -
Example Site ...................... 7-12
7-8 Example Unit Costs for Hydrologic Testing -
Example Site ...................... 7-13
7-9 Example Costs for Slurry Wall Testing ........... 7-14
7-10 Example Unit Costs for Site Clearing ........... 7-16
7-11 Example Unit Costs for Excavation ............. 7-17
7-12 Example Unit Costs for Backfill .............. 7-18
7-13 Example Unit Costs for Borrow ............... 7-19
7-14 Example Unit Costs for Compaction ............. 7-19
7-15 Example Unit Costs for Grading .............. 7-22
7-16 Example Unit Costs for Hauling .............. 7_21
7-17 Example Unit Costs for Mobilization and Demobilization . . 7-23
7-18 Example Costs for Site Dewatering ............. 7-23
xvi
-------
LIST OF TABLES (continued)
Number
7-19
7-20
7-21
7-22
7-23
7-24
7-25
7-26
7-27
Relation of Slurry Cut-Off Wall Costs per Square Foot
as a Function of Medium and Depth
Breakdown of Cost Categories for Cut-Off Trench
Construction
Example Ranges of Unit Costs for Cut-Off Wall
Construction
Example Unit Costs for Monitoring Well and Piezometer
Installation
Example Unit Costs for Slurry Maintenance Activities
Example Unit Costs for Materials
Example Operating and Rental Costs for Earthworking
Equipment
Example Operating and Rental Costs for Concrete and
Mixing Equipment
Example Operating and Rental Costs for General
Construction Equipment
Page
7-24
7-26
7-26
7-27
7-29
7-29
7-30
7-32,
7-33
xvi i
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ACKNOWLEDGEMENTS
This document was prepared by JRB Associates for EPA1s Office of Research
and Development in partial fulfillment of Contract No. 68-03-3113, Task 40-2.
Dr. Walter Grube, of the Municipal Environmental Research Laboratory, Solid
and Hazardous Waste Research Division, was the EPA Project Officer. Philip
Spooner was Task Manager and principal author for JRB. Other major contribu-
tors include Roger Wetzel, Constance Spooner, Claudia Furman, Edward Tokarski,
Gary Hunt, Virginia Hodge, and Thomas Robinson, of JRB. Preparation of this
handbook was aided greatly by the constructive contributions of the following
reviewers:
Herbert Pahren
Douglass Ammon
Jon Herrmann
Richard Stanford
Ann Tate
Dr. David Daniel
S. Paul Miller
Nicholas Cavalli
S. Geoffrey Shallard
George Alther
T. Leo Collins
U.S. EPA MERL
U.S. EPA MERL
U.S. EPA MERL
U.S. EPA OERR
U.S. EPA CERI
Civil Engineer
U.S. Army COE, Waterways Experiment Station
ICOS Corporation of America
Engineered Construction International, Inc.
IMC Corporation
General Electric Company
The technical contributions of the following individuals were greatly
appreciated:
John Ayres
Robert Coneybear
David D'Appolonia
Jeffrey Evans
Donald Hentz
David Lager
Christopher Ryan
Glen Schwartz
Enzo Zoratto
GZA Corporation
Engineered Construction International, Inc.
Engineered Construction International, Inc.
Woodward-Clyde Consultants
Federal Bentonite
Case International Company
Geo-Con, Inc.
Engineered Construction International, Inc.
Engineered Construction International, Incu
Appreciation is also extended to the numerous other individuals from Federal,
State and industry organizations who were contacted on matters related to this
handbook.
xvi 11
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SECTION 1
INTRODUCTION
1.1 Purpose of This Handbook
In recent years, an increased effort has been focused on the problems
caused by the improper land disposal of wastes. The need to clean, up thou-
sands of these disposal sites, and the need to site new, more secure facili-
ties, has resulted in the innovation and adaptation of a wide variety of
engineered measures to waste sites and their remediation. One such engineered
measure is the technique of slurry trenching. By this method, a trench of the
desired configuration is excavated using a bentonite and water slurry to
support the sides. The trench is then backfilled with materials having far
lower permeability than the surrounding ground. The low permeability cut-off,
or slurry wall, has been used as part of the remedial efforts at both hazard-
ous and solid waste disposal sites. This handbook was developed so that the
use of slurry walls for pollution control might be better understood.
This handbook is intended for use by individuals responsible for review-
ing the scientific and technical aspects of slurry walls used for the control
of pollutants. These individuals, from federal, state or local governments,
or from private organizations, may use this handbook to become familiar with
what slurry walls can and cannot be expected to do to help control pollution
migration.
This handbook is not intended to replace the services of a qualified
design engineer, nor is it intended to make inexperienced construction firms
qualified to install slurry walls. Both the design and installation of slurry
walls are as much an art as a science, and the state-of-the-art is evolving
rapidly.
1.1.1 Organization and Use
This handbook is organized primarily to meet the needs of individuals
reviewing the technical aspects of slurry walls included in proposed waste
site remedial action plans. These reviewers will need to become thoroughly
familiar with the entire handbook and probably some of the most commonly cited
references as well. These reviewers may wish to follow the suggested review
procedures given in Section 8 of this handbook, or use the handbook contents
to develop procedures better suited to their own needs.
1-1
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Other users of this handbook may be interested in only certain aspects of
slurry wall use, and can refet to sections on:
Background
Theory
Applications
Related Remedial Measures
Site Characterization
Design and Construction
Monitoring and Maintenance
Major Cost Elements.
These sections cover nearly all aspects of slurry wall use for pollution
control, and show how complex certain of these aspects are. Where differences
of opinion exist on scientific or technical points, they are reported with as
much documentation as possible. Nonetheless, the state-of-the-art in slurry
walls for pollution control is rapidly changing. At this writing,, a committee
of the American Society For Testing and Materials (ASTM) is beginning to
develop new standards for slurry walls which will replace or modify many of
the standards and procedures in use today. Measuring units used in the slurry
wall industry are commonly expressed in the International System of Units
(S.I.), except for hydraulic conductivity which is expressed in cm/sec.
Therefore, S.I. units are used here. A conversion table is included following
Section 8.
1.2 Background
This section provides a brief overview of slurry trench construction;
what it is capable of, what its limitations are, and its history.
1.2.1 Slurry Trench Construction Techniques
As stated earlier, slurry trenching is a means of placing a low permea-
bility, sub-surface, cut-off or wall, near a polluting waste source in order
to capture or contain resulting contamination. ~*These walls are described by
the material used to backfill the slurry trench. Soil-Bentonite (SB) cut-off
walls are composed of soil materials (often the trench spoils) mixed with
small amounts of the bentonite slurry from the trench. Cement-bentonite (CB)
cut-off walls are excavated using a slurry of Portland Cement and bentonite
which is left to set, or harden, to form the final wall. Diaphragm walls are
composed of pre-cast or cast-in-place reinforced concrete panels (diaphragms)
installed using slurry trenching. Each of these, as well as hybrids of the
three, has different characteristics and applications.
1-2
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In general, SB walls can be expected to have the lowest permeability, the
widest range of waste compatibilities, and the least cost. They also offer
the least structural strength (highest compressibility), usually require the
largest work area, and are restricted to relatively flat topography.
Cement-Bentonite cut-off walls can be installed at sites where there is
insufficient work area to mix and place soil-bentonite backfill, and, by
allowing wall sections to harden and then continuing the wall at a higher or
lower elevation, are adaptable to more extreme topography. Although CB walls
are stronger than SB walls, they are at least an order of magnitude more
permeable, resistant to*fewer chemicals, and more costly.
Diaphragm walls are structurally the strongest of the three types as
well as the most costly. Provided the joints between panels are installed
correctly, diaphragm walls have about the same permeability as cement-
bentonite and because of a similarity of materials, about the same chemical
compatibilities. Because of the higher expense and higher permeability of
diaphragm walls, they are seldom used for pollution control.
Combinations of these three major backfill types may be included within
the same wall. For example, a soil-bentonite backfill may be used for the
majority of a wall, with cement-bentonite being used for a portion, such as a
road or rail crossing, that requires greater strength. Being able to combine
the various types of walls makes this technique adaptable to a wider range of
site characteristics.
Depending on the situation in which they are employed, slurry walls may
be keyed into an underlying, low permeability zone, such as a clay layer or
bedrock, or, in the case of floating contaminants, may be only deep enough to
intercept the upper few feet of the water table. These "hanging" slurry walls
have been used to capture and recover floating petroleum products at several
locations.
A number of other construction techniques can be used with slurry walls
to widen their range of applicability. Among these are grouts, used to help
key the wall into fractured bedrock; and sheet piles, used to protect the wall
from stream erosion. Another technique involves placing a synthetic membrane
within a cement-bentonite wall to lower its permeability and increase its
resistance to attack by certain chemicals. This is a newly developed
technique and is not yet documented in the literature. Also, no cost
information is yet available.
1.2.2 History of Slurry Trench Construction
Slurry trench construction originated over 30 years ago in Italy and the
United States. This technique is now in use throughout the world to meet a
variety of engineering needs.
1-3
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1.2.2.1 Technique Development
Slurry trench construction developed out of the use of slurries and muds
in oil well drilling operations. The early 1900"s marked the first use of
clay mud suspensions in the drilling of oil wells (Nash 1976). The next 20 to
30 years involved investigations of slurry properties, such as thixotropy, and
experimentation with additives to control the viscosity of drilling muds
(Xanthakos 1979). In 1929, bentonite clays were first used in drilling opera-
tions to stabilize deep wells in unconsolidated materials and to bring
cuttings to the surface (Nash 1976).
In the late 1930's, Veder, in Milan, Italy, developed the concept of a
continuous diaphragm wall constructed in a slurry-supported trench (U.S. Army
Corps of Engineers 1978). This concept evolved from a combination of two
systems already in use; the mud-filled borehole, and the continuous bored-pile
wall (Xanthakos 1979). By the late 1940's, Veder tested structural slurry
trenches and used slurry trenches in construction of the Milan subway (U.S.
Army Corps of Engineers 1978, Winter 1976).
The U.S. Corps of Engineers also used slurry trench construction in the
late 1940's. At Terminal Island in California, the Corps constructed a slurry
trench backfilled with plastic material to control salt water intrusion into a
freshwater zone (Nash 1974). The Corps also installed slurry trench cut-offs
under Mississippi River levees for control of underseepage and piping (U.S.
Army Corps of Engineers 1978).
The 1950's marked a period of continuous development and improvement of
the slurry trench technique. These activities were accompanied by laboratory
research into the supporting properties of bentonite in excavations (Veder
1963). In the early 1950's, concrete diaphragm walls were installed at dams
in Italy to control seepage flow and support vertical loads (U.S. Army Corps
of Engineers 1978). In the United States, a soil backfilled cutoff trench was
constructed beneath the Wanapum Dam in 1959; this represented the first use of
this technique in the United States for seepage control at a major dam (Meier
1978, Jones 1978, Wilson and Squier 1969).
By the mid-19601s, slurry trench cutoffs had become an established method
for use in earth dam construction as an alternative to traditional foundation
methods. A major improvement in slurry trench cut-off construction occurred
in 1969 with the development of a self-hardening slurry. This slurry is
termed "coulis" and consists of cement, bentonite and water. It is used both
as a stabilizing fluid for trench construction and as the cut-off wall. The
slurry hardens in place to form a continuous, jointless wall (Soletanche
1977).
1.2.2.2 Applications
Early applications of diaphragm walls were as impermeable barriers below
earth dams in sand and gravel, and water barriers to make reservoirs
1-4
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watertight (Veder 1963). Slurry trench methods were first adopted in pile and
caisson construction and eventually led to the development of cast-in-place
continuous concrete diaphragm walls (Soletanche 1977).
The use of slurry trench techniques developed in different directions in
Europe and the United States. Primary uses of slurry trench techniques in
Europe were the construction of structural walls and load bearing foundations
(U.S. Army Corps, of Engineers 1978). Additionally, European slurry trench
methods generally involved cement-bentonite cut-off walls (Miller and Salzman
1980, Sommerer and Kitchens 1980). In the United States, slurry trench appli-
cations have been oriented toward earth-filled cut-offs for seepage control
and dewatering purposes (U.S. Army Corps of Engineers 1978).
In many cases, slurry walls were initially used as temporary structures
with the permanent structure built inside (Regan 1980). Diaphragm walls were
considered dangerous as permanent structures because of their relative
rigidity, susceptibility to cracking, and ability to induce cracking of earth-
fill due to differential settlement. However, the development of procedures
and materials to construct jointless, continuous diaphragm walls that are
impermeable and have physical characteristics compatible with earth materials
has led to the use of slurry walls as permanent structures (Soletanche 1977).
There are many applications for use of slurry walls for seepage control
and for groundwater diversion during site dewatering. Present applications of
slurry trench construction include:
Retaining structures
Load-bearing elements
Underground facilities, transit stations, tunnels, etc.
Docks and waterfront installations
Cut-offs under dams
Repair of leaky dams
Pollution migration control.
For pollution control, slurry trenches have been constructed to control
sewage, acid mine wastes, chemical wastes and sanitary landfill leachate (Ryan
1980a). Slurry cut-off walls have also been constructed to control the
lateral movement of oily wastes. As of 1980, approximately 10 slurry cut-off
walls have been constructed as spill control barriers (Ryan 1980b).
1.3 Limitations
Slurry walls have been installed to retard the movement of groundwater
and leachate at numerous waste sites. These walls have been constructed at
sites having widely divergent geologic, hydrologic, climatic and demographic
characteristics. Slurry cut-off walls are applicable to numerous situations
1-5
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involving hazardous wastes. They are not, however, an answer to all waste
site problems.
Slurry cut-off walls are not impermeable, and some leakage through them
is inevitable. Permeability values range from less than 10 cm/sec for a
well designed SB wall, to over 10 cm/sec for a CB wall. For this reason, and
the fact that they are usually less costly, SB walls are the most commonly
applied to waste site remediation.
A second limitation to the use of slurry cut-off walls is that exposure
to the wastes at some sites may cause increases in wall permeability. Wall
failure may occur if the slurry wall is not designed or installed well enough
to withstand exposure to the chemical constituents of the permeating solutions
or the hydraulic gradients at the site. Certain chemicals have been shown to
have pronounced effects on both bentonite and Portland cement, and even brief
exposure of some walls to high strength leachates can seriously threaten their
integrity.
The use of slurry walls is also limited by the need for heavy construc-
tion equipment, sufficient maneuvering area and suitable access. At some
disposal sites, the degree of complexity in the design and installation of a
slurry wall caused by site conditions may reduce its viability as a remedial
alternative. For example, if a disposal site was located in a congested urban
area, the cost of the added design and construction effort needed to deal with
nearby cultural features or other obstruction, could make some other alterna-
tive, such as excavation and secure reburial, more attractive.
Slurry walls will seldom, if ever, be the only remedial measure applied
to a site. They are usually accompanied by other measures, such as surface
sealing, or drains and collectors, as part of an overall engineered solution
to the site's problems. Some of these measures can extend the effectiveness
of the slurry wall beyond what it would be without them. For example, if a
waste site were to be surrounded by a slurry wall, and the site dewatered by
capping and pumping, the net flow of groundwater would be toward the interior
of the wall. In this way, some waste/wall compatibility problems can be over-
come because the wall is being permeated with groundwater and not leachate.
The amount of leachate in. the enclosed area is greatly reduced, and the life
expectancy of the wall is increased. Also, an extraction well or drain net-
work can act as a back-up containment measure if, for some reason, the wall is
breached.
Most of the slurry walls that have been installed for pollution control
have been in the private sector, and the majority have been in place for a
relatively short time. In most cases, the firms for which these walls have
been installed are not willing to provide the monitoring data that are needed
to evaluate the performance of pollution control slurry walls. Slurry cut-off
walls used in other applications, such as dam projects, have yielded enough
data to evaluate short and long term geotechnical performance. Long term
performance of the walls in the presence of chemical contaminants, however, is
not as well documented. The best indications of the ability of slurry walls
to withstand chemical degradation over time comes from laboratory studies.
1-6
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These studies have begun to better define the range of chemical compati-
bilities but have not, and may never, replace the need for extensive, site
specific testing and long term monitoring.
1.4 Summary
This handbook is intended to provide reviewers of remedial actions with
the means of evaluating technical aspects of slurry walls. Although these
groundwater cut-off barriers have been in use for decades, the last several
years have seen a rapid increase in their use for pollution control. Although
slurry walls are versatile and adaptable control measures, they are not suited
to all waste sites or waste types.
1-7
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SECTION 2
THEORY OF SLURRY AND BACKFILL FUNCTION
During construction of slurry walls, a slurry containing bentonite is
placed in the open trench to support the trench walls. After excavation is
completed, a mixture of bentonite slurry and soil, or a mixture of cement,
bentonite, and water is placed in the trench to form the completed wall. To
become familiar with slurry trench construction techniques, it is helpful to
understand several key theoretical considerations. These can best be pre-
sented in the five questions listed below.
Why is bentonite used in slurries and cut-off walls, and how can
bentonite1s behavior in slurries be explained?
What factors affect bentonite slurry properties?
What factors affect soil-bentonite wall performance?
What factors affect cement bentonite slurry properties?
What factors affect cement-bentonite wall performance?
Each of these questions is addressed in this section. The theoretical
aspects of slurry functioning and slurry wall performance are emphasized. The
practical applications of these theoretical aspects are presented in the
Design portion of Section 5.
2.1 Bentonite
Bentonite is a soft, soapy-feeling rock found in commercial quantities in
several areas of the United States. The rock is composed primarily of the
clay mineral montraorillonite or smectite, as it is frequently called, with
about 10 percent impurities, such as iron oxides and native sediments (Boyes
1975). Finely ground bentonite is mixed with water to form the slurry that is
kept in the trench during excavation. Although there have been attempts to
use locally available native clays in place of commercial bentonites, the
evidence presented below illustrates why these attempts have not met with
great success.
2-1
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2.1.1 Rationale for Bentonite Use
The bentonite performs two functions when used in slurry trench construc-
tion. First, it coats the sides of the trench with a thin, slippery layer
called a filter cake. This low-permeability layer minimizes slurry seepage
out of the trench and groundwater seepage into the trench. It also forms a
plane against which the weight of the slurry can push against the trench
sides. The lateral pressure of the slurry against the filter cake on the
trench walls holds the trench open. Thus, the first function of the bentonite
in the slurry is to form the filter cake.
The second function of the bentonite is to maintain slurry density. The
bentonite particles must not settle out of the water and the slurry must hold
in suspension small particles of soil that inadvertantly fall into the slurry
during excavation. The density of the bentonite is only slightly higher than
that of the water. The density of the trench spoils is, however, much higher.
When the slurry holds particles of trench spoil in suspension, the density of
the slurry is increased. The reason the slurry must be denser than water is
that a higher density slurry pushes against the trench walls with greater
force and assists in maintaining trench stability, particularly where
groundwater levels are high (Xanthakos 1979).
2.1.2 Bentonite Properties
The major properties of bentonite that are of interest in slurry trench
construction are:
Swelling and hydration
Extensive dispersion
Thixotropy.
These properties are expressed when bentonite comes in contact with water.
2.1.2.1 Swelling and Hydration
When finely ground bentonite is mixed with water, both the exterior and
interior of the particles become wetted. Water becomes attached to the sur-
faces of the clay particles in the bentonite through electrochemical inter-
actions that will be described in detail later in this section. Water also
penetrates the interior of the clay particles and forces each clay particle to
expand in volume, or swell. In addition, the cations that are associated with
the clay particles become hydrated. Thus the water reacts with both the
exterior and interior clay surfaces as well as the associated cations. As a
result, the bentonite increases in volume. Dry bentonite can swell as much as
10 to 12 times its original volume when wetted (Case 1982). This swelling
2-2
-------
continues until the bentonite is fully hydrated, which can take as long as a
full week (Boyes 1975).
2.1.2.2 Dispersion
The surfaces of the clay particles in bentonite are predominately
negatively charged. When two of these clay surfaces are in close proximity to
one another, they repel each other due to long-range coulombic forces (Mustafa
1979). The causes of this repulsion will be discussed in Section 2.1.3.1.
The effect of this repulsion is that the clay particles remain for the most
part dispersed throughout the slurry. This dispersion allows the intimate
mixture of bentonite and water to be maintained.
2.1.2.3 Thixotropy
When a mixture containing 5 percent by weight bentonite and 95 percent
water is allowed to stand undisturbed for a few minutes, it changes from a
viscous solution to a gel-like substance. When agitated or vibrated, the gel
reverts to a slurry. The gel will reform each time the agitation ceases.
This behavior is the result of a property called thixotropy.
Thixotropy is important in slurry trench construction because the gel
structure is what keeps the particles of trench spoils in suspension in the
slurry.
Thixotropy is measured by determining how strong of a gel structure is
formed over a set period of time. As the strength of the gel structure
increases and the speed of gel formation increases, the degree of thixotropy
is said to increase. The strength of the gel structure (called the gel
strength) is measured using a Fann Viscometer. Measurements are taken at 10
seconds and 10 minutes. In a high quality bentonite, the 10-minute gel
strength should be only slightly higher than the 10 second gel strength
(Boyes 1975).
2.1.3 Factors Affecting Bentonite Performance
Because bentonite is a natural, rather than manmade substance, its
quality, and therefore its performance, is likely to vary from deposit to
deposit. Several factors influence the performance of bentonites in slurry
trench construction. These factors include:
Montraorillonite content and properties
Relative sodium and calcium concentrations
2-3
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Fineness of grinding of the raw material
Chemical additives.
2.1.3.1 Montmorillonite Content
As mentioned previously, bentonite contains about 90 percent montraoril-
lonite and 10 percent impurities (Boyes 1975). Montmorillonite, or smectite,
is the crystalline material that gives bentonite its unique properties. To
understand the behavior of this mineral, it is necessary to know its general
structure and some of the interactions between montmorillonite crystals, water
molecules, and cations. A description of montmorillonite structure is given
below, followed by a detailed discussion of clay-water and clay-cation
interactions as they affect the physical properties of raontmorillonite.
a. Montmorillonite Crystal Structure
Crystals of this clay are composed of three distinct layers, as shown in
Figure 2-1. The outer layers are a tetrahedral arrangement of silicon and
oxygen molecules. Some of the silicon atoms in these layers have been
replaced by aluminum. Sandwiched between the silica layers is a layer of
aluminum atoms surrounded by six hydroxyl or oxygen atoms in an octahedral
shape. Some of the aluminum atoms in this layer have been replaced by
magnesium. Because of the substitutions in the three layers, unsatisfied
bonds exist within the crystal, resulting in a high net negative charge. To
satisfy this charge, cations and water molecules are adsorbed onto the
internal and external surfaces of the clay crystals. These surfaces comprise
the exchange complex of *the clay. The types of cations adsorbed on the
exchange complex have a great influence on the properties of the clay (Brady
1974).
The characteristics of bentonite slurries are caused to a large extent by
the properties of the montmorillonite they contain. As described previously,
three sets of properties are particularly relevant to slurry function. These
are:
Degree of hydration and swelling
Flocculation and dispersion characteristics
Gel strength and thixotropy.
The extent to which these montmorillonite properties are expressed varies
considerably, depending on the types of cations adsorbed to the surface of the
clay. Although numerous cations and organic molecules can be adsorbed, two
cations are of primary interest in slurry trenching situations. These are
sodium and calcium.
2-4
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Figure 2-1.
Montmorillonite Crystal Lattice, Showing Adsorbed Cations
and Oriented Water Molecules
Outer Limit of Adsorbed
Water Surrounding
Other Clay Crystals
Oriented
Water
Molecules
Cloud of Additional
Water Molecules
Less Oriented
Than Those Directly
Contacting the Crystal
Cations
Note: Not to Scale
Source: Based on Grim, 1968
2-5
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Sodium-saturated montmorillonites behave quite differently than the
calcium-saturated varieties. These differences are summarized in Table 2-1.
Theories governing the reasons for these differences are described in detail
below.
b. Theory of Clay Hydration and Swelling
During hydration of montmorillonite, water molecules are adsorbed to the
clay crystal surface by the attraction between the hydrogen atoms on the water
molecules and the hydroxyls or oxygens on the outer clay surface and in
between the silicate layers. This is illustrated in Figure 2-1. The adsorbed
water is held so strongly by the clay that it may be thought of as a non-
liquid, or a semi-crystalline substance. Even the water molecules that do not
directly contact the clay surface are influenced by the montmorillonite
crystals. This is because the water molecules that are bonded to the clay
surface form partially covalent bonds with a second layer of molecules. In
addition, the second layer of water molecules forms partially covalent bonds
with a third layer, which bonds to a fourth layer, and so on. The water in
these layers surrounding the crystal surface is oriented, forming what may be
thought of as a semi-rigid structure (Grim 1968).
The number of layers of water molecules and the regularity of their
configuration is dependent upon the types and concentrations of cations
associated with the clay. The cations tend to disrupt water adsorption, and
the degree of disruption depends on the size of the hydrated cation, its
valence, and its tendency to disassociate with the clay surface during
hydration (Grim 1968).
Sodium ions disrupt hydration much less than calcium ions. For example,
sodium-saturated montmorillonites have been found to influence the orientation
of water molecules more than 100 Angstroms from their crystal faces. This
corresponds to about 40 molecular layers of water. In contrast, calcium-
saturated montmorillonites have much smaller spheres of influence, on the
order of 15 Angstroms, or about 6 molecular layers of water (Grim 1968).
The observable effects of these sub-microscopic interactions are that
sodium montmorillonites adsorb much more water and swell far more than do
calcium montmorillonites. As a result, as the amount of sodium on the
exchange complex of montmorillonite increases, the amount of swelling
increases (Rowell, Payne and Ahmad 1969). In addition, a 5 percent solution
of highly hydrated sodium montmorillonite has a much higher viscosity than a
5 percent calcium montmorillonite solution. In fact, a 5 percent solution of
sodium bentonite in water can exhibit a viscosity of 15 centipoise, but it
takes 12 percent calcium montmorillonite in a solution to obtain the same
viscosity (Grim and Guven 1978). This is illustrated in Figure 2-2.
2-6
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TABLE 2-1
COMPARISON OF SODIUM AND CALCIUM-SATURATED MONTMORILLONITES
Parameter
Sodium-Saturated
Montmor i11on i t e
Calcium-Saturated
Montmorillonite
Swelling upon hydration,
cm /g of clay
Hydration rate, 5%
solution (2)
Cation exchange
Capacity, meq/lOOg.
Degree of thixotropy
Liquid limit
Plastic Limit (4)
Yield in barrels of 15cP
drilling mud per ton
of clay (4)
Percentage of clay by
weight in water to
produce a 15cP
colloidal suspension (4)
11 (1)
(Wyoming sodium
bentonite)
Hydrated to~9cP
in 10 min., stabilized
at 9.2cP by 20 min.
3% solution of polymer
treated sodium bentonite
hydrated to 17.2cP in
10 min., then stabilized,
80-150 (3)
high (2)
300-700 (4, 5)
75-97
125
2.5 (1)
(4 base-exchanged
bentonites tested)*
hydrated to ^13cP in
10 min., stabilized
at ~14 to 18 cP in
4 hours.*
60-100 (2)
low (6)
155-177 (4)
65-90
18-71
2-7
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TABLE 2-1
COMPARISON OF SODIUM AND CALCIUM-SATURATED MONTMORILLONITES
Sodium-Saturated Calcium-Saturated
Parameter Montmorillonite Montmorillonite
Permeability of a 9:3
quartz to clay mixture- 2.76 x 10 7.2 x 10~7
(cm/sec) (4)
Permeability of a 7:3
lartz to clay
(cm/sec) (4)
If) - R
quartz to clay mixture 5.0 x 10 3.5 x 10
*Base-exchanged bentonites are calcium bentonites that have been treated with
sodium compounds to increase their adsorbed sodium content. They are
commonly used in European slurry trenching construction (Boyes 1975).
References: (1) Baver, Gardner and Gardner 1972, (2) Boyes 1975, (3) Grim
1968 (4) Grim and Guven 1978, (5) Xanthakos 1979, (6) Case 1982.
2-8
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Figure 2-2.
Viscosity and Weight of Mud in Relation to Percentage of Bentonites
and Native Clays in Fresh Water
8.58.6 8.8 9.0 9.2 9.4 9.6 98 10.0 10.2 TO*
Attapulgite
Sodium
montmorillonite
Calcium
montmoriHonite
Range of Typical
Native Clays
M 30 40 go 93 TO 80 90 100 110
Pounds of Clay per Barrel*
15 20 & *>
Percentage of Day by Weight*
200 100 65 50 40 35 30 25 20 18 « 15
Yield in Barrels of Mud per Ton of
Clay Specific Gravity Assumed to be 2.50.
Copyright 1978 by Eisevier Scientific Publishing Company. Used with Permission
2-9
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c. Theory of Flocculation and Dispers
ion
;
are attracted to the cations
of anioas that
reduced, face-to-face contact ca
." (See
can ocur and
' "
s
.
-bf ween
Particles can form "packets,"
rst
.«
d. Theory of Gelation and Thixotropy
suspension to
" "' 'bility °f the
2-10
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Figure 2-3.
Bentonite Particles During Hydration, Gelation, Flocculation,
and Dispersion
Dry
Flocculation
Partly Hydrated
Breakdown of Get Structure
as a Result of Agitation
Dispersion Caused by Phosphates
Replacing Positive End Charges
with Negative Ones
Source: Boyes, London 1975
-------
a "house of cards" structure between positively charged clay particle edges
and negatively charged clay faces, as illustrated in Figure 2-3 (Xanthakos
1979). In practice, the gelation of the bentonite slurry provides support for
small particles of soil to remain in suspension rather than to sink to the
trench bottom (Boyes 1975).
'The amount of thixotropy is determined by measuring the gel strength of
the slurry. The gel strength is "the stress required to break up the gel
structure formed by thixotropic buildup under static conditions" (Boyes 1975).
It is measured using a Fann viscometer, as described in Section 4. The
difference between the gel strength 10 seconds after agitation and the gel
strength after standing for 10 minutes is a measure of the slurry's thixotropy
(Xanthakos 1979).
Measurements of 10 minute gel strengths of bentonite slurries can range
from about 5 to 20 Ib/ft and average 10 to 15 Ib/ft (Xanthakos 1979).
The bentonites used during slurry trench construction behave essentially
like the sodium saturated raontmorillonites described above. The properties of
hydration, flocculation, dispersion and gel strength that are exhibited by the
slurries are a result of the interactions of montmorillonite crystals, water
molecules, and cations. The ability of a bentonite slurry to perform its
functions during slurry trench construction is dependent on these
interactions.
2.1.3.2 Relative Sodium and Calcium Concentrations
Natural sodium bentonite from Wyoming is commonly used in many of the
slurry trenching operations in the United States. These bentonites do not
contain pure sodium montmorillonite. One bentonite was reported to contain
60 percent sodium on its exchange complex, with the remaining sites being held
by calcium and magnesium. However, the average distribution of cations on
Wyoming bentonite is somewhat different. Most of the Wyoming bentonite
currently being sold contains an average of 38 to 50 percent sodium, 15 to
35 percent calcium and 10 to 30 percent magnesium (Alther 1983).
High sodium bentonites should be more effective than the low sodium
grades in many situations. At sites where a high concentration of calcium
salts occurs in the soil or groundwater, or where cement bentonite slurries
will be used, higher sodium bentonites are particularly recommended, for the
reasons described below. The detrimental influence of the calcium from the
cement or the groundwater on the sodium bentonite can be substantial. This is
due to the strong attraction between calcium ions and montmorillonite
crystals. Because this attraction is so strong, calcium ions can easily
displace sodium ions on the clay. The ease of replacement of sodium by
calcium increases as the concentration of calcium in the solution and on the
clay surface increases. After about 30 percent of the exchange sites on the
clay surface become occupied by calcium, the bentonite acts more like calcium
montmorillonite than the sodium variety (Grim 1968).
2-12
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Because there are limited quantities of natural sodium bentonites, some
areas are forced to use specially treated calcium bentonites instead. This
occurs most frequently in Europe. These calcium bentonites are exposed to
sodium-containing materials such as sodium hydroxide to force some of the
calcium ions off of the exchange complex of the montmorillonite and then
replace them with sodium ions (Grim 1968). Sodium carbonate, which is less
expensive and more effective than sodium hydroxide, is also used on some
bentonites (Alther 1983). As long as there is less than 30 percent calcium
and at least 50 percent sodium on the exchange complex of the montmorillonite,
the material will act essentially like a sodium montraorillonite (Grim 1968;
Shainberg and Caiserman 1971).
2.1.3.3 Bentonite Particle Size
This purely physical parameter can influence the performance of the
bentonite in a number of ways. Finely ground bentonite has a larger surface
area per unit weight than coarser bentonite because as particle size
decreases, surface area per unit weight increases. The increased surface area
of the finer particles allows the bentonite to hydrate more readily and form a
gel structure more quickly than coarser particles of the same bentonite. Thus
the average particle size of the bentonite can affect its performance in the
slurry. Typically, the types of bentonite that are recommended for slurry
trenching have been pulverized to yield particles small enough so that 80
percent will pass through a number 200 mesh sieve (Federal Bentonite 1981).
2.2 Bentonite Slurries'
The Wyoming bentonites most commonly used in slurries are mixed at a rate
of from 4 to 7 percent bentonite in 93 to 96 percent water (Boyes 1975). This
muddy mixture stabilizes the sidewalks of the open trench during excavation.
The properties of a well-functioning slurry and the factors that affect
bentonite slurry quality are discussed below.
2.2.1 Bentonite Slurry Properties
To maintain trench stability while exhibiting suitable flow character-
istics, the slurry must have the proper viscosity, gel strength and density.
It must form a thin, tough, low-permeability filter cake rapidly and
repeatedly. The bentonite slurry supplied to the trench may meet or exceed
the quality standards stated in the specifications, however, slurry
properties are altered during trench excavation and slurry quality may either
improve or degrade during use. Table 2-2 presents data on fresh and in-trench
slurries. As shown in this table, the density, viscosity, gel strength, and
solids content of the slurry generally increases during excavation, while the
overall water content decreases, due to the increased solids content. Brief
2-13
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TABLE 2-2.
SPECIFIED PROPERTIES OF BENTONITE AND CEMENT BENTONITE SLURRIES
S)
I
Parameter
3
Density (g/cra )
(p.c.f.)
Viscosity, apparent
(Seconds Marsh)
(centipose)
Viscosity, plastic
Filtrate Loss, ml
pH
Water Content, %
by weight
Bentonite Content, %
by weight
Other Ingredients, %
by weight
Gel Strengths
10 seconds, Pascal
10 minutes, Pascal
10 minutes ,
lb/100 ft2
(24-72 dynes/era )
Bentonite
Fresh-Hydrated
1.01-1.04 (1,2)
65 (3)
38-45 (1,5)
-15
<20* (7)
<30 (7)
range 15-30 (3)
7.5 to 12 (6)
-93-97 (6)
4-7 (6)
sand~130 (7)
30-50 (7)
55-70 (7)
6 (7)
30-45 (7)
10 (7)
22 (7)
*SpecTHcation for construction of tremie concrete diaphragm walls.
References: (1) Case 1982, (2) Xanthakos 1979, (3) Millet and Perez 1981, (4) US Army Corps of Engineers 1976;
(5) Guertin and McTigue 1982b, (6) Boyes 1975, (7) Jefferis 1981a, (8) Ryan 1976.
-------
descriptions of slurry viscosity, thixotropy (gel strength) and density are
given below.
2.2.1.1 Viscosity
The viscosity of a slurry must be maintained at a level high enough to
assist in stabilizing the trench walls, but low enough to avoid interfering
with trench excavation (Ryan 1976). Viscosity is a term used to describe a
fluid's resistance to flow. It is caused by interparticle attraction
(cohesion) and inter-particle friction (Millet & Perez 1981). In bentonite
slurries, about 80 percent of the viscosity is due to the attraction between
montmorillonite crystal edges and faces. The remaining 20 percent is due to
friction (Grim and Guven 1978). Ideally, a fresh bentonite slurry should have
a viscosity equivalent to 40 seconds, as measured on a Marsh cone (D'Appolonia
1980).
Marsh cone readings, which are used to measure slurry viscosity, do not
really measure viscosity. Instead they measure a series of interrelated
properties including density, viscosity, and shear strength (Hutchison et.
al., 1975). The Marsh cone viscometer consists of a standard size funnel. To
measure slurry viscosity, 1 U.S. quart (946 cm ) of slurry is placed in the
funnel. The time taken for this quantity to flow through the funnel is the
viscosity in Marsh seconds. This test indicates the response of the slurry to
conditions found in the trench. For example, Marsh cone readings less than
40 seconds indicate a slurry that has poor filter cake formation and
insufficient trench-supporting ability (D'Appolonia 1980a). Marsh cone
readings also indicate the workability of the slurry. If too high, the slurry
can become too dense and difficult to work with. If too low, trench wall
stability may suffer (Ryan 1976). D'Appolonia (1980a) found that slurry
viscosity has a direct influence on filter cake permeability and is one of the
most crucial slurry characteristics. The relationship between viscosity and
filter cake permeability is shown in Figure 2-4.
2.2.1.2 Gel Strength
The gel strength represents the shear strength of the slurry when it is
not agitated. It is caused by the edge-to-face linkage of clay crystals in
the slurry. Gel strengths are measured using a Fann rotational viscometer, as
described in Section 4. Typical values average around 15 Ib/ft (Xanthakos
1979).
Thixotropy, which is measured as gel strength, is in essence, the shear
strength of the slurry. Stated differently, it is the resistance of the
solids in the slurry to movement, or shearing (Baver, Gardner and Gardner
1972).
It is evident that viscosity and gel strength are ways of measuring the
resistance to movement in liquids and solids, respectively. Because a
2-15
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Figure 2-4.
Relationship between Filter Cake Permeability
and Slurry Viscosity
30
20
10
I
Water
Premium Grade Bentonite
16 Hours = Cake Formation Time
Hydraulic Pressure From 30 Feet Water = Cake
Formation Pressure (Formation Head)
30
40 50
Slurry Viscosity, sec-marsh
60
80
Source: D'Appolonia (1980)
2-16
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bentonite slurry is a thixotropic suspension, it displays properties of both
solids and liquids, thus the consideration of both slurry properties is
appropriate.
2.2.1.3 Density
A fresh slurry of 4-8% bentonite is.only slightly denser than water,
averaging less than 65 Ib/ft (1.04 g/cm ) (Case 1982, Xanthakos 1979). As
particles of excavated materials fall into the trench, they become suspended
in the slurry and cause the slurry's solids content and density to increase.
Densities may increase to 85 Ib/ft (1.34 g/cm ) or higher when excavating in
sandy soils (Shallard 1983).
2.2.1.4 Filter Cakes
The filter cakes that are formed on both trench wall are extremely
important both during excavation and possibly after backfilling as well.
formation, function and desirable characteristics of filter cakes are
described below.
The
a. Filter Cake Formation and Function
When trench excavation is initiated, the slurry is pumped into the trench
to maintain a slurry level at or near the initial ground level (Millet and
Perez 1981). As the slurry is introduced into the trench, it flows into pores
in the strata through which the trench is cut. Leakage of slurry into these
voids continues until the flat clay particles in the slurry begin to
accumulate in layers, which grow large enough to bridge the gaps between the
soil particles, or until gelation of the slurry within the pores occurs.
Figure 2-5 illustrates the relatively rapid initial slurry loss, followed by a
reduction in the rate of loss. This reduction is caused by the formation of a
layer of clay particles "plastered" on the trench sides, which reduces lateral
liquid flow out of the trench and into the adjacent soil. This layer is
called a filter cake.
The filter cake is a thin glue-like membrane composed of closely packed
bentonite particles (Case 1982). The solids content of the newly formed
filter cake ranges from 10 to 50 percent, with higher solids contents found in
filter cakes from calcium than from sodium bentonites (Grim and Guven 1978).
The filter cake from a slurry containing 5 percent bentonite typically
contains 15 percent bentonite (Hutchinson et al 1975). During active
excavation, the filter cake is usually less than 3 millimeters thick. This
thin layer of clay is, however, an effective barrier to water movement as the
permeability of the filter cake can be as low as 10~9 cm/sec (Xanthakos 1979).
2-17
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Figure 2-5.
Fluid Loss During Filter Cake Formation
T_ = Time for Initial Cake Formation
Source: Hutchinson, etal. 1975
2-18
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Formation of the filter cake is of critical importance in slurry trench
construction. This membrane performs numerous functions, including:
Minimization of slurry loss into surrounding soils
* Stabilization of the soil that is in contact with the slurry by
gelling in the soil pores and by plastering the particles against the
trench walls
Providing a plane on each trench wall against which the hydraulic
pressure and dead weight of the slurry can act to stabilize the
excavation.
b. Desirable Filter Cake Characteristics
Desirable characteristics of filter cakes include rapid formation and re-
formation when necessary, resistance to shearing, and low permeability.
Experience has shown that a thin filter cake is an indication of a tough,
impermeable membrane, but a thick "flabby" filter cake is likely to allow high
fluid losses (Boyes 1975).
Filter cakes must be formed rapidly when the initial soil contact occurs,
in order to avoid excessive slurry losses. These membranes must resist
mechanical disruption by the backhoe bucket or clamshell during trench
excavation. If inadvertently scraped off the trench wall during excavation,
the slurry must be of such composition as to allow rapid formation of a new
filter cake to avoid possible collapse of the trench.
A high gel strength is desirable in the filter cake because the gel
structure contributes to shear strength, and the filter cake must resist
shearing forces from both the excavation equipment agitated slurry and the
soil comprising the trench wall. In addition, high gel strength indicates
rapid formation of a gel structure. When the slurry penetrates soil pores,
rapid gelation assists in restricting further slurry flow and thus minimizes
slurry losses (Xanthakos 1979).
A final desirable filter cake characteristic is low permeability.
movement of water through the filter cake should be minimized to:
The
Avoid wetting and thus softening and lubricating unstable layers that
may be present in the soil surrounding the trench (Boyes 1975),
Avoid increasing pore water pressure, because this increases the total
stress on the system and reduces the angle of friction in the soil
surrounding the trench (Hutchinson et al. 1975)
Maintain the slurry level in the trench well above the groundwater
level. This sustains the thrust of the slurry on the trench side
walls by restricting the pressure losses due to filter cake leakage
(Xanthakos 1979).
2-19
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2.2.1.5 Resistance to Flocculation
In addition to having the proper viscosity, gel strength, density, and
filter cake characteristics, a slurry should also have a certain level of
flocculation resistance.
As noted earlier, flocculation is undesirable in slurries because as the
clay particles form clumps, their effective hydrated diameters are greatly
reduced. This increases the size and number of the voids available for water
movement, which increases the permeability of the system (Shainberg and
Caiserman 1971). Using bentonites that contain a high concentration of sodium
or that are chemically treated to resist flocculation can help reduce the
likelihood of permeability increases due to flocculation.
Sodium bentonites resist flocculation more effectively than calcium
bentonites because the sodium bentonites swell more extensively than the
calcium-saturated types. Montmorillonites that have 50 percent of their
exchange complex occupied by sodium ions act essentially like pure sodium
montmorillonites (Shainberg and Caiserman 1971). However, as the sodium
content of the clay decreases, the permeability increases and swelling
decreases proportionately (Rowell, Payne and Ahmad 1969). After replacement
of sodium by calcium on the exchange complex of the clay begins, it can
continue until nearly complete replacement has occurred. In addition, the
sodium tends to become more easily displaced as the amount of sodium in the
clay decreases (Grim 1968). Thus, a large sodium concentration in the clay is
desirable to aid in resistance to flocculation.
2.2.2 Factors Affecting Bentonite Slurry Performance
Numerous factors affect the performance of bentonite slurries. Trench
designers should be aware of these factors to maximize performance and
minimize problems and unnecessary expenditures. The theoretical aspects of
slurry performance are given in this section. The practical applications of
these considerations are discussed in Section 5.
Among the factors that affect slurry performance are:
Filter cake performance
Gel strength
Density
Use of chemical of physical additives.
Each of these factors is described below.
2-20
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2.2.2.1 Filter Cake Performance
The efficiency of the filter cake in performing its functions depends on
numerous factors. These include:
Characteristics of the slurry
Characteristics of the strata surrounding the trench
Time allotted for filter cake formation
Hydraulic gradient between the slurry and the groundwater
Presence of contaminants in the spoils or groundwater.
Bentonite quantity and quality strongly affects filter cake functioning.
Factors such as the bentonite concentration, the mixing methods used, and the
exchangable sodium percentage influence the filter cake thickness, formation
time, and permeability.
a. Bentonite Concentration
Figure 2-6 shows the effect of increasing the bentonite concentration of
the slurry on the amount of initial slurry loss. Slurry loss is reduced
because the higher bentonite concentration allows greater clay particle
interaction. This results in more rapid filter cake formation and higher gel
strength in the soil pores that are wetted by the slurry. To minimize slurry
loss and consequent trench instability, it is recommended that the bentonite
content of the slurry be maintained above 4.5 percent (Hutchinson et al.
1975).
b. Bentonite Quality
The quality of the bentonite used in slurries greatly influences filter
cake formation and performance. Criteria for bentonite quality include sodium
content, fineness of grinding, and type and effects of chemical treatments.
The. sodium content of the montmorillonite determines the hydraulic
conductivity and resistance to flocculation of the filter cake. Montmorillo-
nites containing high sodium contents have been found to swell to a greater
degree upon hydration than those containing less sodium. In addition,
several* researchers have found a strong correlation between the amount of
swelling and the hydraulic conductivity of the soil (Rowell, Payne and Ahmad
1969, McNeal 1968). Based on these data, filter cakes from sodium bentonite
slurries could be expected to have lower permeabilities than those from
calcium bentonites. This hypothesis is supported by tests on clay membranes
conducted by Shainberg and Caiserman (1971).
These researchers found that the hydraulic conductivity of the
montmorillonite tested was very sensitive to the type of cation adsorbed on
the clay. In fact, the hydraulic conductivity of the calcium montmorillonite
membrane was 14.2 times greater than that of the sodium montmorillonite.
2-21
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Figure 2-6.
The Effect of Bentonite Concentration on the
Initial Fluid Loss During Filter Cake Formation
6 -
0 10 20 30 40 50 60
Initial Fluid Loss: cm3
Source: Hutchinson, et al., 1975
70
80
2-22
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Values for the calcium and sodium montmorillonite clay membranes were 9.3 x
10 cm/sec and 0.65 x 10 cm/sec, respectively (Shainberg and Caiserman
1975). These values are similar to the permeabilities reported for filter
cakes. In tests of a bentonite slurry in contact with a London clay, for
example, the permeability of the filter cake was found to be 2.3 x 10 cm/sec
(Xanthakos 1979). The size of the dry bentonite particles affects performance
because the smaller particles hydrate more rapidly and have a larger surface
area to hydrate and swell than do larger bentonite particles. The grades of
bentonite used for slurry wall applications typically have smaller particle
sizes than do bentonites used for other applications, such as pond sealing.
c. Slurry Mixing Methods
The methods used to mix the slurry can also affect the filter cake
formation. Bentonite slurries with high shear strengths penetrate a shorter
distance into the soil pores before gelation occurs (Xanthakos 1979). Thus
high shear strength slurries exhibit faster filter cake formation and less
initial slurry loss. Figure 2-7 illustrates the difference in 10 minute gel
strengths between slurries mixed by two different methods. In this study,
the slurry processed in the high shear mixer initially manifested a higher
10 minute gel strength than the slurry mixed in the anchor stirrer. More
importantly, the difference in gel strengths did not diminish during the
period of time the two slurries were compared. Even 1,400 minutes (23 hours
and 20 minutes) after the initial mixing, the slurry from the high shear mixer
had a higher gel strength than the other slurry tested (Hutchinson et al.
1975).
d. Filter Cake Formation Time
The amount of time allowed for filter cake formation can influence filter
cake performance. As shown in Figure 2-5, a certain period of time is
required for the initial slurry loss to occur before colloidal packing at the
slurry/soil interface produces the filter cake. Studies reported by
D'Appolonia (1980) showed that, at any given hydraulic pressure, the filter
cake permeability is a function of formation time. According to Xanthakos
(1979), the filter cake thickness increases as time passes. Boyes (1975)
found that filter cakes formed under quiescent conditions had higher shear
strengths than those formed during agitation of the slurry. Because formation
time affects filter cake performance so stongly, D'Appolonia (1980a) recom-
mended that 24 hours should elapse between slurry trench excavation and back-
filling to allow complete filter cake formation under quiescent conditions.
e. Strata Characteristics
When the strata surrounding the trench contain numerous large pores,
considerable slurry loss can occur before the filter cake can form. In many
2-23
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Figure 2-7.
The Effect of Mixing Techniques and Times on Hydration of a
5% Suspension of Ca-Exchanged Bentonite
250 '
01
*5
O
200 400 600 800 1,000 1,200 1,400
Time After Initial Mixing: min.
Source: Hutchinson, et al. 1975
2-24
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cases, particles of spoil that were inadvertently mixed with the slurry during
excavation, can assist in clogging soil pores and can reduce the amount of
slurry loss. However, in gravel beds, which allow water movement rates of
1 to 10 cm/sec, the pores are too large to be easily closed (jefferis 1981a).
Thus slurry loss under these conditions continues until rheological blocking
occurs (Hutchinson et al 1975).
Rheological blocking is the gradual inhibition of slurry flow due to the
increase in slurry shear strength as gelation progresses (Boyes 1975). The
effect of this phenomena is a steady but slow decline in fluid loss even in
gravel beds, as shown in Figure 2-8. In this figure, the time necessary for
filter cake formation in a less permeable stratum is also illustrated. The
soil layers containing 0.1 percent sand required rheological blocking coupled
with pore space blockage to stop slurry flow. Even so, deep filtration
occurred before the filter cake formed. As the permeability of the stratum
was reduced by the addition of sand, the time needed for filter cake forma-
tion, the amount of slurry lost, and the depth of filtration were reduced
(Hutchinson et al 1975).
The time required for filter cake formation varies from less than
30 seconds in fine textured soils, to over 3,600 seconds (6 minutes) in
gravel beds. The depth of penetration as shown in Figure 2-8 depends on the
permeability of the strata. In strata of very low permeability, slurry loss
has been found to be minimal (Xanthakos 1979). One set of tests using a
3.2 percent bentonite slurry in contact with a sandstone having permeability
of from 10 to 10 cm/sec showed slurry penetrated into the stone only from
2 to 3 cm (Boyes 1975).
f. Hydraulic Gradient
Hydraulic gradient also influences filter cake formation and performance
(D'Appolonia 1980a). The difference between the hydraulic head of the slurry
and that of the groundwater should be as great as possible to improve filter
cake characteristics (Guertin and McTigue 1982b). A high hydraulic head in
the trench forces the slurry to be packed tightly along the soil/slurry inter-
face. This compresses the filter cake and reduces leakage (Hutchinson et al
1975).
g. Slurry Contamination
Contamination of the slurry can occur due to the presence of salts,
cement, extremely basic conditions in the make-up water used. Slurry
contamination can decrease filter cake performance. For example, slurry
contamination by cement causes portions of the slurry to flocculate. This
leads to the formation of relatively thick, permeable, weak filter cakes, high
fluid losses and low viscosity in the slurry (Hutchison et al 1975). As the
amount of cement is increased, the thixotropic properties of the slurry are
inhibited. This leads to lower slurry viscosity, lower gel strength and
2-25
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Figure 2-8.
The Effect of Added Sand on Filtration of a 5% Suspension
of a Calcium-exchanged Bentonite through a Fine Gravel Bed
2,000
1,500
1,000
500
Bed Cross-section = 11.5 cm2
Bed Depth = 30 cm
Pressure Drop = 21 kN/m2 (3 Ib/sq. in.
k = 0.3 cm/s
Sand - Buckland 50 FG
+ 0.1% Sand. Deep Filtration
Surface Filtration
+ 0.5% Sand
+ 1 % Sand
+ 10% Sand
0123456
Time T: min
Source: Hutchinson, et al. 1975
2-26
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inhibited filter cake formation (Guertin & McTigue 1982b). These effects also
occur in cement bentonite slurries, as is discussed in Section 2.4.1.1. Other
contaminants can also influence slurry and slurry wall properties. These are
discussed in Section 4.
2.2.2.2 Gel Strength
The slurry's gel strength is a measure of its ability to form a gel
structure. This gel structure can influence slurry density. The gel strength
of the slurry allows the fine particle materials that mix with the slurry
during excavation to remain suspended. At a gel strength of 15 Ib/ft , the
slurry has the ability to suspend average sized coarse sand particles. These
particles are up to approximately 1 mm in diameter. Particles smaller than
this, such as fine sands, silts and clays are likely to remain in suspension,
while larger particles sink to the trench bottom and form a heavy mixture of
slurry, coarse sand, and gravel (Xanthakos 1979).
As the slurry's gel strength increases, the maximum diameter of particles
it can support will increase. High gel strength slurries support a large
portion of the soil particles that fall into the trench, while low gel
strength slurries allow more particles to sink to the trench bottom. As the
concentration of suspended solids in the slurry increases, the slurry becomes
more dense. Xanthakos (1979) reported slurry density measurements from tests
at several slurry trench construction sites. These tests revealed that the
sediments that became suspended during excavation increased the slurry density
by an average of about 4 to 5.5 percent.
2.2.2.3 Density
The density of the slurry can influence the stability of the trench wall.
According to Xanthakos (1979), high density slurries resist the pressures
exerted on the trench walls by high water tables and low shear strength soils.
However, slurries that have a low density do not resist these pressures as
effectively as do the higher density slurries. Thus the increase in slurry
density that is caused by the slurry's gel structure can contribute directly
to trench wall stability.
The heavier soil particles that fall into the trench during excavation do
not remain in suspension. Instead, they fall to the trench bottom and
accumulate there. The amount of sand accumulation on the trench bottom is
dependent on the coarseness of the strata being excavated and other factors,
such as the excavation techniques used (Xanthakos 1979). This sand layer does
not have a direct effect on trench stability, however it may have an impact on
the permeability of the completed cut-off wall, and the ease of backfilling.
The density of the slurry taken from the trench bottom (i.e., the sand layer)
must be at least 15 pcf less than the backfill. If the slurry is too dense,
it will not be displaced properly by the backfill (D1Appolonia 1980b).
2-27
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2.2.2.4 Chemical and Physical Additives
Numerous chemical and physical additives have been used in slurries to
improve their viscosity, gel strength, density, or fluid loss rate (Xanthakos
1979). Some of those additives are listed in Table 2-3. It is recommended
that the use of any slurry additives be allowed only with the approval of the
engineer. Some slurry trench excavation specifications forbid the use of
chemically treated bentonites (U.S. Army Corps of Engineers 1975). One
problem with the use of chemically treated bentonites is the possibility of
enhanced interaction with pollutants. Conversely, certain chemical treatments
may render the bentonite less susceptible to chemical attack. Slurry/waste
interactions are discussed in Section 4.
2.3 Soil-Bentonite Walls
SB walls are excavated under a bentonite slurry in a continuous trench.
As excavated materials are removed from the trench, they are mixed with slurry
and replaced in the trench a short distance from the active excavation area.
Techniques used during slurry trench construction are described in detail in
Section 5.
2.3.1 SB Wall Properties
A properly designed and constructed SB wall exhibits the following
properties:
* Low Permeability
Resistance to hydraulic pressure and chemical attack
Low bearing strength and moderate to high plasticity.
2.3.1.1 Low Permeability
Permeabilities of completed soil-bentonite cut-offs have been as low as
5.0 x 10 cm/sec, although higher permeabilities are more common (Xanthakos
1979). Typical permeabilities of SB walls range from over 10"^ cm/sec in
walls composed primarily of coarse, rather than fine materials, to less than
10 cm/sec in walls containing over 60 percent clay (D'Appolonia 1980b).
2-28
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TABLE 2-3
COMMON SLURRY MATERIALS AND ADDITIVES
Weight materials
Colloid materials
Thinners and dispersing
agents
Intermediate-sized particles
Flocculants and
polyelectrolytes
Fluid-loss-control agents
Lost-circulation materials
Barite (barium sulfate) or soil (sand)
Bentonite (Wyoming, Fulbent, Aquagel,
Algerian, Japanese, etc.), basic fresh water
slurry constituent
Attapulgite, for saltwater slurries
Organic polymers and pretreated brands
Quebrancho, organic dispersant mixture
(tannin)
Lignite, mineral lignin
Sodium tetraphosphate
Sodium humate (sodium huraic acid)
Ferrochrome lignosulfonate (FCL)
Nitrophemin acid chloride
Calcium lignosulfonate
Reacted caustic, tannin (dry)
Reacted caustic, lignite (dry)
Sodium acid pyrophosphate
Sodium hexametaphosphate
Clay, silt^ and sand
Sodium carboxmyethyl cellulose (CMC)
Salts
Starches
Potassium aluminate
Aluminum chloride
Calcium
CMC or other flocculants
Pregelatinized starch
Sand in small proportions
Graded fibrous or flake materials; shredded
cellophane flakes, shredded tree bark,
plant fibers, glass, rayon, graded mica,
ground walnut shells, rubber trees, perlite,
time-setting cement, and many others.
Reference: Xanthakos 1979. Copyright 1979 by McGraw-Hill Books. Used with
Permission.
2-29
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2.3.1.2 Resistance to Hydraulic Pressure and Contaminants
An SB wall that exhibits an extremely low permeability is not effective
in the long run if it cannot withstand the hydraulic gradients induced by its
presence or if it disintegrates upon contact with contaminants at the site.
Because of its low permeability, the wall can be used to severely
restrict downgradient water movement. This causes the water level on the
upgradient side of the wall to rise significantly as compared to the
downgradient side. This difference in water levels is termed the hydraulic
gradient. A high hydraulic gradient across the wall is likely to develop
unless groundwater rerouting is accomplished through the use of upgradient
extraction wells, subsurface drains or interceptor trenches (see Section 3).
Despite the use of these ancillary measures, the wall should be designed to
withstand significant hydraulic gradients. The incorporation of a high
concentration of clayey materials into the backfill improves the wall's long-
term resistance to hydraulic gradients up to 200 (D'Appolonia 1980b). Wall
design is discussed in Section 5.
The wall's resistance to degradation by chemical contaminants is also a
primary measure of long term performance. Prior to SB wall construction,
extensive testing of the effects of the site's leachate on proposed backfill
mixtures should be conducted. In general, clayey backfill mixtures withstand
permeation with contaminants more effectively than those that contain less
clay (D'Appolonia 1980b).
2.3.1.3 Strength and Plasticity
The strength of SB cut-off walls is not usually of primary concern when
designing pollution migration cut-offs. These walls are usually designed to
be comparable in strength to the surrounding ground (jefferis 1981b). If
stronger walls are required, coarser material may be added to the backfill,
although this practice results in an increase in wall permeability (Millet and
Perez 1981). In any case, the strength of a soil-bentonite wall is not
usually relevant in hazardous waste applications, except where traffic must
pass over the wall. Design of traffic caps is discussed in Section 5.
The response of the SB wall to lateral earth pressures and earth
movements is an important factor in the design of pollution migration cut-
offs. If the wall is too brittle, shifts in nearby strata caused by
overloading the surface by stockpiles or heavy machinery can result in
cracking and subsequent leakage of the wall. Fortunately, completed SB cut-
off walls behave plastically when stressed. That is, they undergo plastic
deformation rather than crack (Guertin and McTigue 1982b). In contrast, CB
walls have higher strength than SB walls and can be brittle and thus more
easily cracked (Millet and Perez 1931).
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2.3.2 Factors Affecting SB Wall Performance
There are numerous factors that can affect the performance of SB Walls.
These can be divided into four general groups which are:
Design criteria
Backfill composition and characteristics
Backfill placement methods
Post-construction conditions at the site.
2.3.2.1 Design Criteria
The design criteria that affect SB wall performance include wall width,
wall depth, selection of appropriate aquiclude, wall configuration, and use of
ancillary measures. These criteria are discussed in Section 5. The factors
relating to backfill preparation and post-construction conditions are
described below.
2.3.2.2 Backfill Composition and Characteristics
To produce a low-permeability, durable cut-off wall, the backfill must
contain a high concentration of plastic fines (clays), a minimal amount large-
diameter particles, and a suitable concentration of bentonite and water.
Contaminants in the soil or water can also affect the wall's performance.
a. Native Clay and Bentonite Content
A primary requirement for backfill material is that it contain a suitable
particle size distribution. For low permeability, this means the backfill
must have from 20 to 40 percent fine particles, preferably plastic fines.
Fine particles (less than 0.074 mm in diameter or passing a number 200 sieve)
exert a significant influence on backfill permeability, as shown in Figure
2-9. At a given bentonite concentration, the backfill permeability will be
lower when the backfill material contains a higher proportion of fines.
Conversely, increasing the bentonite content of the backfills tested
significantly reduced the wall permeability. The bentonite content of the
mixed backfill should not fall below 1 percent (D1Appolonia 1980b). Where the
strength of the cut-off wall is of primary concern, a higher concentration of
coarse and medium sized particles are required. In any case, material over
6 inches in diameter are not considered desirable for use in backfills
(Federal Bentonite 1981).
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Figure 2-9.
Relationship Between Permeability and Quantity of Bentonite
Added to SB Backfill
10
10
10
10
-5
10
10
10
10
Clayey Silty Sand
w 30 to 50% Fines
0
1
Well Graded
Coarse Gradations
(30-70% ^ 20 Sieve)
w'10 to 25% NP Fines
Poorly Graded
Silty Sand
w/30-50% NP Fines
% Bentonite by Dry Weight of SB Backfill
Source: D'Appolonia, 1980
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D'Appolonia (I980b) found that plastic fines reduce permeability more
effectively than nonplastic fines. This is most likely due to the fact that
plastic fines are composed of smaller particles than nonplastic fines. The
effect of plastic fines on backfill permeability is shown in Figure 2-10.
Fine particles, particularly clays, contribute to low permeability by
assisting in bridging the pores between larger particles and by contributing
to the swelling, viscosity, gelation, and cation exchange capacity of the
backfill (D'Appolonia 1980b, Boyes 1975). Although these properties find
their maximum expression in montmorillonite, other clays exhibit these
characteristics to a lesser degree (Grim 1968). Thus the clay content of the
backfill has a pronounced effect on SB wall permeability.
b. Water Content
The water content of the backfill can also influence the SB wall
performance. The'amount of water in the backfill should be carefully
controlled because the hydraulic conductivity of sodium montmorillonite has
been reported to increase dramatically as the water content increases (Low
1976). There is an effective limit on reducing the water content of the
backfill, however, because the backfill must slump sufficiently to allow
proper placement. The water content of backfills at ideal slumps is from 25
to 35 percent (D'Appolonia 1980a). Even so, the excess water in the backfill
has been found to result in increased permeability (Jefferis 1981b).
If the moisture content of the soil material excavated from the trench is
over 25 percent initially, the addition of bentonite slurry during backfill
mixing results in a very wet backfill that exhibits high permeability. To
remedy this situation, D'Appolonia (1980a) suggests spreading the soil
material in a thin lift over the backfill mixing area, then broadcasting dry
bentonite over the lift at the desired rate. The soil material is then mixed
with the dry bentonite prior to the addition of the slurry. This reduces the
water content of the backfill while simultaneously increasing the bentonite
content.
c. Contaminants in Backfill Materials
The construction of a low-permeability SB cut off walls requires the use
of soils in the backfill that are free of deleterious materials. To be free
of deleterious materials, the proposed soil source must not contain signifi-
cant amounts of soi.l organic matter, including plant and animal debris, high
calcium materials, including gypsum, chalk and caliche, or high concentrations
of soluble salts, including sodium chloride, sodium sulfates or anhydrite.
In addition to the items listed above, other subsurface materials may be
detrimental to backfill quality. For example, at some sites where pollution
migration cut-offs have been constructed, the soil excavated was contaminated
with pollutants. These pollutants may or may not significantly interfere with
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Figure 2-10.
Effect of Plastic and Non-plastic Fines Content on Soil-Bentonite
Backfill Permeability
80
70.
60
50
40
20
10
0
Plastic
Fines
Non-Plastic or Low
Plasticity Fines
0 10-9
Source: D'Appolonia, 1980
10-e 10-7
SB Backfill
1Q-6
10-5
10-"
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cut-off wall performance. D'Appolonia (1980a) suggested preparing a test
mixture to determine compatibility. He further suggested using the con-
taminated soil if equal in quality to uncontaminated soil, even though the
material may decrease the slurry and backfill performance initially. This is
because early exposure of the bentonite to the contaminants reduces the
permeability changes that occur during subsequent exposure to the contami-
nants. This approach must be balanced against the fact that contaminant
breakthrough may occur earlier.
2.3.2.3 Backfill Placement Methods
The mixing and placement of the carefully selected backfill material is
of critical importance in the overall performance of the completed wall. The
bentonite slurry and soil material must be combined to form a relatively
homogenous paste with a consistency similar to that of mortar or concrete. It
must flow easily yet stand on a slope of about 10:1, and must be at least 15
pcf (240 kg/m ) denser than the slurry in the trench (D'Appolonia 1980b). The
methods used to mix the backfill and the tests used to measure its shear
strength, flow characteristics and density are described in Section 5.
2.3.2.4 Post-construction Conditions
Once the backfill has been mixed and placed, the performance of the wall
is dependent on the subsurface conditions surrounding the wall. In particu-
lar, the hydraulic gradient and the presence of contaminants can influence the
wall's ability to function properly.
a. Hydraulic Gradient
The difference in hydraulic pressure between the upgradient and down-
gradient sides of the trench strongly influences the trench's durability as
well as its initial permeability. Little data are available on this factor;
however, it has been shown that high hydraulic pressures within the trench
during filter cake formation result in a lower permeability filter cake. The
long-term effect of high hydraulic pressure differentials across the trench on
wall permeability is, however, likely to be different (D'Appolonia 1980b). A
large difference in hydraulic pressure from one side of the trench to the
other is expected to severely tax the integrity of the wall. Methods used to
combat high hydraulic gradients include increasing wall thickness and/or using
extraction wells or subsurface drains upgradient to assist in equalizing
hydraulic pressures near the wall. These are discussed in Section 5.
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b. Presence of Contaminants
The resistance of soil-bentonite cut-off walls to permeation and
destruction by various pollutants is the subject of much current research.
Bentonite is extremely resistant to degradation from some substances, but
others cause rapid dehydration and shrinkage of the montmorillonite particles.
SB wall performance can be severely inhibited by contact with incompatible
chemical compounds in leachates or wastes.
The wall can be protected from degradation due to chemical incompatabil-
ity in several ways. First, waste/wall contact can be minimized by using
extraction wells or subsurface drains. Second, contaminated soil can be used
in the backfill, as described earlier. Third, the concentration of
non-montmorillonite clay in the backfill can be maximized.
Non-montmorillonitic native clays are not likely to be as severely
affected by chemical contaminants as are bentonites or native montmorillonitic
clays. This is because the non-montmorillonitic native clays do not swell as
extensively as montmorillonite when they are hydrated. Consequently, if they
become dehydrated during chemical interactions, they do not shrink as
extensively as montmorillonite does when it becomes dehydrated. When
shrinkage is minimized, the associated permeability increase is also
minimized. Thus the adverse effects of the chemical interaction can be
decreased.
Different types of wastes affect the clay in the backfill in different
ways. In addition to dehydration and shrinkage, the clay may be dissolved or
its properties can be drastically altered. Data on chemical compatabilities
of wastes and SB walls are summarized in Section 4.
The proper design and construction of an. SB wall can result in a durable,
low permeability cut-off that withstands high hydraulic gradients and
permeation with various contaminants. At some sites, the use of SB walls is
not appropriate (see Section 5). When SB walls cannot be used CB walls can be
installed. These walls are similar to SB walls in that they contain bentonite
and form a relatively low permeability cut-off, but they differ in several
important ways, as described below.
2.4 Cement Bentonite Slurries
When CB walls are being constructed CB slurries are prepared. Techniques
used to construct CB walls are described in Section 5.
2.4.1 CB Slurry Properties
Ceraent-bentonite slurries normally contain about 6 percent by weight
bentonite, 18 percent ordinary Portland.cement (o.p.c.) and 76 percent water
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(Jefferis 1981b). Typical ranges of CB slurry contents are presented in
Table 2-5.
When bentonite slurries are compared to CB slurries, the differences
become evident. Table 2-4 illustrates these differences. Most of these
differences are due in part to the effects of the calcium from the cement on
the sodium montmorillonite in the bentonite. The three most important
differences between the properties of CB slurries and bentonite slurries are:
Physical properties, including viscosity and filter cake formation
Setting times
Filter cake permeability.
2.4.1.1 Differences in Physical Properties
Because of the calcium in the cement, the properties of the sodium
bentonite in CB slurries are permanently altered. For example, the viscosity
is higher due to the flocculation of the slurry and the higher solids
concentration, 15 to 30 percent in CB slurry, opposed to about 6 percent in
bentonite slurries (Millet and Perez 1981, Jefferis 1981b). The results of
the filtrate loss test are also higher. This indicates that the time required
for filter cake formation is longer and the permeability of the cake formed is
higher (Hutchinson et al. 1975; Millet and Perez 1981). A comparison of the
properties of bentonite and CB slurries is presented in Table 2-5.
2.4.1.2 Differences in Setting Times
The primary difference between CB and SB slurries that is of practical
importance in slurry trench construction is the fact that CB slurries begin to
harden within 2 to 3 hours after mixing (Case 1982). This necessitates the
use of construction techniques different from those used during construction
of SB walls, as described below.
CB walls can be constructed either in a series of panels or as a
continuous trench that is backfilled with a CB slurry. When CB panels are
constructed, alternate panels are excavated under a CB slurry, then allowed to
partially set. When they have obtained a sufficient shear strength, the
intervening panels are excavated also under a slurry. A portion of the
initial panel ends are removed during this second stage to ensure continuity
between the initial panels and the intervening ones.
Another result of the rapid setting times of CB slurries as compared to
bentonite slurries, is the fact that construction delays can cause problems in
set up of CB walls. This is because continued agitation of the CB slurry
(that is, more than 24 hours) reduces the ability of the cement in the slurry
to set. In fact, 48 hours of agitation can completely prevent setup of the CB
wall. This effect can be off-set by the use of blast furnace slag. This
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TABLE 2-4
TYPICAL COMPOSITIONS OF CEMENT BENTONITE SLURRIES
Cons ti tuen t Percentage in Slurry
Bentonite 4-7
Water 68-88
Cement
without replacements 8-25
when blast furnace slag added, minimums 1-3
when fly ash added, minimums 2-7
Blast furnace slag, maximums, if used 7-22
Fly ash, maximums, if used 6-18
Reference: Adapted from Jefferis 1981b.
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TABLE 2-5
PROPERTIES OF SOIL BENTONITE AND CEMENT
BENTONITE BACKFILLS
Parameter
Soil-Bentonite Backfill
Cement-Bentonite
Backfill
Density
Water Content, %
by weight
Bentonite Content, %
by weight
Other Ingredients, %
by weight
Strength
Permeability, cm/sec
typically 105-120 p.c.'f, (1)
1680-1920 kg/m (l)
25-35(1)
0.5-2 (1)
Fines 10-20 (3)
Pines 20-40 (1)
Plastic. Very little
strength (4)
normally around 20 p.s.f,
unconfined (5)
minimum reported
5.0 x 10~* (6)
maximum reported
~1 x 10 (1)
maximum likely 1300
kg/m
55-70(2)
6 (2)
Cement 18 (2)
Solids 30-45 (2)
Ultimate strength
Range: 5-55 p.s.i
Normal strength
20-45 p.s.i. (8)
1 to 5 x 10
-6
(2)
References:
(1) D'Appolonia 1980b, (2) Jefferis 1981b, (3) Millet and Perez 1981,
(4) Guerntin and McTigue 1982b, (5) Ryan 1976, (6) Xanthakos 1979, (7)
Case 1982, (8) Cavelli 1982.
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pozzolanic material can replace up to 90 percent of the ordinary Portland
cement in the slurry (Jefferis 1981b).
2.4.1.3 Differences in Filter Cake Permeability
Although there are some basic differences between the use of bentonite
and CB slurries during trench excavation, the critical differences between
these two cut-off wall construction techniques appears in the finished cut-off
wall.
The tub'st important differences between SB and CB walls with regard to
pollution migration cut-offs are that SB walls have much lower permeabilities
and higher resistances to certain pollutantsr than do CB walls (Jefferis 1981b;
Xanthakos 1979). An important factbr contributing to the permeability
differences can be seen by comparing the permeabilities of filter cakes from
each walltype. For bentonite slurries, filter cake permeabilities can be as
low as 10 cm/sec, while permeabilities calculated from filtrate loss tests
on CB slurries ranged from about 1 to 4 x 10 cm/sec (Xanthakos 1979,
Jefferis IJjSlb). The permeability of a SB wall installed in a dam was tested
at 5 x 10 cm/sec (La Russo 1963). Samples of aniristal ledges wall were
tested and found to have permeabilities between' 10 and 10 cm/sec. (Jones
1978). Other important differences between finished CB and SB walls are
listed in Table 2-5. ' :
2.5 Cement-Bentonite "Walls
In contrast to SB walls, CB walls are used where there is a lack of
suitable soils or sufficient backfill mixing areas, or there are excessive
slopes at the site1. CB walls are also used where strength, rather than low
permeability is the primary consideration (Guertin and McTigue 1982b). The
properties of SB and CB walls are compared in Table 2-5. Appropriate applica-
tions of CB walls for pollution migration control are described in Section 5.
Typical compositions of CB slurries were described earlier. This section
describes normal CB wall characteristics and factors affecting the performance
of both CB slurries and CB walls.
2.5.1 CB Wall Requirements
The requirements for CB wall performance include
Strength
Durability
Continuity
2-40
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Set time
Permeability.
2.5.1.1 Strength
CB wall strength is designed to be slightly greater than that of the
surrounding ground, and is typically comparable in strength to stiff clay
(Jefferis 1981b, Millet and Perez 1981). Although strengths of CB walls can
range from 10 to 1,000 R.s.i., ultimate strengths are generally about 20 to
45 psi and are achieved after 28 days (Xanthakos 1979, Cavalli 1982).
At a hazardous waste site, strength may be required where traffic crosses
the wall, or where a weak wall may interfere with the stability of nearby
buildings, storage tanks, or bridge foundations or road or rail subgrades.
2.5.1.2 Durability
CB walls appear to be quite durable under most conditions, as they can
usually withstand compressive strains of several percent without cracking
This is because they are not as brittle as typical concrete walls (Ryan 1976).
Moreover, they can withstand relatively high hydraulic gradients. A CB wall
only 2 to 3 feet wide can satisfactorily withstand at least 100 feet (30
meters) of hydrostatic head (Millet and Perez 1981). A mixture of 50 kg/m3
bentonite, 70 kg/m blast furnace slag, and 30 kg/m3 cement in a CB backfill
was reportedly not damaged after 40 days of exposure to a head differential of
200 feet (Jefferis 1981b). Despite the fact that CB walls are normally quite
durable they are not indestructable, as hydrofracturing of CB walls has been
reported to occur (Millet and Perez 1981). Also, comparatively little is
known of the permanence of CB walls in hostile chemical environments.
2.5.1.3 Continuity
Continuity of CB walls is an important factor in construction of cut-
offs. Because these walls are sometimes constructed in panels rather than in
a continuous trench, there is a possibility for unexcavated portions to remain
between the panels. To prevent this, care is taken during the excavation of
panels, and the clamshell bucket or backhoe is moved vertically and
horizontally throughout each slot at the completion of slot excavation. In
addition, when the connecting area between the initial and subsequent panels
is excavated, a portion of the set panel is removed to ensure that all
intervening soil has been excavated (Guertin and McTigue 1982b) If
unexcavated areas are inadvertantly left between panels, leakage can occur.
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2.5.1.4 Set Time
The time required for CB walls to harden depends on the presence of set
time retarders, cement replacements, and water/cement ratios, among other
factors. The speed of set is of interest because of the construction
techniques employed. During CB wall installation, slow setting of panels can
delay construction. This is because alternating panels are excavated, leaving
unexcavated areas between each. After the CB slurries in the first set^of
panels have set, the areas between them can be excavated. If the slurries
take unduly long to harden, construction can be delayed. This may occur where
fly ash is used as a cement replacement, because a slurry containing fly ash
may harden very slowly. The use of blast furnace slag does not delay CB
slurry set up as long as does fly ash (Jefferis 1981b).
2.5.1.5 Permeability
The permeability of CB walls is normally about 10" cm/sec (Case 1982).
This can be decreased by adding blast furnace slag or additional bentonite.
Jefferis (1981b) reports that permeability can also be decreased as much as an
order of magnitude due to consolidation of the completed wall.
2.5.2 Factors Affecting CB Wall Performance
Factors affecting the performance of CB walls include:
Slurry contents, including bentonite, water, cement, cement
replacements
Mixing methods and speeds.
2.5.2.1 Slurry Constituents
The quality and quantity of ceraent-bentonite slurry constituents can
alter the characteristics of the slurry and the completed wall. Bentonite
quality, water quality cement content and the use of cement replacements all
affect CB slurry and wall performance.
a. Bentonite Content
The low permeability and resistance to chemical attack of CB walls are
contributed by the bentonite in the slurry. Where very low permeability or
resistance to aggressive chemicals are required, the bentonite content of the
slurry should be increased (Jefferis 1981b). Increasing the quality of
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bentonite used in the slurry can also help produce a durable low-permeability
CB wall.
An important characteristic of high quality bentonite is its sodium
content. Bentonites with higher sodium contents are particularly desirable
for use in CB slurries due to the changes in the bentonite slurry caused by
the calcium ions in the cement.
When cement is added to a bentonite slurry, several interrelated changes
occur. First, calcium ions from the cement begin replacing sodium ions on the
exchange complex of the montmorillonite particles. This compresses the
diffuse double layer surrounding each clay flake and reduces the net negative
charge on the hydrated particles (See Figure 2-1). As the double layer
contracts, the pore space between the particles is enlarged, thus increasing
the amount of free water in the mixture. Due to the reduction in net negative
charges, the mutual repulsion between the clay particles decreases, so the
clay flakes come closer together. At this point, the large calcium ions can
serve to link clay particles together, causing them to flocculate, or form
stacks of particles. These stacks, being much heavier than dispersed clay
particles, tend to settle out of the suspension readily. They also have a
decreased ability to form a gel structure or a filter cake (Case 1982, Boyes
1975). All of these changes are detrimental to slurry quality.
b. Water Quality
Water quality can also influence CB slurry characteristics. If water
containing calcium ions or dissolved salts is used to mix with the dry
bentonite, the cement slurry produced exhibits low viscosity, poor filter cake
formation and an increased set time (Guertin and McTigue 1982b). Specifica-
tions for water quality include:
Hardness of <50 ppm (Xanthakos 1979)
Total dissolved solids content of <500 ppm (U.S. Army Corps of
Engineers 1975)
Organics content of <50 ppm (U.S. Army Corps of Engineers 1975)
pH of about 7.0 (U.S. Army Corps of Engineers 1975).
Leachate-contaminated water should not be used to mix with the fresh
bentonite. If the use of poor quality water cannot be avoided, 10 to
12 percent more bentonite than normal may be required and longer mixing times
are recommended (Xanthakos 1979).
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c. Cement Content
The cement content of CB slurries is the chief factor controlling the
strength, deformability, and permeability of the finished wall. Generally,
there is a trade-off in CB walls between strength and permeability, for as the
cement content is increased, a stronger, more brittle wall is formed (Millet
and Perez 1981). The trade-off is due to the detrimental effects of the
cement on the bentonite. Higher cement contents allow higher wall
permeabilities, and, although wall strength can be increased by the addition
of coarse materials, these materials also result in increased permeability
(Jefferis 198Ib).
Due to this trade-off, the ultimate strength of cement bentonite walls is
low, normally ranging from 20 to 45 p.s.i. (Cavalli 1982). The minimum
reported strength was 5 p.s.i. in a relatively young wall, and the maximum
strength was reported at 55 p.s.i. (Case 1982, Millet and Perez61981). The
permeability of CB walls is relatively high, usually around 10 cm/sec (Case
1982). In one completed CB wall tested, the permeability of Shelby Tube
samples was on the order of 10~ to 10 cm/sec., and the strength was
measured at 13 to 15 p.s.i. (Jones 1978).
The ratio by weight of water to cement in the slurry also affects the
characteristics of CB walls. Generally higher ratios produce weaker walls.
Typical water cement ratios range from 3:1 to 11:1. These are much higher
than the ratios found in concrete mixes. The reason the cement and water do
not separate (bleed) to a great extent is the presence of the bentonite in the
mixture. Bentonite absorbs a great deal of water thus minimizing the free
water in the slurry. At the same time, the gel structure of the bentonite
particles in the slurry assists in supporting the cement particles and thus
reduces settling and prevents excessive bleeding. A quality CB slurry should
show bleed rates of less than 1 percent (Jefferis 1981b).
d. Cement Replacements
A fourth slurry constituent that can affect CB slurry and wall perfor-
mance is the use of cement, replacements. Blast furnace slag and fly ash are
two of the materials that have been used to replace a portion of the cement
used in CB slurries. Blast furnace slag can be used to replace up to
90 percent of the cement, and fly ash can replace up to 70 percent (Jefferis
1981b). The resultant cement replacement concentrations are shown in
Table 2-4.
When using these replacements, two important slurry properties are
altered. The slurry's set time is extended, allowing the slurry to remain
fluid and workable longer. Thus construction schedules can be extended and
the risk of problems caused by delays is reduced. In addition, the blast
furnace slag and fly ash that were described by Jefferis (198lb) did not
damage the bentonite as much as cement does. Thus the viscosity, gel
strength, and ability to form filter cakes is less impaired (Jefferis 1981b)
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The^mechanisms by which the cement replacements slow the setting time and
avoid bentonite inhibition are described below.
The fly ash and blast furnace slag require the presence of lime in order
to harden. ^The lime is not available in the CB slurry until most of the
cement particles have fully hydrated. Since the cement does not fully hydrate
upon exposure to water, the blast furnace slag and fly ash delay the setting
time. This delay extends the period of CB workability (Jefferis 1981b).
The use of these cement replacements is less detrimental to the bentonite
because blast furnace slag and fly ash do not release calcium ions as rapidly
as does cement. Thus the mass action effect of calcium ions on the exchange
complex of the montmorillonite clay is reduced (Jefferis 1981b).
Cement replacements also affect the characteristics of the completed
wall. These effects include:
Reduction in bleeding rates
Maintenance of setting ability even though agitated for long periods
Lowered permeabilities in the completed wall
Reduced susceptibility to chemical attack.
According to Jefferis (I981b), the bleeding rates of cement-replaced CB
slurries are less than those of typical CB slurries. This effect is most
likely due to the fact that blast furnace slag and fly ash do not inhibit
bentonite properties as drastically as does cement (Jefferis 1981b).
Another effect of cement replacement is a prolongation of setting time.
When^CB slurries are used without cement replacements, they begin to set up
within^2 to 3 hours (Case 1982). If the slurries are agitated to prevent
hardening, the ability to set up will be diminished and will be lost
altogether if agitation continues for 48 hours. The addition of blast furnace
slag or fly ash to the slurry allow agitation to be continued for up to seven
days without substantial loss of setting ability (Jefferis 1981b).
Blast furnace slag can contribute to low permeability in CB walls. When
conventional CB walls or CB walls with fly,ash replacement are allowed to set
for 7 days, permeabilities of 1 to 5 x 10 cm/sec, result. In contrast, CB
walls containing from_2.5 to 7.5 percent blast furnace slag exhibited perme-
abilities of about 10 cm/sec after 7 days of hardening (Jefferis 1981b).
A final effect of cement replacement on CB walls concerns resistance to
chemical attack. Fly ash replacements can effectively increase a CB wall's
chemical resistance, however, blast furnace slag replacements cannot (Jefferis
1981b).
The cement in CB walls is much more susceptible to chemical attack than
is bentonite. For this reason, CB walls are not normally used where exposure
to detrimental chemicals is likely. However if CB walls are required due to
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site conditions, the addition of protective agents such as fly ash may be
desirable.
e.
Chemical Additives
In addition to bentonite, water, cement, and sometimes cement replace
ments, chemical additives can be mixed into CB slurries to alter their
performance. Several types of additives are used, including deflocculants and
setting time retarders. Deflocculants include chemicals such as sodium
hexametaphosphate, oxidants, acids, and peptizing reagents which are added to
break up the floes formed when calcium is added to the slurry (Xanthakos
1979).
Setting time retarders can be added to extend the period^of CB slurry
workability. The use of chemical set retarders has several disadvantages.
These include the following:
Compared to the amount needed in normal concrete mixes, the quantity
of setting time retarders in CB slurries is much greater, due to the
adsorptive effects of the bentonite
Many chemical retardants lose their effectiveness within 24 hours
Some may actually accelerate setting after 24 hours
Some may reduce the strength of the finished wall (Jefferis 1981b).
2.5.2.2 CB Slurry Mixing Methods
The methods used to mix the cement-bentonite slurry also effect the
performance characteristics of the slurry. To obtain optimum slurry
properties, it is recommended that first the bentonite be given sufficient
time and agitation to hydrate fully, then the slurry be mixed with the cement
as rapidly as possible in a high shear mixer. The reasons for using these
techniques are explained below.
If the cement is present in the water to which dry bentonite is added,
the slurry will behave as if it were mixed in poor quality water, as^described
above. Hydration, swelling, viscosity, and thixotropy will be inhibited (Case
1982, Boyes 1975). For this reason, the bentonite must be fully hydrated
prior to mixing with cement to form a cement-bentonite slurry.
After the slurry and cement are mixed, flash stiffening of the constitu-
ents occurs. If agitated continually for another 5 minutes or so, the mixture
becomes fluid again. The speed of mixing has a pronounced effect on slurry
performance. For example, CB slurries from low shear (i.e., 50 rpm) mixers
were compared to slurries from high shear (i.e., 1400 rpm) mixers. The high
shear slurries showed lower bleed rates, a greater sensitivity to drying
2-46
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problems, lower permeabilities when set, and higher unconfined compressive
strengths when set than those from the low shear mixers (Jefferis I981b) .
From the discussion above, it is evident that the performance of both the
CB slurry and the completed wall are dependent on both the slurry consti-
tuents, and. the mixing procedures used. Thus, both of these variables should
be carefully controlled during design and construction of CB walls.
2.6 Summary
Bentonite slurries can be used to hold open trenches during the construc-
tion of soil bentonite and concrete walls. The bentonite used is composed
primarily of the clay mineral sodium montmorillonite, whose properties deter-
mine the characteristics of the slurry.
The sodium montmorillonite properties that are important during slurry
functioning include extensive hydration and swelling, nearly complete disper-
sion, and thixotropy. These charcteristics allow the slurry to form low
permeability filter cakes on the trench walls.
The filter cakes are an important component of the slurry trench, as they
minimize slurry loss and groundwater inflow, plaster the soil grains together
at the soil/slurry interface, increase the shear strength of the soil into
which they flow, and form a plane against which the hydrostatic force of the
slurry can act to stabilize the trench walls.
Montmorillonite characteristics also allow the slurry to form a gel
structure when agitation is not occurring. This gel structure assists in
suspending small particles of spoil inadvertently dropped into the slurry
during excavation. The suspended sediments increase the slurry density and
viscosity and thus indirectly assist in maintaining trench stability. Chemical
additives are used in some slurries to improve the hydration, dispersion, and
viscosity of the mixture.
Once the excava-t-ion of a slurry trench is complete, the slurry is back-
filled using a homogeneous paste composed of soil mixed with bentonite slurry.
The characteristics of the soil and slurry used, as well as the water content
and placement methods control the permeability of the completed wall.
Cement is added to some bentonite slurries to form a mixture that will
harden in time and form a cut-off wall. Because the calcium in the cement
interferes with the sodium montmorillonite in the bentonite, properties of
cement bentonite slurries differ in some respects from those of bentonite
slurries. The most important differences are the facts that CB slurries
harden to form a CB wall and that the permeability of CB filter cakes and CB
walls is higher than that of bentonite slurries, filter cakes, and completed
SB walls.
When CB slurries harden to form CB walls, the relative proportions of
cement, water, bentonite, and cement replacements and chemical additives,
2-47
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along with the mixing methods used, control the characteristics of the
completed wall.
To determine the suitability of SB or CB walls as a pollution migration
cut-off at a particular site, an in-depth site investigation must be
conducted. This investigation is described in the following section.
2-48
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SECTION 3
SLURRY WALL APPLICATIONS
The effectiveness of a pollution control slurry wall is determined, in
large part, by its horizontal and vertical configuration as well as the
associated remedial measures applied in conjunction with it at a particular
site. These are, of course, highly site specific factors. The site
conditions that determine both configuration and associated measures include
setting, both geologic and geographic waste characteristics, and the nature of
the environmental problems caused by the site. Although these factors are
site specific, generalizations on applications can be useful in understanding
and evaluating a slurry wall alone, and as part of a total remedial effort.
3.1 Configuration
Configuration as used here refers to the vertical and horizontal
positioning of a slurry wall with respect to the pollution source location,
and the groundwater flow characteristics. Although each slurry wall
installation is unique, the vast majority can be described in the terms used
in this section.
3.1.2 Vertical Configuration
Vertical configuration refers to the depth of the slurry wall with
respect to both geologic formations and the water table. Based on vertical
positioning, walls are either "keyed" into a low permeability formation below
the aquifer, or placed to only intercept the upper portion of the aquifer.
This latter type is commonly referred to as a "hanging" slurry wall.
A description of these two general wall types and their uses follows.
3.1.2.1 Keyed-in Slurry Walls
Keyed-in slurry walls are excavated to a confining layer below it, to
contain contaminants that mix with or sink to the bottom of the,aquifer. This
layer may be a low permeability formation such as a clay or silty clay or may
be the underlying bedrock (See Figure 3-1). In either case, the connection
3-1
-------
Figure 3-1.
Keyed-in Slurry Wall
3-2
-------
between the wall and the low permeability zone is very important to the
overall effectiveness of the wall.
From a construction standpoint, wall key-in can be very straightforward
or very complicated. If the low permeability zone is some easily excavated
material, such as a clay layer or weathered rock, basic construction quality
control should be sufficient to ensure a good key-in (see Section 5). In
cases where the low permeability zone is hard bedrock, however, the excavation
process may be much more complicated and costly, and may not be necessary.
In many cases, a sufficient seal can be formed between the wall and the
bedrock by scraping the rock surface clean with the excavation equipment.
Depending on the condition of the bedrock, and the anticipated hydraulic head
the connection must withstand, the weight of the backfill pressing against
bedrock may form a sufficiently tight seal that will meet the designed
permeability requirements (D'Appolonia 1982).
3.1.2.2 Hanging Slurry Walls
Hanging slurry walls are so called because they are not keyed into a low
permeability zone. This configuration is used to control contaminants, such
as petroleum products, which do not mix with the groundwater but float on top
of it. In such cases, the slurry wall need only extend into the water table
to intercept the contaminants (see Figure 3-2). The exact depth of the wall
will depend on the thickness of the floating contaminant layer and the
historically lowest water^able eJLevation. Other considerations^include the
extent to which the weight of the contaminant might have depressed the water
table, and what effect removal of the contaminants would have on the water
table.
3.1.3 Horizontal Configuration
Horizontal configuration refers to the positioning of the wall relative
to the location of the pollution to be controlled and the direction of
groundwater flow (gradient). Based on horizontal configuration, slurry walls
may completely surround the pollution source or be placed up or downgradient
from it.
These configurations and the type of situations to which each can apply,
are described below.
3.1.3.1 Circumferential Wall Placement
Circumferential placement refers to placing a slurry wall completely
around the wastes contained within a site. Although this requires a greater
wall length, thus greater cost, than either upgradient or downgradient
placement alone, it does offer some advantages, and is a common practice. A
3-3
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Figure 3-2.
Hanging Slurry Wall
Leaky
Fuel Tank
Extraction
Weil
Water
Table
Floating
Contaminant
3-4
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circumferential slurry wall, when used with a surface infiltration barrier
(cap), can greatly reduce the amount of leachate generated within a site. If
a leachate collection system is used a waste site can be virtually dewatered.
This offers the advantages of vastly reduced leachate amounts and can help
increase the longevity of the wall by reducing the amount of leachate/wall
contact. As can be seen in Figures 3-3 and 3-4, the direction of flow is from
the exterior toward the interior. Consequently, leachate/wall contact is
minimized while waste containment is maximized. Figure 3-4 illustrates what
can be achieved in the way of site dewatering. Walls used in this fashion
must be very carefully designed. Because the head differential across the
wall is relatively high in these cases, the backfill will be more prone to
piping and hydrofracturing than if the head difference were lower. These
problems are discussed in Sections 5.8.8 and 5.8.9.
3.1.3.2 Upgradient Wall Placement
Upgradient placement refers to the positioning of a wall on the
groundwater source side of a waVte site. This type of placement can be used,
where there is a relatively steep gradient across the site, to divert uncon-
taminated groundwater around the wastes. In such cases, clean groundwater is
prevented from becoming contaminated while leachate generation is reduced. As
can be seen by Figures 3-5 and 3-6, a high gradient is required for Upgradient
placement to be effective. Unless the groundwater can be. diverted around the
site, and be drained to a lower elevation, it can flow around and return to
the same elevation or rise to the surface to overtop the wall.
The use of a wall placed only Upgradient of the wastes is limited in the
types of situations to which it is applicable. Depending on the actual site
setting, and the contaminants involved, an upgradient wall may be keyed in or
hanging. In either case, drainage and diversion structures are likely to be
needed to successfully .alter the flow of clean groundwater.
3.1.3.3 Downgradient Wall Placement
Placement of a slurry wall at a site on the side opposite the groundwater
source is referred to as downgradient placement. This placement configuration
does nothing to limit the amount of groundwater entering the site and so is
practical only in situations, such as near drainage divides, where there is a
limited amount of groundwater flow from upgradient. Such a situation is
illustrated in Figures 3-7 and 3-8. It should be noted that this positioning
does not reduce the amount of leachate being generated, but acts as a barrier
to contain the leachate so it can be recovered for treatment or use. Although
this placement may be used as a keyed-in wall for miscible or sinking contam-
inants, it is most often used to contain and recover floating contaminants.
In either case, compatibility between the wastes and the wall backfill is
important because contact between the two would be difficult to avoid. In
addition, care must be taken in designing a downgradient wall installation to
3-5
-------
Figure 3-3.
Plan of Circumferential Wall Placement
Groundwater Flow
Slurry Wall
Extraction Wells
3-6
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Figure 3-4.
Cut-away Cross-section of Circumferential Wall Placement
^gpiMp^-i-iii Wastes O^yJ^
___
^
O^AV.O-xS .0 <.<(
3-7
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Figure 3-5.
Plan of Upgradient Placement with Drain
3-8
-------
Figure 3-6.
Cut-away Cross-section of
Upgradient Placement with Drain
3-9
-------
Figure 3-7.
Plan of Downgradient Placement
Groundwater Divide
Extraction Welte
Slurry Wall
3-10
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Figure 3-8.
Cut-away Cross-section of Downgradient Placement
3-11
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ensure the build up in head behind the wall does not result in overtopping of
the wall by the contaminated groundwater.
The above discussion centers on slurry walls used to control the
groundwater regime in the vicinity of pollution sources. However, slurry
walls, especially soil-bentonite walls, are also used for the control of
methane and other landfill gases (Lager 1982). It has been shown that both
the water table and fine, moist soils can be effective barriers to gas
migration (Moore 1977). This implies that for gas control, the slurry wall
may be either keyed-in or hanging, whichever is shallower. A gas control
slurry wall will be placed opposite the waste site in the direction or
directions of gas migration, and so could be placed on only one side, or
completely surrounding the site. It should be noted that to be truly
effective in controlling the gas migration, venting, and particularly forced
venting of the gases is recommended (Rovers, Tremblay, & Mooij 1977).
In summary, slurry walls can be applied to a pollution problem in a
variety of ways. Table 3-1 shows the possible combinations and outlines their
typical uses. It should be remembered that the effectiveness of the completed
slurry wall which is a passive measure, will be dependent not only on its
configuration and proper construction, but on the other remedial measures used
in connection with it, in particular, active methods of handling groundwater,
e.g., wells or drains.
3.2 Associated Remedial Measures and Practices
The effectiveness of slurry cut-off trenches can be dramatically
increased by incorporating additional remedial measures into the overall plan
addressing the problem. Some of these measures are:
Groundwater pumping
Surface and subsurface collection
Surface sealing
Grouting, sheet piling or synthetic membrane installation.
These measures affect the hydrologic environment at a site, and can result in
a more efficient and effective control program. The following sections
discuss additional control measures, giving a description, and mentioning
installation considerations"when they are used with slurry walls. More
detailed discussions of each technology can be found in the EPA "Handbook for
Remedial Action at Waste Disposal Sites", EPA-625/6-82-006, June 1982.
3.2.1 Groundwater Pumping
Groundwater pumping involves the installation of a series of wells or
wellpoints located such that the cones of depression formed while pumping each
3-12
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TABLE 3-1.
SUMMARY OF SLURRY WALL CONFIGURATIONS
Vertical
Configuration
Horizontal Configuration
Circumferential
Upgradient
Downgradient
Keyed-in
Most common
and expensive use
Most complete
containment
Vastly reduced
leachate
generation
Not common
Used to divert
groundwater
around site in
steep gradient
situations.
Can reduce
leachate
generation
Compatibility
not critical
Used to capture
miscible or sinking
contaminants for
treatment or use
Inflow not
restricted, may
raise water table
Compatibility very
important
Hang ing
Used for float-
ing contaminates
moving in more
than one direction
(such as on a
groundwater
divide)
Very rare
May temporarily
lower water table
behind it
Can stagnate
leachate but not
halt flow
Used to capture
floating contami-
nants for treatment
or use
Inflow not
restricted, may
raise water
table
Compatibility very
important
3-13
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will intersect. The result is a locally depressed water table in the area
being pumped. Pumped groundwater can be treated, if contaminated, and
reinjected.
There are several ways in which this measure can be applied in concert
with slurry walls. In a situation where a wall is containing a contaminant
plume, pumping can remove trapped, contaminated groundwater. This reduces the
concentration of contaminant in contact with the wall. This may be important
if there exists a possibility of wall degradation due to chemical attack, or
if the recovered contaminants are of some value.
In another situation where a wall is diverting groundwater flow away from
a waste disposal site, pumping can reduce the hydrostatic pressure exerted
against the wall. This reduces the rate of flow through the wall, since
Darcy's law states that the rate of flow is dependent on hydraulic gradient
across the wall. This may be important in situations where a large head
difference is anticipated across the wall. Reducing the head difference also
reduces the possibility of wall failure through piping.
Using cut-off walls and pumping concurrently has several advantages.
First, the reliability of the wall is improved by reducing the probability of
chemical attack or piping. In many instances, pumping systems are used to
protect the wall by keeping leachate away from it. Second, since the slurry
wall is creating a low permeability boundary, the rate of pumping needed to
lower the water table elevation or remove contaminants is reduced.
When a well is pumped, the elevation of the water table is lowered in a
cone-shaped manner (see Figure 3-9). This effect is known as drawdown. In
order to lower the water table over a wide area, the drawdown "cones" for
several wells must intersect (see Figure 3-10).
Drawdown is affected by the following factors:
Pumping rate
Permeability and thickness of water bearing zone
Manner of groundwater recharge
Presence of boundaries
Length of pumping time.
The calculation of drawdown is one of the most important exercises in design-
ing and installing well points or extraction wells. The location of wells or
well points, the depth of the screen, and pumping rate are all variables
determined by the desired amount of drawdown. Extensive geohydrologic testing
of a site is usually required for satisfactory solution of the equations.
3-14
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Figure 3- 9.
Shape of Drawdown "Cone'
Well
Before Pumping
Well
'IT
2 T.
3 .TA
Ti
-------
Figure 3-10.
Intersection of Drawdown Cone of Two Adjacent Wells
Drawdown
3-16
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3.2.1.1 Pumping Systems
There are two types of pumping techniques used for dewatering: well
points and extraction wells. While similar, each technique has special
applications.
a. Well Points
Well points are made to be driven in place, jet placed by water, or
installed in open holes. They consist of a slotted screen, reinforced and
pointed at one end, and connected to a riser pipe or casing of the same or
smaller diameter. The most common practice is to water jet to the desired
depth, flush out fines, and leave the coarser material to collect around the
well point. The point can then be driven into coarser material. Once in
place, the well points are connected to a header pipe (see Figure 3-11).
Well points can be from 1.4 inches in diameter up to 6 inches in
diameter. The size of a well point is generally determined from experience,
and is a^function of the permeability of the aquifer. Fine-grained materials
(e.g., silts and clays) usually require smaller well points. Well points are
not installed to depths greater than 20-25 ft for groundwater pumping purposes
due to the fact that suction pumping is ineffective at depths greater than
20-25 ft. .
Spacing well points is based on the radius of influence of each well and
the composite effects required to achieve the desired drawdown. Once
theoretical drawdowns and spacing are developed using equations, a few well
points can be installed and tested to determine actual values, since equations
assume idealized conditions. Adjustments can be made, which usually require
minor decreases in spacing, thus achieving the desired decline in water table
elevation.
The location of well points is complicated by the presence of the cut-off
wall of much less permeability than the surrounding material. The less
permeable zone has a great impact on the cone of depression formed by well
points (see Figure 3-12). Drawdown is much greater near the wall, because
rate of flow is less through the wall than through the surrounding matrix.
This reduces the area from which water can be drawn during pumping, resulting
in a faster rate of drawdown near the wall. This is useful since it requires
less pumping capacity to achieve desired drawdown near a slurry wall than it
does in the absence of such a barrier.
Locating well points in relation to slurry walls is dependent upon
whether the slurry wall is upgradient or downgradient of a contamination
source. Well points associated with upgradient walls can be used to dewater
behind the wall once it is in place, or they can be used to reduce the water
table upgradient of the wall, reducing the hydraulic gradient across the wall.
These situations are illustrated in Figures 3-13, 3-14, and 3-15.
3-17
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Figure 3-11.
Schematic of a Well Point Dewatering System
Copyright 1975 by Johnson Division, UOP Inc.
Used with Permission
3-18
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Figure 3-12.
The Effect of Drawdown in the Absence
and Presence of a Slurry Wall
A. Drawdown in Absence of Slurry Wall
Slurry Wall
SSJ
To Discharge
/ss
B. Drawdown in Presence of Slurry Wall
3-19
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Figure 3-13.
Well Point Located Behind an Upgradient Slurry Wall Cut-Away View
A. Before Well Points Are Pumped
To Treatment and Reinjection
Direction of Row
B. After Pumping
3-20
-------
Figure 3-14.
Well Points Located Behind an Upgradient Slurry Wall, Plan View
Slurry Wall
Direction of Flow
Well Points
To Treatment and/or Reinjection
C. Map View of Slurry Wall and Well Point System
3-21
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Figure 3-15.
Well Points Located Before an Upgradient Slurry Wall
Well Point
A. Before Pumping
To Discharge
B. After Pumping
3-22
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Well points associated with downgradient walls are commonly used to
remove contaminated groundwater trapped by the wall. Installation can be such
that the concentration of contaminants in groundwater near the wall is sub-
stantially reduced, lowering the possibility of chemical attack and removing
the contaminant from the subsurface environment.
Groundwater pumped from this type of system must be treated before being
reinjected. Treatment systems are discussed in a following section.
The depth and design of the well points is dependent upon whether the
contaminant plume is heavier or lighter than water. Installation of well
points downgradient of a site can result in reducing or virtually eliminating
the flow of groundwater through the slurry wall.
Water pumped from the well points can be reintroduced into the subsurface
environment by one of several means. These include:
Reinjection wells or well points
Surface spraying
Recharge trenches.
Reinjection wells or well points are simply wells or well points where
recovered or treated water is pumped back into the aquifer. Spraying refers
to recharge using large area sprayers, much like irrigators used in arid
farmland. Trenches are simply excavated ditches where water is allowed to
infiltrate into the subsurface.
Of the three methods, a recharge trench is the most cost-effective.
Water is pumped into the trench and infiltrates into the subsurface. The
trench can be dug with a backhoe and often requires little or no maintenance.
Maintenance, when necessary involves removing from the trench materials that
may be clogging it, such as accumulated wind blown material or bacteria and
fungus. The trench results in a local elevation in the water table, but since
water is infiltrating at its own rate, as opposed to reinjection, the impact
is not as severe. Reinjection wells or well points are more costly to
install, incur costs through operation and maintenance, and may result in a
substantial rise in the local water table. Sprayers are also costly to
operate and maintain, and site considerations (such as a need for large areas
of well-drained soil) may prohibit their use.
b. Extraction/Injection Wells
Extraction wells are similar to well points in that they lower the water
table by pumping. They differ from well points in several ways. Wells are:
Able to go much deeper
Not limited to unconfined aquifers
Capable of almost unlimited capacity.
3-23
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Extraction/Injection wells are installed using one of several^drilling
techniques, depending upon the conditions existing at a site. In installing
these wells, a hole is drilled to the desired depth; the hole being of a
diameter larger than that of the well. The well consists of a screen much
like that used in a well point though usually longer, topped by a riser pipe
to the surface. Appropriate size material, e.g., gravel or sand, is added to
fill the annulus between the well and the drill hole. Usually, grout or some
other sealant is used to isolate the formation being pumped from other
formations.
The wells themselves must be of sufficient diameter to house a submer-
sible pump and to accommodate expected flow. Casings and screens must be
sufficient to withstand pressures developed during pumping.
In selecting a location for extraction/injection wells drawdown cones and
radii of influence must again be calculated. In using extraction wells,
larger capacities can result in larger radii of influence, reducing the number
of wells needed. Wells can be placed either upgradient or downgradient of a
slurry wall, much like well points, and will affect the water table in much
the same way. Extraction wells are suitable for use in conjunction with ^
slurry cut-off walls if the depth is beyond the effective range of well points
or if high capacities are required.
c. Skimmer Systems
Skimmer systems have applicability in situations where contaminants are
lighter than water and form layers on top of the water table. Systems have
been developed that rely on a series of pumps and probes which lower the water
table and pump the floating contaminants. The probes detect the concentration
of contaminants being pumped, and automatically shut down or start-up the
system when certain trigger concentrations are reached. These systems have
been used successfully to recover oil or petroleum products which have made
their way to the water table (see Figure 3-16). Proponents of these systems
claim that they are so sensitive that recovered material is virtually 100%
free of water, and can be re-used as is, without additional processing (Oil
Recovery Systems, Inc. 1982).
Skimmer systems are made up of three major components. They are:
Submersible pump to create a cone of depression
Probe assembly to prevent the submersible pump from pumping
contaminants
Recovery unit, which uses its own probe system and pumps high
concentrations of contaminants.
Installation and operation of the system is relatively straightforward.
A well is drilled which intersects the contaminated layer and water table.
The well is cased using a perforated casing allowing the passage of both
contaminant and water. A submersible pump lowers the water table, causing the
contaminants to migrate to the recovery well. A probe which differentiates
3-24
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Water Outlet (After Testing)
Figure 3-16.
Skimmer Systems
A. Floating
B. Pump Recovery
Source: Oil Recovery Systems Inc. (Undated)
3-25
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between water and the contaminant is located above the submersible pump and
automatically shuts the pump down when contaminants are detected. The
recovery unit, also with a probe, pumps the contaminant layer and shuts off
when concentrations fall below a certain level. Contaminants are stored in a
tank for final disposition, and the pumped water can be reinjected after
testing and treatment.
There are two types of recovery units; one which "floats" on top of^the
contaminant layer, and one which pumps from within the layer. The floating
systems require larger diameter wells, usually over 18" in diameter. The
pump system can be used in smaller diameter wells but usually not less than
6 inches in diameter. The pump recovery units have a higher pumping rate than
the floating units, and can be used at greater depths. New units are
available which contain filters to further concentrate the contaminant during
recovery.
Skimming recovery systems can be used in concert with cut-off walls to
effectively remove floating contaminants trapped by a wall. The wall can
prevent further migration of the contaminant, causing a build-up of the
material at the well. Either type of recovery unit can then be used to
recover the "ponded" material.
3.2.1.2 Groundwater Treatment
The type of treatment system used depends on the contaminants present.
Treatment systems may be relatively simple or quite complex. If publicly
owned treatment works (POTW's) are near, they may be used to treat water but
preliminary on-site treatment may be necessary in order to meet certain limits
set by the POTW. Using an existing facility will substantially reduce the
cost of treatment. If the water must be reinjected, more complete on-site
treatment may be necessary prior to reinjection. In any case, treatment
systems must be designed to meet site-specific situations. Treatability
studies should be conducted to determine effectiveness of the treatment system
options. Mobile labs and treatment modules are available to perform on-site
studies and treat groundwater.
3.2.2 Collectors and Drainage Systems
Collectors and drainage systems can be used in conjunction with slurry
walls to control surface and subsurface waters. Surface water control
measures, for example, can be used at a slurry wall site to prevent water from
infiltrating into the disposal area. In combination, surface water controls
and a slurry wall can often serve together to curtail leachate generation.
Surface trenches can be used as recharge points during the dewatering of a
3-26
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slurry wall site. There are numerous types of surface water collectors among
which are the following:
Dikes and berms
Ditches, diversions, waterways
Terraces and benches.
Subsurface drainage systems are designed to intercept and collect shallow
subsurface water flow. In conjunction with a slurry wall, as with surface
water controls, these systems can intercept water before it reaches the
disposal area, thus controlling leachate generation. The interception and
re-direction of groundwater in the vicinity of a slurry wall may also serve to
relieve water pressure on the wall itself. The basic components of a subsur-
face water drainage or collection system are listed below:
Gravel trenches or permanent drains
Drain filter material
Basin, sump or pit.
The design and use of surface and subsurface water control measures is
always dependent upon site specific conditions. For a more complete dis-
cussion of these technologies and their applications, see U.S. EPA (1982).
3.2.3 Surface Sealing
Surface sealing, or capping, is the process by which surface areas are
covered to minimize surface water infiltration, control erosion, and contain
contaminated wastes and volatiles. A variety of low permeability cover
materials and sealing techniques are available for such purposes.
Surface sealing can be particularly important in conjunction with slurry
walls Because one of the major routes of infiltration is vertically along the
wall itself^and this could seriously alter the effectiveness of a wall. To
prevent infiltration, a surface cap can be installed which is below grade
sloping upward away from the trench. This cap should be added after the
slurry trench has been backfilled, and should consist of low permeability clay
or other suitable material. If the area in which the trench lies is expected
to accommodate traffic, especially if the wall is an initial step in a larger
remedial action project, the surface cap must be constructed to distribute
weight and avoid placing a stress directly upon the wall. To accomplish this,
a special "traffic cap" can be installed. This cap also is V shaped sloping
up and away from the trench, but is backfilled with alternating layers of
clay, gravel, and geotextiles. The geotextiles can be anchored with soil away
from the trench to provide additional strength.
There is a variety of materials available for surface sealing purposes.
Fine-grained soils such as clays and silty clays have low permeabilities and
are therefore best suited for capping because they resist infiltration and
3-27
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percolation of water. Very often different soil types can be blended together
to broaden the grain size distribution and minimize the infiltration capacity
of the soil cover. Chemical stabilizers, cements, lime, or fly ash can also
be added to cover soils to create stronger and less permeable surface
sealants. Finally, synthetic membrane liners and asphalt mixtures can also be
used as surface sealants.
3.2.4 Ancillary Measures
A site requiring remedial action, is seldom the ideal setting for any one
remedial technique and in the case of slurry cut-off walls there are several
additional measures that can be used to reinforce the integrity of the^ wall.
These techniques include grouting, sheet piling and the use of synthetic
membrane liners. The following section will describe these three techniques
and discuss how they can be used in conjunction with slurry cut-off wall
installations.
3.2.4.1 Grouting
Grouting is the practice of pressure injecting a fluid material into
soil, rock or concrete so as to decrease the soil/rock permeability and/or
strengthen the formation. Three major types of grouting techniques are
practiced in the construction industry:
Area Grouting - Low pressure blanket or area grouting performed to
seal and consolidate soils near the surface
High-Pressure Grouting - Grouting at depth to seal fissures or small
void spaces
Contact Grouting - Injection of a slurry at the outer surface of an
excavation to seal possible passages for water flow.
The latter two are the more commonly used techniques when grouting^is^
necessary as part of a slurry cut-off wall installation. One of the principle
elements in the design of a cut-off wall is the connection between^the wall
and the underlying aquiclude. Keying a cut-off trench into the existing
aquiclude requires depth enough to penetrate any weathered zones, pervious
lenses, desiccation cracks or any other geological features that might allow
seepage under the cut-off wall. In many situations trench excavation can be
accomplished using standard excavation equipment, however, excavating into
rock can pose problems. Even in a situation where the rock mass^is jointed^or
fractured, it is often next to impossible to excavate without using percussion
tools or heavier machinery that can further fracture the rock mass
(D'Appolonia 1980). If the trench construction equipment on-site includes a
crane, no additional equipment would be needed other than a chisel and
clamshell which are suspended from the end of the crane. Use^of heavier
equipment can often be quite costly and time consuming. Consideration must
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also^be taken for the area surrounding the site in a decision to use heavy
machinery. Often there is a problem with either site access or physical
constraints on the site itself and heavy equipment is impractical. In many
cases, as an alternative to using heavier and more expensive equipment and
attempting to advance, the cut-off into the rock, the decision is made to grout
the interface between the cut-off wall and the rock (Figure 3-17). This can
be accomplished by using either one of two grouting techniques. The a^uiclude
can be sealed^during trench excavation, prior to wall' installation by the
contact grouting method or the wall-aquiclude contact can be reinforced after
the wall has been emplaced. The latter case entails the use of high-pressure
grouting.
Bottom key grouting is one application of grout injectioti associated with
slurry cut-off wall,installations. Additional practices include grouting one
or both ends of the completed wall to some existing structure on site and,
area grouting of soil material along sections of a constructed wal'l that need
extra reinforcement.
It is sometimes practical to utilize structures already existing on-site
as^part of a pollutant containment solution. An example of such a pre-
existing structure might be a flood protection wall or dike. If a remedial
action project required the construction of a slurry cut-off wall on
the river side of a dike to prevent contaminant migration into the surface
waters, it might be possible to contain the contaminant plume using the dike
as part of the containment wall by grouting the wall-dike contact at one or
both ends (Figure' 3-18). There are, of course, many factors not discussed
here that must be considered before arriving at such a decision, such as the
permeability of the dike material, depth of the contaminant plume, or
direction of groundwater flow. It might be necessary, for example, to grout
only the downstream end of the cut-off wall depending upon the areal
groundwater--flow pattern and contaminant movement.
Area grouting of soil or loose material along a slurry cut-off wall might
be necessary to retard erosional processes along the length of the wall. A
situation in which this type of grouting technique could be applied, might be
where a wall has been installed along a steep slope. Erosion rates can be
significantly reduced by grouting loose material surrounding the wall.
grouting materials fall into three groups: cement, bituminous, and
chemical. Some specific grout mixtures include Portland cement, sand-cement,
clay-cement, clay-bentonite, bituminous emulsions, sodium silicate, and
acrylamide. The applicability of each material is based on grain size or
fissure size, and the anticipated area of grout penetration. The subsurface
environment must be investigated thoroughly prior to the design of a grouting
program. Initially, it must be determined with tests whether or not a site
is, in^fact, groutable. Areas of extremely low permeability or great
variability may not be groutable. Other tests and investigations will provide
the necessary hydrogeologic information in order to choose the best-suited
grout or grouts.
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Figure 3-17.
Bottom Key Grouting
Cut-off Wall
3-30
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Figure 3-18.
Cut-off Wall-Dike Contact
Stream
Grouted
Wall-Dike
Tie-In
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3.2.4.2 Sheet Piles
Sheet piles are typically used to brace trenches and other excavations,
or to support retaining walls and bulkheads (U.S. EPA 1982). Their main
utility is to hold earth materials in place. The use of sheet piling in
conjunction with slurry cut-off walls is yet another alternative remedial
measure, in which it can be used as an erosion control. Examples of this type
of sheet pile utilization, however, have not been identified in the reviewed
literature. The following is a discussion of sheet piles and their most
common applications, in addition to possible applications in adjunct with
cut-off walls.
Sheet piles can be fabricated from three materials; wood, precast
concrete, and steel (U.S. EPA 1982). Wood is generally an ineffective water
barrier and because slurry cut-off walls are generally constructed at sites
where leachate or groundwater containment is the objective, it would not be
advisable to use wooden structures. Concrete sheet piles are used primarily
in situations that require a great amount of strength, such as is needed
during dam construction. Sheet piling strength, in this degree, most likely
would not be a requirement in a leachate containment remedial project. Steel
sheet piling, in a case where sheet piling was chosen to be an additional
remedial measure, would be, in comparison, the most effective in terms of both
cost and potential for groundwater cut-off.
Steel sheet pilings are installed by driving the interlocking piles into
the ground with a pneumatic or steamdriven pile driver. In some cases, the
piles are pushed into pre-dug trenches (U.S. EPA 1982). The lengths of piles
range between 4 and 40 feet and their widths range between 15 and 20 inches.
Many steel pile manufacturers offer their own shape of piling and often their
own form of interlock. Steel sheet piling can be used in conjunction with a
slurry cut-off wall as an erosion control or resistance mechanism. In slurry
cut-off wall projects where contaminant containment is of utmost importance,
and sheet piles are to be used as erosion controls in a location where there
may be leachate or groundwater build up, it is crucial that the piles are as
well locked as possible to minimize seepage through the interlocks. For this
reason, the pilings should be assembled at their edge interlocks before they
are driven into the ground. When initially placed in the ground, sheet piling
is permeable. The edge interlocks, which are necessarily loose to facilitate
placement, allow water passage. With time, however, soil particles are washed
into the pile seams and water cut-off is effected to a greater extent
(U.S. EPA 1982). The time required for sealing to take place depends on the
rate of groundwater flow and the soil texture involved. In very coarse, sandy
soils, the wall may never seal. Additionally, steel sheet piles should not be
considered for use in extremely rocky soils. Even if enough force can be
exerted to drive the piles around or through cobbles and boulders, the damage
to the piles might render them ineffective.
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3.2.4.3 Synthetic Membrane Liners
Synthetic membrane liners can also be used in conjunction with slurry
cut-off walls and comprise the third group of associated remedial measures.
In certain situations, it may be possible to reinforce the integrity of the
cut-off wall with the addition of a synthetic liner. The liner materials are
available in a variety of compositions with relatively well known chemical
compatibilities.
Placement of a synthetic liner into the ground as a vertical barrier is a
relatively difficult procedure. The liner is suspended vertically in and
along one side of a slurry filled trench and the trench is then backfilled.
Placement of a liner within a cut-off wall does have disadvantages. The liner
material is extremely heavy and is stored and purchased on large rolls.
Frequently, suspending the liner vertically in the trench is next to impos-
sible because of the size and weight of the sheet. Also, once the liner is
successfully placed in the trench, there is always the possibility that a
bottom corner of the liner will be uplifted during the backfilling process
(Villaume 1982). If this were to happen, water might be permitted to flow
around the barrier. Careful inspection should be enforced during the
installation process.
A possible way of overcoming problems that might arise due to the weight
and bulk of the liner material itself, would be to install the liner in
smaller sections. However, care must be taken to ensure that there is
sufficient overlap of the sections so as not to permit water seepage. In
conclusion, the use of liners as secondary containment measures can reinforce
the integrity of the wall and create additional protection against possible
seepage due to chemical attack on the backfill material.
3.3 Summary
The effectiveness of a slurry wall, with respect to both costs and
technical performance, is determined in large part by the configuration
employed. This configuration is determined by the specific remedial goals of
a specific site. For most uncontrolled hazardous waste sites, a circular,
keyed-in slurry wall offers the most complete containment. Nonetheless,
slurry walls installed for hazardous materials containment are nearly always
used in conjunction with some other remedial techniques. These range from
relatively simple measures such as surface sealing, to complex groundwater
extraction and treatment systems. Effective use of slurry walls for waste
site remediation is dependent on the selection of the most appropriate
configurations and materials, and on the selection of the other remedial
measures employed with it.
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SECTION 4
SITE INVESTIGATION AND CHARACTERIZATION
The data from a site investigation are used by a design firm to formulate
specifications for the remedial measures to be implemented at the site. These
specifications are then used by the firms installing the slurry cut-off wall
and other remedial measures. Because all later actions are dependent on the
quality and completeness of the site investigation, the effectiveness of the
entire remedial action is directly determined by the thoroughness of the site
investigation.
This section discusses the types of data that should be acquired during
the site investigation, possible data sources to be researched, and methods
used to conduct field and laboratory analyses. The physical constraints
imposed by the site1s surficial features are discussed, followed by a
description of subsurface site investigative procedures. Finally, information
is presented on the procedures used to characterize wastes and leachates and
their effect on wall quality and durability.
4.1 Physical Constraints
There are a number of physical considerations regarding both the site and
the working area that affect the applicability of slurry wall types and the
techniques used for their construction. Table 4-1 summarizes the various
physical constraints that may be encountered and must be resolved. These
constraints include:
Topography
Vegetation density
Land drainage patterns
Availability of water
Location of utility crossings
Proximity of property lines, major residential areas and
transportation routes
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TABLE 4-1.
TYPES OF PHYSICAL CONSTRAINTS AND THEIR EFFECTS
ON SLURRY WALL CONSTRUCTION
Physical Constraints
Possible Affected Areas
Approach Required
Topography: Irregular contours
Steeply sloping terrain
Necessary equipment
Site access and work space
Type of wall selected
Selection of equipment capable of
operating in site specific terrain
Extensive site preparatory work -
leveling of areas for site entry and
work space
Use of CB wall in panels or diaphragm
wall
-o
i
Site Access and
Work Space: Site congestion/
traffic
Steep terrain
Dense vegetation
Lack of head room
Insufficient space
for mixing
Extent of site prepara-
tion and pre-construction
Type of equipment selected
Type of wall selected
Wall construction process
Special equipment needs; construction
of access road; leveling of working
area; clearing of dense vegetation
Amount of head room affects type of
equipment selected or needed to
relocate obstruction
Amount of work space affects wall type
selected; SB wall requires space for
mixing; CB wall requires less area for
operations, but it is more expensive;
SB can be mixed away from trench but
this approach may mean CB is cheaper
for Che site
(continued)
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Table 4-1. (continued)
Physical Constraints
Site Access and
Work Space:
(continued)
Utilities:
Abandoned sewers
Pipelines
Leakage from water
mains, sewers
Power/telephone cables
i
UJ
Possible Affected Areas
Approach Required
Equipment selection
Construction process
(operations)
Problem control methods
Sudden slurry loss and
possible trench collapse
if unanticipated pervious
zone, i.e., sewer piping
is encountered and
ruptured
Cultural Features:
Old foundations
Nearby structures
Overhead structures
* Equipment selection
Construction process
(operations)
Problem control methods
Extra time needed for site pre-
paration and construction
Appropriate easement clearances
Special equipment necessary for
excavation around piping and
sewer lines; or need for manual
excavation
Sequence of trench segment
excavation may change if
utility discovered; excavate
other areas first
Watermain or sewer leakage may
cause slurry contamination and
loss of trench stability; a control
plan necessary at outset of project
Sudden slurry loss requires immediate
placement of solid materials (soil,
debris) into trench
Foundation penetration to isolate site
Excavation around foundations, or
incorporate foundation into wall; if
foundation support needed CB or dia-
graphm may be required
(continued)
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Table 4-1. (continued)
Physical Constraints
Possible Affected Areas
Approach Required
Cultural Features:
(continued)
Other: Availability of water
£. Time of year; water table
£ fluctuations, temperature
Subsurface geology; large
subsurface boulders
Type of wall backfill
Headroom
Special equipment needed if breaking
old foundations
- Tall equipment,-e.g., cranes, may be
restricted
More time may be necessary for
operations
Experienced problem control personnel
necessary
Equipment selection
Slurry mixing
Time needed and
available for project
completion
Site preparation
Problem control methods
Equipment selection for boulder
destruction or excavation
Site may need de-watering system
if water table is high or is expected
to rise
SB backfill cannot be mixed in sub-
freezing temperatures
CB will not set in certain temperature
ranges
Experienced problem control-personnel
necessary
Transport of water to site if none
available
References: (1) Ryan 1980a, (2) U.S. Army Corps of Engineers 1978, (3) Xanthakos 1979, (4) Wetzel 1982,
(5) Tamaro 1980, (6) Ryan 1980b, (7) Namy 1980.
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Site accessibility
* Other man-made features.
4.1.1 Topography
Topographic features that should be noted during on site investigation
include the steepness of the slopes on the site, the types of land drainage
patterns present, and the proximity of the site to major bodies of water.
These features can be discerned from topographic maps but should also be noted
during site investigations. - >
4.1.2 Vegetation Density
Areas having extremely dense vegetation should be expected to require
extra site preparation prior to field investigations or construction
activities. Dense vegetation can inhibit access and hide important surficial
features such as small outcrops or erosion gullies. Surveying of the proposed
trench line and location of the bore holes is also delayed by dense
vegetation. '
4.1.3 Land Drainage Patterns
As noted previously, land drainage patterns at the site can be
preliminarily assessed using topographic maps. Additional information on the
presence of erosion gullies, heads of drainageways, and anomolous features
must be obtained during site visits. Because surficial land drainage patterns
directly affect subsurface water movement, this site feature should be
carefully assessed.
4.1.4 Availability of Water
A great deal of water is necessary during slurry cut-off wall
installation. The source amount, quality of the water available should be
ascertained during the site investigation.
4.1.5 Location of Utility Crossings
If water electric, gas, telephone, sewer or other utility lines cross the
site, the exact locations should be determined and marked so that they are
disturbed as little as possible during the site investigation and subsequent
trench excavation. If these lines cross the excavation site, provisions for
re-routing must be made.
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In addition data on the availability of water, sewer, electricity and
telephone service should be noted.
4.1.6 Proximity to Property Lines
The location of the site relative to property lines, major residential
areas, and transportation routes is an important element to be considered
during the site investigation. If existing structures are very close to the
site, the effects of trench excavation on structural stability must be
assessed, and provisions made for interim structural support. Residential
structures require special provisions, such as noise control and fencing of
the site. Major transportation routes transecting the site must be re-routed
If the site characteristics make it necessary to consider installing the
slurry cut-off wall close to property lines, permission of nearby property
owners for access road use or land disturbance may be necessary. These
factors should be noted in the site investigation report.
4.1.7 Site Accessibility
In remote areas or extremely congested locations, site access may be a
problem. Remote areas may require the construction of roads or bridges and
bringing water and other utilities to the site. Construction in congested
environments may be complicated by access roads, such as driveways or alleys,
or overhanging obstructions, such as signboards or utility lines. In these
areas, equipment mobility may be seriously hampered.
4.1.8 Presence of Other Man-made Features
Certain man-made features can seriously affect the design and instal-
lation of slurry cut-off walls. These features should be thoroughly
investigated during the site characterization. Included in this category of
physical site constraints are: mines, dams, irrigation ditches and tunnels.
The location, size and other characteristics of these man-made features should
be determined during the site investigation.
Some of the slurry walls installed to date have been placed in sites
having a number of interesting site constraints. Two examples of the
procedures used are given below.
During excavation dewatering at a large industrial plant, Ryan (1980b)
reported that the following constraints were identified for this site:
Concrete foundations under site
Tight access conditions throughout site.
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In addition to these constraints, the owner needed to maintain access across
the site, the foundations had to be penetrated to isolate the site and its
wastes, and it was hoped that the old foundations could be used as much as
possible as part of the wall. The final solution to this site involved the
following:
Constructing an open-cut along the foundation alignment
Using a hydraulic ram to break the old foundations
Filling the opencut.
After these procedures were accomplished, the slurry trench was excavated to
its design depth and a CB slurry was installed.
At another site, an SB slurry trench was to be installed around a waste
lagoon. The installation took place on top of a dike which was 20 feet wide.
The base of the backhoe was 14 feet wide so there was no problem using the
backhoe for excavating the trench. A bulldozer was normally used to mix and
place the backfill,-however, since the bulldozer could not work in the limited
area on top of the dike, a crane with a clamshell bucket was used for backfill
mixing instead (Wetzel 1982).
4.2 Subsurface Investigations
As an initial step in obtaining information on the subsurface conditions
existing at a site, all available sources of hydrogeologic data should be
gathered. These include: geologic and topographic maps, hydrogeologic
reports, aerial photographs, well drilling logs, and soil surveys. By
reviewing published and other historical data, a preliminary characterization
of the site can be made concerning the subsurface environment. Table 4-2
summarizes the principal sources of available geotechnical data.
A description of the hydrogeologic framework of an area should include a
discussion of the following factors:
Structural attitude and distribution of bedrock and overlying strata
Chemical and physical properties of these strata including mineralogy,
and permeability
Weathering of these strata including the degree of alteration, the
pattern and depth of weathering and any evidence of incompetent rock
Groundwater regime, including water table depths, aquifer types, flow
gradients and groundwater quality
Soil characteristics including soil type and distribution, particle
size distribution and permeability.
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TABLE 4-2.
PRINCIPAL SOURCES OF AVAILABLE GEOTECHNICAL DATA
Published Data
1. U.S.G.S. Surficial Geology Maps
2. U.S.G.S. Bedrock Geology Maps
3. U.S.G.S. Hydrological Atlases
4, U.S.G.S. Basic Data Reports
5. State and County Geologic and Hydrologic maps
and reports.
6. National and Local Technical Journals,
Magazines and Conference Proceedings
7. U.S.S.C.S. Soil Maps
Unpublished Data
1. Local test boring and well drilling firms
2. Local and State highway departments
3. Local water departments
4. State well permit records
5. State and Local transportation departments
6. State and Federal Environmental Agencies
7. State and Federal Mining Agencies
8. Army Corps of Engineers
9. Local consulting, construction and mining
companies
10. Geologists, hydroleogists, and engineers at
local universities
11. Historical records
12. Interviews
Notes: U.S.G.S. - United States Geological Survey
U.S.S.C.S. - United States Soil Conservation
Service
Reference: Guertin and McTigue 1982c.
4-8
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Through the examination and analysis of this information specific data
gaps can be identified and programs of further exploration can be planned and
implemented to broaden or add to existing knowledge of site conditions.
There are three major issues involved in slurry wall design and
construction that require an accurate and detailed hydrogeologic assessment.
They include (l) the type of excavation equipment to be used (2) the depth to
which the trench and subsequent wall will extend, and (3) the extent to which
soils found onsite can be used in the backfill. These three issues must be
continuously considered throughout the subsurface investigation in order to
develop an appropriate and adequate slurry wall design for a particular
situation. The design of a slurry wall project should progress in stages as
investigations proceed and more detailed subsurface information is gathered
and analyzed (Guertin and McTigue 1982c) ,
The following sections discuss the geologic, hydrologic and soil data
necessary for the proper design and construction of a slurry wall.
4.2.1 Geology
The types of geologic information needed to properly design and construct
a slurry wall include: rock depth, rock locations, location of structural
discontinuities and the degree to which weathering in each rock type has
occurred.
The existing rock types and the nature of the weathered zone will often
determine the type of equipment used during excavation of the trench (Goldberg
1979). The depth to which a trench must be excavated will also play a major
role in determining the type of equipment necessary at a particular site. The
types of excavation equipment used during slurry trench excavation are
described in Section 5.
The existence of an impervious geologic formation at a particular site
into which a slurry wall can be keyed is the geologic characteristic that most
strongly influences the vertical extent of a slurry wall. The depth to such
an aquiclude frequently determines the excavation depth of the trench. Trench
excavation for a keyed-in wall must extend into the aquiclude at all points
along the trench length in order to avoid seepage zones that can easily breach
the cutoff (Namy 1980). The elevation of the aquiclude1s surface, however, is
not necessarily constant. For this reason, the contour of the aquicludes
surface must be carefully mapped.
The distance that the cut-off wall must penetrate into the aquiclude is
determined by the composition and geochemistry of the impervious layer. If
the aquiclude is a competent impervious bedrock, a minor penetration may be
satisfactory. In contrast, an excavation that is carried into a clay
formation may employ deeper penetration into the aquiclude as a safety factor
(Millet and Perez 1981). It should be noted that although the presence of an
aquiclude is an important consideration in the design of a slurry wall, there
are cases in which it is unnecessary to key into an impervious layer, such as
4-9
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a scenario involving floating organic wastes, e.g., coal tar residuals. In
this case it is only necessary to extend the wall to some specified depth
below the base of the waste column.
Structural discontinuities and anomalous subsurface conditions that might
interfere with either the wall continuity as it is being excavated or the
tie-in with the aquiclude should be identified and methods for dealing with
the discontinuities should be developed prior to excavation activities. Even
if there is an acute awareness of potential problems, complications can arise
and possible solutions for closures of pervious areas (called windows) in the
wall should be evaluated.
The site-specific data needed to evaluate an area for the purposes of
design and construction are seldom available solely from published sources.
Thus the determination of'the character and condition of the existing rock
must be made through site investigative techniques, such as:
Test borings
Test pit excavations
Rock coring
Geophysical surveys
Laboratory analyses.
Detailed descriptions of these techniques and the types of data that can be
collected through their use can be found in ASCE (1976) and Ash et al (1974).
4.2.2 Hydrology
In addition to understanding the areal geology, an understanding of the
groundwater system and its interactions with surface water is necessary prior
to designing a slurry wall system for installation at a site. A detailed
description of the groundwater regime is needed to more clearly define
pollutant migration at a site. Only with a thorough understanding of
potential plume configurations can the optimum slurry wall system be selected
and implemented. Evaluations must also be made to determine the necessity for
site dewatering during construction and if so, what the effects might be to
surrounding land and structures. The types of hydrologic information that are
typically required to design and construct an effective slurry wall are:
Determination of boundary conditions, e.g., hydraulic head distribu-
tions, recharge and discharge zones, locations and types of boundaries
Determination of material constants, e.g., hydraulic conductivity,
porosity, transmissivity, area extent and thickness of geologic units,
location of geologic units (accomplished during geologic investiga-
tion)
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Analysis of ground and surface water quality, e.g., background water
quality, waste constituent concentrations, boundary conditions of
wastes.
As with the initial stages of a geological investigation, preliminary data
needed for determining the hydraulic system at a site can be obtained from
numerous published and unpublished sources (see Table 4-2). Those sources
most applicable to the hydrologic investigation are:
Federal and state Geologic Surveys (USGS)
Soil Conservation Service (SCS)
Environmental Protection Agency (EPA)
Local Water Control Boards (i.e., state and county)
Drillers logs
Operators plans and permits.
These data could include reports on the local geology, surface and groundwater
quantity and quality, hydraulic properties of materials, and location of
groundwater users. Based on the amount and quality of information obtained
from these sources, it may be necessary to perform an additional on-site data
collection program. This program could be very simple, where existing local
wells are sampled for water quality and yield. An on-site data collection
program may also be very complex, if new wells are drilled, geologic materials
are sampled, pump tests are performed, and water quality and quantity samples
are taken and analyzed. The level of effort spent in field collection
programs should be adequate to fill the needed knowledge gaps so that a
working slurry wall can be designed at a minimum cost.
Discussions of the various methods and testing techniques available for
the accurate definition of the groundwater regime at a site can be found in
numerous references, however, a recommended source of information is Guertin
and McTigue (1982c).
In addition to the investigation of the movement and distribution of
existing groundwater, it is equally important to study groundwater quality.
Water quality has important influences on selection of the bentonite slurry
and backfill, on the selected equipment and on the environmental concerns
relative to disposal of uncontarainated groundwater from extraction wells
(Guertin and McTigue 1982c). Depending upon the suspected or known waste
types at a site, laboratory analyses of groundwater and/or surface water
samples, will involve tests for different chemical constituents. The testing
conducted relies entirely on site specific conditions. These issues are
discussed in Section 4.3.
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Using the data obtained for a site, numerous hydrogeologic and slurry
wall design specifications can be produced. These could include the
development of:
Potentiometric surface maps and flow nets for the hydrologic system
Geologic cross-sections with water table levels
Depth and extent required for slurry wall
Type of slurry wall that can be constructed based on contaminant
compatibility and intended use.
For each site where a pollution migration cut off wall is to be con-
structed, a detailed hydrologic analysis must be performed to ensure success.
Failure to perform this analysis could result in walls which allow contami-
nants to migrate past them (i.e., either under or around), walls that
deteriorate because of contaminat interaction or walls that are over designed
causing increased -costs.
4.2.3 Soils and Overburden
Examination of the soil conditions existing at a site is another
important part of the subsurface investigation. Soil information, in addition
to being required for excavation and construction planning purposes, is needed
primarily for the backfill design. The design of the backfill is probably the
most important factor in the design of a slurry wall system (Case 1982). The
backfill composition that is selected for a particular slurry wall site will
consist of a designated percentage of small sized particles called fines. The
desired particle size distribution of the backfill is determined during the
design stages of the wall (Case 1982). It is, therefore, necessary to
evaluate the presence of fines in the area of excavation. If sufficient fines
are not available on-site, then a borrow source must be identified and plans
for transportation of the material to the site should be incorporated into the
construction contract. Additional soil parameters that should be measured
during a site soil survey for the purpose of designing the backfill material
are the following:
Soil water content
Permeability
Horizontal and vertical distribution
Chemical properties (e.g. organic content)
Gradation (discussed above as percent of fines).
If suitable soils are not available nearby, a decision may be made to use
alternative backfill.
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Though maps and aerial photographs of an area may provide useful soil
information, a slurry wall construction project should have on-site subsurface
soil explorations conducted to obtain the necessary detailed information on
soil types. This information is typically obtained through the use of the
same investigative measures used to obtain hydrogeoiogic information. These
include soil borings, test pits, and geophysical investigative methods
followed by laboratory analyses of the samples collected. When properly
correlated, the data obtained by utilizing these techniques, can be used to
accurately define the type and extent of the soil strata underlying a site
area. Two references are recommended as sources of information regarding soil
investigative techniques. They are McCarthy (1977), and Ash et al (1974).
There are two points with respect to site investigation and character-
ization that require reiteration. The first point involves the measures used
to examine site conditions. It is important that the subsurface investiga-
tions involve both direct and indirect methods of exploration, particularly
where conditions are suspected to be complex. Neither of the two types of
techniques are solely capable of providing all of the information required to
adequately describe the subsurface environment of a site area. Their use in
conjunction with one another, however, can provide the detail and level of
certainty necessary to properly characterize a site.
The second point is directed towards slurry wall application and
effectiveness in a particular situation. The more thoroughly a site is
characterized by investigation, i.e., the more detailed information available
on surface and subsurface conditions, the more effectively a slurry wall may
be designed and employed to control a pollution problem. Even the most
experienced professionals cannot properly design and construct a slurry wall
without having a thorough understanding of the situation at hand.
4.3 Wastes and Leachates
The presence of organic or inorganic compounds in the groundwater can
have a detrimental effect on the bentonite slurry used during wall construc-
tion as well as on the ability of the finished wall to restrict pollutant
migration. These chemicals can affect the physical/chemical properties of the
bentonite and the backfill material, leading to failure of the wall either
during construction or during its operational lifetime. Thus, before a slurry
wall is considered as an appropriate remedial action at a site, the effects of
the leachate on the bentonite slurry and the finished wall must be determined.
A compatability testing program is necessary to provide the information needed
to properly select the type of bentonite and backfill material that should be
used in the slurry wall construction.
The following sections outline the effect that chemicals have on
bentonite and backfill material, and laboratory tests that can be used to
establish the potential effectiveness of the slurry wall.
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4.3.1 Effects of Groundwater Contaminants on SB Walls
Different chemicals can affect the physical/chemical properties of the
bentonite and backfill material, leading to:
Flocculation of the slurry
Reduction of the bentonite's swelling capacity
Structural damage of the bentonite or backfill material.
These changes can in turn lead to failure of the trench during excavation
and/or increase the permeability of the finished cut-off wall. These
potential adverse effects on the performance of slurry walls are discussed
further in the following sections.
4.3.1.1 Effects of Groundwater Contamination on Bentonite Slurries
Contaminated groundwater can come in contact with the bentonite slurry^
during trench excavation or if it is used to hydrate the bentonite. The major
problems presented by the contaminants is flocculation of the slurry and/or a
reduction in swelling capacity of the bentonite, leading to poor filter cake
formation and potential collapse of the trench (Alther [no date],^Xanthakos
1979). This is caused usually by the presence of high concentrations of
electrolytes, such as sodium, calcium, and heavy metals, in the groundwater^
(Matrecon 1980, Alther [no date]). These ions can produce several changes^in
the betonite/water system that will lead to flocculation or reduced hydration
of the bentonite.
Monovalent sodium ions on the surface of the bentonite can be readily
exchanged with multivalent ions, such as calcium or other metal ions, con-
tained in the leachate. The replacing multivalent ions will have^a smaller
radius of hydration than the sodium ion, thus reducing the dimension of the
double layer (see Section 2). This will in turn greatly reduce the swelling
capacity of the bentonite. (Alther [no date], D'Appolonia and Ryan, 1979).
Thus the bentonite will not fully hydrate, and can settle out of suspension
(Alther [no date]). Ions in solution can also compete for available water
with the clay surface, causing a decrease in the thickness of the water shell
around the clay particle (Matrecon, 1980). This can also impede the full
swelling potential of the bentonite (Metrecon 1980, Hughes 1975).
These mechanisms will cause a compression of the double layer of water
molecules surrounding each clay particle. This results in a decrease in the
repulsive interactions between clay particles that can lead to an increased
potential for particle aggregation resulting in flocculation of the bentonite
suspension (Weber 1972).
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4.3.1.2 Effects of Groundwater Contaminants on the Permeability of
Cut-off Walls
Recent studies have shown the effects of a variety of inorganic and
organic compounds on SB slurry walls (D'Appolonia and Ryan 1979, D'Appolonia
1980a). As Table 4-3 illustrates, slurry walls can withstand the attack of a
number of chemicals commonly found in leachates. The soil bentonite mixtures
utilized in these studies contained from 30 to 40 percent fines. The results
of a large number of permeability tests utilizing a wide range of pollutants
indicated that a well graded soil bentonite mixture containing more than 30
percent fines and about 1 percent bentonite will show only a small increase in
permeability when leached with many common contaminants (D'Appolonia 1980a).
The commercial bra'nds of bentonite used in preparing the slurry or
backfill does not seem to have a significant effect on the ability of the
bentonite to withstand the effects of leachate or permeability. Table 4-4
illustrates the effects of several different types of chemicals on the
permeability of four brands of commercially available bentonite hydrated with
fresh water. For the brands of bentonite tested, there was not a significant
difference between their ability to withstand the effects of various chemicals
(D'Appolonia 1980a).
Increase in the permeability of the finished SB slurry wall can be caused
by chemical and physical changes in the structure of the bentonite and
backfill material. These changes, caused by the compounds contained in the
leachate, can affect the swelling potential of the bentonite as well as alter
the structure of the bentonite and backfill material.
Numerous organic and inorganic compounds can, through a variety of
mechanisms, cause bentonite clay particles to shrink or swell. All of these
mechanisms affect the quantity of water contained within the interspatial
layers of the clay structure. Inorganic salts can, as discussed above, reduce
the double layer of partially bound water surrounding the hydrated bentonite,
thus reducing the effective size of the clay particles. (D'Appolonia and Ryan
1979). Upon disassociation, organic bases can be sorbed into the internal
surfaces of clay particles thus affecting the interlayer spacings (Anderson
and Brown 1981). Neutral-nonpolar and neutral-polar compounds can replace the
water contained in the clay particle interlayers, thus affecting the size of
the bentonite particle (Anderson and Brown 1981). This can lead to increased
permeability of the finished slurry wall, possibly resulting in breaching of
the wall. For example, a decrease in the amount that hydrated bentonite has
swelled increases the amount of pore space in. the backfill, thus increasing
the permeability of the wall. In the worst case, a large reduction in the
effective particle size can result in the physical erosion of the soil/
bentonite matrix under the seepage pressure (D'Appolonia and Ryan 1979). This
can lead to piping failure of the wall. The probability of this type of
failure occurring can be reduced, if the backfill material contains at least
20 percent plastic fines (D'Appolonia 1980b).
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TABLE 4-3.
SOIL BENTONITE PERMEABILITY INCREASES
DUE TO LEACHING WITH VARIOUS POLLUTANTS
Pollutant
Ca or Mg++ @ 1000 PPM
Ca++ or Mg++ @ 10,000 PPM
NH,N03 @ 10,000 PPM
Acid (pH>l)
Strong Acid (pHll)
HCL (1%)
H2S04 (1%)
HCL (5%)
NaOH (1%)
CaOH (1%)
NaOH (5%)
Benzene
Phenol Solution
Sea Water
Brine (SG=1.2)
Acid Mine Drainage (FeSO,
pH~3)
Lignin (in Ca solution)
Organic residues from
pesticide manufacture
Alcohol
Backfill0
N
M
M
N
M/H*
N/M
M/H*
N
N
M/H*
M
M
M/H*
N
N
N/M
M
N
N
N
M/H
N - No significant effect; permeability increase by about a factor of 2 or
less at steady state,
M - Moderate effect; permeability increase by factor of 2 to 5 at steady
state.
H - Permeability increase by factor of 5 to 10.
* - Significant dissolution likely.
- Silty or clayey sand, 30 to 40% fines.
Reference: (1) D'Appolonia (I980a), (2) D'Appolonia and Ryan 1979.
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TABLE 4-4.
INCREASE IN THE PERMEABILITY OF FOUR BRANDS
OF BENTONITE CAUSED BY LEACHING WITH VARIOUS POLLUTANTS
Final Permeability/Initial Permeability
Permeant Slurry National Premium Saline Seal
Ben 125 Brand 100
Lignin in
Ca++ solution 1.9 1.5 2.5
NaCl based salt
solution
(conductivity
170,000) 2.7 1.8 2.7
Ammonium nitrate
(10,000 ppm) 1.8 N/A 2.8
Acid mine drainage
(pH~3) N/A 1.5 1-3
Calcium and
magnesium salt
solution
(10,000 ppm) 2.9 3.2 3.2
Do we 11
Ml 79
1.4
N/A
N/A
N/A
N/A
N/A: Data not available.
Reference: D'Appolonia 1980a.
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Strong organic and inorganic acids and bases can dissolve or alter the
bentonite or soil portion of the backfill material, leading to large permea-
bility increases (D'Appolonia and Ryan, 1979, Alther [no date]). Aluminum and
silica, two of the major components of bentonite, are readily dissolved by
strong acids or bases, respectively. (Matrecon 1980). Strong bases, though,
usually produce a greater increase in permeability than acids due to the
dissolution of silica (D'Appolonia and Ryan 1979). Laboratory studies
have shown that a well graded soil-bentonite backfill containing more than
20 percent plastic fines and about 1 percent bentonite shows only a small
increase in permeability when exposed to a solution in the pH range of
2 to 11 (D'Appolonia and Ryan 1979).
While a properly designed slurry wall can withstand the effects of many
chemicals, it is essential to know the long terra effects that the compounds
found in the groundwater will have on the permeability of the wall. Thus it
is^essential to test any pollutant with the actual backfill material that is
going to be used. (D'Appolonia and Ryan 1979). The following sections
address testing methods which can be used to determine the impact of leachate
on SB slurry walls.
4.3.2 Compatibility Testing
To test the compatibility of compounds contained in the groundwater with
Che material used in the construction of slurry walls, a series of laboratory
tests should be performed. Since there are, as yet, no standard tests and
testing procedures established for determining the compatibility of chemicals
with slurry walls; the types of tests and their associated testing procedures
can vary^widely between laboratories. Through discussions with both private
and public laboratories and a review of the literature, several quantitative
testing methods were identified as being applicable. These include:
Viscosity test
Filter-press test
Permeability test
Examination of Bentonite Mineralogy.
In performing any of these tests, representative samples of the leachate and
backfill material must be collected. Procedures for groundwater and soil
sample collection can be found in several publications, such as U.S EPA
(1981).
4.3.2.1 Viscosity Test
The^viscosity of the bentonite slurry can be an important factor in
determining the effect of the compounds contained in the groundwater on the
slurry. Groundwater contaminants can change both the viscosity and gel
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strength of a bentonite slurry. Thus by testing the changes in slurry
viscosity caused by the addition of leachate, the effects can be established
and potential remedial responses can be sought.
One device that is recommended by the American Petroleum Institute (API)
for testing slurry viscosity is the direct indicating viscometer, as
illustrated in Figure 4-1. By utilizing this device the plastic viscosity,
yield point, and apparent viscosity can be easily determined. The procedures
for performing this test and the required calculations are outlined in API
(1982).
4.3.2.2 Filter-press Test
The standard filter-press test that is commonly used to evaluate drilling
mud has been utilized to indicate the effects of leachate on slurry and filter
cake performance. In the filter press test, bentonite slurry is introduced
into a high pressure filter apparatus, as in Figure 4-2. A pressure is
applied to the filter apparatus, and the resulting filtrate is collected
(Xanthakos 1979; API 1982). The quantity of filtrate should be within
established limits. By introducing a representative quantity of leachate into
the bentonite slurry before the test is performed, the short-term effects on
fluid loss can be established. If there are any detrimental effects, the
fluid loss will be outside the bounds of the established limits. The
procedures and apparatus needed in order to conduct this test are outlined in
API (1982).
4.3.2.3 Examination of Bentonite Mineralogy
An examination of the mineralogy of the bentonite backfill before and
after it has been exposed to the leachate in the permeameter test could be
used to determine short-term chemical effects on the clay structure. By
utilizing standard laboratory tests, such as X-ray diffraction the effects of
the leachate on the clay structure can be determined. This can provide an
indication of the long-term stability of the bentonite backfill.
4.3.2.4 Perraeability
The effect of leachate on the permeability of a soil/bentonite cut-off
wall can be ascertained by several standard soil testing procedures. By
comparing the permeability of a soil/bentonite mixture when it is permeated
with leachate to that obtained when it is permeated with water, a deter-
mination can be made of the leachate1s potential affect on the permeability of
the SB cut-off wall.
Laboratory procedures for performing permeability tests can be divided
into several general categories depending on how the level of liquid used in
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Figure 4-1.
Rotational Viscometer
Scale
\
Slurry Level
Pointer
Copyright 1979 by McGraw-Hill, Inc.
Used with permission.
Helical Torsion Spring
Splash Guard
Rotor Sleeve
Cup
Source: Xanthakos, 1979
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Figure 4-2.
Filter-Press Test Apparatus
,
Regulated Pressure Source
Gas Pressure
i Pressure Cell
Leach ate (or Slurry)
Filter Cake
Filter Paper
Porous Stone
Collection Cup
Filtrate
Source: D'Appolonia & Ryan 1979
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the apparatus is maintained during the course of the test, e.g., constant or
falling head permeability tests, and the type of apparatus used to contain the
soil/bentonite sample, e.g., fixed wall or triaxial permeameters.
Any of the established permeability testing procedures can be utilized,
such as those outlined in OCE (1970). While all permeability tests can
potentially be affected by a number of problems, the evaluation of the results
of a particular peraeameter test hinges not so much on the type of equipment
utilized but the test and quality control procedures followed during the
study. Data currently available show that the use of fixed wall or triaxial
type devices does not affect the results of the permeability tests on slurry
trench cut off wall backfill materials (Ayres 1982). Thus more attention
needs to be paid to the test procedures than the type of equipment used.
4.4 Summary
Both the feasibility of a slurry wall for remediation at a particular
waste site, and the degree of success with which it is used, are dependent on
thorough investigation and characterization efforts. These efforts define and
delineate those site-specific factors affecting wall design specifications,
ease of installation, and the overall performance of the final cut-off.
There are many physical factors that in some way would constrain the use
of a slurry wall at a given site. Most of these factors would not preclude
the use of a slurry wall, but would require additional engineering and
construction measures to overcome. Investigation and characterization of
physical site constraints would reveal and define the need for such
pre-trenching activities as site grading, or rerouting of fences, utilities or
roads. All of these factors could have an impact on construction costs and so
must be well characterized.
A thorough investigation and characterization of the sub-surface
conditions at a site is essential to slurry wall feasibility, design and
construction. A detailed delineation of a site's geology and hydrology will
help define the proposed slurry wall depth, and ease of excavation, and
indicate potential construction problems. Soils and overburden characteriza-
tions will also reveal potential excavation and construction problems, but are
most important for determining the suitability of on-site materials for use as
backfill material in a SB wall.
The importance of a complete characterization of site wastes and leachate
should not be underestimated. Several waste types have been shown to be
destructive to both SB and CB walls and could seriously affect their
integrity. Permeability testing of proposed backfill materials, with actual
site leachate is currently the most accurate method for predicting the
longevity of the wall.
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The site investigation and characterization efforts are essential in
developing a wall design suited to controlling that site's contamination.
They also play a major role in identifying those factors that pose problems
for wall installation and developing methods for dealing with them.
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SECTION 5
DESIGN AND CONSTRUCTION
This section addresses the design and construction procedures that are
used to install a soil bentonite or cement bentonite wall. Each step, from
the pre-design stage to the clean up of the site is described.
First, site specific factors that should be evaluated prior to slurry
wall design are discussed. These factors affect the feasibility and
conceptual design of slurry wall use at a particular site, and the relative
applicability of the'two slurry wall types.
Next, slurry wall design types and components are described. This
description illustrates the various portions of a design package and the types
of data that typically are contained in each.
Slurry wall construction requirements, including materials, quality
control, equipment, methods, and dimensional parameters are listed in the
third part of this section.
Following this a brief discussion of preconstruction steps is given. In
addition to evaluating the site and producing the design, the bid package must
be prepared and bids evaluated before the construction contract can be
awarded.
Once the contract is awarded, wall construction can begin. The
techniques used during construction of soil bentonite walls are described in
detail. CB and diaphragm walls are also discussed. The final portion of this
section addresses problems associated with slurry trench construction.
Typical solutions to these problems are also discussed.
5.1 Design Procedures and Considerations
The process of designing a slurry wall requires assessment of site
specific data and consideration of numerous design variables, to determine the
feasibility of a slurry wall and to select the most appropriate wall type for
use at the site. This section describes these processes.
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5.1.1 Feasibility Determination
Using the data from the site investigations, the designer must determine
the feasibility and applicability of installing a slurry wall at the site.
Factors to consider include:
Potential waste incompatibility
Anticipated hydraulic gradients and maximum allowable permeability in
the completed wall
* Aquiclude characteristics - depth, permeability, continuity, and
hardness
Wall placement relative to wastes and leachates
Costs and time considerations.
5.1.1.1 Waste Compatibility
Waste and leachate compatibility with proposed slurry wall backfill
mixtures can be determined using the laboratory tests described in Section 4.
Where long-term permeability is crucial, clay mineralogy and geochemical
testing is advisable. These tests provide an indication of which proposed
backfill mixtures show the greatest resistance to long and short term
permeation by the pollutants at the site.
5.1.1.2 Permeability and Hydraulic Gradient
Data on the anticipated hydraulic pressures on either side of the wall
indicate the range of hydraulic gradients to which the wall may be exposed.
When projected permeabilities and wall areas are known, the rate of subsurface
movement through the wall, can be determined using Darcy1s law. Darcy's law
states that
q s kia
where q is the volume of water flowing through the wall, k is the coefficient
of permeability of the wall, i is the hydraulic gradient, and a is the cross-
sectional wall area (Mitchell 1976).
To illustrate how Darcy's law can be used to estimate a slurry wall's
effects on groundwater flow at a hypothetical site, consider the following
hypothetical situation: a proposed slurry wall is designed to be 164 feet
(50 meters) long, 82 feet (25 meters) deep, and 3.2 feet (1 meter) thick. The
hydraulic gradient at the site is estimated at 2, and the wall's permeability
is designed to be less than 2.12 x 10~3 gpd/ft2 (1 x 10~7cm/sec). According
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to Darcy's law, the amount of water that will move through the wall is 57 gpd
(0.216 m /day. Before the slurry wall was installed, the permeability of the
same area was about 2.12 gpd/ft2 (1 x 10~4 cm/sec), a low permeability for
undisturbed soils. The amount of water flowing through3the area each day
prior to slurry wall installation was 57,000 gpd (216 m /day. Thus, this
particular slurry wall would reduce the volume of water flow through this area
by 99.9 percent. In areas having higher initial permeabilities, the effect of
the slurry wall would be even greater.
5.1.1.3 Aquiclude Characteristics
Another factor to consider when evaluating the feasibility of a slurry
wall is the aquiclude at the site. Ideally, it should also be thick,
impermeable, and unfractured, and should be soft enough for a backhoe or
clamshell to excavate a 1- to 3-foot key-in to prevent seepage under the
slurry wall. In areas where aquicludes are very hard, slurry walls will be
more expensive to install or the aquiclude wall union less certain. Where the
aquiclude is thin, discontinuous, or fractured, slurry walls can be expected
to be less efficient in pollution migration control due to seepage through the
aquiclude and other remedial measures may be called for.
5.1.1.4 Wall Configuration and Size
An early assessment of possible wall locations and configurations can
indicate the overall amount of wall exposure to wastes and leachates, the
types and placements of auxiliary measures and the actual length and depth of
wall required. These factors, along with an estimate of the necessary wall
durability, can assist designers in projecting the cost of the construction
effort. Detailed data on wall applications and configurations are presented
in Section 3.
5.1.1.5 Cost and Time Factors
The need for rapid response at some sites necessitates an evaluation of
the construction time required. According to Miller (1979) soil bentonite
slurry walls are normally installed at a rate of 25 to 100 linear feet per
day. Thus, the wall described previously could be installed in 2 to 7 days,
assuming the work is accomplished in a nonhazardous environment. Hazardous
conditions can more than double on site work time.
5.1.2 Selection of Slurry Wall Type
If it is determined that a slurry wall is feasible, the type of wall
(SB or CB) that is required should be established. To decide whether a
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soil-bentonite or cement-bentonite cut-off wall should be installed at a
particular site, several factors need to be considered. The following factors
affect the suitability of SB or CB walls:
Required permeability and hydraulic pressure
Leachate characteristics
Availability of backfill material
Required wall strength
Aquiclude depth
Site terrain
Cose.
5.1.2.1 Permeability and Hydraulic Gradient
Where low permeability is required, SB walls are used, and the wall width
is determined by the hydraulic head across the trench. Case (1982) recommends
that the trench should "have a width of 0.5 feet to 0.75 feet per 10 feet of
hydrostatic head on the wall. Thus, for a 100 foot head loss, wall thickness
should range from 5 to 7.5 feet." In comparison, a CB wall only 2 to 3 feet
thick will stand up to the same hydrostatic force (i.e., 100 feet). Deeper
walls are generally wider then shallower ones because larger excavation
equipment is used for deep walls and this equipment generally digs a wider
trench (Millet and Perez 1981). Generally, CB walls are designed to be
narrower than SB walls due to the greater shear strength and the higher cost
of cement bentonite walls (Millet and Perez 1981, Ryan 1976).
5.1.2.2 Leachate Characteristics
SB walls exhibit a lower permeability and a greater resistance to
chemical attack, particularly to acids, than CB walls. For this reason, SB
walls are favored for use as pollution migration cut-offs (Jefferis 1981b and
Xanthakos 1979).
Where floating rather than sinking contaminants are encountered, the
slurry wall does not have to be extended down into the aquiclude. Instead, a
"hanging" wall is installed. These walls are usually soil bentonite types.
5.1.2.3 Availability of Backfill Material
In some sites, the material excavated from the trench is contaminated due
to contact with polluted groundwater. This contaminated soil may be unsuit-
able for use in the backfill. Samples of this material should be mixed with
bentonite slurry and tested to determine the effects of the contaminants on
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the SB wall permeability. Testing of SB wall/leachate compatibility is
discussed in Section 4. In some cases, the contaminants will increase the
permeability of the completed wall, but the increase may be less than the
increase that could be caused by the sudden exposure of the wall to the
polluted groundwater. At these sites, it may be advisable to use the
contaminated material in the backfill, providing the gradation is adequate
(D'Appolonia 1980b). If, however, the contaminated soil is discovered to be
inappropriate for use as backfill material, a suitable borrow area should be
found. Where no such borrow area is available nearby, CB walls may be more
appropriate.
5.1.2.4 Wall Strength
Generally, CB walls are used where heavy vertical loadings are
anticipated and large lateral earth movements are not expected. This is
because CB walls have a higher shear strength and lower compressibility than
SB walls. CB walls are, however, more likely to crack than relatively plastic
SB walls (Millet and Perez 1981). If the wall must be extended beneath roads,
rail tracks or in close proximity to existing foundations, CB walls can be
used. In addition, CB walls can be used in localized areas requiring
strength and tied into SB walls for the rest of the trench distance.
5.1.2.5 Aquiclude Depth
CB walls are more expensive than SB walls due to the cost of the cement.
For this reason, CB walls are not generally used where the aquiclude is deep,
or where very long cut-off walls are required (Ryan 1977).
5.1.2.6 Site Terrain
At sites where slopes are steep and the areas for backfill mixing are
limited or non-existent, and low permeability is not critical, CB walls may be
preferred. In general, SB walls are limited to areas where the maximum slope
along the trench line is on the order of 2 percent or less. At many sites,
hills can be leveled and depressions backfilled with compacted soil prior to
trench construction. The lack of sufficient backfill mixing areas can be
overcome by hauling trench spoils to a central backfill mixing area, then
hauling mixed backfill back to the trench. Pug mills can also be used for
backfill mixing. These operations site leveling use of pug mills and
central backfill mixing result in slower construction rates and higher
costs.
In contrast to SB walls, CB walls can be constructed in areas of steeper
terrain by utilizing the CB panel construction technique described later in
this section.
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5.1.2.7 Cost
As mentioned earlier, a CB wall is typically more expensive than a SB
wall of the same volume due to the cost of the cement. Where thick or deep
walls are planned, CB walls will, in most cases, be more expensive than SB
walls. Where wall thickness can be minimized and very low permeability is not
essential, CB walls be can considered.
After the type of slurry wall has been selected, the preparation of the
design can begin.
5.2 Specification Types and Design Components
The objective of the design phase is to produce accurate specifications
for wall construction. Normally, the design of a slurry wall for pollution
migration control involves producing either a performance type specification
or a materials and methods specification.
5.2.1 Differences in Specification Types
Performance specifications consist of the performance standards which
spell out what the owner or engineer expects to receive in exchange for
payment. These specifications stipulate the results desired by the owner and
leave the achievement of the results the responsibility of the contractor.
This type of specification provides the widest latitude to the contractor but
still maintains the quality desired for the end product. Historically,
performance specifications allow innovation on the part of the contractor to
achieve results at reasonable costs.
The most commonly used measure of slurry wall performance is the
permeability of the completed cut-off. Often, the maximum permeability is
specified at 10~ cm/sec (Lager 1982). Materials requirements are also
specified. Most design engineers and bentonite producers use performance type
specifications.
The materials and methods type of specifications, which are normally used
for major construction projects are typically very long because requirements
for both materials and methods are spelled out in great detail. Although this
type of specification is applicable for slurry trench installations, the
general consensus of design and construction firms with much slurry trenching
experience is that construction costs are typically increased without
improving the quality of the installation if materials and methods types of
specifications are used.
In some situations, this specification type may be favored over
performance specifications. For example, where there are not qualified
bidders for a project or where special structural considerations are involved,
5-6
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a materials and methods type specification may be necessary. For this reason,
components of both design and performance specifications are described below.
5.2.2 Components of Design
In slurry wall designs, the following items are typically addressed:
Scope of work
Construction qualifications
Construction requirements of the trench and slurry wall
Materials
Equipment and facilities
Performance
Clean up
Quality Control and Documentation
Measurement and Payment.
Each of these items are briefly described below.
5.2.2.1 Scope of Work
This section describes in general terms what the contractors will be
required to accomplish including material quantities and performance period.
5.2.2.2 Construction Qualifications
This should describe the prior experience required of the contractor and
his personnel on site. This could be expressed in terms of a specific number
of similar jobs, in years of experience in slurry trench construction, or
both. This requirement is not always stipulated.
5.2.2.3 Construction Requirements of the Trench and Wall
Among the items included in this section are the width and depth of the
trench, the aquiclude to be penetrated, and the depth of penetration, the
location, continuity, verticality, and permeability of the completed wall.
The two most important design considerations, which are the selection of the
aquiclude and the design of the backfill, must also be described (D'Appolonia
1980a).
5-7
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5.2,2.4 Materials
This section should specify the material standards to be maintained
during construction. It usually covers water quality, bentonite type, slurry
quality, backfill characteristics, and additives, if any. Each of these
materials should be separately addressed to ensure compliance with design
requirements for the particular site.
5.2.2.5 Equipment
This section should be used to ensure that the contractor has the proper
equipment on site to perform the following:
Trench Excavation
Slurry Mixing
Slurry Placement
Backfill Mixing
Backfill Placement
Site Clean-up.
5.2.2.6 Methods
This section describes the acceptable methods of slurry mixing, trench
wall stabilization, trench excavation, backfill mixing, backfill placement and
clay cap construction. The design engineer must ensure that the slurry is
properly hydrated prior to use, that the slurry is pumped into the trench at
the start of excavation, and that sufficient slurry is kept in the trench to
maintain trench wall stability.
The trench for an SB or CB wall must be excavated so that it is
continuous to the required depth along the specified line of excavation.
Unexcavated areas within the trench are prohibited, as these interfere with
wall integrity. To ensure continuity in CB panel walls, sufficient overlap
between adjacent panels must be required (Geo-Con, Inc. 1979).
The consistency of SB backfill material (as measured by slump) must be
specified in order to maintain the desired flow properties during backfill
placement. The required backfill properties are described in Section 5.3.2.5.
The method of backfill placement should also be addressed in order to avoid
entrapping pockets of slurry or pervious materials during backfilling that
interfere with wall performance. After completion of the trench backfilling
process, the trench must be protected from desiccation by means of a clay cap.
Techniques for construction of this cap must be described, particularly if the
cap is designed to withstand traffic loads.
5-8
-------
section
should also include the
* °in tne excavation,
--
.2.2.7 Quality Control and Documentation
The contractor should be "quir-d to p.rfo»
Wtion and in some i""nce. c.rtxfic.^ on dur, ng ^ materials used In
^r.^ ~/S£^ ^"-"h QA/QC procedure8 x P
the slurry wall contractor.
5.2.2.8 Drawings
to estimate the
. Any earth-.ovins retired be.ore actual trench con.truction can
. A P,an vie» o« slurry trench with areas .or slurry preparation and
equipment
. A cross section of the trench to show depth and location o* any
utility or road crossings
. Soil boring, locations, and depths.
nay
also be delineated
The cross
section of the trench
. .
iping- wastewater plping>
are shown
given area
5-9
-------
5.2.2.9 Measurement and Payment
5.3 Slurry Wall Requirement
usxng slurry trench nethod f
hazardous waste applications will be to
water flow to a specific degree"^ ?
The design factors affectino
completed CB and SB walls are:
Wall location
Wall depth
Wall width
Wall continuity and vertically
Connection to surface structure
Material quality
Methods and procedures used.
a logical flow of
"xteri. for cut-off walls
ive.. The objective in most
ground
ft=
effectiveness and durability of
the
such as
permeability, continuity
5.3.1 Location
^8th and grade of
the
s. Factors affect
locates and hydrogeologic conditfons
a
and grades shown
'°
5-10
-------
5.3.2 Depth
Slurry wall depth is controlled by the depth to the aquiclude. The
selection of the aquiclude is one of the most important items in the slurry
wall design. Although several relatively impervious zones may be encountered,
the aquiclude used as a cut-off wall foundation should be continuous,
relatively free of fractures and other pervious zones, and within the reach of
currently available excavation equipment. Selection of a suitable aquiclude
is based on the data obtained during site investigations, as described in
Section 4. Usually the cut-off wall is extended 2 to 3 feet into the
aquiclude past the zone of "pervious lenses, weathered zones, desiccation
cracks or other geological features that might permit seepage under the
cut-off" (D'Appolonia 1980a). If the acquiclude to be penetrated is of
questionable integrity in the excavation area, the base of the cut-off can be
grouted to seal pervious zones beneath the wall (D'Appolonia [unpublished]).
In the case of a hanging slurry wall, the depth of the seasonally lowest water
table determines wall depth.
5.3.3 Width and Permeability
In addition to wall location and depth, the wall width and permeability
are stipulated in materials and methods specifications.
Design width depends on several factors, including:
The required cut-off effectiveness
Head loss across the wall
The hydraulic gradient
The s.ize of the available excavating equipment.
The permeability of the completed cut-off is usually stated in
performance specifications. For adequate pollution control, ie is necessary
to maintain a wall permeability of 1 x 1CT7 cm/sec or less (D'Appolonia
1980a). This permeability is achievable using SB walls with high fines
contents. Cement bentonite walls usually have higher permeabilities; on the
order of 1 x 10~6 cm/sec (Jefferis 1981b).
The relationship between required wall thickness and hydraulic gradient
is given in Section 5.1.2.6. Data on equipment sizes and excavation depths
are presented in Section 5.3.7.
5.3.4 Continuity and Verticality
The continuity and verticality of the completed wall can significantly
affect wall performance and must be carefully specified. Soil bentonite or CB
5-11
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trench walls are excavated in a continuous trench, and continuity is tested by
passing the backhoe bucket or clamshell vertically and horizontally along each
segment before it is backfilled. When a circumferential slurry wall is
constructed, a segment of the earliest-backfilled trench is reexcavated to
ensure complete continuity.
In CB panel walls, each panel is excavated under a CB slurry then allowed
to set before excavation of the intervening panels is begun. The overlap at
each panel joint is dependent on wall depth and the type of equipment used. A
minimum of 3 feet of each end of each partially set CB panel is excavated when
the intervening panel is excavated. The intervening panel is allowed to
harden and that particular wall segment is then complete (Geo-Con, Inc. 1979).
Soil bentonite walls are usually not required to be strictly vertical, as
these walls are seldom,"if ever, used in a load-bearing capacity as part of a
structure. Verticality can affect wall continuity, particularly at corners.
If one wall is vertical but the other slants outward at the base, a large
unexcavated area may exist in the corners. For this reason, some specifica-
tions call for nearly vertical walls, or a 5-foot overlap at wall corners
(U.S. Army Corps of Engineers 1975). Good field quality control will insure a
nearly vertical wall and good continuity.
5.3.5 Surface Protection
Another requirement of completed soil bentonite walls that they be
protected from consolidation and compaction as well as from erosion. The
completed wall must not be allowed to consolidate unevenly and thus form deep
cracks, or to consolidate enough to form a seepage path or a depression at the
ground surface that follows the original trench excavation (Millet and Perez
1981). Consolidation is a function of backfill gradation and water content,
and the ratio of trench width to depth. In most cases, it can be predicted in
advance and so not cause unforeseen problems. Wider walls have been found to
consolidate more than narrow ones, and excessive fines content is said to
result in a greater degree of consolidation. In trenches wider than 8 feet,
having a depth from 50 to 90 feet, consolidation was reported to average 1 to
6 inches (Xanthakos 1979). One 3-foot wide wall backfilled with material
containing an average of about 60 percent fines consolidated about 6-8 inches
over the course of about 6 months. The surface of the SB wall was dry and
cracks less than 1 inch wide and a few inches deep were evident prior to
placement of a clay cap (Coneybear 1982).
To protect the surface of finished SB walls, clay caps are often
installed (Millet and Perez 1981). These can be designed to support traffic
by interspersing geotextiles (construction fabric) between a series of clay
lifts and by covering the surface with gravel (Zoratto 1982), As
consolidation occurs during the first few months after trench construction,
additional clay layers may be added (U.S. Army Corps of Engineers 1975).
5-12
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5.3.6 Materials, Quality Control, and Documentation Requirements
The types of materials that are acceptable during slurry trench
construction are often specified in great detail. Quality requirements for
the following items are commonly listed:
Dry bentonite
Water
Fresh slurry
In-trench slurry
Backfill materials
Mixed backfill.
Table 5-1 presents a materials quality control program for soil bentonite
walls. The types, frequencies and results of the tests are specified.
Additional requirements are described below.
5.3.6.1 Dry Bentonite
The quality of the dry bentonite should
by checking the pH, viscosity and fluid loss
bentonite. Certification of compliance with
the bentonite manufacturer must be obtained.
include physical and chemical purity and dry
number 200 mesh sieve).
be tested frequently, for example
of a slurry made from the
the material specification from
Other criteria for dry bentonite
fineness (percent passing a
Another bentonite quality criteria is the type of additives allowed.^
Additives are reportedly present in most, if not all, commercial "natural"
sodium bentonite sold in the U.S. (D'Appolonia and Ryan 1979). Among the
types of additives used in bentonite are peptizers, bulking agents, softening
agents, dispersatits, retarders and plugging or bridging agents (Corps of
Engineers 1975; IMC., [no date]). Some of these are listed in Table 2-3.
Although some specifications categorically prohibit the use of additives,
others require the engineer's prior approval and manufacturer's certification
of compliance with stated characteristics before the additives can be used
(U.S. Army Corps of Engineers 1976; Geo-Con Inc. 1979). Specifications
requiring approval of additives usually refer to additives other than those
allowed in "natural" bentonites.
5.3.6.2 Clay
At some sites, the use of native clay materials has been attempted.
There are several drawbacks to the use of other clays in the slurry during
trench construction. The primary ones are the difficulty in meeting slurry
5-13
-------
TABLE 5-1.
MATERIALS QUALITY CONTROL PROGRAM FOR SB WALLS
Quality
Control Item
.Subject
Standard Name
Type of Test
Frequency
Specified Values
Water
Materials
I
1'
-p-
Additives
Bentonite
API Std 13
Standard Procedure
for Testing
Drilling Fluids
-pH
-Total Hardness
Manufacturer certificate
of compliance with stated
characteristics
Manufacturer certificate
of compliance
Selected soils obtained
Per water
source or-as
changes occur
As required to properly
hydrate bentonite with
approved additives.
Determined by slurry viscosity
and gel strength teats.
As approved by Engineer
Premium grade sodium cation
montmorillonite
Slurry
Backfill
Mix
Backfill - from a borrow area approved
Soils by the Engineer
Roll to 1/8" thread
Prepared API Std 13 - Unit Weight
for Place- Standard Procedure - Viscosity 1 set per shift or
ment into for Testing per batch (pond)
the Trench Drilling Fluids - Filtrate Loss
- PH
In Trench API Std 13B1 - Unit Weight
Standard Procedure
for Testing
Drilling Fluids - Slump
- Gradation
At Trench ASTM C 143
Slump Cone Test
1 set per shift at
point of trenching
1 set per 200 cu
yds
35 to 853! passing #20 Sieve
15 to 35Z passing #200 Sieve
Unit Weight 1.03 gm/cc
V 15 centipose of
_ 40 sec-Marsh 9 68"
Loss 15 cc to 25 cc in 30 min
@ 100 psi
pH 8
unit~~weight - 1.03 - 1.36 gm/cc
Slump 2 to 6 inches
65 to 100% passing 2/8" Sieve
35 to 85% passing #20 Sieve
15 to 35% passing #200 Sieve
Reference: Federal Bentonite 1981.
-------
specifications and controlling slurry quality (Boyes 1975). As Table 5-2
shows, a 6 percent solution of commercial montmorillonite gives a viscosity of
15 centipoise (cP). To obtain an equivalent slurry using typical native
clays, a solution containing about 25 to 36 percent clay would be necessary
unless they are very high in montmorillonite (Grim and Guven 1978). At this
clay content, the slurry would be denser than is desirable for trenching
slurries: dense slurries are detrimental because thay may not be displaced
properly by the backfill.
Like montmorillonite, native clays are composed of very small particles
that will pass through a 200 mesh sieve. Most native clays are found mixed
with larger-sized particles such as silt, sand, and gravel. These larger
particles are difficult to separate from the clay. The amount of clay in the
native deposits normally will vary both horizontally and vertically. Because
of this variation, it will be difficult to control the clay concentration in
slurries made from native clays. A third problem with non-montmorillonitic
native clays is that they are much less thixotropic than montmorillonite.
They may take days to develop a gel structure rather than minutes (Boyes
1975). For these reasons, typical clays do not perform as well or as
consistently in slurries as montmorillonite; therefore, the use of
non-montmorillonitic clays is not recommended for most applications.
5.3,6.3 Water
Water quality must be tested for each water source used. Tests include
pH, total hardness and content of suspected deleterious substances (U.S. Army
Corps of Engineers 1976). Reported water quality requirements include:
Hardness of < 50 ppm
Total dissolved solids content of < 500 ppm
Organics content of < 50 ppm
Other deleterious substances (i.e., oil or leachate) < 50 ppm
pH of about 7.0 (Xanthakos 1979, U.S. Array Corps of Engineers 1975).
5.3.6.4 Fresh Slurry
The fresh hydrated bentonite slurry should have a minimum viscosity of
40 seconds Marsh, a unit weight of about 65 p.c.f.) a pH of from 7 to 10, a
bentonite content of from 4 to 8 percent (Case 1982, Xanthakos 1979, Boyes
1975 and Alther 1982). The factors affecting bentonite and slurry quality are
discussed in Section 2. Common slurry properties and the tests used to
measure them are presented in Table 5-3.
One test that is often conducted on fresh slurries is the filtrate (or
fluid) loss test (API 1982). This test is supposed to simulate formation of
the filter cake (Millet and Perez 1981). The filtrate test involves measuring
5-15
-------
TABLE 5-2.
COMPARISON OF SELECTED PROPERTIES OF CLAYS
Parameter
Montmorillonite
Kaolinite
Illite
Other Clays or
Sheet Silicates
Amount of water the 200-300% (1)
dry clay can absorb,
% of dry weight (1)
Volume change
due to hydration
under similar
conditions
Hydration rate
Particle shape
Theoretical
Specific surface
area, m /g
Cation Exchange
Capacity,
meq/lOOg
Liquid Limit (%)
Plastic Limit (%)
2-11 cufVg (2)
Water sorption
continues for
about 1 week (2)
thin, flat, irregular
plates (3)
700-800 (2)
60-150 (3)
150-700 (4,5)
65-97 (4)
irregular, flat
six-sided
shapes (2)
5-20 (2)
3-15 (2)
29-75 (4)
26-35 (4)
100-200 (2)
10-40 (2)
59-90 (4)
34-43 (4)
100%
vermiculite >montraorillonite
>beidellite >kaolinite
>halloysite (2)
water sorption for
most colloidal clays is
complete in 1 to 3 days (2)
attapulgite-fibrous,
sepiolite-fibrous,
most others irregular
and flat (3)
vermiculite 300-500 (3)
vermiculite 100-150 (2)
attapulgite/sepiolite
3-15 (3)
attapulgite 160-230 (6)
attapulgite 100-120 (6)
(continued)
-------
TABLE 5-2. (continued)
Parameter
Montmorillonite
Kaolinite
Illite
Other Clays or
Sheet Silicates
Shrinkage Limit (%)
Percentage of clay
by weight in
water to produce
a 15 cP colloidal
suspension
Density of charge
meq/ra x 10 (2)
Layer thickness
in A (6)
Particle density,
g/cin (2)
8.5-15 (6)
^5.5-12 (4)
25-29 (6)
1.1-1.9 6-7.5
expansive >10 7.15
air dry 15
2.5 Wyoming bentonite (5)
2.2 Japanese bentonite (5)
15-17 (6)
1,0-2.0
10
~25-36 for typical
native clays (4)
attapulgite-same as
montimorillonite (4)
vermiculite 3,0-3.3
vermiculite 14
muscovite 10
biotite 10
halloysite 10
mica 2.8-3.2 (2)
References: (1) Case 1982, (2) Baver, Gardner and Gardner 1972, (3) Grim 1968, (4) Grim and Guven 1978,
(5) Xanthakos 1979, (6) Mitchell 1976.
-------
TABLE 5-3.
COMMON SLURRY PROPERTIES AND TESTING METHODS
Property
Definition
Current test method
Concentration
Ib bentonite/100 Ib water
kg bentonite/100 kg water
Ib bentonite/ft water
Density
Mass of given volume of
slurry
Mud Balance
Plastic viscosity
apparent
viscosity,
yield stress
For a slurry behaving as a
Bingham body, the flow
law is
Fann V-G viscometer
T - yT
N D
0
where T = shear stress
T = yield stress
.0
Np = plastic viscosity
D = rate of shear
TD = apparent viscosity
Marsh cone
gelation
Time for 946 cm (1 U.S.
quart) or 1500-cm
volume to drain
from a standard cone or
time for 500 cm of the
500-cm volume to drain
from cone (Japan)
Marsh funnel viscometer
Initial gel
strength
Minimum shear stress to
produce flow; designated
as T,,
Rotational viscometer
10-min
gel strengh
Shear strength obtained
by allowing 10 min to
elapse between stirring
and reading
Rotational viscometer
pH
Logarithm of reciprocal of
hydrogen-ion concentration
pH electrometer; pH
papers usually not reliable
Filtration or
fluid loss
Volume of fluid los t in
given time from fixed
volume of slurry when
filtered at given pres-
sure through standard
filter
Filter-press test (but this
procedure does not permit
exact estimation);
stagnation-gradient test
more appropriate
Filter cake
Thickness and strength of
filter cake for standard
or actual conditions
Thickness measured in fluid-
loss test, strength esti-
mated from triaxial tests
Sand content
Percentage of sand greater
than 200mesh in suspension
API standard sand-content
test using a sand-screen set
Reference:
Xanthakos 1979.
Permission.
Copyright 1979 by McGraw-Hill Books. Used with
5-18
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the amount of water lost from a 15cP slurry through filter paper when
subjected to a pressure of 100 p.s.i. for 30 minutes (Grim and Guven 1978).
Some controversy exists as to whether or not this test accurately reflects the
ability of the slurry to form a filter cake.
Despite the fact that the test may serve as a useful indicator, the
filtrate loss test has several innate flaws, including the facts that the
filter paper used has very little similarity to the strata at the slurry/soil
interface, and that the pressures used are not representative of in-place
pressures found in slurry trenches (Hutchison et al. 1975).
In addition, there is little if any relationship between the results of
the standard API filtrate loss test and the permeabilities of filter cakes
from the same slurry (D'Appolonia 1980b). Thus the filtrate loss test is
regarded by some persons involved in slurry trench construction as
inappropriate for slurry specification (D'Appolonia 1980b, Millet and Perez
1980).
5.3.6.5 In-Trench Slurry
The requirements for the in-trench slurry are few and simple. The
in-trench slurry becomes denser due to the suspension of soil particles in the
slurry. For the in-trench slurry, two measurements are important. A slurry
sample taken from the trench bottom near the toe of the backfill should be at
least 15 p.c.f. less dense than the backfill material, and must be capable of
passing a Marsh funnel. This is to allow complete and rapid displacement of
the slurry during backfilling (D'Appolonia I980b).
5.3.6.6 Backfill Materials
The gradation of the soil material used for the backfill must be tested.
For a low permeability cut-off, at least 20 to 60 percent fines should be
presented (D'Appolonia 1980). Although plastic fines yield lower
permeability, non-plastic fines will show a greater ability to withstand
chemical attack. Larger particles, such as cobbles or clay lumps greater than
5 inches across should be prohibited (IMC [no date]).
5.3.6.7 Mixed Backfill
Once the backfill material has been selected, it must be mixed with the
slurry in the proper proportions so that it has the following characteristics
A bentonite content of 1 to 2 percent
A moisture content of 25 to 35 percent
A fines content of 20 to 60 percent.
5-19
-------
In addition, the mixed backfill should have a:
Slump from 2 to 7 inches on the ASTM C143-74 "Slump of Portland
Concrete" Test
Density at least 15 pcf greater than that of the slurry in the trench
Shear strength low enough to allow ready flow, and preferably lower
than that of the filter cake.
a. Slump
A typical backfill used in a pollution migration cut-off wall has a water
content between 25 and 35 percent, a bentonite content ranging from 0.5 to 2
percent and a fines content of from 20 to 40 percent. The soil excavated from
the trench typically has an initial moisture content of from about 10 to 20
percent (D'Appolonia 1980b). This moisture aids in mixing the slurry with the
backfill by softening the materials (Coneybear 1982). When the backfill is
mixed in these proportions, the backfill forms a thick paste that will flow
easily (D'Appolonia 1980b). The slump of the backfill, which is an expression
of its propensity to flow, is of great practical importance during backfilling
operations.
If the backfill slumps too much, a very flat backfill slope occurs. This
interferes with the efficiency of excavation. Conversely, backfill with too
little slump allows voids and honeycombs to form and may cause entrapment of
pervious materials, leading to the formation of high permeability "windows" in
the finished wall (Millet and Perez 1981).
When the backfill folds over and traps pockets of slurry, another problem
results. The slurry does not become mixed with the backfill. Instead, it
gradually rises to the top of the trench, due to its lower density. This may
lead to wall weakness in the areas where the slurry pockets were initially.
To avoid the problems listed above, a slump of 2 to 7 inches is usually
specified (Case 1982). Figure 5-1 shows a cross-sectional view of a completed
trench, showing the successive layers of a well placed backfill.
b. Density
Backfill densities typically range from 105 to 120 p.c.f. At these
densities, the backfill easily displaces the slurry in the trench (D'Appolonia
1980). Even so, samples of the slurry taken from the trench bottom should be
tested for density prior to initiation of backfilling operations.
5-20
-------
Figure 5.1.
Typical Backfill Profile in Trench with Irregular Bottom
8 + 00
Source: D'Appolonia 1980
5-21
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c. Shear Strength
The shear strength of the backfill must be high enough to allow it to
stand on a 5:1 to 10:1 slope (Millet and Perez 1981). Preferably its shear
strength is lower than the shear strength of the filter cake to avoid
disrupting it. Data on the relative shear strengths of the backfill and the
filter cake were not located. However, Boyes (1975) stated that the shear
strength of bentonite filter cakes is greater than that of concrete emplaced
in slurry trenches during concrete panel wall construction, and that the shear
strength of one filter cake was measured at 0.00051 N/m .
The fate of the filter cake during backfilling has been the subject of
controversy. Some people involved in slurry trench construction feel that the
filter cakes on both of the trench walls remain intact not only during
backfilling but also during subsequent permeation with groundwater.
D'Appolonia (I980b) suggested that the downstream filter cake may degrade
under the influence of high hydraulic gradients across the trench. The
bentonite particles may then be forced into the soil pores if the soil
permeability is high enough. Jefferis (1981a) proposed that the upstream
filter cake would be more likely to decompose and be forced into the cut-off
wall. This is because the hydraulic pressure on the upstream side of the wall
is so much higher than the pressure on the downstream side. Experimental
evidence cited by D'Appolonia (1980b) indicates that, as long as the backfill
contains at least 15 to 20 percent fines, this will not occur.
5.3.7 Equipment
Appropriate equipment must be available to accomplish the following tasks
associated with slurry wall construction:
Slurry mixing
Slurry supply to the trench
Slurry density control
Trench excavation and aquiclude key-in
Backfill mixing
Backfill placement
Hauling of backfill or spoils, if necessary.
The type and size of this equipment is job dependent and is usually
selected by the contractor rather than specified.
5-22
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5.3.7.1 Slurry Mixing
The slurry mixing and placement equipment must be capable of supplying
adequate quantities of slurry during excavation. For slurry mixing, hydration
and control, the following equipment and facilities are needed:
Mixing apparatus such as a venturi (flash) mixer or a paddle (high
vortex) mixer
Pumps, valves, pipes, hoses, fittings and small tools
Slurry hydration and storage ponds (or paddle mixer for small jobs)
Slurry cleaning equipment, including airlift pumps, valves, pipes, and
desanders, or mudshakers, if sand removal is desired.
5.3.7.2 Trench Excavation
For trench excavation, it is important to ensure the equipment used can
maintain a continuous excavation line to the total depth required. Table 5-4
lists types of excavation equipment commonly used for slurry trenching. If
the aquiclude is composed of a hard type of rock, the backhoe or clamshell may
not be able to rip it out. In this type of situation, the slurry wall can be
terminated at the top of the rock layer or special equipment such as drills or
chisels may be used for key-in. If rock fractures are noted during key-in
excavation, grouting may be necessary.
5.3.7.3 Backfill Mixing
Bulldozers or graders are commonly used for backfill mixing, although
mechanical batchers or pugmills may be employed at sites where backfill mixing
areas are not available. When a single centralized backfill mixing area is
planned, sufficient flat area must be set aside for this operation.
5.3.7.4 Backfill Placement
Backfill placement equipment normally consists of a bulldozer that slowly
slides the mixed backfill into the trench at a point slightly in advance of
the peviously-placed backfill. (Backfill placement methods are described in
detail in Section 5.4.6.) Clamshells are also used at some sites. At sites
where the trench spoils are unsuitable for backfill mixing, soils from a
borrow area, along with bentonite slurry, must be hauled to the trench. To
assist in placement of mixed backfill from trucks a metal trough-like device
can be used to direct it into the trench at the proper point. The trough is
5-23
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TABLE 5-4.
EXCAVATION EQUIPMENT USED FOR SLURRY TRENCH CONSTRUCTION
Type
Standard
backhoe
Modified
backhoe
Clamshell
Dragline
Rotary drill,
percussion
drill or large
chisel
Trench Width
(feet)
1-5 (1)
2-5 (1,3)
1-5 (3)
4-10
Trench Depth
(feet)
50 (2)
80 (2)
>150 (3)
>120
Comments
Most rapid and least costly
excavation method (1)
Uses an extended dipper stick,
modified engine & counter-
weighted frame; is also rapid
and relatively low cost (1)
Attached to a kelly bar or
crane; needs > 18 ton crane; can
be mechanical or hydraulic (3)
Primarily used for wide, deep
SB trenches (4)
Used to break up boulders and to
key into hard rock aquicludes.
Can slow construction and result
in irregular trench walls (3)
References: (1) Case 1982, (2) D'Appolonia 1980, (3) Guertin and McTigue
1982, and (4) Shallard 1983.
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advanced to coincide with the advancing backfill placement area (Zoratto
1982).
5.3.8 Facilities
The facilities necessary for slurry wall construction include a trailer
or small building for supervisory operations and quality control procedures.
Necessary testing equipment includes a Fann viscometer, mud balance, moisture
tester (or sample cans, a balance and an oven), pH meter, sieves, Marsh
funnel, and slump testing equipment (Xanthakos 1979, Zoratto 1982).
5.3.9 Methods
In a materials and methods specification, the methods to be followed
during each stage of construction are spelled out in great detail. This
section lists the steps that are typically described in the specifications.
Discussions of each step are given in Section 5.4. The steps for which
methods are described include:
Slurry mixing and hydration
Trench excavation
Backfill mixing
Backfill placement
Protective capping construction
Site clean up.
5.3.10 Safety Procedures
At non-hazardous sites, safety procedures for slurry wall construction
are very similar to those for most other construction sites. The personnel
involved in trench construction at hazardous waste sites must, however, be
protected from exposure to contaminants from the trench spoils, the wastes or
leachates and the area surrounding the excavation. Hard hats, as well as
rubber boots, gloves, and protective coveralls may be required. Where
volatile toxins are suspected, air-supplied respirators may be necessary.
Personnel required to use safety equipment include equipment operators,
inspectors, QA/QC personnel and all other personnel near the trench.
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5.4 Preconstruction Activities
Preconstruction activities typically follow a chronology that includes:
Designing the installation
Estimating the costs
Assembling the bid package and advertising
Evaluating proposals
Awarding the construction contract.
5.4.1 Slurry Wall Design
The design of a slurry wasl requires consideration of numerous
site-specific conditions, as described earlier. The data that are gathered
during the design phase are carefully evaluated and organized to produce the
slurry wall specifications and plans, along with drawings of the site and the
proposed slurry wall. These documents are used by the owner or engineer and
by potential contractors to estimate the complexity and cost of the slurry
wall construction. The two types of slurry wall design specifications most
frequently encountered are described in Section 5.2. The procedures and
considerations involved in slurry wall design were detailed in Section 5.1.
5.4.2 Cost Estimates
The designer must develop a cost estimate for slurry trench installation
that can be compared with bids received from potential construction
contractors. Although a rough figure of $3-5/square foot of soil bentonite
wall is generally accepted, it is not specific enough for sites where special
problems are anticipated. To prepare a cost estimate, detailed information on
costs and cost variation between sites is necessary. These data are presented
in Section 7.
5.4.3 Bid Package Preparation
The bid package consists of a number of documents that will make up the
eventual contract. Components of the bid package are typically the following,
which are the same topics covered for most construction contracts:
Invitation to bid
Instructions to bidders
Contractor's bid sheets or proposal
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The agreement between owner and contractor
Performance bonds and insurance
General and special conditions of the contract
Specifications (general and special technical requirements)
Plans and drawings (Jessup and Jessup 1963) .
The bid package should give bidders enough information, both technical
and contractual, to prepare and submit bids which accurately reflect the
effort that will be required to complete a quality, cost effective slurry
wall.
The bid package is.usually sent to prospective contractors who have
responded to an advertisement. In some cases, only pre-qualified firms are
invited to bid. Bids are then submitted by those interested firms for
evaluation.
5.4.4 Bid Evaluation and Contract Award
Public works financed by Federal, State, or local funds are usually
required to be awarded to the lowest responsible bidder, as determined by the
opening and reading aloud of sealed bids at a specifically designated time
(Jessup and Jessup 1963). Therefore,' much emphasis is placed on bid prices.
After proposal evaluation, and bid examination have been completed, the
most qualified firm submitting the lowest bid is determined. The construction
contract is then awarded to the firm selected in the bidding process.
5.5 Soil Bentonite Wall Construction
Following award of the construction contract, the selected firm will
proceed with construction of the slurry wall. The major activities include:
Preconstruction Assessment and mobilization
Site preparation
Slurry preparation and control
Slurry mixing and hydration
Slurry placement
Backfill preparation
Backfill placement
Site cleanup and demobilization.
Discussions of these activities follow.
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5.5.1 Preconstruction Assessment and Mobilization
Three major activities occur during the mobilization phase of slurry
trench construction. These are:
Layout site plan
Determine the equipment, type, amounts of materials, and facilities
required
Determine number and source of personnel required.
5.5.1.1 Plan Layout
A preliminary layout is prepared based on drawings supplied by the
engineer. Once the preliminary layout is developed, a close examination of
the proposed construction site must be performed in order to ensure that all
details of the plan are practical. After the onsite examination, a final
layout of the worksite can be prepared. A diagram of a typical slurry wall
construction site is shown in Figure 5-2.
5.5.1.2 Equipment Requirements
The specifications and drawings, test boring records, subsurface
exploration reports, and records of utility lines are the first sources of
information for determining equipment, materials and facilities needs. In
addition, an on-site inspection may be required to gain the detailed
understanding needed for planning the construction activities.
The major work elements and equipment and facilities typically associated
with each work element are:
Excavation
- hydraulic backhoe
- mechanically operated clamshell
- hydraulically operated kelly-mounted clamshell
Slurry preparation and control
high speed colloidal batch mixer for small projects
- flash mixer for large projects
- pumps, valves, pipes and tools
- hydration ponds
- desanders, hydrocylones or screens
Slurry placement
- pumps
- placement hoses and piping
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Figure 5- 2.
Typical Slurry Wall Construction Site
Bentonite
Storage
Backhoe
Backfilled
Trench
Backfill
Placement
Area
Area of Active
Excavation
Proposed Line
of Excavation
\
Slurry
Storage
Pond
Slurry
Pumps
Slurry
Preparation
Equipment
Bentonite
Storage
ooo
Water Tanks
Access
Road
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Backfill preparation
dozer or grader
Backfill placement
- dozer
- mechanically operated clamshell
trucks and trough
Supervision and quality assurance
- shed or trailer
- Marsh funnel
- mud balance
- standard sieves.
Numerous factors influence the types of equipment required as well as the
final plan layout and the relative difficulty of construction activities.
Some of these factors, along with their potential effects on slurry trench
construction operations are listed in Table 4-1.
Determining the correct equipment applications for a particular project
is based upon construction requirements and the constraints imposed by the job
site. For example, choice of excavation equipment depends upon the depth of
the slurry wall and the soil in which the wall is placed. The maximum exca-
vation depth for a standard backhoe is about 50 feet, but larger, extended
models are available to reach up to 80 feet in depth (D'Appolonia 1980a).
Both clamshell excavation systems will reach depths more than 150 feet, with
the hydraulic model sometimes preferred in more difficult digging conditions
(Guertin and McTigue 1982c) (See Table 5-4).
Consideration must also be given to site access and obstructions. Access
roads might limit the size of equipment that can be brought to the site, while
obstructions at the site might preclude the use of some types of equipment.
5.5.1.3 Personnel Requirements
When determining the sources of personnel for use at slurry trench
construction site, two choices face the construction firm. The firm can send
their own equipment and personnel to the construction site or they can rent
equipment and hire personnel locally. Most firms will use varying
combinations of each approach. For larger jobs and critical small jobs, it is
frequently more efficient to send equipment and personnel directly from the
construction firm. Small jobs can often be handled effectively by using only
specialized company-owned equipment, such as a extended backhoe arm,
accompanied by supervisory personnel. Other equipment such as bulldozers,
cranes, clamshells, and large backhoes can be rented near the job site.
Laborers and certain equipment operators can be hired locally for the specific
job. However, there are no set rules, and each construction contractor will
tailor his approach on a site-by-site basis.
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Slurry trench construction contractors are best able to judge appropriate
equipment and personnel needed at the job site. However, site owners and
their representatives should be aware of the approach to be taken by the
construction contractor, and should be satisfied that appropriate equipment
and personnel are available at the job site.
5.5.2 Pre-excavation Site Preparation
Once site planning has been completed, necessary permits and clearances
have been obtained, and required utility, water and other services have been
arranged, preparation of the construction site can proceed. The work site can
then be cleared if necessary, security fences erected, utility and water
hook-ups made, equipment and facilities brought in and set-up, and construc-
tion materials delivered. At this time, work can proceed to move or remove
obstructions if necessary.
5.5.3 Slurry Preparation and Control
Before excavation begins, the slurry must be prepared. To do this,
bentonite and water quality must be tested, hydration ponds must be
constructed, lines laid, pumps placed, and the mixing area prepared. The
slurry is then mixed in a venturi of paddle mixer and allowed to hydrate fully
prior to placement in the trench for SB slurry trench cut-off construction or
mixing with cement for CB cut-off construction.
5.5.3.1 Testing Bentonite and Water
Bentonite quality is critical to the quality of the slurry. Bentonite is
usually shipped to the job site accompanied by laboratory test results showing
that it meets quality criteria. These criteria include physical and chemical
purity, pH, gel strength, dry fineness (percent passing the number 200 sieve)
and filtrate loss. At the job site, these criteria are checked frequently,
such as by testing every truckload of bentonite delivered. It is important for
the site owner to require field testing of delivered bentonite, because
deliveries are occasionally rejected by field testing. At a minimum, testing
of pH, viscosity and fluid loss should be conducted in the field for bentonite
delivered to the site. (Quality control tests are described in Section
5.3.2.5.)
Slurry quality decreases significantly if the quality of make-up water is
poor. Make-up water should be relatively low in hardness, near neutral or
slightly higher pH and low in dissolved salts. Water suitable for drinking is
not necessarily suitable for mixing with bentonite. Water of inadequate
quality will result in higher bentonite consumption and a lumpy slurry that is
difficult to mix and that contains above average amounts of free water. In
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some instances, poor quality water can be chemically treated to make it
suitable for mixing (Ryan 1977).
5.5.3.2 Slurry Mixing and Hydration
Two types of mixing systems are most frequently used. These are batch
mixing and flash mixing. In the batch system, specified quantities of water
and bentonite are placed in a tank and are mixed at high speeds with a circu-
lation pump or paddle mixer. Mixing continues until hydration is complete and
the batch is ready for use in the trench. Hydration is usually complete in a
matter of minutes for the two to five cubic yard batch produced by this
system. Because of the low output of the batch system, its use is limited to
small jobs.
The second type of mixing system is the flash or venturi mixer. For this
system, bentonite is fed at a predetermined rate into a metered water stream
as it is forced through a nozzle at a constant rate. The slurry is subjected
to high shear mixing for only a fraction of a second, which is not always
adequate for hydration. Therefore, the slurry is often stored until hydration
is complete. This is determined by periodically measuring the Marsh Funnel
viscosity.
When Marsh Funnel viscosity readings stabilize, hydration is considered
complete. Flash mixing is a process that can be operated at high production
rates. Because a majority of cut-off walls require continuous production of
large amounts of slurry, flash mixing is the more common of the two mixing
methods (Ryan 1977).
The type of mixing system used has been found to affect the quality of
the slurry produced. High shear (or high speed batch mixers) produce slurries
with higher gel strengths (Xanthakos 1979). Section 2 discusses the influence
of gel strength on slurry quality.
The grade of bentonite dictates the percent needed for a given slurry and
hydration time. For example, a grade 90 (bbl/ton) bentonite may have to be
mixed at a 6.3 percent concentration, while a grade 125 would require 4.5
percent concentration for an equivalent slurry. Hydration times for higher
grades are likewise lower, which may result in higher slurry production rates
for a given slurry preparation facility (Ryan 1977). However, the higher cost
of the higher grade bentonites requires that the selection of bentonite grade
to be made on a site by site basis. Frequently a mud balance test is run on
slurry from the hydration pond as a quality control check of the bentonite
content. Viscosity and pH are also checked frequently (Cavalli 1982).
5.5.4 Slurry Placement
From the hydration pond, slurry is pumped on an as needed basis to the
open slurry trench. Slurry level in the trench must be maintained at least
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several feet above the water table and normally within a foot or two of ground
level. This slurry level is maintained to provide the hydrostatic pressure
necessary to hold open the trench.
Once a slurry trench installation is underway, backfill and excavation
are being performed simultaneously, with a minimum amount of trench remaining
open under the slurry. Figure 5-3 illustrates the excavation and backfill
placement operations. The amount of trench remaining open at any one time
depends on the properties of the backfill material and the characteristics of
the excavation equipment, which are discussed in the following sections.
Samples removed from the trench for QC checks must be representative, that is
they should not be taken only from the top surface of the slurry but should be
taken at various depths in the trench. Characteristics of the slurry, while
geared toward keeping the trench open during excavation and backfilling, must
also allow displacement by the backfill material. That is the reason for
maximum as well as minimum values for parameters such as density, viscosity,
and sand content. Those requirements are listed in Table 5-1. During the
excavation operation, some of the spoil becomes incorporated in the slurry.
This increases slurry density and sand content. A high sand content indicates
a high density and a likelihood of problems with eventual displacement of the
slurry by the backfill (D'Appolonia and Ryan 1979).
5.5.5 Trench Excavation
Excavation of a slurry trench proceeds much as any trench excavation
except that only the portion of the trench above slurry level can be visually
inspected for continuity.
Trench excavation is usually accomplished with appropriately sized back-
hoes with adequate boom length and bucket capacity. Frequently, boom lengths
are extended by construction contractors to meet the needs of the trench
installation. Counterweights are often required to offset the movement
created by the long boom lifting a full bucket from the trench. The backhoe
is the favored means of excavating a slurry trench because it is much faster
than other equipment, such as the crane and clamshell. However, boom lengths
are currently limited to 70 to 90 feet. For greater depths, the crane and
clamshell are normally used. Drag lines have been used in the past, but have
been used rarely for recent installations (D'Appolonia 1982).
Trench continuity is critical to a successful installation. For checking
continuity of slurry trenches, several approaches have been used. All of them
may be employed at a given site to insure that the trenching is continuous
from the ground surface to the aquiclude key-in.
The field inspector should have boring logs and a cross-sectional drawing
of the trench so that visual inspection of excavated material and degree of
extension of the backhoe boom will indicate approximate depth and whether the
aquiclude has been reached. Soil boring data can also be used to quantify the
aquiclude key-in. By watching excavated material for a change in color or
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Figure 5-3.
Cross-section of Slurry Trench, Showing Excavation
and Backfilling Operations
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texture, construction personnel can determine when the subsurface layer which
is to be keyed into is reached.
Sounding of depth with a weighted line or a rod should be performed
frequently to ensure an even trench bottom and to detect any irregularities.
Finished trench depths should be recorded for preparation of a drawing showing
trench cross section.
5.5.6 Backfill Preparation
Standard practice during backfill preparation for soil bentonite walls is
to use excavated material mixed with slurry from the trench for backfill. In
this case, the slurry provides moisture necessary for backfill mixing. When
trench spoils are used, the material excavated from the trench is usually
placed nearby, slurry is added and a bulldozer is used to track and blade the
material until it is thoroughly mixed.
A relatively level working surface is needed for backfill mixing. At
sites that are too steep, backfill mixing areas can be excavated. These
should be at least as wide as the width of the excavating equipment track.
Where no backfill mixing areas are available, batch mixers or pugmills can be
used, although these are slower than using a bulldozer for backfill mixing.
A number of quality control checks are necessary for backfill preparation
activities. These include tests of the:
Fines content
S lump
Wet density
Presence of contaminants.
5.5.6.1 Fines Content
^A key parameter in the design of a backfill is the sieve analysis and
particularly the amount of fines. The content of fines in the backfill is
directly related to the permeability of the finished SB wall and its ability
to withstand chemical attack. A standard practice is to perform permeability
testing in the pre-construction phase to define the range of acceptable grain
size distributions that will provide the design permeability. During
construction, frequent grain size distributions and less frequent permeability
testing^is performed on the backfill material to ensure that the design
permeability requirements will be met by the completed slurry trench. Grain
size distributions can be performed in a field laboratory and are much less
expensive to run than permeability tests, which are usually run in a
laboratory.
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5.5.6.2 Slump
Slump cone testing should be performed frequently on backfill material
after mixing to make sure the backfill is wet enough to slump in the trench
without trapping pockets of slurry yet dry enough to displace the slurry
easily. High slump also indicates a gentler slope of backfill in the trench,
which would require keeping more of the trench open. A slump of 2 to 6 inches
is adequate .(D1Appolonia 1980). Additional information on^the slump of the
backfill and its effect on the finished wall can be found in Section 2.
5.5.6.3 Wet Density
Mud balance testing should be performed frequently on the backfill before
it is placed in the trench. Mud balance tests indicate the wet density of the
backfill. This shows for certain that the backfill will or will not readily
displace the slurry. The wet backfill sample used for the mud balance test
can be dried and reweighed to determine the water content. A very high water
content can result in excess water infiltration into the trench walls and
excess settlement of the backfill. The separation of water from an
excessively wet backfill can also dilute the slurry in the trench.
5.5.7 Backfill Placement
Once trench excavation has proceeded for a distance that will not result
in backfill material being re-excavated, backfill placement can begin. First,
samples of slurry at the base of the trench are collected and tested for wet
density. The slurry should be at least 15 pcf less dense than the backfill
mixture (D'Appolonia 1980b). If the slurry is too dense, it will not be
displaced properly during backfill placement. The dense slurry or coarse
material on the trench bottom must be removed via airlift pumps, a clamshell
bucket or other method. This slurry can be used for backfill mixing or it can
be desanded via desanders, hydrocyclones or screens, and returned to the
trench.
To place the initial layer of backfill, a clamshell is often used. The
backfill must not be allowed to drop freely through the slurry, as this may
cause segregation of the backfill particles or entrapment of slurry pockets
within the backfill. For this reason, the clamshell lowers the initial
grabfull of backfill to the trench bottom. The next grabfull is placed on top
of the first, and so on until the backfill is visible at the ground surface.
Thereafter, the backfill is pushed into the trench by bulldozers or graders.
The point of trench backfilling progresses towards the area of active
excavation (D'Appolonia and Ryan 1979). Figure 5-2 illustrates this process.
The slope of the emplaced backfill is normally 5 to 10:1. The distance
to be maintained between the toe of the backfill and the area of active
excavation varies greatly, depending on soil types and backfill slopes.
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Ideally this distance is kept to a minimum to avert trench stability problems
(D'Appolonia 1980b). Some specifications have set the distance at from 30 to
200 feet (U.S. Army Corps of Engineers 1975, Ryan 1976).
Sounding the placed backfill should be conducted to show the slope at
which the backfill is coming to rest, and to indicate possible problems with
trench wall collapse and entrapment of pockets of slurry. Depths should be
recorded and plotted on a cross sectional drawing of the trench. Trench
excavation and backfilling progress can also be recorded in this manner.
When a slurry wall is constructed to entirely surround a waste site, the
excavation must end with enough overlap to ensure that all material designated
for excavation is removed. The .inspector must determine that the backhoe
bucket is removing backfill materials from the full trench depth to verify
that the trench is continuous for its entire length.
5.5.8 Capping
To protect the finished SB wall, either a dessication cap or a traffic
cap is applied to the slurry wall surface.
Once backfill has been completed, cracking is soon observed on the top of
the slurry wall unless it stays, wet; to complete the installation, the top i
to 3 feet of wall is removed to eliminate the cracks and a high quality
backfill material replaces it. The material used is usually required to have
a high clay content and to be compacted in lifts over the trench. This forms
a low permeability cap to protect the cut-off wall from excessive dessication
(U.S. Army Corps of Engineers 1976). This is followed by topsoil and seeding
or a gravel layer to prevent water and wind erosion.
Where traffic over the wall is anticipated, a traffic cap can be
constructed to reduce the load on the completed cut-off. To do this, lifts of
compacted clay are interspersed with geotextile layers. Gravel can be used
over the final geotextile layer at the surface. At one facility where a soil
bentonite cut-off wall was close to a heavily traveled gravel road, the cap
consisted .of an 18-inch thick compacted clay lift topped by a geotextile
sheet. This was overlain by another 18-inch thick compacted clay layer topped
by another geotextile sheet. Inches of gravel were placed over the final
sheet of geotextile to distribute the weight and bear the load of the vehicles
(Coneybear 1982).
5.5.9 Clean Up Activities
After wall construction is complete, the excess slurry and mixed backfill
must be disposed of in a manner that avoids erosion and disruption of sewer
lines. The slurry should not be allowed to enter sanitary or storm sewer
lines due to the potential for pipe blockage from the slurry. It also should
not be left as a thick layer on the soil surface, as this may result in
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excessive ponding of surface water. One method of slurry disposal would be to
mix the slurry with dry coarse soil to produce as dry a mixture as possible.
This material could then be either buried or spread in a thin layer over
disturbed areas, then fertilized and seeded. Any contaminated soil from the
excavation must be disposed of in accordance with site requirements. All
disturbed areas should be stabilized and site maintenance procedures, as
outlined in Section 6, should be instituted.
There are a number of differences in construction activities that vary
with construction materials. The following is a brief summary of cement
bentonite and diaphragm wall construction techniques.
5.6 Cement Bentonite Wall Construction
The discussion that was presented above is an outline of a soil bentonite
(SB) cut-off wall construction. Modifications to the construction specifica-
tions are necessary when constructing a cement-bentonite (CB) cut-off wall.
These modifications include:
The requirements for backfill materials are eliminated.
A description of the standards for cement and cement storage is added
in the materials section.
A description of the CB slurry requirements and the cement/water (C/W)
ratio to be maintained for desired compressive strength is added to
the materials section.
The methods to be used for tie in of adjacent CB panels (if used) are
addressed in the performance section.
Under the Quality Control section, a requirement is added for the
manufacturer's certification of the cement and the testing of cement/
water ratio for each batch of mix is required.
Cement bentonite (CB) slurry walls involve the use of a slurry consisting
of water, bentonite and cement. The advantages of CB walls are that backfill
material is not needed and they exhibit some structural strength. In
addition, by excavating a section (panel) at a time, a CB wall can be
installed on a site with more extreme topography.
Two types of CB walls are being used. The in-place method involves
simply excavating under a CB slurry and leaving the slurry in place. The
slurry eventually sets to provide some structural strength. For the
replacement method, excavation takes place under a bentonite slurry. Once
excavation of the section of wall is complete, the bentonite slurry is pumped
out of the trench and the CB slurry is pumped in and allowed to set. The
replacement method is used only when setting of the CB slurry could possibly
occur while excavation is being completed.
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Cement bentonite slurries begin to set within 2 to 3 hours after the
cement and slurry are mixed. If the slurries are agitated for over 48 hours,
they lose their ability to set (Jefferis 1981b). For this reason, when it
appears that the excavation of a single CB panel will take longer than a day
or so to complete, cement retarders are added to the slurry or the replacement
method is used. Examples of this situation would be very deep excavations,
when rock is encountered in the excavation, or when keying the trench into
bedrock.
Quality control procedures for CB walls are identical to those for soil
bentonite walls. However, composition of the CB slurry is more critical,
therefore care^must be taken when weighing and mixing components of the
slurry. This is because the calcium in the cement causes an irreversible
decrease in slurry quality, as described in Section 2. Table 5-5 shows
typical materials quality control standards for cement-bentonite cut-off
walls.
5.7 Diaphragm Wall Construction
Construction of diaphragm walls also involves the use of bentonite or CB
slurries, although these walls are not normally used for pollution migration
cut-offs, except where high strength is required. Diaphragm walls are
composed of either precast concrete panels or cast-in-place concrete sections.
Unlike SB and CB walls, these walls develop a great deal of strength over time
and can be used as structural components. A brief description of the
techniques used for construction of diaphragm walls is given below.
Precast concrete panel walls are cast offsite in segments from 1 to 3
feet thick, from 10 to 20 feet wide and from 30 to 50 feet long. The panels
are lowered into a trench containing bentonite slurry and secured in place.
Due to their dimensional limitations, concrete panel walls are usually only
employed where depths of 50 feet or less are required (Guertin and McTigue
1982b). An exception to this general rule occurs where a CB slurry is used in
the trench. The panel is lowered into the trench and secured, and the CB
slurry that remains in place is allowed to set up around the panel. Using
this technique, the trench can be extended lower than 50 feet, and the panel
suspended in the CB slurry. The CB slurry then forms a cut-off both below and
on either side of the diaphragm panel. The CB slurry also forms the joints
between the panels (Jefferis 1981b).
Cast-in-place concrete walls are constructed by excavating a short trench
(or slot) under a bentonite slurry. When the slot is completed, the slurry is
desanded if necessary to reduce its density and to avoid problems with sand
accumulations on the reinforcing bars (Guertin and McTigue 1982b). The
maximum recommended slurry density is 75 p.c.f. and the maximum recommended
sand content is 5 percent (Millet and Perez 1981).
The reinforcing bars are lowered into place, then the concrete is tremied
into place, using a funnel-like apparatus that directs the concrete to the
trench bottom. The apparatus is raised as the concrete level rises. Slurry
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TABLE 5-5.
MATERIALS QUALITY CONTROL PROGRAM FOR CEMENT/BENTONITE WALLS
Subject
Standard Type of Test Frequency
Specified Values
Water
. pH Per water source
Total Hardness or as changes
(Ca & Mg) occur
As required to properly
hydrate bentonite with
approved additives.
Determined by slurry
viscosity and gel strength
tests.
Materials Bentonite
Cement
API STD 13A Manufacturer certificate of
compliance
ASTM C 150 Manufacturer certificate of
compliance
Unaltered sodium cation
montmorillonite
Portland, Type 1 (Type V or
Type II for certain applica
tions)
Bentonite
Slurry
C-B
Slurry
Prior to API STD 13B - Viscosity
Addition of
Cement
Upon API STD 13B - C/W Ratio
introduction API STD 10B - Viscosity
in the
trench
.
1 set per shift or V >_ 34 sec-Marsh @ 68°
per batch (pond) pH >_ 8
Each Batch C/W = 0.20
5 per shift V = 40 to 50 sec-Marsh
. , . .
Reference: Federal Bentonite 1981.
-------
is pumped out as the concrete is tremied in. The slurry is then filtered and
used in the next panel. This process is illustrated in Figure 5-4. A
cast-in-place wall, when set, is composed of a concrete panel sandwiched
between two bentonite filter cakes.
These walls are not normally used for pollution migration control due to
their susceptibility to leakage through panel connections, their high
permeability relative to SB walls and their greater expense. In addition,
relatively minor earth movement can cause leakages through panel connections,
cracking of the relatively brittle concrete, and differential settlement of
the panels (Guertin and McTigue 1982b).
5.8 Potential Problems During and After Construction
There are several mechanisms or processes that can affect the
construction or functioning of slurry walls and cause construction delays,
trench collapse or wall leakage. These are usually the result of either
excavation and installation procedures, or unforseen subsurface conditions.
Wall disruption may occur during excavation and installation and requir-
ing re-excavation of the slurry trench. Once the wall is in place, improper
construction techniques or adverse physical and chemical processes can affect
the integrity of the wall and its impermeability. The following discussion
focuses on the various problems that may be encountered and methods used to
overcome them. Types of problems include:
Unstable soils
High water tables
Hard rock in excavations
Sudden slurry losses
Slurry flocculation
Trench collapse
Inadequate backfill placement
Cracking
Chemical disruption.
5.8.1 Unstable Soil
At some sites, the surface soils are too soft to support heavy construc-
tion equipment. A work platform can be constructed of compacted soil along
with the proposed line of trench excavation. The construction equipment can
maneuver on this platform and the trench can be excavated directly through it.
5-41
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Figure 5-4.
Schematic of Conventional Cast-in-Place Diaphragm Wall
Slurry Supply Pipe
Clamshell Bucket
a) Excavate Soil and Replace with
Bentonite Slurry
Tremu
Pipe
Concrete - * -. -4
'.- ' v ' .''
Ill *^M^v "
'. r^ &f.'
"itf«** ''
Pour Tremie Concrete to Displace Slurry,
Remove Stop-End Tubes
&
Bentonite
Slurry
Reinforcing Steel
b) Place Stop-End Tubes and Reinforcing Steel
into Fully Excavated Panel
Paru:l ti> bt; Concreted
St,T< nullify
with Slurry Secondary
d) Different Construction Phases
Source: Guertin and MuTiyue, 1982
-------
5.8.2 High Water Table
A work platform can also be used to maintain sufficient hydraulic head in
the slurry trench to offset high groundwater pressures (Namy 1980). The
slurry level must be maintained at least several feet above groundwater
levels, as described earlier.
When groundwater levels suddenly rise close to the surface, the trench
may^collapse unless measures are quickly taken. The U.S. Army Corps of
Engineers (1976), recommends in their specifications that, should this occur,
the contractor should stop excavating and begin backfilling any open trench
sections as rapidly as possible. After the groundwater level decreases and
the wall has set sufficiently, the hastily backfilled sections can be
re-excavated and properly backfilled.
5.8.3 Rock in Excavation
When a slurry trench must be excavated through material containing
numerous or large boulders, hard rock layers, or into a hard aquiclude,
construction delays are likely. The use of large equipment such as cranes may
be required to remove very large boulders. Rotary or percussion drills may
also be used to break up boulders prior to the use of smaller excavation
equipment. All rock fragments should be removed from the trench bottom prior
to backfilling.
The presence of boulders near the trench bottom, or the presence of an
unrippable aquiclude may lead to variations in trench depth and inadequate
aquiclude key-in. If trench excavation is not extended far enough into the
aquiclude, the bottom of the slurry wall is inadequately sealed, and
underseepage may result. When a permeable layer exists beneath the slurry
wall, migration of water, wastes, or other materials is likely to occur
(D'Appolonia 1980b). This reduces slurry wall efficiency and may lead to
piping failure, as described later in this section.
5.8.4 Sudden Slurry Loss
Occasionally, slurry levels within a slurry trench will drop rapidly.
This situation, termed sudden slurry loss, can be caused when the excavation
encounters previous layers, such as gravel lenses or subsurface pipes.
5.8.4.1 Pervious Zones in Excavation
When^sand^or gravel layers are encountered, rapid slurry loss can occur.
In this situation, lost circulation materials are used and large quantities of
slurry are pumped into the trench to maintain high slurry levels. Factors
5-43
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that can reduce the flow of slurry into pervious soil such as rheological
blocking and filter cake formation, are discussed in Section 2.
5.8.4.2 Pipes and Conduits
At some sites, subsurface pipes or other conduits have been encountered
unexpectedly. This results in rapid loss of slurry from the trench. Two
methods have been used to plug up the pipes and thereby slow or stop the^rapid
slurry loss. The first is applicable only to corrugated metal pipes, which
can be pinched shut, using the excavation tool like a giant pair of pliers.
The second method involves the rapid introduction of lost circulation
materials, including "shredded cellophane flakes, shredded tree bark^ plant
fibers, glass, rayon, graded mi'Ca, ground walnut shells, rubber tires,
perlite, time-setting cement and many others" (Xanthakos 1979). Coarse sand
and crushed brick have also been used. These materials clog the pipe and
allow the slurry to reseal the trench. In any case, additional fresh slurry
must rapidly be pumped into the trench to avoid the loss of trench wall
stability (Guertin and McTigue 1982b).
5.8.5 Slurry Flocculation
This situation may occur when cement is added to a bentonite slurry to
form a CB slurry or when bentonite slurries come in contact with other high
calcium materials. Several approaches can be taken when the slurry begins to
form flattened clumps. Various thinners or dispersing agents can be added, as
listed in Table 2-4. Additional fresh slurry may be added, or additional
bentonite may be added, depending on the severity of the flocculation. A
discussion of flocculation and its causes is presented in Section 2.
5.8.6 Trench Collapse
Trench collapse is caused by the loss of stability in the trench walls
during excavation and before backfilling or CB slurry hardening. Causes of
trench collapse include:
Insufficient slurry head above groundwater
Sudden or rapid loss of slurry due to contact with gravel, large
pores, fissures, etc.
Surface runoff into open cracks
Insufficient agitation of slurry
Overloading of the ground surface with stockpiles or heavy equipment
in close proximity to the trench.
5-44
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The underlying causes of trench collapse is the failure of the slurry to
form and maintain a low permeability filter cake. Factors that interfere with
filter cake formation or functioning were discussed previously.
Trench collapse can be either total or partial. Sometimes only partial
collapse occurs, and the material from one wall slips partially into the
trench without bridging the entire trench width, as shown in Figure 5-5. In
this situation, the trench may still be salvageable.
If severe collapse of the trench side walls occurs, the trench can be
backfilled as much as possible and left as is. Another trench parallel to the
collapsed trench and at least 15 feet away is excavated using typical slurry
trench construction techniques (Boyes 1975).
5.8.7 Inadequate Backfill Placement
Improper placement of the backfill can result in underseepage, excessive
consolidation, or wall leakage. Specific backfill problems are described
below.
5.8.7.1 Sediments in Trench Bottom
The slurry can suspend small particles of sand as well as silt and clay
particles due to the shear strength and gel structure of the slurry. Coarse
sand and larger particles are not suspended. Instead they sink to the trench
bottom and accumulate there. Excessive accumulations of heavy sediments mixed
with slurry can interfere with backfilling if these sediments are close enough
in density to the density of the backfill. Cuttings from broken boulders or
gravel can also interfere with backfilling. The backfill cannot displace
these sediments, so they remain after backfilling as a layer of sandy or
gravelly slurry beneath the backfill. Studies of this type of material have
shown that slurry-laced sand and gravel layers are much more susceptible to
failure, leakage, and chemical degradation than are backfills containing a
higher percentage of fines (D'Appolonia 1980b). At many sites, the removal of
the sand from the trench bottom prior to backfilling is not necessary. This
is because the slurry-encapsulated sand has a lower initial permeability and a
higher density than the backfill. Thus the sand layer is not likely to
interfere with backfilling operations.
The ability of a pure sand/bentonite layer to withstand permeation over
time is, however, questionable. Even if a sand/bentonite backfill contained
from 2 to 3 percent bentonite by weight, failure could occur due to permeation
by calcium-rich solutions (D'Appolonia 1980b).
This situation is similar to the conditions at the trench bottom where
bentonite concentrations may be minimal. Bentonite slurries usually contain
from 4 to 7 percent by weight bentonite when introduced to the trench. As
spoil particles become mixed with the slurry, the slurry density increases and
5-45
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Figure 5-5.
Trench Collapse, Showing Plane of Weakness (a)
and Block Slippage (b)
^ * J. - ^ . . , PL -r*J° °
A^^AwAwfc^A^ '-*iVXi»/>.;
paPP
V J"*^ fJJ-W*»
J Oi\r,
nc-.«
^^^roCS
Line of Slurry Visible QM
at Ground Surface a Few °
t Feet from the Trench
r-rV Along Plane of Weakness
(a)
-------
the weight percent of bentonite is consequently reduced. The slurry is most
dense at the trench bottom due to the presence of the settled sand and gravel
layer. Thus the weight percentage of bentonite in the slurry at the trench
bottom may conceivably drop to 3 percent or less, leaving the cut-off wall
susceptible to failure should prolonged permeation with cation-rich solutions
occur.
To prevent piping failures of soil-bentonite cut-off walls, D'Appolonia
(1980) recommends that the backfill contain at least 20 percent fines. These
fine particles effectively resist intergranular stresses and reduce the
erosion of the bentonite particles from the backfill matrix. The backfill
should be homogenous, both horizontally and vertically, to avoid piping
failures and underseepage. Removal of the slurry-encapsulated sediments from
the trench bottom prior to backfilling helps ensure vertical homogeneity and
can contribute to the long term integrity of the cut-off wall.
5.8.7.2 Slurry Pockets in the Backfill
If the slump of the backfill is too high, the backfill does not flow to
the trench bottom properly, but folds over itself during backfill placement
and may entrap pockets of slurry. These slurry pockets remain in the wall and
act like compressible layers with lower resistance to hydraulic gradients and
chemical attack than the surrounding backfill. Because the slurry is less
dense than the backfill, it gradually rises through the paste-like backfill
until it reaches the trench surface. There, it is subject to desiccation and
cracking if the clay cap has not yet been placed, or it may interfere with the
connection between the clay cap and the cut-off. If the amount of entrapped
slurry is excessive, the wall may need to be re-excavated and backfilled.
5.8.8 Cracking
It is well known that soils containing an appreciable concentration of
clay can shrink, forming cracks, when allowed to dry. Even cement bentonite
walls can crack "drastically" when allowed to dry (Jefferis 1981b). Some
soils can, however, shrink and crack even when nearly fluid enough to flow.
Nash (1976) and Tavenas et al (1975) noted the formation of cracks in silty
clay soil mixtures only slightly dry of their liquid limits.
These cracks can be caused by several processes, including:
Consolidation
Hydraulic fracturing
Syneresis.
5-47
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5.8.8.1 Consolidation
Consolidation of soils occurs when water is squeezed from the soils pores
(Hirschfeld 1979). This process is accompanied by a decrease in the volume of
the soil mass due to a decrease in the volume of voids in the soil. (Baver,
Baver and Gardner 1972) The amount of consolidation is maximal in fine-
textured soils and minimal in coarse textured materials (Xanthakos 1979). The
rate of consolidation depends on the soils permeability, the thickness of the
layer being loaded and the magnitude of the soil mass's volume decrease. In
fine textured soils, consolidation occurs at a much slower rate than in coarse
textured ones. Most of the consolidation occurs rather quickly, however fine
textured soils can continue slow minor consolidation for several months
(Hirschfeld 1979). The soil-bentonite backfill in slurry walls has been found
to continue consolidating"for about 6 months, with minimal decreases in volume
thereafter. (Xanthakos 1979)
The amount of consolidation in slurry walls is limited by the backfill's
arching action along the trench walls and by grain to grain contact in the
backfill (Xanthakos 1979, D'Appolonia 1980b). The amount of slurry wall
consolidation depends on trench width, as well as on the amount of fines in
the backfill. As the fines content increases, consolidation increases because
fine particles are more compressible than coarse ones (Mitchell 1976). Wider
trenches have been found to consolidate more than narrow ones. An 8-foot wide
trench, for example, was reported by Xanthakos (1979) to have consolidated
from 1 to 6 inches during the first few months after construction. In
contrast to soil bentonite walls, CB walls consolidate very rapidly (Ryan
1976).
The process of consolidation in SB walls can produce many small cracks
along shear zones in the soil mass (Mitchell 1976). Horizontal cracks that
extend through the backfill can also be produced by the arching action
mentioned earlier (Nash 1976). This cracking can occur even when the backfill
is still quite fluid. Normally, the backfill contains from 25 to 30 percent
water. This moisture content is slightly more than the liquid limit of the
backfill (D'Appolonia 1980a). When soil is at its liquid limit, it contains
so much water that it will flow under the influence of an applied stress
(Baver, Baver and Gardner 1972). Even so, Nash (1976) observed the formation
of horizontal cracks in a silty clay backfill that was only slightly dry of
its liquid limit. The details of Nash's laboratory tests on this soil
material were not given.
The arching that accompanies consolidation can also lead to the formation
of another type of cracking due to a phenomenon called hydrofracturing. The
relationship between hydrofracturing and consolidation is described below.
5.8.8.2 Hydro frac turing
When soils or rocks are subjected to excessive hydraulic pressures,
cracks may form through which the excess water can flow. The pressure at
5-48
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which this cracking occurs can be less than the effective overburden pressure
on the rock or soil in situ. This phenomenon has been used by petroleum
geologists to fracture petroleum-containing strata and thereby increase the
yield of oil wells (Bjerrum et al 1972).
The amount of pressure required to induce fracturing depends on the depth
of the point receiving the pressure, and the ratio of vertical to horizontal
pressures at that point. If is likely that other factors are also involved.
Values used by petroleum geologists are 1 psi per foot of depth where the
vertical pressures are less than the horizontal pressures, and 0.64 psi per
foot of depth where the vertical pressures are greater than the horizontal
pressures (Bjerrum et al 1972).
The cracks caused by this pressure extend vertically where the vertical
pressures are greatest and horizontally where the horizontal pressures are
greatest (Tavenas et al 1975). These cracks can continue to increase in lengh
as long as the excess head is applied until they reach an area having greater
permeability (Bjerrum et al 1972). When the pressure is decreased, the cracks
will partially close, but will reopen when the pressure is again increased.
(Tavenas et al 1975)
The effects of hydrofracturing on the functioning of a slurry wall can be
severe. Where the vertical pressure exceeds the horizontal pressure (which is
normally the case) the pressure exerted on the aquiclude can cause it to
fracture. The vertical cracks will be immediately filled with slurry if this
occurs during construction. Continued fracturing can, however, occur during
backfilling. The backfill is less likely than the slurry to completely flow
in and fill the cracks in the aquiclude, and the aquiclude's permeability may
thus be increased. The overall effect of this type of fracturing may be
minimal except at sites where the aquiclude is thin and/or overlies a very
permeable strata.
A more detrimental effect of hydrofracturing may occur where the
horizontal pressures on the wall are greater than the vertical pressures. If
more than 1 psi of pressure per foot of depth is applied to the wall,
horizontal cracks may form. These could allow significant amounts of water or
leachate to flow through the wall. Hydrofracturing has been reported in both
SB and CB walls (Bjerrum et al 1972 and Miller and Perez 1981).
The likelihood of slurry wall damage due to hydraulic fracturing is
highest where:
Significant amounts of consolidation occur
Piezometers are installed in the wall to monitor the wall's
permeability using constant head tests
Large vertical loads are applied to the soils on either side of the
trench
Large hydraulic gradients are allowed to develop across the wall.
5-49
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a. Consolidation
When a great deal of consolidation and subsequent arching occur after
backfilling, the vertical loadings on the wall can be reduced to levels less
than the horizontal loadings. This allows the wall to become susceptible to
horizontal cracking (Bjerrum et al 1972). The amount of consolidation can be
minimized by reducing the content of fine particles, in the backfill (Mitchell
1976). However, when additional coarse material is included in the backfill,
the wall's permeability is likely to be increased (D'Appolonia 1980b). The
amount of permeability increase expected due to the inclusion of additional
coarse material in the backfill should be weighed against the risk of
hydrofracturing due to the anticipated pressure differential across the
trench.
b. Piezometers
Where piezometers are placed in the wall to test the wall's permeability,
hydrofracturing may be induced when the excess head is applied. Hydro-
fracturing due to the use of piezometers occurred when permeability tests were
being conducted on a series of dikes with soil-bentonite cores that were
installed in Israel. The constant head permeability tests were designed so
that the maximum head applied did not exceed the effective weight of the soil
above the piezometer. However, even at very low hydraulic pressures,
hydrofracturing occured. This fracturing resulted in a thousand-fold increase
in the measured permeability (from 10 cm/sec initially to 10 cm/sec after
fracturing) (Bjerrum et al 1972).
Tests on an in situ, normally consolidated clay were conducted to see if
hydrofracturing were caused by arching along the walls of the soil bentonite
cores. The normally consolidated clay that was not influenced by arching
along trench walls also fractured under the influence of an applied hydraulic
pressure that was less than the effective overburden pressure (Bjerrum et al
1972). This indicates that the occurrence of hydrofracturing is not dependent
upon arching (and consolidation) along the walls of slurry trenches.
When large vertical loads are applied along the trench walls, the
horizontal stresses on the slurry wall can be greatly increased. The vertical
loading can occur due to placement of stockpiles or heavy equipment along the
sides of the trench. Depending on the other stresses acting on the wall at
the site, the horizontal stress may become greater than the vertical stress,
thus making the wall susceptible to horizontal hydrofracturing.
c. High Hydraulic Gradients
A fourth potential cause of hydrofracturing is the presence of an
excessive hydraulic gradient across the wall. If the pressure on the
upgradient wall exceeds 1 psi per foot of depth, and the horizontal pressures
5-50
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acting on the wall are greater than the vertical pressures, it is quite
possible that horizontal fracturing of the wall could occur. Excessively high
hydraulic gradients could be induced by:
Failing to provide subsurface drains or extraction wells upgradient of
the wall
Installing extraction or injection wells too close to the wall
Dewatering a site without deflecting groundwater around the site and
away from the wall (via drains, ditches or extraction wells).
For this reason, the proper use,and placement of auxiliary measures, such
as wells, should receive careful attention during the wall's design stages.
5.8.8.3 Syneresis
Another process that can result in the formation of cracks in a slurry
wall is syneresis. According to Mitchell (1976) syneresis is a "mutual
attraction between clay particles" that causes the particles "to form closely
knit aggregates with fissures between." It is the contraction that occurs in
a gel that, results in the extrusion of liquid (water). Syneresis is often
observed,in gelatin after aging (Mitchell 1976). Syneresis may take place in
slurry walls, however, the extent to which this phenomenon affects the
performance of slurry walls is not known.
5.8.9 Tunnelling and Piping
Two processes that can result in extensive breaching of a slurry wall are
tunnelling and piping. Both of these processes involve the formation of
channels through the wall. However, the causes and solutions for the two
problems are different. .
5.8.9.1 Tunnelling
Several dams have failed due to formation of very large pores that extend
completely through the dam, from the upstream to the downstream face. These
failures occurred where the earthen dams were constructed of low to medium
plasticity native clays that contained appreciable amounts of sodium montmor-
illonite (Mitchell 1976). The process by which the failures occurred is
termed tunnelling, and it can be described as a series of interrelated steps,
as are listed below.
1. Differential consolidation of the wet and dry portions of several
earthen dams led to the formation of stress cracks below the water
line.
5-51
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2. Water that contained calcium ions flowed into the cracks.
3. Calcium ions from the water replaced sodium ions on the exchange
complexes of the clay particles in the dam. (See Section 2 for a more
detailed description of cation exchange.)
4. The calcium ions caused the clay particles to decrease in size
(shrink) and to form packets, or "floes."
5. As the clay particles formed floes, they became less dispersed, and
the space formerly occupied by the dispersed clay particles became
filled with water.
6. As the sizes of water-filled spaces (pores) increased, the rate of
water movement in the pores increased.
7. The increased rate of water movement allowed the water to carry more
particles in suspension.
8. As the particle-carrying capacity of the water increased, the number
of clay particles eroded by the flowing water increased.
9. As the number of clay particles eroded from the dams increased, the
sizes of the pores in the dams increased.
10. As the pore space size increased, the speed of the tunnelling process
increased until extensive tunnelling had occurred (Mitchell 1976).
This tunnelling process has been found to take place in earthen dams and
embankments that had initial permeabilities as low as 10 cm/sec. One method
that has been used to reduce the likelihood of tunnelling is to mix the soil
with lime prior to dam construction. This causes the clay particles to shrink
and become less easily dispersed before any cracking or particle erosion can
occur (Mitchell 1976).
The risk of tunnelling failures in slurry walls is greatest where ground-
waters contain high concentrations of calcium. At these sites the calcium
ions from the groundwater can disrupt the soil bentonite backfill and cause
tunnelling failures that are similar to the ones experienced with the earthen
dams described above. High calcium concentrations is groundwater are most
commonly found in sedimentary aquifers, particularly limestone ones. Water
from these aquifers can contain over 50 ppm dissolved calcium (Freeze and
Cherry 1979).
The presence of stress cracks, high hydraulic gradients and permeable
backfills are not prerequisites for the tunnelling process but these factors
are likely to speed failure rate considerably. The causes of stress cracking
were described previously. The hydraulic gradients across slurry walls at
hazardous waste sites are not normally as high as the gradients across dams,
so the rate of tunnelling in slurry walls should be much slower than in
earthen dams. Another factor that operates in favor of slurry walls is their
5-52
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low permeability, which is normally 1 to 3 orders of magnitude less than the
minimum permeability of the materials through which tunnelling had been
reported (Mitchell 1976, D'Appolonia 1980b). This low permeability indicates
that the initial rate of water movement through a slurry wall will be much
less than through the earthen dams, consequently the particle carrying
capacity will also be severely restricted, and the erosion rate will be
minimal.
Despite the fact that tunnelling failures, if they occur in a SB wall are
expected to require long time periods to develop, the potential for slurry
wall disruption due to the presence of calcium ions should be kept in mind
when evaluating the feasibility and design criteria for a slurry wall
installation at a particular site.
5.8.9.2 Piping
Unlike tunnelling, which starts at the upstream side of the wall, piping
begins at the downstream face and proceeds towards the upstream face (Mitchell
1976). It occurs due to the use of improper backfill materials or procedures.
Variable wall thicknesses, poorly 'mixed backfill, or extensive quantities of
coarse materials in the backfill can all contribute to piping failure.
Piping occurs where a high hydraulic gradient causes the rate of water
movement through the wall to increase as the water nears the downgradient side
of the wall. If the water movement is rapid enough, it could conceivably
force the downstream filter cake into the pores in the soil along the trench
wall. As the rate of water movement out of the wall increases, it can begin
to erode the easily-dispersed backfill, creating even-larger pores and
allowing the water movement rate to increase further (Anderson and Brown
1980). To avoid piping failures, the quality of the filter cake should be
maximized, as described in Section 2, the backfill materials should be
properly selected and mixed, the backfill should be carefully placed to avoid
fold-overs and permeable areas, and the hydraulic gradient across the wall
should be monitored and kept within designed levels.
5.8.10 Chemical Disruption
Chemical substances in soil and groundwater can affect the durability of
the slurry wall once it is in place. Chemical destruction can affect the
cement in CB slurry walls as well as the bentonite in CB and SB walls. The
effects of alkali salts on bentonite slurries were described previously.
The action of the chemicals on cement or bentonite are similar to that of
the tunnelling process. The cement may become slurry solubilized and the
bentonite may become entrained in the solution as the chemicals eventually
create a solution channel through the wall into surrounding soil. Thus,
chemical destruction processes may create as well as accelerate the tunnelling
process. Chemicals may also prevent the slurry from forming an adequate
filter cake along the sides of the slurry trench by interfering in the slurry
5-53
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gelation process. Additional information on chemical attack of slurry walls,
and on testing compatibilities is contained in Section 4.
5.9 Summary
Design and construction activities for slurry trenches for the most part
are relatively simple as long as thorough site investigation results are
available and design and construction firms involved are experienced with
slurry trench construction techniques. Although many of the slurry quality
control procedures are more applicable to drilling muds than slurry walls,
experience has shown that they are adequate until improved procedures are
developed. ASTM is studying these procedures and is expected to make recom-
mendations for changes. However, trench excavation and backfilling processes,
which many experienced design and construction people consider significantly
more important than the slurry testing procedures, are governed chiefly by
techniques that are practical and field proven. Many depend on the physical
dimensions and continuity of the trench and backfill properties which can be
verified by good field inspection.
Therefore, when adequate foresight is used in the design stages, and good
field inspection is practiced in the construction phase, existing slurry
trench construction techniques should result in a quality installation that
meets the criteria of providing a low permeability barrier to the migration of
contaminants from waste sites.
5-54
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SECTION 6
SLURRY WALL MONITORING AND MAINTENANCE
Upon completing the design, preparation and actual construction of a
slurry cut-off wall, the next concern becomes the continuing effectiveness of
the wall in the subsurface environment. The main question that arises
subsequent to a wall installation is whether or not the wall material will
remain resistant to the flow of the substances it is meant to contain. All
three of the commonly used backfill materials are subject to attack by certain
substances which can lead to an increase in permeability. In addition,
failures may result from structural disconformities within the wall. The
ability to detect and measure these types of performance failures is an
important part of the slurry wall installation process. Performance moni-
toring is essentially the only means of determining the effectiveness of a
cut-off wall over time, and some type of monitoring program should be
instituted at all walls installed for pollution control.
The following section describes the different types of monitoring instru-
mentation used to evaluate wall effectiveness and discusses the various
maintenance and restoration techniques which can be used to prevent and remedy
the deterioration or destruction of the wall.
6.1 Effectiveness Monitoring
It is not possible to provide hard guidelines for the selection of a
monitoring program at a particular cut-off wall site. Such a selection is
dependent upon two aspects of a project: (1) the questions remaining in the
designer's mind after completion of the design and (2) those problems that
were encountered either during or after the construction phase. Every slurry
wall design and subsequent installation has individual problems that are cause
for unanswered questions. The monitoring program chosen for a project should
reflect these questions, and the instrumentation selected should act as tools
for providing data upon which additional judgements can be made.
Despite the emphasis that must be placed upon site specific characteris-
tics in selecting a monitoring system, a few general guidelines are
recommended and they are as follows:
A solid knowledge should be attained of problems with the wall design
and of those problems that were encountered during construction.
6-1
-------
A monitoring system should never be solely selected on the basis of
what was arranged at a similar site.
Selection should be preceded by as thorough an understanding as
possible with current data, of the nature of contamination, both
physically and chemically.
There are basically four types of potential geotechnical problems that
require consideration after a slurry cut-off wall has been installed. These
relate to the following parameters:
Basal stability
Ground movement behind the wall
Groundwater level and chemistry
Surface water chemistry.
Table 6-1 summarizes these parameters and possible measurement methods
for each.
6.1.1 Basal Stability
A common method of determining the basal stability of an area is to
measure the subsurface horizontal movement, either of the slurry wall itself
or of the ground behind the wall. The instrument used for this purpose is an
inclinometer (Dunnicliff 1980). An inclinometer system consists of a pipe
installed in a vertical borehole, with internal longitudinal guide groves. A
torpedo containing an electrical tilt sensor is lowered down the pipe on the
end of a graduated electrical cable, the orientation being controlled by
wheels riding in the guide grooves. The electrical cable is connected to a
remote readout device indicating tilt of the torpedo with respect to the
vertical. Tilt readings and depth measurements allow alignment of the grooved
pipe to be determined. Charges measured in the alignment of the pipe provide
horizontal movement data (Dunnicliff, 1980).
6.1.2 Ground Movement
There are several methods for measuring ground movement behind a cut-off
wall and they are categorized according to the type of movement being meas-
ured. Horizontal ground movement can be measured using one of the following;
an optical survey, a horizontally installed multi-point extensometer, a
piezometer or an inclinometer system.
Vertical movement of the ground surface is yet another potential problem
in the vicinity of a slurry wall construction site. There are two techniques
available that can provide information on this type of movement; the optical
survey and the subsurface settlement gauge.
6-2
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TABLE 6-1.
POTENTIAL PROBLEMS RELATED TO SLURRY WALL
EFFECTIVENESS AND POSSIBLE ASSOCIATED MONITORING METHODS
Potential Problem
Parameters
Possible Measurement/
Monitoring Method
Basal Instability
Horizontal Move-
ment of Ground
Inclinometer
Ground Movement
Behind Wall
Horizontal Movement
of Ground
Vertical Movement
of Ground
Optical Survey
Inclinometer
Horizontally Installed
Multi-Point Extensometers
Piezometer
Optical Survey
Subsurface Settlement Gauge
Groundwater
Surface Water
Groundwater Level
Pore Pressure
Chemistry
Chemistry
Observation Well
Piezometer
Sampling Wells
Direct Sampling
Reference: Dunnicliff 1980
6-3
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In addition to the separate measurement of vertical and horizontal ground
movement, it is also possible to combine instruments and gather both types of
data simultaneously.
Detailed discussions of all these techniques can be found in Dunnicliff
(1980).
The parameters discussed thus far are measured in order to determine the
possibility of structural wall failure occurring due to changes in ground
stability in the surrounding area. Basal instability and ground surface move-
ment are both possible causes for premature wall deterioration. The moni-
toring of groundwater in the slurry wall area, on the other hand, serves a
slightly different purpose and is probably the most important parameter to be
discussed here. Data obtained through groundwater monitoring provides infor-
mation on the present efficiency of the wall, instead of on possible reasons
for its future ineffectiveness.
6.1.3 Groundwater Level and Chemistry
The slurry cut-off wall is by no means a totally impermeable structure.
In practice, it is impossible to achieve complete water tightness (Telling et
al 1978). There are, however, varying degrees of cut-off efficiency and the
following section discusses the use of groundwater monitoring and pump-in
testing methods to assess wall effectiveness.
6.1.3.1 Groundwater Monitoring
The major steps necessary in establishing a groundwater monitoring
network at a slurry wall site include the following:
Measure groundwater contamination levels
Design well and piezometer placement
Design groundwater sampling and laboratory analysis program
Data interpretation.
In assessing wall effectiveness, both the quality of groundwater prior to
slurry wall installation and the background level of the groundwater quality
immediately following wall construction are of primary importance. A solid
groundwater background data base is necessary to ensure the validity of future
comparative studies of sampling and chemical analysis results. Proper inter-
pretation of chemical analyses conducted on samples taken on opposite sides of
a wall is crucial in determining the ability of a wall to contain a contam-
inant plume. Differences in groundwater chemistry across a wall can be one
indication of whether or not the wall is sufficiently containing the
contaminant.
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Another method of measuring wall efficiency involves the relative
hydraulic head drop across the wall. The hydraulic head difference is
generally measured using data obtained from piezometer readings, although
observation well data can also be used. Equal numbers of piezometers or
observation wells are normally placed on each side of the wall. Piezometer or
well depth and distance from the wall will vary depending upon the surface and
subsurface characteristics of the site. The optimal placement scheme would
entail varying distances and depths to scan as large an area on either side of
the wall as possible. With data collected over a large area, the formulation
of a detailed groundwater flow diagram is possible. Using data obtained from
piezometer readings, the increase in volume of water inside the cut-off during
a particular time period and the effective permeability can be computed (U.S.
Army Corps of Engineers 1978).
The design of a groundwater monitoring system to assess wall effective-
ness at a slurry wall site can involve either groundwater monitoring wells or
piezometers where monitoring wells are installed, the designated locations
will depend upon the number and extent of the water-bearing zones to be
monitored. A single well design is used only in the case where one aquifer
system is to be monitored. If more than one aquifer or water-bearing zone is
to be monitored, a well cluster system may be required at each well location.
The most easily used piezometer design for monitoring a cut-off wall in
terms of groundwater levels across the wall is the open standpipe, the
simplest of these being a cased or open observation well. The water level is
measured directly with a small probe. However, the open standpipe does not
function well in impervious soils because of time lag or in partially sat-
urated soils because there is a problem with evaluating pore-air or pore-water
effects. The simplicity, sturdiness, and over-all reliability of this type,
however, dictates its use in many situations.
The electrical piezometer, consisting of a tip with a diaphragm that is
deflected by the pore pressure against one face, is also used at slurry wall
sites. This type of piezometer is most suitable for installation in quite
impervious and very clayey, plastic materials'. The remaining two piezometer
types, the hydraulic and pneumatic piezometers are less frequently used for
the purpose of monitoring groundwater levels across a slurry wall and will not
be discussed here. However, a good source of information regarding piezom-
eters and their applications is Wilson and Squier (1959). The choice of a
groundwater sampling method will depend upon the frequency of sampling, the
number of monitoring wells that have been installed and the specific
conditions at the site.
The type of laboratory analyses to be performed for samples will be
dictated by the type of wastes contained at the site.
6.1.4 In Situ Permeability Tests
Another means of monitoring wall effectiveness is by conducting in situ
permeability tests. These involve sinking a vertical hole in the center of
6-5
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the wall, and using this backhole for conducting pump-in or slug type
permeability tests. This procedure is not recommended for two reasons.
First, these tests are designed for use in permeable materials, and may give
erroneous values for low permeability SB backfill. Second, these tests have
been shown to cause hydraulic fracturing in fine-grained soil materials. Not
only would this give a falsely high permeability for the wall, it could create
planes of weakness, and lead to wall failure.
6.1.5 Surface Water Chemistry
The quality of surface water can be used as indication of cut-off
effectiveness if a contamination problem is located in close proximity to a
surface water body. An example of such a situation is where a cut-off wall
has been installed to prevent a contaminant plume from entering a stream. At
this type of site, monitoring the quality of stream water is an important and
necessary part of the monitoring program instituted. As with groundwater
monitoring, the background water quality must be established in order to
determine the effectiveness of the remedial action program undertaken. A
location far enough upstream to be unaffected by the site is necessary to
provide the most reliable background data. The number and location of
sampling points will depend primarily upon the suspected configuration of the
contaminant plume and also the level of criticality judged from knowledge of
the chemical characteristics of the contaminant. However, there should be a
minimum of three stream sampling sites:
Background upstream
Closest stream point to plume boundary
Location downstream from site.
6.2 Maintenance
One claim that is very often seen in the literature, concerns the number
of advantages that the slurry wall construction method enjoys over competitive
systems. Among these advantages is that there is very little required
maintenance. The slurry wall system eliminates the mechanical problems that
are often involved with other remedial actions, such as pump breakdowns
electrical power failures risks due to worker strikes, etc. There do exist,
however, other possible causes for wall deterioration and there are measures
that can be taken to protect the wall from premature breakdown (see
Table 6-2).
A slurry cut-off wall's maintenance needs are very often determined
during the design and installation stages. Pervious zones in slurry walls are
possible, for example, due to improper mixing of the backfill, which then
results in pockets of permeable material within the wall. Failure to excavate
and subsequently key into the aquiclude properly, can also be the cause for
6-6
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TABLK 6-2.
POTENTIAL CAUSES FOR PREMATURE WALL DETERIORATION
AND ASSOCIATED MAINTENANCE TECHNIQUES
Potential Cause for Premature
Wall Breakdown
Maintenance Method
Loading pressures
Traffic capping
Redistribution of load
Erosion
Re-vegetation
Capping
Hydraulic head
Groundwater pumping
wall inefficiencies, such as underseepage. Proper design and installation
will greatly reduce the possibility that failures such as these will occur.
Chemical breakdown of the wall backfill material is another possible
failure mechanism. However, if the proper steps are taken, during the design
stages, including extensive compatibility testing, the chances of breakdown
are greatly diminished.
Normal loading pressures and even catastrophic events such as earth-
quakes, are not generally seen as causing problems with slurry wall stability.
The compressibility of slurry wall backfill is designed, in most situations,
to allow for deformations without cracking (Ryan 1976). In addition, slurry
walls tend to retain sufficient moisture to remain somewhat plastic. This is
due to the fact that some portion of their structure is located below the
water table. Stress and strain forces that change subsurface pressures would
merely cause the wall material to flow, filling any cracks that might have
otherwise developed (SCS 1981). In the rare case where lesions form due to
excessive pressures, grout, of some type, can be used to seal the openings.
Despite the fact that loading pressures are not generally a major
concern, the top of a wall should be covered with some type of vegetation or
capping material to prevent application of concentrated loads (Ryan 1977).
The cover or cap can also serve to reduce surface infiltration and control
runoff and erosion around a wall particularly if the wall is located along a
steep slope or where precipitation is normally high. The degree to which the
wall cover should be maintained depends on the site location, i.e., whether
6-7
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the area is heavily trafficked, and on the physical and chemical nature of the
contaminants.
The pressure exerted against the outer wall face by a large hydraulic
head can be another cause for gradual wall deterioration. To counter this
force, groundwater pumping can be used and is quite effective in diminishing
potentially destructive pressures (Figure 6-1).
6.3 Wall Restoration
The need for wall restoration will arise due to reasons identical to
those that necessitate wall maintenance. Wall failure can be due to:
Chemical reaction processes between the wall and contaminant
Stress/strain forces causing structural failure
Improper design and/or construction methods.
It will be re-emphasized at the beginning of this discussion that in most
cases, wall failure is due to poor design and construction specifications or
lack of supervision during installation. Many problems can be avoided
entirely with the proper knowledge and the ability to utilize it.
A breach in a wall caused by chemical attack usually originates in one
small area of the wall. The cause for deterioration can be due to one of two
factors: (1) there exists an area of weakness in the wall, such as the type
produced by inadequate mixing of the backfill material during construction or
(2) the contaminant concentration is greatest at one location, e.g., a
floating solvent layer present in the groundwater column. In either case, the
bentonite becomes dehydrated in one portion of the wall which causes an
increase in porosity, as described in Section 2. These can result in a piping
failure and an eventual breach in the wall (Figure 6-2). In the case where
the cause for the breach is the nature of the contaminant and the wall
material has the permeability specified in the design requirements, there is
little that can be done to permanently restore the wall. A slurry wall is
probably not the proper solution for that particular problem and a revision of
the engineered solution should be required.
On the other hand, if a breach is due to a hole in the wall and the hole
can be located with some accuracy, two restorative possibilities exist: (l) a
synthetic liner can be placed along one side of the wall and (2) the breached
area can be re-excavated and re-backfilled. There is one stipulation that
must be made concerning the second option. In the case of a soil-bentonite
wall, the soil-bentonite mixture tends to slide into any re-excavation,
requiring that a long section of the trench be dug out and rebuilt (Ryan
1977). Cement-bentonite walls, however, are easily re-excavated by sections.
Failure of a cement-bentonite wall would actually stabilize the surrounding
soil, making it easy to excavate that section of the trench (Ryan 1977). In
addition, new cement-bentonite slurry added to the breached section seals the
wall to pre-existing segments (Ryan 1977).
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Figure 6-1.
Groundwater Pumping to Reduce Hydraulic
Head Pressure on a Slurry Wall
Extraction Wells
Wall
*"*T»"T**""^»»"**/*-**it»" V,**"*" !**-"*.*- *^
-.*.**.-* -*-*«»* V« -" -*** *,*-**V **
6-9
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Wall failures related to physical stress/strain forces do not usually
result in a breach. Instead, physical stresses can cause cracking, which then
allows leachate seepage through the wall. As mentioned in the previous sub-
section, this type of failure rarely occurs (Ryan 1977). If it does, however,
there are three restorative actions that can be taken (1) grouting of the
cracks, (2) the re-excavation and re-backfilling of the wall (if the wall
material consists of cement-bentonite), or (3) placement of a synthetic liner.
The third type of failure is not a failure of a wall, but failure to
properly construct a wall. This is due to either inadequate excavation and
keying into the aquiclude, or poor backfill design and/or mixing. The most
frequent result of not keying into the underlying aquiclude properly is the
seepage of leachate under the wall. This can be remedied by re-excavation and
re-backfilling, if the problem area can be located. The restrictions for this
procedure are described above. Wall failure due to permeability higher than
the design requirements is a problem that should never occur. Proper con-
struction supervision and backfill design specifications will prevent this
type of failure. Table 6-3 summarizes the available restorative methods for
various wall failure problems.
TABLE 6-3.
POSSIBLE RESTORATIVE METHODS FOR
VARIOUS WALL FAILURE PROBLEMS
Mechanism for Wall
Failure
Resulting Problem
Possible Restorative
Methods
Chemical reaction
between contaminant
and wall
Wall breach
Re-excavation and
re-backfill
Second slurry wall
installation
Stress and strain
forces
Lesions or cracking
Grouting
Re-excavation
and re-backfill
Improper design and
installation practices
Low wall permeability
> contaminant
penetration rate high
Inadequate key-in
> underseepage
Re-excavation and
re-backfill
Grout key-in
6-10
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Figure 6-2.
Wall Breach Due to Localized Chemical Attack
Wall
Surface
- Water Level
Solvent Layer
6-11
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6.4 Summary
The state-of-the-art of slurry wall use for pollution control, and of
hazardous waste site remediation, in general is such that few if any remedial
measures can be assured of long term effectiveness without some degree of
monitoring and maintenance. Such programs should be planned in advance of
wall installation and continue through the design life of the wall. A
monitoring program should be tailored to each specific site and should be able
to detect significant changes in the wall's ability to serve its designed
purpose. Although slurry walls require little maintenance, a program should
be established to ensure the wall is not damaged by surface activity. If
portions of a slurry wall are damaged or found to be incapable of serving
their intended role, they may have to be restored by reexcavation and
reinstallation.
6-12
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SECTION 7
MAJOR COST ELEMENTS
7.1 Introduction
The objective of this section is to present unit costs for activities and
items associated with slurry cut-off wall construction. These costs may be
used to prepare preliminary estimates for trench construction for comparison
with alternative remedial measures at waste disposal sites. They can also be
used to examine engineering cost estimates provided by contractors. Slurry
trench construction is a specialized field of expertise, and the expense of
installing a wall is highly dependent upon site conditions. Because of the
dependence on factors which vary between sites, cost items presented herein
should be used to develop costs for comparison purposes only. Qualified
contractors experienced in this field should be contacted to provide more
complete, detailed estimates for specific cut-off wall testing, design, and
installation.
It should be noted that the costs presented here are examples only. Many
site-specific factors, which could have significant impact on wall costs are
not addressed here. Health and safety programs for protecting workers in a
contaminated environment, for example, can more than double the time involved
in on-site work. Additional information on the costs of working in a
hazardous environment can be found in U.S. EPA (1983). These costs, and
regional price differences must be considered in evaluating overall wall
costs.
In developing unit costs, information presented in other sections of this
report was used to develop specifications for material and equipment. These
sections were also used to outline construction activities carried out during
wall installation. This resulted in a list of items associated with cut-off
wall construction. Industry representatives were asked to provide costs for
specific items. This sometimes resulted in a range of costs for a specific
item, along with a number of factors affecting costs. Industry representa-
tives were contacted whenever an item had specific application to slurry
trenches (e.g. bentonite manufacturers provided costs for several types of
bentonite). Costs for equipment, materials or activities common to the
construction industry and not significantly affected by factors peculiar to
slurry trenches, were developed using standard sources. These include Means
(1982), Dodge (1982), and NCE (1981).
7-1
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The unit costs presented include standard overhead and profit. This does
not include additional subcontractors overhead and profit. In several
instances, EPA (1982) provides estimates for construction at hazardous waste
sites.
7.1.1 Developing Preliminary Cost Estimates
There are several steps in developing cost estimates from unit costs.
They are:
1. Develop conceptual design including type and size of slurry wall.
2. Develop plan listing all activities to be conducted as part of the
job.
3. Analyze activities, and determine type of equipment to be used and
the size of the activity (e.g. number of cubic yards of earth
excavated).
4. Look up unit price for activity and size of equipment.
5. Multiply size of activity (step 3) by unit cost (step 4).
6. Add costs for each activity.
7. Multiply total by contingency fee; usually between 5 and 20%, to
account for unforeseen difficulties or problems.
For this report, unit costs are divided into 6 major categories to
facilitate use:
1. Feasibility Testing
2. Construction Activities
3. Slurry Wall Installation
4. Maintenance and Monitoring
5. Materials
6. Equipment.
In developing a plan for installing a slurry wall, activities can be
divided into four phases:
1. Feasibility testing, which includes soil testing and hydrogeologic
testing for the site, but does not include compatibility testing of
the wall with local groundwater.
2. Site preparation, which consists of all activities ancillary to wall
installation. These include site clearing, grading and preparation
of work areas, excavation of slurry holding ponds and backfill
preparation areas, and temporary alterations in the site such as road
construction.
7-2
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3. Wall installation, including mobilization and demobilization of
equipment, labor, shipping and mixing of bentonite, backfill
preparation, trench excavation and site cleanup. Wall installation
costs as provided by contractors are a function of the size and type
of wal1.
4. Site clean-up, including re-grading, re-seeding and security.
Once this plan is complete, unit costs presented in this section can be
used to develop total estimated costs for the slurry wall installation.
Cost estimates for associated remedial actions as discussed in
Section 2.4 are not provided in this report. Assistance in developing costs
for these items can be found in the EPA (1982),
7.1.1.1 Cost Estimation Example
An example of the cost estimation process is provided below.
Based upon preliminary information, a slurry wall appears
feasible as a remedial action at a given site. It is thought that
the wall must be at least 50 feet deep to key into a clay layer, and
that it must be between 700 and 800 feet long. Local materials will
be used as a source of fines in the backfill. Access to the site
will be difficult, as no road leads directly to the work area. The
nearest access is 1200 feet from the site. There are few large
trees in the work area; which is relatively flat and covered with
tall weeds and grasses^ After construction is complete, the area
will be graded and revegetated.
Using this description, a preliminary estimate of costs can be made.
First, a list of activities can be developed. This list includes:
Feasibility testing
Temporary road construction
Site clearing and preparation
Slurry wall excavation and completion
Site re-grading and revegetation.
Costs for each activity are described below.
a. Feasibility Testing
Testing is required to determine the continuity and depth of the clay
layer to be keyed into, as well as to obtain data on the type of native
7-3
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material to be used as backfill. It is estimated that soil borings of between
60 and 70 feet depth be set 50 feet apart in a line along that of the
anticipated trench. Specific tests to be conducted include:
Sieve analysis, all locations, at 10 feet depth intervals
Atterberg limits, all locations, at 10 feet depth intervals
Permeability tests, all locations, depths of 20, 40, and 60 feet.
Costs are given in Table 7-1.
b. Temporary Road Construction
The road must extend at least 1200 feet from the nearest access.
Vegetation and trees less then 6-inch diameter must be cleared and a gravel
base laid. It is estimated that the road must be 40 feet wide to accoraodate
heavy equipment. Costs are presented in Table 7-2.
c. Site Clearing and Preparation
Brush and grass must be cleared to provide working space. An estimated
10 acres must be cleared, and approximately 3 acres must be regraded to serve
as a backfill preparation and slurry mixing and storage area. Costs are given
in Table 7-3.
d. Slurry Wall Excavation and Installation
Costs for slurry wall excavation and completion includes all activities
associated with wall installation. This includes such items as:
Backhoes, dozers, and trucks, including mobilization/demobilization
Bentonite and water
Slurry mixing and preparation including mixers and hoses
Backfill preparation and installation
Site clean-up.
Costs are derived based upon discussion with industry representatives and
published data. Cost ranges for some slurry wall installation are shown in
Table 7-4.
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TABLE 7-1.
ESTIMATED COSTS FOR FEASIBILITY TESTING - EXAMPLE SITE
Activity
Costs
SOIL BORINGS
14 Sites (700 ft./50 ft. Intervals) x 60 ft. x
$7.80/ft. - 8.89/ft
16 Sites (800 ft./50 ft. Intervals) x 60 ft. x
$7.80/ft. - j8.89/ft
SOIL TESTS
Sieve Analysis - 6/site x 14 sites x $40.00 - $70.00
Sieve Analysis - 6/site x 16 sites x $40.00 - $70.00
Atterberg Limits - 6/site x 14 sites x $25.00 - $50.00
Atterberg Limits - 6/site x 16 sites x $25.00 - $50.00
Split Spoon - 6/site x 14 sites x $15.00
Split Spoon - 6/site x 16 sites x $15.00
Permeability Tests - 3/site x 14 sites x $50.00
Permeability Tests - 3/site x 16 sites x $50.00
Low
Estimate
High
Estimate
$ 6,552.00 $ 7,468.00
$ 7,488.00 $ 8,534.04
$ 3,360.00 $ 5,880.00
$ 3,840.00 $ 6,700.00
$ 2,100.00 $ 4,200.00
$ 2,400.00 $ 4,800.00
$ 1,250.00
$ 1,440.00
$ 2,100.00
2,200.00 $ 2,200.00
$15,362.00 $17,368.00
Reference: Means 1982; Dodge 1982; and Smith 1982.
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TABLE 7-2.
ESTIMATED COSTS FOR TEMPORARY ROAD CONSTRUCTION - EXAMPLE SITE
Activity Cost
1. CLEAR AND GRUB
1200 ft. x 40 ft. x sq.yd./9sq.ft. x $.26/sq.yd. $ 1,386.00
2. TEMPORARY ROADWAY - 4" GRAVEL FILL - NO SURFACING
1200 ft. x 40 ft. x sq.yd./9sq.ft. x $2.91/sq.yd. $15,520.00
$16,906.00
Reference: Means 1982, and Dodge 1982.
TABLE 7-3.
ESTIMATED COSTS FOR SITE CLEARING AND PREPARATION - EXAMPLE SITE
Activity Cost
1. CLEAR AND GRUB WITH DOZER
10 acres x 4840 sq. yds./acre x $.21/sq.yd. $10,164.00
2. REGRADE AVERAGE 2 FT. 75 H.P. WITH DOZER
3 acres x 43,560 sq.ft./acre x 2 ft. x
cu.yds./270 ft. x $211/cu.yd. $20,424.00
$30,588.00
Reference: Means 1982, Dodge 1982.
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TABLE 7-4.
ESTIMATED COSTS FOR SLURRY WALL INSTALLATION - EXAMPLE SITE
Activity Cost
1. SLURRY WALL INSTALLATION
SOFT TO MEDIUM SOIL
DEPTH: 60 FT.
SOIL BENTONITE BACKFILL
700 ft. x 60 ft. x $4.00 - $8.00/sq.ft. $168,000.00 - $336,000.00
800 ft. x 60 ft. x $4.00 - $8.00/sq.ft. $192,000.00 - $384,000.00
Reference: Ressi de Cervia 1979.
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e. Site Re-grading and Revegation
After the wall is complete, the site must be re-graded and revegetated
Typical costs are given in Table 7-5.
f. Total Costs
To arrive at total costs, all items are added. A general contingency
factor is added to account for unforeseen problems. This usually is between 5
percent to 20 percent, depending upon the type of work conducted. A contin-
gency factor of 15 percent is usually more than sufficient. Total costs are
given in Table 7-6.
7.2 Unit Costs
Unit costs are presented under the following catagories:
* Feasibility testing
Construction activities
Slurry wall installation
Maintenance and monitoring
Materials
Equipment.
Unit costs are based upon the discussions already presented. The
presentations here attempt to follow previous discussions as closely as
possible.
The use of unit costs to develop estimates is relatively simple, but the
result is still only an estimate. In addition to the variations inherent in
estimating costs for such a site specific construction technique as slurry
cut-off walls, unit prices can vary depending upon a variety of factors.
These include wage rates, labor efficiency, union regulations and material
costs. Other factors which may affect costs are weather, season of year,
contractor management, and unforeseen difficulties.
Unit costs presented in the following pages give a range of equipment
sizes and types which can accomplish the same activity. Judgement must be
used in deciding what type of equipment may be used. Unit costs are given for
a wide variety of activities which may be required as part of a slurry cut-off
wall. If a special activity is required and is not included in this section,
one of the reference sources cited in this Section will probably contain the
necessary information.
7-8
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TABLE 7-5.
ESTIMATED COSTS FOR SITE REGRADING AND REVEGETATING - EXAMPLE SITE
Activity
1. REGRADE AT AVERAGE 2 FT. DEPTH WITH 75 H.P. DOZER
2. REVEGETATE AND HYDROSEED, INCLUDING FERTILIZER
10 acres x 4840 sq.yd. x $.46/sq.yd.
Reference: Means 1982, Dodge 1982.
Cost
$ 7,414.00-
$20,424.00
$22,264.00
$42,688.00
TABLE 7-6.
ESTIMATED TOTAL COSTS - EXAMPLE SITE
Activity
Feasibility Testing
Temporary Road Construction
Site Clearing and Preparation
Slurry Wall Excavation and Completion
Site Re-grading and Revegetation
15% Contingency
Cost
$ 17,210.00 - $ 19,480.00
$16,900.00
$30,588.00
$192,000.00 - $384,000.00
$42,688.00
$299,386.00 - $493,656.00
$ 44,907.00 - $ 74,048.00
$344,294.00 - $567,704.00
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7.2.1 Feasibility Testing
To determine the feasibility of using a slurry cut off wall at a
particular site, three types of tests are conducted. They are:
Geologic and soils tests
Hydrologic tests
Slurry wall tests.
The first two catagories provide a characterization of the site while the
last determines the compatibility of the technique with specific site condi-
tions, most notably the effect of the constituents present in the local^
groundwater upon the slurry cut off wall. In general, feasibility testing for
a slurry wall can run between 8 to 18 percent of costs (JRB 1979).
7.2.1.1 Geologic and Soils Testing
Geologic and soils testing is accomplished using a drill rig to extract
samples from various depths. Any type of drill rig may be used, but one of
the more common type makes use of hollow stem augers to penetrate the sub-
surface and withdraw samples. Costs presented here assume the use of hollow
stem augers.
Factors which affect the cost of geologic and soils testing include: the
number and type of samples, the depth to which samples are taken, mobilization
and demobilization of equipment, weather conditions, and the presence of
contamination which may require careful handling techniques and special equip-
ment. Based on the presence of contamination, cuttings material^brought to
the surface by the advance of the auger may have to be disposed in secure
landfills. Drilling in contaminated areas can significantly affect the rate
of sampling as well as the overall cost.
It is possible that a good deal of information is already available on
many of the hazardous waste sites where slurry cut-off walls are^being
considered. However, it will still be necessary to conduct special testing to
determine the location and extent of the less permeable layer to^be keyed
into, and to fully characterize the content of the overburden which will be
excavated. Overburden characterization is especially important if native
material is to be used as a source of fines for the backfill. In this
instance, test points relatively close together, both in^the horizontal and
vertical sense, may be necessary. Where a cement-bentonite backfill is
considered, soil testing can be kept to a minimum but cannot be discarded. It
is still important to know the characteristics of the key-in layer, and^
information on the overburden could prove useful at a later date if additional
remedial measures are considered.
In general, test points of between 50 and 150 feet should provide more
than sufficient information to characterize the subsurface material. The test
7-10
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borings should extend at least several feet into the impermeable layer to
confirm its suitability. Costs for geologic and soils tests are given in
Table 7-7.
7.2.1.2 Hydrologic Testing
Hydrologic testing is designed to determine the characteristics of
groundwater quality and flow at a site. Some hydrologic tests involve
altering the condition of the aquifer and measuring the response to the
change. Others merely monitor the existing conditions. In any event, it is
necessary to install wells to conduct these tests.
Factors which affect*hydrologic tests are almost identical to those which
affect soils tests. Mobilization and demobilization, weather conditions, type
and location of sample points and the presence of contamination all affect the
cost of hydrologic testing.
Many hazardous waste sites have been studied to a degree that very good
characterizations of the hydrology of the area already exist. Additional
hydrologic testing may not be required at very many sites. Where more
information is required, existing monitoring wells may be used to reduce
costs. Costs for conducting hydrologic tests are given in Table 7-8.
7.2.1.3 Backfill Testing
Tests conducted to determine probable performance characteristics of a
planned cut-off wall are important. Tests can be carried out by one of
several contractors who specialize in slurry wall installation.
There are two types of tests carried out which are used to determine wall
characteristics:
Compatibility testing
Permeability testing.
These tests determine the optimum mix of backfill materials to maximize
wall strength and stability and minimize wall permeability. Usually, samples
of soil and groundwater are taken from the site and used for the tests. These
can be gathered as part of the soil and hydrologic testing programs. The
tests are then run using these native materials. Ranges of costs as provided
by a number of contractors are provided in Table 7-9.
7-11
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TABLE 7-7.
EXAMPLE UNIT COSTS FOR GEOLOGIC AND SOILS TESTING
Activity
Cost
Soil Borings
Mobilization and Demobilization
over 100 miles, additional per mile
Auger Holes, 2.5" diameter
4" diameter
Cased Borings, 2.5" diameter
4" diameter
Split Spoon Samples, 2 foot drive
Soil Testings
$110.00
$ 1.00/Mile
$ 6.70/LF - 8.89/LF
$ 7.80/LF
$ 9.45/L.F. - 11.41/LF
S 16.15/L.F.
$ 15.00 - $50.00 each
Atterberg Limits, Liquid and Plastic Limits
Hydrometer Analysis and Specific Gravity
Sieve Analysis - Washed
- Unwashed
Moisture Content
Permeability, Variable or Constant Head
Proctor Compaction, 4" Standard Mold
$ 25.00
$ 50.00
$ 23.00
$ 40.00
$ 7.00
$ 50.00
$ 80.00
- $90.00
- $70.00
- $15.00
- $95.00
each
each
each
Reference: Means 1982, Dodge 1982, Smith 1982
7-12
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TABLE 7-8.
EXAMPLE UNIT COSTS FOR HYDROLOGIC TESTING
Activity
Cost
Well Installation
1. Boring and well installation, without casing
2.5" auger
4" auger
6" auger
2. Casing
2"
1
*1
4" pvc.
2" steel
4" steel
3. Well Screens .010" Slot, 10' length
2" pvc.
4" pvc.
2" steel, 5 foot length
4" steel, 5 foot length
4. Submersible pump,.5 gallons/minute
at 180 ft. lift
$ 7.23 - $ 9.00/foot
$11.50 - $14.40/foot
$17.34 - $21.60/foot
$ 3.00/foot
$ 5.00/foot
4.50-5.50/foot
7.00-9.00/foot
$ 6.47/foot
$ 18.98/foot
$144.80 ea. + 26.OO/
additional foot
$238.70 ea. + 41.10/
additional foot
$750.00 each
Hydrologic Tests
5. Water Quality Tests
and analysis
6. Pump Tests
7. Slug Tests
- Includes sampling
highly variable
depending upon site-
specific conditions
References: (1) JRB 1979, (2) Smith 1982.
7-13
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TABLE 7-9.
EXAMPLE COSTS FOR SLURRY WALL TESTING
ACTIVITY COST
1. Compatibility tests $800.00-$!,200.00 each test
2. Permeability tests $800.00-$!,200.00 each test
Reference: Various industry sources.
7.2.2 Construction Activities
Construction activities include all activities conducted which are not
directly related to slurry wall installation. Therefore, such items as slurry
preparation, mixing and introduction into the trench; backfill preparation and
placement; and slurry disposal are not covered in this section. These items
are covered in a following section.
Costs for the following activities are included in this section:
Site clearing
Excavation
Backfill (excluding slurry trench backfill)
Borrow
Compaction
Grading
Hauling
Mobilization and Demobilization
Site Dewatering.
Costs for these activities are shown in Tables 7-10 through 7-18.
7.2.2.1 Site Clearing
Slurry wall installation is made easier if a working area is cleared of
trees, shrubs and bushes. In urban areas, or where other factors may prevent
7-14
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the removal of obstacles, the cost of slurry cut-off wall installation
increases dramatically. Slurry wall installation or implementation of addi-
tional remedial actions is made easier if a working area can be prepared by
clearing the site of obstructive objects. In rural areas the predominant
obstack is vegetation, ie, trees, shrubs, bushes, and site clearing can
usually be undertaken with minimal difficulty and at relatively low cost. In
urban areas, however, where the obstacles are power lines, underground water
lines etc., site clearing is often difficult and as a result slurry cut-off
wall installation and any additional site work increases dramatically. Costs
for site clearning are shown in Table 7-10. Costs for clearing a site
containing wastes should not be significantly greater than normal costs,
however, if decontamination of personnel and equipment is required, costs may
rise significantly.
7.2.2.2 Excavation
Excavation may be required to prepare the work area. Slurry may be
stored in excavated ponds to insure complete hydration of bentonite and to
provide a sufficient reserve of slurry. In addition, "benches" may need to be
cut into hillsides to accomodate excavation equipment. The type of equipment
chosen is dependent upon the size of the job, the type of excavation and site
conditions.
Costs for excavation are given in Table 7-11. Costs will be affected by
the type of material being exhumed, and also by the presence of contaminants.
Contaminated material must be properly disposed, adding to overall costs. In
addition, decontamination of equipment and health and safety precautions will
slow work, and increase costs. Rental costs for equipment are presented under
the equipment discussion. Unit costs provided in Table 7-11 include rental of
equipment.
7.2.2.3 Backfilling
Backfilling is required to "fill in" areas to insure level grade for
excavation equipment. In slurry trench construction, below grade depressions
may need to be filled and compacted to create the level grade. Backfill may
also be required to fill in slurry ponds after the job is complete.
Costs for backfilling are presented in Table 7-12. The presence of
contaminants may influence costs, but not significantly if clean fill is used,
Health and safety precautions and decontamination of equipment may slow the
rate of backfilling operations.
7-15
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TABLE 7-10.
EXAMPLE UNIT COSTS FOR SITE CLEARING
ACTIVITY
Clear and Grub - Trees
Light trees to 6" diam., cut and chip
grub stumps
Medium trees to 10" diam., cut and chip
grub stumps
Heavy trees to 16" diam., cut and chip
grub stumps
Trees over 16" diam., using chain saws and chipper
For machine load, 2 miles haul to dump add
Clearing-Brush
With brush saw and rake
by hand
With dozer, ball and chain, light clearing
medium clearing
UNIT COSTS ($)
2,125/acre -
22.00 each
565.06 - 825/acre
2,450/acre -
31.00 each
1,100/acre -
1130/acre
2,850/acre -
56.00 each
1,375/acre -
2260.00/acre
$100.00 ea. -
128.00 each
$18.00 - 38.00
each
,27/square yard
(S.Y.)
,55/S.Y.
,27/S.Y.
,31/S.Y.
Reference: Means 1982, Dodge 1982.
7-16
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TABLE 7-11.
EXAMPLE UNIT COSTS FOR EXCAVATION
ASSUMPTIONS:
Medium earth piled or truck loaded
No trucks or haul added
1.
2.
3.
4.
5.
6.
7.
EQUIPMENT AND CAPACITY
Backhoe, hydraulic, crawler mounted
1.0 Cubic Yard
1.5 Cubic Yard
2.0 Cubic Yard
3.5 Cubic Yard
Backhoe, wheel mounted
.5 Cubic Yard
.75 Cubic Yard
Clamshell
.5 Cubic Yard
1.0 Cubic Yard
75 H.P. Dozer, 50' haul
300 H.P. dozer, 50' haul
75 H.P. dozer, 150' haul
300 H.P. dozer, 150' haul
0.75 Cubic Yard Dragline
1.5 Cubic Yard Dragline
Front end loader, track mounted.
1.5 Cubic Yard
2.5 Cubic Yard
3.5 Cubic Yard
4.5 Cubic Yard
Wheel mounted
0.75 Cubic Yard
1.5 Cubic Yard
5.0 Cubic Yard
Shovel
0.5 Cubic Yard
0.75 Cubic Yard
1.5 Cubic Yard
UNIT COST (per
$2.17
$1.96
$1.93
$1.48
$3.76
$2.62
$4.34
$2.93
$1.17
$ .57
$1.43
$1.08
$2.47
$1.76
$ .98
$ .91
$ .72
$ .88
$ .99
$ .84
$ .73
$2.95
$1.66
$1.25
Cubic Yard)
- 2.71
- 2.19
-1.98
- 1.79
- 3.95
- 2.92
- 4.42
- 3.39
- 1.43
- .98
-2.34
- 1.48
- 1.05
- 1.26
- .82
- .99
- 1.30
- .89
- 1.88
-1.39
For soft soil or sand, deduct 15% for heavy soil or clay add 60%.
Reference: Means 1982, Dodge 1982.
7-17
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TABLE 7-12.
EXAMPLE UNIT COSTS FOR BACKFILL
ACTIVITY
UNIT COST
(Dollars Per Cubic Yard)
1.
2.
3.
Backfill by hand, no compaction, light soil
heavy soil
Compaction in 6" layers, hand tamp
roller compaction
air tamp
vibrating plate tamp
Compaction in 12" layers, hand tamp
roller compaction
air tamp
vibrating plate tamp
$8.83 - $11.10
$12.95
7.55 -
3.90 -
5.70
1.55 -
4.56
2.81
4.10
2.63
11.02
4.67
3.76
4. Dozer backfill, bulk, up to 300" haul
Compaction air tamped
- 6" - 12" lifts vibrating roller
- Sheepsfoot roller
5. Dozer backfilling, trench, up to 300' haul
Compaction air tamped
- 6" - 12" lifts vibrating roller
- Sheepsfoot roller
0.85 - 1.04
4.81 Additional
1.77 Additional
1.92 Additional
1.09
4.92 Additional
2.11 Additional
2.39 Additional
REFERENCE: Means 1982, Dodge 1982.
7-18
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7.2.2.4 Borrow
Borrow is material taken from a nearby source to be used as a fill
material on-site. Borrow can be used as a backfill, and as a source of
material to construct berms, dikes, levees, or ramps. Borrow can also serve
as a source of fines used in the preparation of cut-off trench backfill.
Cost for borrow are given in Table 7-13. These costs assume that a
nearby source of material is available. If not, costs could rise
substantially.
7.2.2.5 Compaction
Backfill or borrow used for haul roads, should be compacted to impart
some strength to the material. This is especially important if heavy
equipment is going to be moving over areas of loose or uncompacted material.
Unless backfill or borrow is used at a site, compaction may not be necessary
Costs for compaction are given in Table 7-14.
7.2.2.6 Grading
Slurry trench construction requires a relatively flat working surface.
In areas of sloping or uneven terrain, graders may be used to level the
working area.
There are two types of graders, self-propelled and those towed by a dozer
or some other suitable piece of machinery. Either can be used, but large,
motorized graders are used primarily for larger jobs. Costs for various
graders are presented in Table 7-15.
7.2.2.7 Hauling
Hauling material to the worksite may be a major cost factor if sources of
material are not located nearby. Hauling may be required to bring borrow
material to a site. Hauling costs presented in Table 7-16 do not give cost
for rail or truck transportation of material for great distances (i.e., the
shipping of large quantities of bentonite by rail). Manufacturers should be
contacted to provide exact transport costs for these types of material.
The selection of equipment to haul material is dependent upon several
factors. Of primary importance is the quantity of material to be hauled. For
larger quantities, a larger capacity trailer should be used as they are
usually more efficient. Limiting the type of equipment used are the physical
constraints of the roads over which material will be hauled. Back roads or
7-19
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TABLE 7-13.
EXAMPLE UNIT COSTS FOR BORROW
Assumptions:
Buy and load at pit, haul 2 miles to site, place and spread with 180
H.P. dozer with no compaction.
MATERIAL
UNIT COST
1. Bank run gravel
2. Common borrow
3. Crushed stone 1.5"
Crushed stone 3/4"
Crushed stone 1/2"
Crushed stone 3/8"
4. Sand-washed-
Dead or bank sand
5. Select structure fill
6. Screened Loam
7. For 5 mile haul, Add
$6 - 6.25/Cubic Yard
S4.63/C.Y.
$6.65 - $10.70/C.Y.
$6.65 - $10.55/C.Y.
$11.88/C.Y.
$12.60/C.Y.
$ 6.26 - $10.15/C.Y.
$ 8.30/C.Y.
$ 6.87 - $8.35
$ 8.80 - $12.35/C.Y.
$1.40 - $2.30/C.Y.
Reference: Means 1982, Dodge 1982.
TABLE 7-14.
EXAMPLE UNIT COSTS FOR COMPACTION
ACTIVITY
1. Compaction, Rolling with road roller, 5 tons
10 tons
2. Sheepsfoot or wobbly wheel roller, 8" lifts
for select fill
3. Terraprobe, deep sand
Mobilization-Demobilization
4. Vibratory Plate, 8" Lifts, select fill
UNIT COST
$45.00/hr
$55.00/hr
$ 1.56/C.Y.
$ 1.31/C.Y.
$.92 - 1.31/C.Y.
$4,650.00 - $6,100.00
SI.55 - $2.56/C.Y.
Reference: Means 1982, Dodge 1982.
7-20
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TABLE 7-15.
EXAMPLE UNIT COSTS FOR GRADING
Assumptions:
Site excavation and fill, not including mobilization and demobilization
or compaction
Activity
Unit Cost (Dollars/Cubic Yard)
1. Dozer 300 foot haul-75 H.P.
* -300 H.P.
2. Scraper, towed, 7 C.Y.-300'
-10001
10 C.Y.-3001
1000'
3. Self propelled scraper, 15 C
25 C
4. Dozer with ripper- 200 H.P.
- 300 H.P.
haul
haul
haul
haul
.Y. 1000'
2000'
.Y. 10001
2000 f
haul
haul
haul
haul
1.06 -
1.08 -
3.53
7.75
1.88 -
1.96 -
1.95
2.64
1.02
1.27
0.49
0.99
2.92
2.11
2.33
3.96
Reference: Means 1982, Dodge 1982.
TABLE 7-16.
EXAMPLE UNIT COSTS FOR HAULING
Activity
1. 6 C.Y. Dump truck, 1 mile round trip
2 mile round trip
3 mile round trip
4 mile round trip
2. 12 C.Y. Dump truck 1 mile round trip
2 mile round trip
3 mile round trip
4 mile round trip
3. 16.5 C.Y, Dump truck 1 mile round trip
2 mile round trip
3 mile round trip
4 mile round trip
4. 20 C.Y. Dump truck 1 mile round trip
2 mile round trip
3 mile round trip
4 mile round trip
5. Hauling in medium traffic, add 20%
Hauling in heavy traffic, add 30%
Unit Cost
(Dollars/Cubic
2.03 -
2.64 -
3.42 -
4.28
1.71 -
1.96 -
2.21 -
2.65 -
1.50
1.85
2.17
2.42
1.31
1.64
2.94
2.19
Yard)
2.28
2.91
3.64
2.00
2.11
2.47
2.96
Reference: Means 1982, Dodge 1982.
7-21
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dirt roads may not stand up to larger trailers. In addition, weight
restriction on roadways and bridges will limit the size of vehicle.
7.2.2.8 Mobilization and Demobilization
Mobilization and demobilization refers to the transportation of vehicles
and equipment to and from the job site. These are primarily a factor of the
distance from the equipment storage site to the job location. Mobilization
and demobilization costs for specialized, heavy equipment may be very high.
Average costs for representative pieces of equipment are shown in Table 7-17.
These figures assume a local source of equipment.
7.2.2.9 Site Dewatering
Dewatering may be required at sites where excavations intersect the water
table. It is not anticipated that dewatering will be required during slurry
trench installation as little or no deep excavation is required other than
that of the trench. Costs for dewatering systems are shown in Table 7-18.
7.2.3 Completed Wall Costs
Contractors have provided average costs for completed slurry cut-off wall
construction and installation. These estimates may vary widely however, based
upon a number of site-specific factors. Some of these factors are:
Distance bentonite must be transported.
Presence of contamination or high salt content in groundwater
requiring special bentonite and excavation procedures.
Type of overburden being excavated.
Depth of excavation.
Presence of physical constraints upon working area (i.e., buildings or
other structures which must be worked around).
Suitability of native materials for use as backfill constituents.
Type of backfill (either cement or soil-bentonite).
Ressi di Cervia (1980) developed a chart (Table 7-19) which related
cut-off wall construction costs to type of backfill used, depth of excavation
and soil type present. Costs are given in terms of square foot of wall since
the width of the excavation is a factor of the excavation equipment. Although
7-22
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TABLE 7-17.
EXAMPLE UNIT COSTS FOR MOBILIZATION AND DEMOBILIZATION
Equipment
Cost
1. Dozer 105 H.P.
300 H.P.
2. Scraper-towed (include Tractor) 6
3. Self-propelled scraper
4. Shovel, Backhoe or dragline
5. Tractor shovel or front end loader
C.Y.
10 C.Y.
15 C.Y.
24 C.Y.
3/4 C.Y.
1.5 C.Y.
1 C.Y.
2.25 C.Y.
$ 90.00
$125.00
$ 95.00
$130.00
$180.00
$290.00
$130.00
$190.00
$ 90.00
$125.00
Reference: Means 1982, Dodge 1982.
TABLE 7-18.
EXAMPLE COSTS FOR SITE DEWATERING
De script ion
1. Wells - large
dewatering excavation 10* - 201 deep, 2"
steel casing
submersible pump, 6", 1590 GPM
2. Wells - small
dewatering excavation
4" - 6" with casing
submersibile 3"-300 GPM
4"-560 GPM
3. Wellpoints
Complete installation, operation,
equipment rental, full and removal
of system with 2" wellpoints
100 foot header 6" diameter
200 foot header 6" diameter
100 foot header 6" diameter
cost
$5.00-22.00/Linear Foot
$1250.00/month
$12.95/Linear Foot
$300/month
$400/month
$24,500-$32,000/month
$25,000-$27,000/month
$31,000-$69,000/month
Reference: Means 1982, Various Other Sources.
7-23
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TABLE 7-19.
RELATION OF SLURRY CUT-OFF WALL COSTS
PER SQUARE FOOT AS A FUNCTION
OF MEDIUM AND DEPTH
Slurry Trench Prices Unreinforced Slurry Wall
in 1979 Dollars Prices in 1979 Dollars
Soil Bentonite Backfill Cement Bentonite Backfill
(Dollars/Square Foot) (Dollars/Square Foot)
Depth Depth Depth
<_ 30 30-75 75-120
Feet Feet Feet
Soft to Medium Soil
N _< 40 2-4 4-8 8-10
Hard Soil
N 40 - 200 4-7 5-10 10-20
Occasional Boulders 4-8 5-8 8-25
Soft to Medium Rock
N >_ 200 Sandstone, Shale 6-12 10-20 20-50
Boulder Strata 15-25 15-25 50-80
Hard Rock
Granite, Gneiss, Schist*
Depth
£ 60
Feet
15-20
25-30
20-30
50-60
30-40
95-140
Depth
60-150
Feet
20-30
30-40
30-40
60-85
40-95
140-175
Depth
> 150
Feet
30-75
40-95
40-85
85-175
95-210
175-235
Notes:
N is standard penetration value in number blows of the hammer per foot of
penentration (ASTM D1586-67)
*Normal Penetration Only
For Standard Reinforcement add $8.00 per sq. ft.
For Construction in Urban Environment Add 25% to 50% of Price
Reference: Ressi di Cervia 1980.
7-24
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costs are given in 1979 dollars, contractors maintain that slurry trench costs
have remained stable because of increased experience and new technology.
A breakdown of total cut-off wall costs according to several categories
is shown in Table 7-20. As can be seen, costs for each element may vary
widely.
Table 7-21 gives examples of costs for several walls as reported in the
literature or otherwise available. A brief description of site characteris-
tics which may have affected costs is included. This table also demonstrates
the wide variation in unit costs depending upon site specific factors.
7.2.3.1 Monitoring
Once a slurry cut-off wall has been installed, monitoring should be
conducted to assure that the wall is performing as designed. If the
monitoring program indicates that the wall is not containing or isolating
contaminants, maintenance to restore the integrity of the wall should be
instituted.
There are two types of monitoring systems which are used to indicate the
integrity of a cut-off walls; wall-stability monitoring and water quality
monitoring. Effects on wall integrity due to ground movement is not
considered a major problem as both SB and CB walls are normally flexible
enough to withstand typical deformation. This type of monitoring is usually
important if construction activities place a major load on the wall. Since it
is not expected that slurry walls associated with hazardous wastes sites will
be called upon to support major structures, costs are not provided for this
type of monitoring.
Water quality monitoring will be the most important indicator of wall
performance: the wall will either contain and isolate waste materials or
migration of constituents will continue. Water quality monitoring systems
usually consist of multiple wells installed at suitable locations up and
downgradient of a wall. These walls can be sampled and analyzed to determine
water quality. In addition to groundwater monitoring, surface water sources
can also be sampled. Finally, water levels obtained from piezometers can be
used as an indicator of wall integrity.
Table 7-22 presents estimated costs for monitoring well and piezometer
installation. Costs for sampling and analysis are highly variable and depend
upon the type of parameters analyzed, and therefore, have not been included.
7.2.3.2 Maintenance
Maintenance of a slurry wall begins immediately after installation. The
wall should be capped with a clay or other capping material to reduce surface
infiltration and control erosion. The cap should be properly graded.
7-25
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TABLE 7-20.
BREAKDOWN OF COST CATAGORIES
FOR CUT-OFF TRENCH CONSTRUCTION*
Activity
of Total Costs
Testing - Hydrologic, Geotechnical,
and Lab Tests
Equipment Mobilization
Slurry Trench Excavation and
Backfill
8% - 18%
8% - 18%
65% - 83%
* Assumes soil-bentonite backfill with moderate soil conditions and depth not
greater than 40 feet.
Referemce: EPA 1982.
TABLE 7-21.
EXAMPLE RANGES OF UNIT COSTS FOR CUT-OFF WALL CONSTRUCTION
Unit Cost
Conditions
Site I. (1982)
Site II. (1981)
Site III. (1982)
Site IV. (unk)
$3/sq. ft.
27.00/sq. ft
$5/sq. ft.
$6/sq. ft.
50 ft. x 11,000 ft. SB Backfill
17 ft. x 700 ft. CB Backfill
52 x 3900 ft. SB Backfill
30 ft x 300 ft. SB Backfill
Reference: Various Industry Sources,
7-26
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TABLE 7-22.
EXAMPLE UNIT COSTS FOR MONITORING
WELL AND PIEZOMETER INSTALLATION
1. Monitoring well installation
Boring and well installation, without casing
2.5" auger
4" auger
6" auger
Casing
2" PVC
4" PVC
2" Steel*
4" Steel
Screen .010" slot, threaded flush joint
2" PVC
4" PVC
2" Steel , 5-foot length
4" Steel2, 5-foot width
Submersible pump, 5 GPM at 180 ft. lift
2. Piezometer installation
$ 7.23-9.00/foot
$11.50-14.40/foot
$17.34-21.60/foot
$ 3.00/ft
$ 5.00/ft
$ 4.50-5.50/ft
$ 7.00-9.00/ft
$ 6.47/ft
$18.98/ft
$144.80 ea. + 26.10/
added ft
238.70 ea. + 91.10/
added ft
$750.00/each
Boring and installation (same as monitoring walls)
Piezometers, 24" screen, polyethylene $65.54 each
Reference: (1) JRB 1979, (2) Smith 1982.
7-27
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Other maintenance techniques include grouting, re-excavation of the
trench or installation of a synthetic liner along one side of the wall. Costs
for maintenance operations are shown in Table 7-23. Due to the technical
difficulties in installing a synthetic liner near a slurry trench, unit costs
could not be derived for this activity.
7.2.4 Materials
Materials essential for slurry cut-off wall construction includes
Slurry consisting of a mixture of bentonite or suitable replacement
and water
Backfill consisting of a mixture of soil, bentonite and water and/or
cement.
In many instances, borrow may be required on-site to serve as a source of
fines or to be used'in support of other construction activities. Costs are
also incurred for disposal of spoils.
Supplies of bentonite can be obtained from a number of sources. Each
vender has available a variety of bentonite types suitable for use in slurry
cut-off walls. The costs for bentonite will be affected by the location of
the site as transportation is a major expense item. Costs for bentonite as
well as other materials such as concrete, sand, borrow, rip-rap etc. are also
included in Table 7-24.
7.2.5 Equipment
Large construction projects like the installation of a slurry wall
require varied types of equipment. Contractors usually provide much of this
equipment themselves, especially specialty items like a modified backhoe. In
some instances, equipment rented locally may prove more cost effective than
shipping the same equipment for large distances. To provide a means of
comparing equipment costs, the following tables present hourly operating costs
and daily and monthly rental rates for earthwork equipment (Table 7-25), con-
crete and mixing equipment (Table 7-26), and general equipment (Table 7-27).
No estimate of equipment requirements for a typical slurry wall are available
as each project is site-dependent.
7.3 Summary
The development of costs for a slurry wall installation is an involved
process and is highly site specific. This section has presented example costs
that may be used to generate estimated total costs. Care must be taken in
applying these example costs to a specific site. It is especially important
7-28
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TABLE 7-23.
EXAMPLE UNIT COSTS FOR SLURRY WALL MAINTENANCE ACTIVITIES
1.
2.
3.
Activity
Grouting - soil stabilization with
phenolic resin
Unit Cost
$200.00-465.00/C.Y,
Re-excavation of soil bentonite backfill with
backhoe (Hydraulic crawler 1.5 C.Y. capacity) $ 1.96/C.Y.
re-excavation of cement-bentonite backfill highly variable
Capping - buy, load, 2-mile haul, grade and spread
Borrow - select fill $ 6.97 - 8.35/C.Y,
Borrow - topsoil $ 5.70 - 9.05/C.Y,
Compaction $0.92-$1.31/C.Y.
4. Revegetation - hydroseed
$0.46 S.Y.
Reference: Means 1982, Dodge 1982.
TABLE 7-24.
EXAMPLE UNITS COSTS FOR MATERIALS
1. Bentonite
Natural, "untreated" sodium bentonite
Bulk rail (min 30 tons)
Bulk truck (min 30 tons)
Bag rail (min 21 tons)
Bag truck (min 21 tons)
2
2. Cement, Portland, truckload, U.S. average
Cement Portland, less than truckload U.S. average'
Portland Cement trucked in Bulk, U.S. average
2
3. Borrow, bank run gravel
common borrow
crushed stone, 3/4"
sand - washed
select, structural fill
screened loam
topsoil
$42.00/ton
$43.00/ton
$50.00/ton
$51.00/ton
$ 4.62/Bag
$ 5.55/Bag
$ 3.57/1001bs.
$ 6.00-$6.25/C.Y.
$ 4.63/C.Y.
$ 6.65-$10.55/C.Y,
$ 6.26-$10.15/C.Y,
$ 6.87-$8.35/C.Y.
$ 8.80-$12.35/C.Y,
$ 9.05/C.Y.
Note: Does not include transportation costs.
Reference: (1) Various industry sources: does not include transportation,
(2) Means 1982, Dodge 1982.
7-29
-------
TABLE 7-25.
EXAMPLE OPERATING AND RENTAL COSTS FOR EARTHWORKING EQUIPMENT
Hourly Operating Rent Rent
(Cost Without Operator) Cost Day Month
1.
2.
3.
4.
5.
6.
7.
8.
9.
Augers for truck/trailer mounting, ver-
tical drilling 4" to 36" diam. 101 travel
Backhoe diesel hydraulic, crawler mounted
5/8 C.Y. capacity
3/4 C.Y. capacity
1 C.Y. capacity
2 C.Y. capacity
3 1/2 C.Y. capacity
Backhoe loader, wheel type
40 to 45 H.P. 5/8 C.Y.
80 H.P, 1 1/4 C.Y.
Bucket, clamshell, all purpose
3/8 C.Y.
1/2 C.Y.
1 C.Y.
2 C.Y.
Bucket, dragline, medium duty
1/2 C.Y.
1 C.Y.
2 C.Y.
3 C.Y.
Compactor roller, 2 drum
Vibratory plate, gas, 13" plate, 1000 Ib blow
Grader, self propelled, 25,000 Ibs .
40,000 Ibs.
55,000 Ibs.
Roller, towed type, vibratory, 2 ton
Sheeps foot self propelled, 140 H.P.
Pneumatic tire, 12 ton
$0.56
$6.55
$9.40
$12.15
$23.70
$43.40
$4.23
$7.65
$0.38
$0.50
$0.69
$1.13
$0.32
$0.44
$0.63
$0.88
$2.12
$0.48
$10.60
$19.15
$25.00
$2.05
$9.95
$7.40
$73
$450
$495
$735
$1075
$1650
$295
$400
$30
$35
$45
$75
$21
$33
$47
$73
$70
$39
$325
$540
$845
$80
$285
$120
$645
$3100
$3500
$3925
$7850
$13,800
$1100
$2075
$270
$335
$415
$680
$180
$290
$425
$605
$735-
1500
$330
$3000-
3450
$4200-
4850
$5100-
7550
$700
$2750
$1100-
1500
(continued)
7-30
-------
TABLE 7-25. (continued)
(Cost Without Operator)
10. Scrapers, towed 7 to 8 C.Y.
Self propelled 14 C.Y.
Self loading 22 C.Y.
11. Tractor, dozer, crawler 75 H.P.
105 H.P.
200 H.P.
300 H.P.
410 H.P.
700 H.P.
12. Loader crawler 1.5 C.Y. 80 H.P.
1.75 C.Y. 95 H.P.
2.25 C.Y. 130 H.P.
5 C.Y. 275 H.P.
13. Truck, dump, tandem, 12 ton payload
3 axle, 16 ton payload
Dump trailer only, 15.5C.Y.
Flatbed, single, 1.5 ton rating
3 ton rating
Off highway, rear dump 25 ton
35 ton
Hourly Operating Rent Rent
Cost Day Month
$2.45
$34.00
$46.00
$6.20
49.50
$17.20
$24.00
$34.50
$60.30
$9.35
$11.10
$14.50
$31.00
$9.85
$14.35
$3.47
$3.34
$3.83
$21.85
$30.25
$115
$1050
$1000
$375
$505
$765
$1025
$1250
$1950
$445
$475
$575
$1100
$325
$410
$73
$45
$56
$4760
$1025
$1025
$8800
$6675
$1850
$3525
$6950
$9000
$11,750
$19,250
$2550
$2925
$3950
$9850
$1625
$2150
$675
$415
$505
$6600
$9300
Reference: Means 1982, Dodge 1982.
7-31
-------
TABLE 7-26.
EXAMPLE OPERATING AND RENTAL COSTS FOR CONCRETE AND MIXING EQUIPMENT
1.
2.
3.
4.
5.
6.
7.
8.
Bucket, concrete, lightweight 0.5. C.Y.
1 C.Y.
Conveyor, concrete, 10" wide, 26f long
Core driller, electric, 2.5 H.P. 1" - 8"
Grinder
Mixer, powered, mortar and concrete
Pump, concrete, truck mounted
4" line, 80' boom
5" line, 110* boom
Mud jack 47 cubic feet/hour
225 cubic feet/hour
Portable Concrete Batch Plant
Hourly Operating
Cost
$0.12
$0.12
$1.10
bit $0.56
$.82
$1.92
-
$0.99
$2.84
Rent
Day
$17
$20
$64
$39
$35
$26
$640
$450 -
$800
$20
$125
$1,500
Rent
Month
$125
$165
$505
$290
$305
$255
$4,750
$180
$1075
$15,500
Reference: Means 1982, Dodge 1982.
7-32
-------
TABLE 7-27.
EXAMPLE OPERATING AND RENTAL COSTS FOR GENERAL CONSTRUCTION EQUIPMENT
1. Air compressor, portable gas 60 cfm
160 cfm
diesel engine, rotary screw
250 cfm
360 cfm
600 cfm
2. Barricade barrels with flashers
3. Generator, electrical 1.5 kw to 3 kw
5 kw
10 kw
diesel 20 kw
100 kw
4. Hose, water suction with coupling 20' long
2" diameter
4" diameter
8" diameter
discharge hose with coupling, 50' long
2" diameter
4" diameter
8" diameter
5. Light towers, portable with generator
1000 watt
2000 watt
6. Pumps, centrifugal gas pump, 1.5" diameter
2" diameter
3"diameter
6"diameter
diaphragm, gas, single, 1.5" diameter
3 ' diameter
double 4" diameter
Trash, self-priming 4" diameter
7. Trailers, platform, flushdeck, 25 ton
40 ton
3 axle, 50 ton
Hourly Operating
Cost
$3.19
$3.87
$6.20
$9.30
$15.45
-
$0.48
$0.78
$1.85
$3.11
$7.73
-
-
. -
-
.
-
$1.10
$1.63
$0,39
$0.95
$0.95
$1.52
$0.41
$0.86
$1.05
$1.16
$0.93
$1.13
$2.14
Rent Rent
Day Month
$34
$48
$78
$100
$150
$.65
$20
$27
$45
$54
$180
$7
$12
$34
$7
$12
$40
$66
$94
$15
$20
$25
$75
$13
$28
$45
$55
$78
$115
$145
$290
$430-
840
$700
$880
$1325
$5.70
$180 *
$280
$450
$525
$1300
$45
$100
$255
$45
$90
$180
$540
$845
$125
$175
$210
$655
$115
$235
$400
$520
$715
$1025
$1290
(continued)
7-33
-------
TABLE 7-27. (continued)
Hourly Operating
Cost
Rent Rent
Day Month
8. Water tank, engine-driven discharge, 5000 gallon $8.40
10,000 gallon $9.95
9. Decontamination shower cost
10. Pipe, for excavation drainage
(installation not included)
$225
$325
$2,400.00 each
PVC 13"lengths
Vitrified clay
4"diameter
8" diameter
12" diameter
15" diameter
4" diameter
8" diameter
10" diameter
15" diameter
24" diameter
$2.10 -
$2.59/linear foot
$5.20 -
5.56/linear foot
$10.05
11.98/linear foot
$15.25/linear foot
$3.25 -
$4.10/linear foot
$4.85 -
$7.20/linear foot
$6.99 -
$9.45/linear foot
$19.75/linear foot
3-6.98/linear foot
11. Security fence
galvanized steel, 12 ga., 2" x 4" mesh with
posts 5' o.c., 51 high
12' high, prison grade
12. Snow fence on steel posts 4' high
13. Storage building, bulk, 180; diameter
$3.25/linear foot
$32.00/linear foot
$3.36/linear foot
$20/s.f.
floor
14. Survey, conventional topographic
aerial survey including ground control,
10 acres
$165.00-$1150/
acre (total)
$2200
/acre (total)
200-600/
acre (total)
100-2300
acre (total)
6700 + 70.00
/acre (total)
15. Winter protection, reinforced plastic or wood $0.63/acres
tarpaulin over scaffold without scaffold cost $0.29/s.f.
topographic 2 ft. contours 10 acres
20 acres
50 acres
$1975
$2875
Reference: Means 1982, Dodge 1982.
7-34
-------
that site specific factors such as excavation obstructions or extremely
hazardous working conditions be considered in costing. Factors such as these
can double or triple the time spent on-site and can drastically affect total
costs.
7-35
-------
-------
SECTION 8
EVALUATION PROCEDURES
Numerous factors must be taken into consideration in evaluating proposed
remedial actions, just -as numerous factors must be considered in their design
and installation. To evaluate slurry walls as remedial measures, an under-
standing of the currently accepted theories on the nature and function of the
materials and techniques involved is essential. The early phases of the
remedial planning process will center on characterization of the nature and
extent of the environmental problems caused by the site in question. The
later phases will focus on solutions to those problems. When a slurry wall is
being considered as a remedial measure for a particular site, an evaluation
must be made on the type and configuration of slurry wall to be used, as well
as the other measures that must be taken to resolve the problems. In
addition, the proposed construction techniques, quality control measures, and
monitoring and maintenance programs must be evaluated carefully. Finally, the
costs, both for the slurry wall and the related remedial measures must be
analyzed, keeping in mind the degree of effectiveness or the safety factor
gained for each additional cost incurred.
Each slurry wall installed for pollution control will be unique in many
respects. It is essentially impossible to foresee every potential contingency
of each site, however, the major site planning considerations can be listed.
For this reason, this section presents a series of questions indicative of the
type of thought process that should accompany the evaluation procedure. The
evaluation procedures presented here parallel sections 3 through 7 of this
handbook and are illustrated in Figure 8-1.
8.1 Site Characteristics
Both the surface and subsurface characteristics of a site influence the
design and construction of slurry walls.
8-1
-------
Figure 8-1.
Flow Chart of the Evaluation Procedures
for a Pollution Control Slurry Wall
00
I
hJ
Site
Investigation
and
Characterization
and
Wall
Feasibility
^^^
^^
^
L^
*1
Selection of
Wall Type
and
Configuration
1
f
r ~~^
Selection of
Other
Remedial
Measures
L
1
1
1
1
1
1
J
Selection of
Wall Type
and
Configuration
Wall Design,
Construction,
&QA/QC
A A
1 1
1 1
Wall
Monitoring
and
Maintenance
Walt
Costs
* *
1 1
1 I
r
1
I
I
1
i
L
Other Measure
Design,
Construction,
&QA/QC
-1
1
1
r-
1
_l
r1"
1
i
1
i
i
L
Other
Monitoring
and
Maintenance
~\
\
1
r
i
I
-j
1
1
1
t
1
1
Walt
Costs
*
1
1
1
' "1
Other
Measure
Costs
J
*
and Durability
of Watt
and Entire
Remedial Action
-------
8.1.1 Surface Characteristics
Important surface factors that can affect slurry wall installations
include:
Topography
Soils and vegetation
Property lines, rights-of-way, and utilities
Roads and structures.
Questions that pertain to a site's surfacial characteristics include the
following:
1. Is the topography steep? If so, CB walls can be used in steeper
areas. SB walls can be used where slopes are less than 1 percent, or
site is graded to near level.
2. Is sufficient area available for SB backfill mixing? If not, back-
fill can be mixed in mechanical batchers or pugmills, or it can be
mixed in a central mixing area then trucked to the trench. This will
increase costs.
3. Is the terrain too rugged to allow access of heavy construction
equipment? If so, access roads may need to be constructed prior to
trench excavation. This too will increase costs.
4. Is there a preponderence of rock outcrops or boulders that will
interfere with trench excavation? If so, construction delays should
be expected and drills and chisels should be available for rock
fracturing. In addition, the trench bottom should be cleaned of rock
fragments prior to backfilling.
5. Are the soils at the site stable enough to support heavy construction
equipment? If not, a work platform of compacted soil can be
constructed along the proposed line of the trench.
6. Are the areas to be excavated or used for hydration ponds and
backfill mixing free of vegetation? If not, these areas must be
cleared and the organic matter removed so as not to contaminate the
backfill.
7. Have all property lines, rights-of-way and utility lines been
located? Have utilities been re-routed as necessary? Have pipes
such as sewers been closed off to avoid sudden slurry loss? These
procedures should be followed prior to excavation.
8. If roads are to be crossed during trench excavation, have alternative
routes been devised? Has the structural strength of the cut-off in
the area of the road been designed to withstand traffic loads? CB
8-3
-------
walls, concrete panel, or cast-in-place walls may be used where
structural support is required. Alternatively, a "traffic cap" may
be used. This cap is composed of compacted clay layers interspersed
with layers of geotextiles and topped with gravel.
9. If structures are nearby, what special design considerations were
taken? Can groundwater levels be lowered? Must nearby foundations
require reinforcement, such as by grouting or installing tiebacks?
Are alternative remedial measures more cost effective?
8.1.2 Subsurface Characteristics
The subsurface characteristics of a site that affect slurry trench
cut-off design include:
Properties of the subsurface strata
Aquiclude type and location
Groundwater regime.
Questions pertinent to a site's subsurface characteristics include the
following:
1. Does the material to be excavated from the trench have a favorable
gradation? If not, suitable borrow areas must be located and
provisions made for excavating the borrow and for hauling it.
Ideally, the mixed backfill should contain from 20 to 60 percent
fines.
2. Are the spoils contaminated? If so, will the contaminants interfere
substantially with trench wall stability, or can the contaminated
material be used? It has been suggested that if contaminated spoil
is equal in quality to other available backfill material, that the
contaminated soil be used in the backfill to minimize the detri-
mental effects of later permeation with pollutants.
3. Is all of the backfill contaminated, or are portions still
uncontaminated? If only portions are contaminated with volatile
organic chemicals to the point where they are not usable in the
backfill, these portions can be disposed of and the uncontaminated
material can be used. These materials can be distinguished by using
an organic vapor detector to "sniff" each backhoe bucket or clam-
shell load. The contaminated material can then be dumped into
trucks for haulage to disposal sites. The uncontaminated material
can be used in the backfill.
4. Do the spoils contain other materials that may be detrimental to the
integrity of the backfill? These materials include organic debris,
construction debris such as pieces of concrete, and certain minerals
such as caliche and anhydrite.
8-4
-------
5.
10,
11
12
Will the trench be excavated through highly pervious zones such as
gravel or coarse sand layers? If so, stipulations may be made in
the design criteria to require the availability of lost circulation
materials, as listed in Section 5. In addition, care must be taken
in maintaining high slurry quality to avoid excessive slurry loss
through the pervious layers. If certain fine grained sands become
sufficiently lubricated, they may suddenly lose their stability and
contribute to trench collapse.
How permeable is the aquiclude? Are there fissures, desiccation
cracks, or other pervious zones within the aquiclude? If the
aquiclude has permeable zones, the base of the backfilled trench can
be grouted to decrease the walls permeability.
What is the hardness of the aquiclude? If it is extremely hard,
removal of the rock for the key-in may be very difficult. Chisels
or drills may cause the aquiclude to fracture, thus allowing under-
seepage. In this situation, it may be better to scrape clean the
surface of the aquiclude and install the backfill directly on it.
The weight of the emplaced backfill should be sufficient to ensure
wall integrity.
How deep is the aquiclude?
equipment required.
This determines wall depth and the
What is the key-in depth required in the design? Normally, it is
between 2 and 3 feet in rippable aquicludes.
What is the nature of the groundwater contamination? Are the con-
taminants the same as those in the wastes or are they substantially
different? At a minimum, the groundwater should be tested for pH
and hardness. If any contaminants are suspected, tests should be
run to determine their presence and concentrations. Similar tests
should be conducted on all water sources to be used in the slurry.
What is the depth, volume, flow rate, and flow direction of the
groundwater? These and other data on the groundwater regime should
be obtained through site investigations as described in Section 4.
If groundwater levels get too high, such as during a flood, what
contingency plans are included in the design to protect the trench
prior to backfilling? One set of specifications recommended that
excavations cease immediately and that the trench be immediately
backfilled. After groundwater levels have returned to normal, the
hastily backfilled portion of the trench should be re-excavated,
then properly mixed and placed.
8-5
-------
8.1.3 Waste Characteristics
Before slurry trench cut-offs are selected for use at a given hazardous
waste site, the waste must be carefully characterized and the influences of
the waste on the cut-off determined. These interactions are described in
Section 4.
Specific questions regarding waste characteristics are given below.
1. What are the types and volumes of wastes known to be deposited at the
site? Are other wastes suspected at the site? Certain types of
wastes, such as pure xylene have been found to severely damage cut-
offs. If only small portions of these wastes are present, a slurry
trench cut-off may still be feasible, however if major concentrations
are present, other methods of isolating the site must be seriously
considered.
2. What, if any, are the known waste interactions? Are dangers, such as
explosions or sudden releases of toxic gases, likely? Safety
measures and trench wall siting must take these factors into account.
3. How long have the wastes been at the site? This information can
contribute to the knowledge of plume characteristics and waste
degradation, both of which are necessary for proper wall siting.
4. How soluble are the wastes and how dense are they? Immiscible wastes
of low density can form lenses on the surface of the groundwater. If
these are present, skimmers can be used to remove them from the
groundwater, thus lowering groundwater pumping and treatment require-
ments, as described in Section 3. The presence of floating contami-
nants also affects wall design, in that aquiclude key-in may not be
necessary at these sites. If, however floating contaminants come in
contact with the slurry trench cut-off, they are likely to be at high
concentrations and thus are likely to adversely affect the wall.
Waste/wall interactions are described in Section 4. Similar adverse
effects may occur if extremely dense wastes are present. Under these
circumstances, aquiclude key-in may be of extreme importance.
8.2 Slurry Wall Applications
A critical evaluation of the type of slurry wall and how it is applied is
very important. Because slurry walls are rarely used alone, this evaluation
must also focus on the other remedial measures to be used.
8-6
-------
8.2.1 Wall Configuration and Type
All of the factors discussed above must be taken into account when
selecting the location, configuration, and type of cut-off wall. Questions
regarding wall configuration are listed below.
1. Is the wall configuration consistent with what is known about the
waste types and the groundwater regime at the site? As described in
Section 3, walls can be placed downgradient if very limited
groundwater flow is present; and upgradient if the groundwater can be
effectively diverted from the site. Circumferential walls are used
where maximum containment is required. In any case, the wall must be
protected from contact with the wastes that cause increases in wall
permeability.
2. Are surface water diversions (i.e., dikes, ditches, berms, terraces,
etc.) planned to protect the site from surface runoff? These
features are particularly critical in humid areas and areas with
sudden, extreme rainfall events.
3. Where extremely low permeability is required, are SB walls with a
high percentage of fines specified? Are the fines plastic or non-
plastic? Research has shown that maximum cut-off effectiveness
occurs in walls having 20 to 50 percent plastic fines and 1 to 2
percent bentonite, with 25 to 35 percent water.
4. Where structural strength is required, is the wall designed to take
the stresses applied? SB walls are more flexible and lower in
strength than CB walls. Cast-in-place and panel walls have the
highest strength, however they also have the highest permeability and
cost.
5. Are the site factors favorable for the use of SB walls? Site
factors, described previously, include availability of suitable
backfill and backfill mixing area, suitable terrain, and low strength
requirements.
8.2.2 Associated Remedial Measures
Cut-offs are seldom used alone in controlling hazardous waste sites
types of remedial measures used with cut-offs include:
Surface Sealing
Groundwater Pumping and Subsurface Collection
Surface collection and runoff diversion
Measures used to increase wall efficiency.
The
8-7
-------
Questions useful in evaluating associated remedial measures are listed
below.
1. Is surface sealing of the site planned? This reduces the influence
of precipitation on the volume of groundwater at the site and thus
can reduce the hydraulic pressure on the interior of the wall.
2. If well points or extraction wells are to be installed, are they
close enough to the wall to influence wall stability? This is most
important during trench excavation. If drawdown cones intersect the
wall, the trench wall stability may suffer.
3. Is collection and diversion of surface water planned? If so, this
water must not be allowed to erode, soften, or otherwise degrade the
clay cap placed over the completed trench.
4. Is the use of other measures, such as grouting, sheet piling, or
lining the trench with impervious membranes planned? If so, the
location, construction considerations, and materials used must be
compatible-with the slurry, the backfill, and the wastes at the site
8.3 Construction Techniques and QA/OC Requirements
The most carefully designed remedial action plan will not function
properly if poor construction or QA/QC techniques are used. These factors
must be carefully considered to ensure efficient wall installation. Questions
regarding these criteria are given below.
1. Are the criteria used for selection of the contractor relevant and
stringent enough?
2. Are the specifications overly stringent? Experience has shown that
performance type specifications allow greater opportunity for
innovation on the part of contractors. Because slurry trench
construction is an evolving technique, there are many facets of the
technique that are likely to be improved as greater field experience
is obtained. For this reason, some leeway in the specifications is
reasonable and may even be beneficial.
3. Do the specifications cover all the important design criteria, such
as aquiclude selection and key-in, backfill composition and slurry
viscosity? These specifications are described in Section 5.
Theories explaining the importance of these factors are discussed in
Section 2.
4. Are adequate QA/QC and documenation requirements included? Typical
requirements are listed in Section 5.
8-8
-------
5. Are adequate safety precautions taken during cut-off construction?
These are particularly important in a contaminated environment,
especially during excavation.
8.4 Monitoring and Maintenance
Despite the care taken in design and construction, the overall perfor-
mance of a slurry wall, as well as the other remedial measures, must be
established through a monitoring program. The nature, extent, and frequency
of the monitoring must be determined for each individual site. Although
slurry walls require little maintenance, certain measures must be taken to
ensure short and long-term effectiveness. Section 6 of this handbook presents
these subjects in detail.
8.4.1 Monitoring
The monitoring program must provide answers to several geotechnical and
geochemical performance questions. Some important geotechnical questions
include the following.
1. Does the monitoring data indicate that the wall is continuous and of
the required permeability?
2. Is the wall stable? If not what are the causes, impacts, and
remedies?
3. Has the groundwater regime been altered in the intended manner? If
not, is this due to a poorly functioning cut-off, or an incomplete
initial characterization of groundwater flow?
Given careful design and construction practices, there should be few
geotechnical problems with a slurry wall installation. Nonetheless, these
performance characteristics must be verified.
Because the chemistry and geochemistry of a site have a significant
influence on successful slurry wall installation, questions similar to the
following should be asked.
1. Have the slurry wall and related measures halted or reduced to an
acceptable level, the spread of contamination by groundwater and
surface waters?
2. Has leachate/wall contact been prevented? If not, is there evidence
of chemical destruction of the wall?
8-9
-------
The answering of such questions will help evaluate cut-off effectiveness
and durability, and may alter the character of future site monitoring for that
site.
8.4.2 Maintenance
^Maintenance of a slurry wall primarily involves protecting the upper
portion from damage. Pertinent questions include:
1. Is the wall protected from cracking by desiccation?
2. Is the wall protected from breach by root penetration?
3. Is the wall protected from traffic loading?
All of these are answered by investigating the integrity and maintenance
of the capping method used and the condition of the vegetation or other
material used to protect the cap.
8.5 Costs
Costs are a major concern with any project. The cost of a completed
slurry wall is dependent on a number of factors including the square footage
of the wall, the characteristics of the site, and the materials used for
backfill. Section 7 presents costs for slurry walls and associated remedial
measures. Some relevant questions on costs should include the following.
1. Is the total cost per square foot of the proposed slurry wall within
the ranges for depth and excavation ease given in Table 7-19?
2. If not, are there site characteristics which explain a higher or
lower unit cost?
The quality of the finished product is often related to the cost, and it
is important to understand the compromises involved between cost and quality,
Although the cost for a proposed wall will likely be submitted as a fixed
price lump sum, it is important to have any discrepancy between anticipated
and bid cost adequately explained. The possibility of an inferior product
exists, particularly if a site has not been well-characterized.
The nature and extent of the other remedial measures needed to address a
particular site's problems are difficult to generalize simply because they are
so site specific. Again, a compromise must be struck between the funds
expended and the degree of pollution control achieved. Some basic questions
on the related measures used with slurry walls include the following.
1. If costs are critical, where can costs be cut while reducing the
effectiveness the least?
8-10
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2. If additional funds are available, where can they be best expended to
increase the effectiveness or longevity of the remedial program?
In the majority of cases, the bids for a slurry wall installation will be
quite close to one another. If a wide discrepancy is found, it should be
explained. It should be kept in mind that greater costs for a wall do not
necessarily indicate greater effectiveness of the final cut-off.
This section has provided a summary of the many aspects of slurry walls
that must be examined during evaluation of slurry wall design and instal-
lation. These factors should be carefully considered to ensure that the
proposed installation will perform its intended purpose over its design life
in a cost-effective manner.
-------
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MEASURING UNIT CONVERSION TABLE
S.I. UNITS
inch (in)
foot (ft)
mile (mi)
LENGTH
x 2.54
x 0.3048
x 1.609
METRIC
= centimeter (cm)
- meter (m)
- kilometer (km)
U.S. gallon (gal)
cubic feet (ft )
acre-foot (ac.ft)
VOLUME
x 0.0038
x 0.0283
x 123.53
cubic meter (m"
cubic meter
cubic meter
square inch (in )
square foot (ft )
acre (ac)
AREA
x 6.452
x 0.09
x 0.4047
square centimeter
square meter (m )
hectare (ha)
(cm )
ounce (02)
pound (Ib)
short ton
MASS
X 28
x 0.45
x 0.9
* gram (g)
= kilogram (kg)
= metric ton (t)
DENSITY
3
Pounds per cubic foot (pcf) x 0.016 - grams per cubic centimeter (g/cm )
gallons per day per .
square foot (gpd/ft )
Darcy
HYDRAULIC CONDUCTIVITY
.-5
x 8.58 x 10 ^ centimeters per second
x 4.72 x 10 ~" - centimeters per second (cm/sec)
-4
9-1
-------
-------
GLOSSARY
Adsorption complex: The adsorption complex is the group of substrates in soil
capable of attracting and exchanging other materials. Colloidal
particles account for most adsorption in soils.
Apparent viscosity: The apparent viscosity of a fluid is the viscosity it
exhibits under a given rate of shear and is equal to shear stress/rate of
shear. This quantity can be measured using a Fann viscometer.
Aquiclude: An aquiclude is a body of low permeability rock or other earth
material that does not transmit sufficient groundwater to supply a well
or a spring.
Aquifer: An aquifer is a formation, group of formations, or part of a
formation that contains sufficient saturated permeable material to yield
significant quantities of water to wells and springs.
Attapulgite: Attapulgite is a chain-lattice clay mineral having a distinctive
rod-like shape. Syn: Palygorskite.
Backfill: Backfill is earth or other materials used to replace material
removed during construction or mining operations. For slurry walls, the
backfill is soil-bentonite, cement-bentonite, or concrete.
Bedrock: Bedrock is the more or less solid, undisturbed rock in place either
at the surface or beneath superficial deposits of gravel, sand, or soil.
Bentonite: Bentonite is a light-colored rock consisting largely of colloidal
silica and composed mostly of crystalline clay'minerals. It is produced
by the weathering of glassy igneous materials, usually a tuff or volcanic
ash. When wet, it is soft and plastic.
Bleed water: Bleed water is the water that separates from a cement or
concrete mixture before and during hardening.
Blowout gradient: Blowout gradient refers to the hydraulic gradient at which
a slurry will be forced out of soil or other voids.
Borrow: Borrow is soil material taken from a distant source to be used as
fill material on-site.
Cation exchange capacity: Cation exchange capacity refers to the sum total or
exchangable cations that a soil can adsorb.
10-1
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Confined groundwater: Confined groundwater is under pressure significantly
greater than atmospheric, and its upper limit is the bottom of a bed of
distinctly lower hydraulic conductivity than the aquifer.
Confining bed: A confining bed is a body of less permeable material overlying
or underlying an aquifer. "Aquitard" is a commonly used synonym. The
terms "aquiclude" and "aquifuge" are generally considered obsolete.
Confining beds have a high range of hydraulic conductivities and a
confining bed of one area may have a hydraulic conductivity greater than
an aquifer of another area.
Capillary fringe: The capillary fringe is the zone immediately above the
water table in which all or some of the interstices are filled with water
that is under less than atmospheric pressure and that is continuous with
the water below the water table. The thickness of the capillary fringe
is greater in fine-grained material than in coarse-grained material. It
ranges in thickness from a fraction of an inch to tens of feet.
Dewatering: Dewatering is the removal of groundwater from an area by means of
pumps or drains.
Discharge zone: Discharge zone is a zone in which subsurface water, including
water from both the saturated and unsaturated zones, is discharged to the
land surface or to the atmosphere.
Dispersion: Dispersion is the breaking up of compound particles into
individual component particles. It also refers to the distribution and
suspension of fine particles in or throughout a dispersing medium, such
as water.
Effective porosity: Effective porosity refers to the amount of interconnected
pore space available for transmitting water.
Fann viscometer: A Fann viscometer is a device used to measure the viscosity
of a slurry. In this device the slurry is sheared between two rotating
cylinders. From the results of this test the plastic and apparent
viscosities can be calculated.
Filter Cake: Filter cake refers to the thin, very low permeability layer
formed on porous media by slurry filtration. As the slurry is forced into
the pores by the hydraulic head, the slurry particles plug the pores and
build up a "cake".
Flocculation: Flocculation is the aggregation of soils or colloids into small
lumps called floes, which settle from a suspension.
Flow net: A flow net is a set of intersecting equipotential lines and flow
lines representing two-dimensional steady-state flow through porous
med ia.
Gel strength: Gel strength is the stress required to break up a gel structure
formed by thixotropic build up under static conditions.
10-2
-------
Grout: Grout is a fluid material that is pressure injected into soil, rock,
or concrete to seal openings and to lower permeability and/or provide
additional structural strength. There are four major types of grouting
materials: chemical (silicates and polymers), cement, clay, and
bituminous.
Hanging wall: A hanging slurry wall is one that is completed several feet
into the lowest water table level but is not tied into a low permeability
zone. These are used mostly to control floating contaminants.
Head (Hydraulic): The height above a datum (sea level) to which a column of
fluid can be supported by the static pressure at that point.
Hydraulic conductivity: Hydraulic conductivity K, replaces the term
"coefficient of permeability" and is a volume of water that will move in
unit time under a unit hydraulic gradient through a unit area measured at
right angles to the direction of flow. Dimensions are LT with common
units being centimeters per second.
Hydraulic gradient: Hydraulic gradient is the change in head per unit of
distance in the direction of maximum rate of decrease in head.
Keyed-in wall: A keyed-in slurry wall is one which has been connected along
its base to a low permeability zone such as a clay layer or hard bedrock.
Leachate: Leachate is contaminated liquid discharge from a waste disposal
site to either surface or subsurface receptors. It is created by fluid
percolation through and from waste materials. The contaminated water
then moves either into the ground below or as surface runoff or seepage.
Lithologic unit: A lithologic unit is a stratigraphic unit having a sub-
stantial degree of lithologic homogeneity consisting of a body of strata
that is unified with respect to adjacent strata by possessing certain
objective physical features observable in the field or subsurface or
consisting dominantly of a certain rock type or combination of rock types
and considered completely independent of time.
Marsh funnel: A Marsh funnel is a device used to measure the viscosity of a
slurry. The Marsh viscosity, in seconds, equals the time it takes 1
quart (946 cm ) of slurry to pass through the funnel.
Montmorillonite: Montmorillonite refers to a group of expanding-lattice clay
minerals characterized by high cation exchange capacity and high swelling
and shrinking.
Performance: Used herein to describe a slurry wall's ability to function at
or above design specifications.
Permeability: Intrinsic permeability, k, is a property of the porous medium
and has dimensions .of LT. It is a measure of the resistance to fluid
flow through the medium and is often used to mean the same thing as
hydraulic conductivity.
10-3
-------
Permeameter: A permeameter is an apparatus used in the laboratory to measure
a material's permeability or hydraulic conductivity.
Piping: Piping occurs as a result of seepage erosion in which flowing water
has enough force to erode or carry away soil particles, creating
localized channels and/or cavities in the soil.
Plasticity: Plasticity is the quality of having the capacity to be molded or
altered and the ability to retain a shape attained by pressure
deformation.
Plastic viscosity: Plastic viscosity is a measure of the resistance to flow
caused by mechanical friction. It is measured on a Fann viscometer and
is dependent on solids concentration, size and shape of solids, and the
amount of shearing within the liquid phase.
Porosity: Porosity of a rock or soil is its property of containing
interstices and is the ratio of the volume of interstices to the total
volume. It is expressed as a decimal, fraction, or percentage. Total
porosity is comprised of primary and secondary porosity. Porosity is
controlled by shape, sorting and packing arrangements of grains and is
independent of grain size.
Potentiometrie surface: Potentiometric surface is an imaginary surface
representing the static head of groundwater and defined by the level to
which water will rise in a well. The water table is a particular
potentiometric surface.
Pozzolana: Finely divided siliceous or siliceous and aluminous materials used
to make strong, slow-hardening cements. Pozzolanic cements are resistant
to saline and acidic solutions.
Recharge zone: A recharge zone is a zone in which water is absorbed and added
to the zone of saturation, either directly into a formation, or
indirectly by way of another formation.
Rheological blocking: Rheological blocking refers to the inhibition of slurry
flow into a soil or rock body due to the onset of gelation of the slurry
in the large pores.
Saprolite: Saprolite is a general term for earth materials formed by the
disintegration and decomposition of rock in place. "Saprolitic zone"
refers to that zone where saprolite is present.
Saturation: Water saturation is the percentage ratio of the volume of water
to the volume of void space.
Saturated zone: The saturated zone is that part of the water-bearing material
in which all voids are filled with water under pressure greater than
atmospheric.
10-4
-------
Shear strength: Shear strength is the maximum internal resistance of a
substance to movement of its particles due to intergranular friction and
cohesion.
Slump: Slump refers to the vertical distance a cone-shaped mass of concrete
or other plastic material will settle. It is measured using a slump cone
as specified in ASTM Book of Standards, Part 14.
Slurry: Slurry refers both to colloidal suspensions of bentonite in water as
well as mixtures of Portland cement and water.
Soil gradation: Soil gradation refers to the frequency distribution of the
various sized grains that constitute a particular soil.
Specific storage: Specific storage, S , is defined as the volume of water
that a unit volume of aquifer releases from storage because of expansion
of the water and compression of the grains under a unit decline in
average head within the unit volume. For an unconfined aquifer, for
all practical purposes, it has the same value as specific yield.
Note the dimensions are L . It is a property of both the medium
and the fluid.
Specific retention: Specific retention of a rock is the ratio of the volume
of water a saturated rock will retain against the pull of gravity to its
own volume.
Specific yield: Specific yield is the water yielded by gravity drainage as
occurs when the water table declines. It is the ratio of the volume of
water yielded by gravity to the volume of rock. Specific yield is equal
to porosity minus specific retention.
Storage coefficient: The storage coefficient, S, or storativity is defined as
the volume of water an aquifer releases from or takes into storage per
unit surface area of aquifer per unit change in the component of head
normal to that surface. Note it is dimensionless.
Structural discontinuity: Structural discontinuity refers to a sudden or rapid
change in one or more of the physical properties of a rock mass.
Thixotropy: Thixotropy is the property of various gels to become fluid when
disturbed, and, later, to regain strength at constant water content.
Transmissivity: Transmissivity, T, is defined as the rate of flow of water
through a vertical strip of aquifer one unit wide extending the full
saturated thickness of the aquifer under a unit hydraulic gradient.
Unconfined groundwater: Unconfined groundwater is water in an aquifer that
has a water table.
Underseepage: Underseepage is the passage of water beneath a slurry wall as a
result of an inadequate wall aquiclude key-in.
10-5
-------
Unsaturated zone: The unsaturated zone is the zone between the land surface
and the water table. It includes the capillary fringe. Characteristi-
cally this zone contains liquid water under less than atmospheric
pressure, with water vapor and other gases generally at atmospheric
pressure.
Viscosity: Viscosity refers to the ability of a fluid to resist shearing or
flow due to counteracting, internal forces.
Weathering: Weathering refers to the various chemical and mechanical
processes acting at or near the earth's surface that bring about the
disintegration, decomposition, and comminution of rocks.
Waste Plume: Waste plume refers to a body of water, either surface or
groundwater, that is contaminated by toxic or otherwise hazardous
substances and moves as a coherent mass.
Water table: The water table is an imaginary surface in an unconfined water
body at which the water pressure is atmospheric. It is essentially the
top of the saturated zone.
10-6
-------
REFERENCES
Alther, G. R. The Role of Bentonite in Soil Sealing Applications.
International Minerals and Chemical Corporation, Des Plaines, Illinois.
[no date]
Alther, G. R. IMC Corporation. Personal Communication with C. E. Spooner of
JRB Associates. April 1983.
ASCE. American Society of Civil Engineers. Subsurface Investigation for
Design and Construction of Foundations of Buildings. ASCE No. 56.
Headquarters of the Society, NY, NY. 1976.
Anderson, D., and K. W. Brown. Organic Leachate Effects on the Permeability
of Clay Liners, in Land Disposal of Hazardous Waste. Publication No.
EPA-600/9-81-002. U.S. Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati, Ohio, 1981.
API. American Petroleum Institute. API Recommended Practice: Standard
Procedure for Testing Drilling Fluids. API RP13B. American Petroleum
Institute, Dallas, Texas, 1982.
API. American Petroleum Institute. API Specification for Oil Well Drilling
Fluid Materials. API Spec. 13A American Petroleum Institute, Dallas,
Texas, 1981.
Ash, J.L., B.E. Russel and R.R. Ronmmel. Improved Subsurface Investigation
for Highway Tunnel Design and Construction, Vol. 1. U.S. Department of
Transportation, Federal Highway Administration, Washington, D.C., 1974.
Ayres, J. GZA Corp. Personal Communication with G. E. Hunt of JRB
Associates. September 3, 1982.
Baver, L.D., W.H. Gardner and W.R. Gardner. Soil Physics. John Wiley & Sons,
Inc., New York, 1972.
Bjerrum, L., J. K. T. L. Nash, R. M. Kennard, and R. E. Gibson. Hydraulic
Fracturing in Field Permeability Testing, Geotechnique, Vol. 22, No. 2,
pp. 319-332, 1972.
Boyes, R. G. H. Structural and Cut-off Diaphragm Walls. Applied Science
Publishers Ltd., London, England. 1975.
Brady, N.C. The Nature and Properties of Soils. Macmillan Publishing Co.,
Inc., New York, 1974.
11-1
-------
Brown, K. W., and D. Anderson. Effect of Organic Chemicals on Clay Liner
Permeability. A Review of the Literature. In: Disposal of Hazardous
Waste. Proceedings of the Sixth Annual Research Symposium. EPA
600/9-80-010. USEPA, Municipal Environmental Research Laboratory,
Cincinnati, Ohio. 1980.
Case International Company. 1982. Case Slurry Wall Notebook, Manufacturers
Data. Case Intn'l. Co., Houston, Texas.
Cavalli, N. J. ICOS Corporation of America. Personal Communication with P.
A. Spooner of JRB Associates. December 1982.
Coneybear, R. Engineered Construction International, Inc. Personal
Communication with C. E. Spooner of JRB Associates. September 1982.
D'Appolonia, E. Consulting Engineers Company. Results of Long Term
Permeability Testing Rocky Mountain Arsenal.. U.S. Army Waterways
Experiment Station, Vicksburg, Mississippi. 1979. Contract No. DACW
39-78-M-3705.
D'Appolonia, D. J. Slurry Trench Cut-off Walls for Hazardous Waste Isolation.
Technical Paper. Engineered Construction International, Inc.,
Pittsburgh, Pennsylvania. April 1980a.
D'Appolonia, D. J., J. and C. Ryan. Soil-Bentonite Slurry Trench Cut-off
Walls. Engineered Construction International, Inc. Presented at the
Geotechnical Exhibition and Technical Conference, Chicago, Illinois,
March 26, 1979.
D'Appolonia, D. J. Soil-Bentonite Slurry Trench Cutoffs. J. Geot. Eng. Dir.
ASCE, 106(4):399-417, 1980b.
D'Appolonia, D. J. Engineered Construction International, Inc. Personal
Communication with P. A. Spooner of JRB Associates. June 23, 1982.
Dodge Manual. 1982. Building Construction Pricing and Scheduling. Edition
No. 17, McGraw-Hill Cost Information Systems, Princeton, NJ, 302 pp.
Dunnicliff, J. Performance of Slurry Wall Construction. In: Proceedings of
a Symposium on Design and Construction of Slurry Walls as Part of
Permanent Structures. 1980.
Federal Bentonite. 1981. Suggested Standard Guideline. Technical
Specifications; Soil/Bentonite Slurry Trench Cut-off Wall. Federal
Bentonite, Montgomery, Illinois.
Freeze, R. A. and John A. Cherry. Groundwater. Prentice-Hall, Inc.
Englewood Cliffs, N.J. 1979. 604 pp.
Geo-Con, Inc. Typical Specifications for CB Walls. January 1979.
Grim, R.E. Clay Mineralogy. McGraw-Hill Book Company, Inc., New York, 1968.
11-2
-------
Grim, R. E., and N. Guven. Bentonites: Geology Mineralogy, Properties and
Uses. Developments in Sedimentology 24. Elsevier Scientific Publishing
Company, Amsterdam, 1978.
Guertin, J. D., and W. H. McTigue. Groundwater Control Systems for Urban
Tunneling. Vol. 1. U.S. Department of Transportation, Federal Highway
Administration, Washington, B.C., 1982a.
Guertin, J. D., and W. H. McTigue. Preventing Groundwater into Completed
Transportation Tunnels, and Recommended Practice. Vol. 2. U.S.
Department of Transportation, Federal Highway Administration, Washington,
D.C., 1982b.
Guertin, J. D., and W. H. McTigue. Groundwater Control in Tunneling. Vol. 3.
U.S. Department of Transportation, Federal Highway Administration,
Washington, D.C., 1982c.
Hirschfeld, R.C. Soil Mechanics. In the Encyclopedia of Soil Science, Part
I, Physics, Chemistry, Biology, Fertility, and Technology, pp 462-469,
Edited by R. W. Fairbridge and C. W. Finkl Jr. 1979. Dowden,
Hutchinson, and Ross Inc., Stroudsburg, PA., 646 pp.
Hughes, J. Use of Bentonite as a Soil Sealant for Leachage Control in
Sanitary Landfills. American Colloid Company, Skokie, Illinois, 1975.
Hutchinson, M. T., G. P. Daw, P. G. Sholton, and A. N. James. 1975. The
Properties of Bentonite Slurries Used in Diaphgragm Walling and Their
Control. Paper in Diaphragm Walls and Anchorages. Institute of Civil
Engineers, London. 1975. pp. 33-39. These are proceedings of the
conference: (date of conference Sept. 18-20, 1974, London).
IMC, Iracore Division, International Minerals & Chemical Corporation.
Bentonite product literature, specification and engineering reports:
Mundelein, Illinois [no date].
Jefferis, S. A. Discussion of Soil-Bentonite Slurry Trench Cutoffs. J.
Geotech. Eng. Div. 107:1581-1583, 1981a.
Jefferis, S. A. Bentonite - Cement Slurries for Hydraulic Cut Offs. In:
Proceedings of the Tenth International Conference on Soil Mechanics and
Foundation Engineering, Stockholm June 15-19, 1981b. A. A. Balkema,
Rotterdam, pp. 435-440.
Jessup, W.E., Jr. and W.E. Jessup. Law and Specifications for Engineers and
Scientists. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1963.
Jones, J. C. Design and Safety of Small Earth Dams. In: Proceedings of the
26th Annual Soil Mechanics and Foundation Engineering Conference,
Minneapolis, Minnesota. February 1978.
JRB Associates. Assessment of Alternatives for Upgrading Navy Solid Waste
Disposal Sites. Final Report. JRB Contract No. 2-800-04-187-00. U.S.
Navy Civil Engineering Laboratory, Port Hueneme, California, 1979. pp.
1-14-92.
11-3
-------
Lager, D. C. Case International Co. Personal Communication with
P. A. Spooner of JRB Associates. September 13, 1982.
La Russo, R. S. Wanapum Development-Slurry Trench and Grouted Cut-off. In:
Grouts and Drilling Muds in Engineering Practice. William Clowes and
Sons, Ltd., London, England, 1963. pp. 196-201.
Low, P. F, Viscosity of Interlayer Water in Montmorillonite, Soil. Sci. Soc
Amer. J. 40:500-504, 1976.
Matrecon, Inc. Lining of Waste Impoundment and Disposal Facilities.
Publication No. SW-870. U.S. Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati, Ohio, 1980. 385 pp.
McCarthy, D. F. Essentials of Soil Mechanics and Foundations. Reston
Publishing Company, Inc., Reston, Virginia. 1977. 505 pp.
McNeal, B. L. Prediction of the Effect of Mixed-salt Solutions on Soil
Hydraulic Conductivity. Soil Sci. Soc. Amer. Proc. 32:190-193, 1968.
Means, R. S. Building Construction Cost Data. 1982. Robert Snow Means
Company, Inc. Kingston, MA. 1981.
Meier, J. G., and W. A. Rattberg. Report on Cement-Bentonite Slurry Trench
Cutoff Wall: Tilden Tailings Projects. In: Tailing Disposal Today.
In: Proceedings of the Second International Tailing Symposium, Denver,
Colorado, May 1978.
Miller, E. A., and G. S. Salzraan. Value Engineering Saves Dam Project. Civ.
Eng. 1980.
Miller, S. P. Geotechnical Containment Alternatives for Industrial Waste
Basin F, Rocky Mountain Arsenal. Denver, Colorado; A Quantitative
Evaluation. U.S. Army Engineer Waterways Experiment Station, Vicksburg,
Miss. Technical Report GL-79-23. September 1979.
Millet, R. A., and J. Y. Perez. Current USA Practice: Slurry Wall
Specifications. J. Geot. Eng. Div. 107(8):1041-1056, 1981.
Mitchell, J. K. Fundamentals of Soil Behavior. 1976. John Wiley & Sons,
Inc. New York. 422 pp.
Moore, C. A. Design Criteria for Gas Migration Control Devices. In:
Proceedings on Management of Gas and Leachate in Landfills, Cincinnati,
OH. September 1977.
Mustafa, M. A. Dispersion Phenomena. In the Encyclopedia of Soil Science
Part I, Physics, Chemistry, Biology, Fertility and Technology, pp.
124-127. Edited by R.W. Fairbridge and C.W. Finkl Jr. 1979. Dowden,
Hutchinson, and Ross Inc. Stroudsburg, PA., 646 pp.
11-4
-------
Namy, D. L. Site Conditions Specific to Slurry Wall Construction. Soletanche
and Radio, Inc. In: Slurry Walls for Underground Transportation
Facilities. Proceedings of a Symposium at Cambridge, Massachusetts,
August 1979. Final Report. U.S. Department of Transportation, Federal
Highway Administration. March 1980.
Nash, K. L. and G. K. Jones. The Support of Trenches Using Fluid Mud. In:
Grouts and Drilling Muds in Engineering Practice. William Clowes and
Sons, London, England. 1963. pp. 177-180.
Nash, K. L. Stability of Trenches Filled with Fluids. J. Const. Div.
100(4):533-542, 1974.
Nash, J. K. T. L. Slurry Trench Walls, Pile Walls, Trench Bracing. In:
Sixth European Conference on Soil Mechanics and Foundation Engineering,
Vienna, Austria. March 1976. pp. 27-32.
NCE. National Construction Estimator. Edited by R. Saviel, ASPE. Craftsman
Book Company. Solana Beach, CA 1981. 285 pp.
OCE. Office of the Chief of Engineers. Laboratory Soil Testing. EM
1110-2-1906. U.S. Department of the Army, Office of the Chief of
Engineers, Washington, D.C. 1970.
Oil Recovery Systems, Inc. Product literature on equipment for recovering
petroleum products and hydrocarbons from water. Needham, MA. 1982.
Regan, T. J., Jr. Critical Assessment of Slurry Wall Construction in the
United States. In: Slurry Walls for Underground Transportation
Facilities. Proceedings of a Symposium at Cambridge, Massachusetts.
August 1979. Department of Transportation, Federal Highway
Administration. Final Report. March 1980.
Ressi di Cervia, A. L. Economic Considerations in Slurry Wall Applications.
From: Slurry Walls for Underground Transportation Facilities
Proceedings. U.S. Department of Transportation, FHA. 1979.
Rolfe, P. F., and L. A. G. Aylmore. Water and Salt Flow Through Compacted
Clays. I. Perraability of Compacted Illite and Montraorillonite. Soil
Sci. Soc. Am. J. 41:489-495, 1977.
Rovers, F. A., J. J. Tremblay and H. Mooij. Procedures for Landfill Gas
Monitoring and Control. From: Proceedings of an International Seminar,
Canada, 1977.
Rowell, D. L., D. Payne, and N. Ahmad. The Effect of the Concentration and
Movement of Solutions on the Swelling, Dispersion, and Movement of Clay
in Saline and Alkali Soils. J. Soil Sci. 20(1):176-188, 1969.
Ryan, C. R. Slurry Cut-Off Walls: Design and Construction. Geo-Con, Inc.,
Pittsburgh, PA. 1976.
11-5
-------
Ryan, C. R. Slurry Cut-off Walls: Design Parameters and Final Properties.
Presented at: Technical Course on Slurry Wall Construction, Design,
Techniques, and Procedures, Miami, Florida. February - March, 1977.
Ryan, C. R. Slurry Trench Cut-offs to Halt Flow of Oil-polluted Groundwater.
Presented at: American Society of Mechanical Engineers, Energy and
Technology Conference and Exhibition, New Orleans, Louisiana. February
1980a.
Ryan, C. R. Slurry Cut-off Walls. Methods and Applications. Presented at:
Geo-Tec. 1980, Chicago, Illinois. March 18, 1980b.
SCS Engineers. Memorandum on bentonite slurry walls as remedial actions at
hazardous waste land disposal facilities. Covington, Kentucky. December
1981.
Shainberg, I., and A. Caiserman. Studies on Na/Ca Montmorillonite Systems.
The Hydraulic Conductivity. Soil Sci. 3(3):276-281, 1971.
Shallard, S. G. Engineered Construction International, Inc. Personal
Communication with P. A. Spooner of JRB Associates. January 1983.
Smith, D. Soil Consultants, Inc. Personal Communication with E. F. Tokarski
of JRB Associates. July 1982.
Soletanche Corp. Product Literature. Use of Slurry Trench Cut-off Walls in
Construction and Repair of Earth Dams. Soletanche, 6 Rue de Watford B.P.
511, 92005 Nanterre Cedex, France. 1977.
Sommerer, S., and J. F, Kitchens. Engineering and Development Support of
General Decon Technology for the DARCOM Installation Restoration Program.
Task 1. Literature Review on Groundwater Containment and Diversion
Barriers. Draft. U.S. Amry Hazardous Materials Agency, Aberdeen Proving
Ground, Maryland. 1980.
Tamaro, G. Slurry Wall Construction, Construction Procedures and Problems.
In: Slurry Walls for Underground Transportation Facilities. Proceedings
of a Symposium at Cambridge, Massacuhsetts. Final Report. U.S.
Department of Transportation, Federal Highway Administration. March
1980.
Tavenas, F. A., G. Blanchette, S. Leroueil, M. Roy, and P. LaRochelle.
Difficulties in the In Situ Determination of K in Soft Sensitive Clays.
Proceedings of a Specialty Conference on In Si?u Measurement of Soil
Properties, Vol I., North Carolina State University, Raleigh, NC., June
1-4, 1975, PP 450-476.
Telling, R. M., B. K, Menzies, and H. E. Simmons. Cut-off Efficiency
Performance and Design. Ground Eng. 1978. pp. 30-43.
US EPA. NEIC Manual for Groundwater/Subsurface Investigations at Hazardous
Waste Sites. EPA-330/9-81-002.
11-6
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US EPA. Environmental Protection Agency. Office of Research and Development.
Handbook for Remedial Action at Waste Disposal Sites. EPA-625/6-82-006.
U.S. Environmental Protection Agency, Cincinnati, OH. 1982. 497 pp.
US EPA. Costs of Remedial Actions at Uncontrolled Hazardous Waste Sites, EPA
Contract No. 68-03-3028, MERL, SHWRD, Cincinnati, OH. 1983.
U.S. Army Corps of Engineers. Excerpt from Aliceville and Columbus Locks
(Alabama). Bid Package, Mobile District. 9. Quality Control. February
1975.
U.S. Army Corps of Engineers. Excerpt from Lake Chicot P.S. (Mississippi).
Bid Package, Vicksburg District, Section 2 Slurry Trench. June 1976.
U.S. Army Corps of Engineers. Foundation Report. Design, Construction, and
Performance of the Impervious Cutoff at W. G. Huxtable Pumping Plant,
Marianna, Arkansas. Volume I. April 1978.
Veder, C. Excavation of Trenches in the Presence of Bentonite Suspensions for
the Construction of Impermeable and Load-bearing Diaphragms. In: Grouts
and Drilling Muds in Engineering Practice. Butterworth1s, London,
England. 1963. pp. 181-88.
Villaume, J. Philadelphia Power and Light Co. Personal Communication with
C. A. Furman of JRB Associates. August 4, 1982.
Weber, W. J. Physiochemical Processes for Water Quality Control.
Wiley-Interscience, New York. 1972. 640 pp.
Wetzel, R. Memorandum. Trip to Slurry Trench Installation (Soil-Bentonite).
JRB Associates, McLean, Virginia. August 4, 1982.
Wilson, S. D., and R. Squier. Earth and Rockfill Dams. In: State of the Art
Volume. Seventh International Conference on Soil Mechanics and
Foundation Engineering, Mexico. 1969. pp. 137-223.
Winter, C. D. Slurry Trench Construction. Mil. Eng. (446):437-440, 1976.
Xanthakos, P. P. Slurry Walls. McGraw-Hill Book Company, New York. 1979.
621 pp.
Zoratto, E. M. Engineered Construction International, Inc. Personal
Communication with C. E. Spooner of JRB Associates. September 1982.
11-7
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