United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Center for Environmental
Research Information
Cincinnati OH 45268
Technology Transfer
EPA/625/6-88/018
v>EPA
Guide to Technical
Resources for the
Design of Land
Disposal Facilities
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EPA/625-6-88/018
December 1988
Guide to Technical Resources
for the
Design of Land Disposal Facilities
Risk Reduction Engineering Laboratory
and
Center for Environmental Research Information
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
Printed on Recycled Paper
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Notice
This document has been reviewed in accordance with the U.S. Environmental
Protection Agency's peer and administrative review policies and approved for
presentation and publication. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
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Acknowledgments
This document was developed by EPA's Office of Research and Development under
the direction of Clarence A. demons, Center for Environmental Research Information
and Robert E. Landreth, Risk Reduction Engineering Laboratory. The document was
prepared by A.T. Kearney, Inc., under contract no. 68-01-7374. Special thanks are
given to Dr. Robert M. Koerner of Drexel University and Dr. David Daniel of the
University of Texas, and Dr. Gregory N. Richardson of S&ME/Westinghouse for their
input in reviewing and commenting on the document.
in
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Table of Contents
Acknowledgments iii
List of Exhibits . . x
1.0 Introduction 1
1.1 Purpose 1
1.2 Scope 1
1.3 Use 2
1.4 Update 2
2.0 Foundations 3
2.1 Regulations and Performance Standards . 3
2.2 Site Investigation 3
2.2.1 Foundation Description 4
2.2.2 Subsurface Exploration Programs 4
2.2.3 Laboratory Testing Data 5
2.2.4 Seismic Conditions 6
2.3 Design 6
2.3.1 Waste and Structure 6
2.3.2 Settlement and Compression 6
2.3.3 Seepage and Hydrostatic Pressures 7
2.3.4 Bearing Capacity 7
2.4 Excavations . 8
2.5 Quality Assurance 8
2.5.1 General Quality Assurance Procedures 8
2.5.2 Materials 8
2.5.3 Subgrade Requirements 9
2.5.4 Compaction Requirements 9
2.5.5 Concrete Requirements 9
2.5.6 Placement 9
2.5.7 Compaction Equipment 9
2.5.8 Field Density Testing . . .' . . .- 9
2.6 References 10
3.0 Dike Integrity and Slope Stability 11
3.1 The Regulations and Performance Standards 11
3.2 Design and Materials Selection 11
3.2.1 Subsurface Exploration Program 12
3.2.2 Design 14
3.2.3 Stability Analyses 15
3.2.3.1 Rotational Slope Stability Analysis 15
3.2.3.2 Translation^ Slope Stability Analysis 15
3.2.3.3 Settlement Analysis 15
3.2.3.4 Liquefaction Analysis 16
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Table of Contents (continued)
3.2.3.5 Geotechnical Analysis for Review of Dike Stability (GARDS)
16
3.3 Materials/Specifications ' '17
3.3.1 Subgrade Requirements 17
3.3.2 Borrow Materials 17
3.3.2.1 Selection 17
3.3.2.2 Test Fill 17
3.4 Embankment Construction 17
3.4.1 Compacted Fill Construction 17
3.4.2 Drainage Systems Installation 18
3.4.3 Erosion Control Measures 18
3.5 Quality Assurance/Quality Control (QA/QC) 18
3.5.1 Compaction , 18
3.5.2 Backfill Material Inspection 19
3.6 References i • . • 19
4.0 Liner Systems 21
4.1 The Regulations and Performance Standards 21
4.1.1 Low-Permeability Soil Liners 22
4.1.2 Flexible Membrane Liners 23
4.1.3 Leachate Collection and Removal Systems 23
4.2 Design and Materials Selection 23
4.2.1 Low-Permeability Soil Liners 23
4.2.1.1 Site and Material Selection 24
4.2.1.2 Thickness 25
4.2.1.3 Hydraulic Conductivity 25
4.2.1.4 Strength and Bearing Capacity 26
4.2.1.5 Slope Stability 26
4.2.2 Flexible Membrane Liners (FMLs) 26
4.2.2.1 Performance Requirements of the FML 26
4.2.2.2 Selection of the FML 26
4.2.2.2.1 Polymers used in FMLs 27
4.2.2.2.2 Seaming of FML Sheeting 27
4.2.2.2.3 Properties and Characteristics of FMLs 27
4.2.2.2.4 Permeability 28
4.2.2.2.5 Mechanical Compatibility 28
4.2.2.2.6 Durability : 28
4.2.2.2.7 Chemical Compatibility ' . . 28
4.2.2.2.7.1 EPA Method 9090 - Compatibility Test
for Wastes and FMLs 29
4.2.2.2.7.2 FLEX - Flexible Membrane Liner
Advisory Expert System 30
4.2.2.2.8 Fingerprinting of FMLs 30
4.2.2.2.9 Effects of Exposure on FMLs 30
4.2.2.3 Effect of FML Selection on Design 30
4.2.2.4 FML Layout . 31
4.2.2.5 Appurtenances and Penetrations 31
4.2.3 Leachate Collection and Removal Systems (LCRS) 31
4.2.3.1 Layout of System Components 32
4.2.3.2 Sidewalls 33
4.2.3.3 Grading and Drainage 33
4.2.3.3.1 Granular Drainage Layers 33
vi
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Table of Contents (continued)
4.2.3.3.2 Geosynthetic Drainage Layers-Geonets . . . 33
4.2.3.3.3 Piping 33
4.2.3.3.4 The HELP Model 34
4.2.3.4 System Compatibility 34
4.2.3.5 System Strength 34
4.2.3.5.1 Stability of Drainage Layers 34
4.2.3.5.2 Pipe Structural Strength 34
4.2.3.6 Prevention of Clogging 35
4.3 Materials/Specifications •. ^ 36
4.3.1 Low-Permeability Soil Liners 36
4.3.1.1 Sources 36
4.3.1.2 Soil Properties 36
4.3.1.3 Test Methods . 36
4.3.1.4 Hydraulic Conductivity 36
4.3.1.5 Liner-Leachate Compatibility 37
4.3.1.6 Mechanisms of Soil Liner Failure 37
4,3.2 Flexible Membrane Liners 37
4.3.3 Leachate Collection and Removal Systems (LCRS) 38
4.3.3.1 Granular Drainage Layer Materials 38
4.3.3.2 Geonets 38
4.3.3.3 Granular Filter Layers < 38
4.3.3.4 Geotextiles 38
4.3.3.5 Piping 38
4.3.3.6 Sumps and Pumps 38
4.4 Construction/Installation 39
4.4.1 Low-Permeability Soil Liners 39
4.4.1.1 Soil Liner Test Fill 39
4.4.1.2 Compaction ; . . 39
4.4.1.3 Placement on Side Slopes 40
4.4.1.4 Final Preparation for FML Placement 40
4.4.2 Flexible Membrane Liners 40
4.4.2.1 Materials and Construction Specification Document for FMLs .... 40
, 4.4.2.2 Construction Procedures 41
4.4.3 Leachate Collection and Removal Systems 42
4.4.3.1 Sump Construction and Pump Installation 42
4.4.3.2 Piping Installation 42
4.4.3.3 Placement of Granular Materials 42
4.4.3.4 Placement of Geonets 42
4.4.3.5 Granular Filter Layer Placement 42
4.4.3.6 Geotextile-Placement 43
4.5 Quality Assurance 43
4.5.1 Low-Permeability Soil Liners 43
4.5.2 Flexible Membrane Liners . 43
4.5.3 Leachate Collection and Removal Systems 44
4.5.3.1 Inspections 44
4.5.3.2 Testing 44
4.6 References 45
5.0 Cover Systems , 47
5.1 Regulations and Performance Standards . . 47
VII
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Table of Contents (continued)
5.2 Design . . : 47
5.2.1 Site Characterization 48
5.2.1.1 Topography 48
5.2.1.2 Precipitation 48
5.2.1.3 Other Climatological Data . 48
5.2.1.4 Soils 48
5.2.1.5 HELP Model 48
5.2.2 Waste Characterization 49
5.2.3 Settlement/Subsidence 49
5.2.4 Slope/Stability , 50
5.2.5 Erosion Potential 50
5.2.6 Cover Systems Elements^ 51
5.2.6.1 Foundation, Backfill '. 51
5.2.6.2 Low-Permeable Soil Layer . . 51
5.2.6.3 Geomembrane (Synthetic Membrane) 51
5.2.6.4 Drainage Layer 51
5.2.6.5 Filters 52
5.2.6.6 Vegetated Topsoil 52
5.2.6.7 Boundaries . 52
5.3 Materials 53
5.4 Construction 53
5.4.1 Excavation ' 54
5.4.2 Soil Material Preparation 54
5.4.3 Soil Placement 54
5.4.4 Soil Compaction 54
5.4.5 Geomembrane Installation 54
5.5 Maintenance 54
5.6 Quality Control 54
5.7 References 55
6.0 Run-On/Run-Off Controls 57
6.1 The Regulations and Performance Standards 57
6.2 Design and Operation 57
6.2.1 Design Overview , 53
6.2.2 Design Approach 59
6.2.2.1 Identify Design Storm 59
6.2.2.2 Determine Peak Discharge Rate/Calculating Run-Off:
SCS Method . 59
6.2.2.3 Rational Method 59
6.2.3 Control System Structures 59
6.2.3.1 Dikes/Berms 60
6.2.3.2 Swales, Channels and Waterways 60
6.2.3.3 Terraces 60
6.2.3.4 Chutes and Downpipes 60
6.2.3.5 Seepage Basins and Ditches 61
6.2.3.6 Sedimentation Basins 61
6.3 Materials 61
6.4 Construction/Equipment .' 63
6.5 Quality Assurance (QA)/Quality Control (QC) 63
6.5.1 Materials 63
6.5.2 Erosion Control Systems 63
viii
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Table of Contents (continued)
6.5.3 Operations/Maintenance QA 63
6.6 References 63
IX
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List of Exhibits
Page
2-1 Types of Information Used to Demonstrate that the
Performance Standard for Foundations is Met 4
3-1 Information Typically Submitted to Demonstrate Satisfaction
of Performance Standards for Dike Integrity 12
3-2 Conceptual Slope Failure Modes 13
3-3 Recommended Minimum Values of Fafctor of Safety for Slope Stability Analyses 14
3-4 Minimum Data Requirements for Stability Analysis Options 16
4-1 Schematic of a Single Clay Liner System for a Landfill 22
4-2 Schematic of a Double Liner and Leak Detection System for a Landfill 22
4-3 Types of Information Used to Demonstrate that the
Performance Standard for Soil Liner is Met 24
4-4 Methods for Testing Low-Permeability Soil Liners 25
4-5 Types of Information Used to Demonstrate that the
Performance Standards for Flexible Membrane Liners are Met 26
4-6 Polymers Currently Used in FMLs for Waste Management Facilities 28
4-7 Data Requirements of FLEX 31
4-8 Information Typically Submitted to Demonstrate Satisfaction of LCRS Requirements . . . 32
4-9 Schematic of a Test Fill 40
4-10 Non-Destructive Test Methods from FML Seams 44
5-1 Landfill Cover System Components 47
5-2 Thickened Cover for Tolerance of Settlement 50
5-3 Typical Elements of Maintenance Program 55
6-1 Information Commonly Used in Demonstrating that
Performance Standards for Run-Oh/Run-Off Controls are Met 58
6-2 Hydrograph 59
6-3 Surface Water Diversion and Collection Structures 60
6-4 Typical Temporary Diversion Dike 60
6-5 Typical Channel Design .. 61
6-6 Typical Terrace Design ...' '.'..'.'.'. 61
6-7 Typical Paved Chute Design . . . . 62
6-8 Typical Seepage Basin Design \\\ 62
6-9 Typical Sedimentation Basin Design '..'.'.'.'.'.'.'.'. 62
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CHAPTER 1.0
Introduction
Subtitle C of the Resource Conservation and
Recovery Act (RCRA) of 1976, as amended by the
Hazardous and Solid Waste Amendments (HSWA) of
1984, establishes requirements for landfills and
surface impoundments to ensure that land disposal of
hazardous waste in such units is conducted in a
manner protective, of human health and the
environment. Performance standards and minimum
technology requirements have been promulgated by
EPA at Title 40 Code of Federal Regulations (CFR)
Part 264 for new and existing units. These standards
and requirements are implemented through permits
issued by authorized states or EPA in accordance
with the regulations of .40 CFR Part 122 through 124.
Specific information requirements for RCRA permit
applications have been.promulgated at 40 CFR Part
270.
1.1 Purpose
EPA has issued numerous technical documents
intended to assist preparers and reviewers of permit
applications for hazardous waste land disposal
facilities, including RCRA Technical Guidance
Documents, Permit Guidance Manuals, and Technical
Resource Documents. Some of these documents
provide extensive detail on a select number of
specific technical subjects related to the design and
operation of land disposal facilities, while others
provide broad guidance on RCRA permitting issues.
The objective of these documents is to facilitate the
expeditious preparation and processing of RCRA
permit applications and to achieve consistency in
permitting decisions.
EPA is concerned, however, that permit applicants
and reviewers may not be familiar with all of the
technical documents which the Agency has issued
and may not be taking advantage of the information
which they offer. EPA recognizes a need to provide a
concise directory of information resources which are
available and to suggest how these resources may be
effectively used in the RCRA permitting process.
The purpose of this Guide is to direct permit
applicants and permit application reviewers to the
EPA documents which may be helpful in answering
specific technical questions which often arise during
permit application preparation and processing. Non-
EPA technical literature has also been included in this
Guide as appropriate. It should be noted that the list
of non-EPA documents for any one subject may not
be all inclusive. Other literature, i.e., books, may be
as appropriate. The Guide does not provide detailed
guidance on each regulatory standard for RCRA
permit applicants but rather provides an overview of
technical considerations.
1.2 Scope
Since this is a Guide to other information sources, it
contains very limited primary information itself. To
maximize its usefulness as a Guide, emphasis has
been placed on brevity and conciseness. While the
Guide generally does not discuss in detail the
information provided in the primary sources, it directs
the reader to the locations within these sources
where specific technical subjects are addressed. (The
references, as shown in the text, generally refer to
the paragraph in which they appear. Where
references appear within a paragraph, they refer to
the text that immediately precedes them.)
The topics included in this Guide are limited to key
performance standards and minimum technology
requirements specified in 40 CFR Part 264 for
hazardous waste landfills and surface impoundments.
Each topic is addressed in an individual chapter as
follows:
• Foundations (Chapter 2.0)
• Dike Integrity and Slope Stability (Chapter 3.0)
• Liner Systems (Chapter 4.0)
• Cover Systems (Chapter 5.0)
• Run-on/Run-off Controls (Chapter 6.0)
While the subjects addressed in this Guide are those
which frequently arise in preparing and reviewing
permit applications, the information and references
provided in each chapter may also be useful in
designing and operating other types of land disposal
units (i.e., waste piles and land treatment units) and
land disposal facilities for non-hazardous wastes.
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1.3 Use
It is important to note that environmental performance
standards in the regulations are qualitative objectives
designed to protect human health and the
environment and to guide the evaluation of permit
applications. There may be a variety of technical
approaches to designing, constructing and operating
landfills and surface impoundments to accomplish
these objectives.
Therefore, permit applications must provide a
demonstration that the design, construction and
operation of the specific land disposal units covered
will meet these objectives. Such demonstrations'must
be made on a facility-specific basis. The general
process for these demonstrations is summarized as
follows:
1. Identify and justify the specific technical
parameters which are important to attainment of
the performance standard;
2. Identify and justify the methodologies used in
determining whether such technical parameters
are within an acceptable range of values. This
could include, for example, test procedures,
mathematical calculations, and/or references to
commonly accepted engineering standards or
EPA guidance; and
3. Demonstrate that each technical parameter falls
within an acceptable range using the
methodologies selected.
This Guide is organized in accordance with this
process. The first section in each chapter provides a
brief summary of the existing regulations (as of May
31, 1988) under 40 CFR Part 264 (the performance
standards and minimum technology requirements)
and 40 CFR Part 270 (RCRA permit application
informational requirements) which correspond to the
technical area covered in the chapter. The first
section of each chapter also presents major technical
parameters which are commonly considered in
evaluating permit applications with respect to each
performance standard. In subsequent sections of
each chapter, the reader is referred to those technical
documents which can be helpful in selecting
evaluation methodologies and in determining
acceptable ranges for these parameters. Each of the
chapters are meant to be free-standing documents,
hence, there may be some duplication of information
from chapter to chapter. This duplication is necessary
in order for the individual chapters to flow properly.
1.4 Update
This document is intended to be practical and
informative. It is requested that Guide users submit
ideas and/or suggestions to EPA, at the following
address, regarding ways this document can be
improved, including additional information sources
they have found to be helpful in the preparation and
review of permit applications:
Risk Reduction Engineering Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
Requirements for liners and leachate collection and
removal systems are currently being revised. On
March 28, 1986, EPA proposed a rule implementing
minimum technology requirements of the Hazardous
and Solid Waste Amendments of 1984 (HSWA) for
double liner systems and leachate collection systems.
Following proposal of these regulations, EPA
collected data characterizing and comparing the
performance of compacted soil bottom liners and
composite (soil/flexible membrane liner) bottom liners.
The data indicated that the use of a flexible
membrane liner improved the performance of a
composite bottom liner over that of a compacted soil
liner, with respect to leachate collection efficiency,
leak detection capability, and leakage both into and
out of the bottom liner.
On April 17, 1987, EPA made available the
background document presenting the data on bottom
liner performance and draft minimum technology
guidance documents on single- and double-liner
systems. EPA proposed stronger regulations for
double liners and leak detection systems on May 29,
1987.
When these regulations are promulgated, it may be
necessary to update this Guidis accordingly. Chapter
4.0, which addresses low-permeability soil liners,
flexible membrane liners, and leachate collection/leak
detection systems is most likely to be affected by
these regulatory changes.
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Chapter 2.0
Foundations
Proper subsoil foundation design of a land disposal
system is critical because liner system components,
especially leachate collection pipes and sumps, can.
be easily damaged by stresses caused by foundation
movement. Permit applications must include a
comprehensive evaluation of subsoil foundation
conditions, followed by a demonstration that the
foundation design will minimize the effects of
foundation movements on the rest of a unit's
components.
2.1 Regulations and Performance
Standards
The regulations governing foundations provide a
performance standard rather than a design standard.
The performance standards in 40 CFR'264.221 (a)(2)
for surface impoundments and 40 CFR
264.301 (a)(1)(ii) for landfills state that foundations
must be "capable of providing support to the liner and
resistance to pressure gradients above and below the
liner to prevent failure of the liner due to settlement,
compression or uplift."
Foundations for hazardous waste land disposal
facilities should, provide structurally stable subgrades
for the overlying facility components. The foundations
also should provide satisfactory contact with the
overlying liner or other system components. In
addition, the foundation should resist settlement,
compression and uplift resulting from internal or
external pressures, thereby preventing distortion or
rupture of overlying facility components (Reference 1,
p. 12)
In addition, likely seismic activities at the location
must be confirmed. The jurisdictions where the
seismic location standard is applicable are designated
at 40 CFR 264.18(a). However, regardless of whether
the facility is located in one of these jurisdictions, it is
advisable to design foundations, capable of
withstanding maximum likely earthquake events.
The foundation analysis presented in a permit
application should assess the potential for, and
present calculated estimates of, settlement,
compression, consolidation, shear failure, uplifts,
liquefaction of the foundation soil, and the potential
for hydraulic and gas pressures on the foundation.
Typically, the analysis should provide geologic data,
geotechnical data, hydrogeologic data, and seismic
setting information. The following sections will
describe the type of information and analyses needed
to evaluate the foundation. The references that are
cited describe how to evaluate the analyses.
Exhibit 2-1 summarizes the types of information and
technical parameters commonly included in RCRA
permit applications for landfills and surface
impoundments to demonstrate that the foundation
performance standard is met.
The steps normally taken to prepare such a
demonstration are as follows:
• Preparation of a final design of the units, including
design drawings showing their location on the
site, their depth, configuration and dimensions,
and their position relative to existing and final
grade;
Performance
investigation;
of a location-specific site
• Laboratory analyses of soil samples obtained
during the site investigation;
• Analysis, as appropriate, of settlement potential,
bearing capacity, hydrostatic or gas uplift
pressures, liquefaction potential, and subsidence
and sinkhole potential; and
• A Construction Quality Assurance Plan that
identifies the level of inspection and testing
necessary to construct the foundation to the
specifications used in the design.
The following sections discuss these steps in detail,
with specific instructions on how to evaluate the
information provided in a permit application:
2.2 Site Investigation
Adequate site investigations are necessary to ensure
that the foundation design is developed to
accommodate expected site conditions. Site
investigations are designed to establish the in-situ
subsurface properties, site hydrogeologic
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Exhibit 2-1. Types of Information Used to Demonstrate
That the Performance Standard for
Foundations Is Met
Information Typical Parameters
Exhibit 2-1. Continued
Information
Typical Parameters
Description of Foundations
Subsurface Exploration
Data
Subsurface Exploration
Data
Laboratory Testing Data
Engineering Analyses
Analysis of Settlement
Potential
Analysis of Bearing
Capacity
Description of:
• General foundation design
• Foundation materials
• Include geological and
construction drawings
indicating bearing elevations
Detailed engineering
characteristics of:
• Subsurface soil
• Bedrock
• Hydrogeologic conditions
Engineering characteristics of
foundation materials verified
through procedures including:
Historical data
Test borings
Test pits or trenches
In situ tests
Geophysical exploration
methods
Test results for
• Index testing
• Hydraulic conductivity
• Shear strength
• Compressibility
Engineering analyses using data
obtained through subsurface
explorations and laboratory testing
including, as appropriate:
• Settlement potential
• Bearing capacity
• Stability of cut or constructed
slopes
• Potential for excess
hydrostatic or gas pressure
• Seismic conditions
• Subsidence potential
• Sinkhole potential
• Liquefaction potential
Estimates of total and differential
settlement, including:
• Elastic settlement
• Primary consolidation
• Secondary compression
Analysis of allowable bearing
capacity and comparison of
required bearing capacity based
on actual loading
(continued)
Analysis of Stability of
Landfill Slopes
Analysis of Potential for
Hydrostatic Pressures
Analyses of static and dynamic
cases for:
• Excavated slopes
• Embankment slopes
• Slopes including liners and/or
cover
• Drained and/or undrained
conditions
Estimates of potential for bottom
heave or blow-out due to
unequal hydrostatic or gas
pressures.
characteristics and the area seismic potential, all of
which are critical to facility design (Reference 1,
p.13).
2.2.1 Foundation Description
Foundation design procedures are site specific and
very often are an iterative procedure. A typical
preliminary foundation description should include:
• the geographic setting;
• the geologic setting;
• ground-water conditions;
• soil and rock properties;
• surface-water drainage conditions;
• seismic conditions; and
• basis of information.
Site plans should include the unit locations within the
site; the unit depths, configurations, and dimensions;
and whether the unit will be completed below or
above grade. It is particularly important that the
investigation borings, test pits, and other procedures
be performed as near as possible to the units, if not
within their boundaries. Some other critical elements
of the foundation design that need to be addressed
prior to completion of the site investigation are the
foundation design alternatives, the foundation grade,
the loads exerted by the unit or the foundation, and
the preliminary settlement tolerances.
2.2.2 Subsurface Exploration Programs
Subsurface exploration programs are conducted to
determine a site's in-situ subsurface properties, as
well as its geology and hydrogeology. The in-situ
subsurface properties and hydrogeologic
characteristics have a significant influence on the
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bearing capacity settlement potential, slope, stability,
and uplift potential for the site. The site's subsurface
geology may impact the settlement and seismic
potential at the site and exert an influence on the
site's hydrogeologic characteristics.
Reference 2, p. 5-3 and Reference 13 provide a
strong guidance for planning subsurface exploration
programs. The following list provides details of the
elements of a subsurface exploration program:
• Relate the site geology to the regional geological
setting;
• Provide analysis of the engineering properties of
representative subsurface samples;
• Establish in-place subsurface characteristics that
include depth to bedrock, and the presence of
features that can act as failure planes or
hydrogeologic pathways;
• Identify bedrock characteristics such as lithology,
orientation, extent of weathering, fractures, joints
and solution cavities; and
• Establish the hydrogeologic site characteristics
such as depth of the water table; horizontal and
vertical flow components; hydrogeologic
pathways; seasonal variability; and the location,
use, and type of aquifers present.
Subsurface exploration programs can utilize both
indirect and direct methods. Indirect investigation
methods include geophysical techniques (e.g.,
electrical survey methods, ground-penetrating radar
and seismic refraction). These methods do not
require drilling or excavation. The selection of the
proper geophysical techniques is dependent on the
geologic settings. (Reference 2, pp. 5-4 and 5-5).
While geophysical procedures can provide large
amounts of data at a lower cost, they require careful
interpretation which must be done by qualified experts
only. Furthermore, geophysical data must be verified
by direct procedures such as borings or test pits.
Direct investigation methods include drilling boreholes
and wells and. excavating pits and trenches. Direct
methods allow the site geologic conditions to be
observed and measured. Typically, boring logs should
provide descriptions of the soil strata and rock
formations encountered, as well as the depth at which
they occur.' In addition, the boring logs should provide
standard penetration test results for soils and rock
quality designation results for rock core runs. The
boring logs should also record the intervals for, and
the results of, any field hydraulic conductivity testing
conducted in the borings.
Direct methods allow the investigator to obtain
samples of subsurface material for laboratory testing
of engineering properties. Soil samples can be
obtained either by split spoon or thin-walled tube:
Split spoon samples are disturbed and are of limited
value other than for identification and water content.
The thin-walled tube sample provides an
undisturbed sample that can be used for a wide
variety of laboratory tests; however, its use is limited
to certain soil types and conditions.
The scope of the subsurface exploration program will
vary depending upon the complexity of the
subsurface geology, seasonal variability in site
conditions and the amount of site information
available. Typically, the investigator should drill an
adequate number of borings across the site to
characterize the underlying deposits and bedrock
conditions and to establish a reasonably accurate
subsurface cross-section. Depth of borings is highly
dependent on site-specific conditions. However,
typically, the borings should extend below the
anticipated site base grade, or below the water table,
whichever ,is deeper. A sufficient number of water
table observation wells and piezometers should be
installed to define both the horizontal and vertical
ground-water flow directions. When subsurface
heterogeneities are encountered that could lead to
seepage or loss in strength in the foundation,
additional subsurface exploration is sometimes
necessary to identify and determine the extent of
these features (Reference 2, p. 5-6). Typically the
hydrogeoiogic conditions are identified as part of the
ground-water monitoring program.
2.2.3 Laboratory Testing Data
Laboratory testing for foundations may include the
following (Reference 2, Chapter 3):
• Atterberg Limits,
• Grain size distribution,
• Shrink/swell potential,
• Cation exchange capacity,
• Mineralogy,
• Shear strength,
• Dispersity,
• Compressibility,
• Consolidation properties,
• Density and water content, and
« Hydraulic conductivity tests.
Soil index properties are simplified tests that provide
indirect information about the engineering properties
of soils beyond what can be gained from visual
observations. Although the correlation between index
properties and engineering properties is not perfect, it
is generally adequate for QC purposes (Reference 2).
Index property tests commonly used to screen soils
are described below.
-------
Atterberg Limits include tests to establish the Ijquid
limits and the plastic limit of a soil (Reference 8).
These tests are commonly used along with grain-
size distribution, for monitoring changes in soil type. A
significant change in Atterberg limits usually reflects a
change in important engineering properties, such as
the relationship among water content, density,
compactive effort, and hydraulic conductivity.
Grain-size analysis is another important screening
test for changes in soil composition. The percentage
of silt and clay-size particles and the overall particle
size distribution of a soil affects its engineering
properties, especially hydraulic conductivity and
strength. Rough estimates of grain size may be
obtained through manual estimates (Reference 3;
ASTM D 2488) and may be sufficient for screening. A
200-mesh sieve may be used to separate coarse
(sand and gravel) and fine (soil and clay) particles.
More detailed grain-size distributions may be
obtained by sieving the coarse fraction and by using
several settling methods (hydrometer, decantation, or
pipette) for the fine fraction (Reference 3; ASTM D
422). Again, it is important to monitor carefully for
soil-type changes as backfill material is being placed
(Reference 2).
Soil index properties, Atterberg limits and grain-size
distribution, are simplified tests that provide indirect
information about the engineering properties of soils
beyond what can be gained from visual observations.
Although the correlation between index properties and
engineering properties is not perfect, it is generally
adequate for construction QC purposes (Reference
1). These two index property tests commonly used to
screen soils are described below.
2.2.4 Seismic Conditions
Seismic analysis is particularly critical when there is a
high potential for liquefaction to occur, such as in
seismically-active areas underlain by loose,
saturated sands and silts. Many regions in the country
that have experienced earthquake activity should
have information on the frequency and magnitude of
earthquakes. There may also be established Jocal
standards for the design of structures. Generally,
earth structures can be designed to withstand! the
vertical and horizontal accelerations experienced
during such design earthquakes. A more detailed
discussion of methods for evaluating site seismic
parameters is presented in Chapter 3.0 of this Guide.
2.3 Design
Foundations are designed to provide structural
support and to control settlement. Foundations must
also be designed to withstand hydrostatic and gas
pressures.
2.3.1 Waste and Structure
The engineering analysis for foundations is based on
subsurface conditions; however, the results of these
analyses are based on loading conditions. In order to
perform the appropriate engineering analysis to
demonstrate the adequacy of the foundation, the
permit application should provide an accurate
estimate of the loadings (including both structure and
waste), plans showing the structure's shape and size,
the expected waste characteristics and volumes, and
the foundation elevations.
2.3.2 Settlement and Compression
The performance standards require that the
foundation be capable of preventing failure of the liner
system due to settlement and compression.
Therefore, it is important that the permit application
provides an analysis estimating total and differential
settlement/compression expected due to the
maximum design loadings. The results of this analysis
are then used to evaluate the ability of the liner
system and leachate collection and recovery systems
to maintain their integrity under the expected
stresses.
A settlement analysis will provide an estimate of
maximum settlement. This maximum settlement can
be used to aid in estimating the differential settlement
and distortion of a land disposal unit. Allowable
settlement is typically expressed as a function of total
settlement, rather than differential settlement,
because the latter is much more difficult to predict.
However, the differential settlement is a more serious
threat to the integrity of the structure than total
settlement (Reference 4 and Reference 10).
i
Total settlements of a few inches or less are usually
not a problem for soil liner foundations, since most
are sufficiently thick and flexible to withstand some
differential settlement of the foundation. As long as
the topography is fairly uniform and significant
subsurface heterogeneities are not present,
differential settlement should be minimal. Foundation
settlement analyses based on the site's subsurface
conditions (determined during site investigation)
should be conducted during the design of the facility.
These analyses should take into account the loadings
of all facility components on the foundations, including
footings for pile-type structures such as leachate
collection risers, which, if improperly designed, .can
be forced into or through the liner. Compensated
foundation, which implies thaf: the weight of soil
extracted from the site balances the weight of fill
material, also can be used as part of the design to
minimize subgrade settlement. In addition, the
expected differential settlement should be compared
to the design slope of the leachate collection system
-------
to ensure the letter's adequacy is maintained
(Reference 2, p. 5-14).
Landfill design calculations should include estimates
of the expected settlement, even if it is expected to
be small. Small amounts of settlement, even a few
inches, can cause serious damage to leachate
collection piping or sumps. The ability to predict the
extent of settlement depends upon the type of
process anticipated to cause settlement. There are
several settlement processes, each of which should
be considered in a land-based unit design. These
are discussed in the following paragraphs and
include:
• Primary consolidation,
• Secondary compression, and
• Elastic Compression.
Primary consolidation, which is typically a reduction in
void ratio due to removal of pore fluids by mechanical
loading, generally occurs in saturated fine-grained
soils according to the consolidation theory developed
for soil. Basically, the theory states that the rate and
amount of compression is equal to the rate and
amount of pore fluids squeezed out of the soil
(Reference 5). The classic Terzaghi theory for one-
dimensional consolidation of a soil is discussed in
Reference 6, at page 17.
Primary consolidation of soils by lowering of the water
table has been identified as an additional cause of
ground subsidence in some locations. The effect of
lowering the water table in a soil is to surcharge the
soil particles by increasing the effective stress (the
vertical stress minus the pore water pressure)
through a decrease in pore pressure (Reference 6, p.
17).
Secondary compression is the gradual settlement
from creep under essentially constant load. It
depends upon the applied load and the chemical and
physical nature of the soil particles. Secondary
compression is more irregular and less predictable
than primary consolidation and may be significant in
settlement of plastic clay soils, heterogeneous fill
materials, organic materials, and other compressible
materials. Although secondary compression occurs at
the same time as primary consolidation, secondary
compression is usually taken into account at later
times in the loading history of the fill when primary
consolidation is complete and the applied stress is
transferred from the pore fluids to the soil skeleton
(Reference 2, p. 5-15 and Reference 11).
Elastic compression occurs when the volume of
solids is reduced. The effect of elastic compression
of mineral soils is minimal; however, it may be a
major concern with organic soils, soluble materials,
and materials subject to chemical attack. This type of
compression is highly irregular and is influenced by a
number of environmental factors that make it difficult,
if not impossible, to predict and are, therefore, not
typically considered in geotechnical practice
(Reference 2, p. 5-15).
Both theoretical and empirical approaches are utilized
for predicting settlement. The theoretical methods are
based on elastic theory and summation of strains.
Empirical or semi-empirical methods include
performing load tests and penetration tests in the
field. Theoretical methods should be used only in
conjunction with empirical methods that provide field
verification (Reference 4, Chapters 8 and 14, and
Reference 6, Chapter 9:3).
The elastic theory applies to soil only in a very
approximate way. Soil itself is not elastic; however,
elastic theory provides a convenient means to
estimate stresses induced within a soil mass by
applied loads. Knowing these stresses allows the
engineer to compute the strains, and by adding up
the strains along any vertical line, the settlement of
the surface can be computed (Reference 4, Chapters
8 and 14).
2.3.3 Seepage and Hydrostatic Pressures
Foundations should be designed to control seepage
and hydrostatic pressures. Heterogeneities such as
large cracks, sand lenses, or sand seams in the
foundation soil offer pathways for leachate migration
in the event of a release through the liner and could
cause piping failures. In addition, soft spots in the
foundation soils due to seepage can cause differential
settlement possibly causing cracks in the liner above
and damaging any leachate collection or detection
system installed. Cracks can also be caused by
hydrostatic pressure where the latter exceeds the
confining pressure of the foundation and liner
(Reference 2, p. 5-15).
Solutions to these problems include various systems
that are available to lower the hydraulic head at the
facility. These systems include pumping wells, slurry
walls and trenching. Other methods to control
foundation seepage include grouting cracks and
fissures in the foundation soil with bentonite and-
designing compacted clay cut-off seals to be
emplaced in areas of the foundation where lenses or
seams of permeable soil occur (Reference 2, p. 5-
16).
2.3.4 Bearing Capacity
For waste disposal units, the major issue of concern
for foundations is differential settlement. However, for
structures such as tank foundations, leachate risers,
etc., an additional area of concern is bearing capacity
failure (Reference 6, Chapters 9:2 and 9:3).
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The basic criterion for foundation design is that
settlement must not exceed some permissible value.
This value varies, dependent on the structure and the
tolerance for movement without disruption of the
unit's integrity. To ensure that the basic criterion is
met, the bearing capacity must be established for the
foundation soil. The bearing capacity of a soil, often
termed its stability, is the ability of the soil to carry a
load without failure within the soil mass. The load
carrying capacity of soil varies not only with its
strength, but often with the magnitude and distribution
of the load. Reference 7, Chapter 10, provides
information regarding the evaluation of bearing
capacities and typical ranges of key parameters. After
the bearing capacity is determined, the settlement
under the expected load conditions should be
estimated and compared to the permissible value.
The foundation design should be such that the actual
bearing stress is less than the bearing capacity by an
appropriate factor of safety (Reference 4, Chapter 14;
Reference 6, Chapter 9:2 and Reference 12).
Several types of structural foundation failures can
occur that are highly ^site specific. These failures
depend on subsurface conditions and loading type
and conditions. The various types of foundation
failures that can occur are discussed in Chapter 9 of
Reference 7, Chapter 9:2 of Reference 6 and Chapter
14 of Reference 4. In addition, for cases where the
foundation consists of soft soils, special care must be
taken to ensure that local shear failures dp not occur
due to equipment movement during placement of the
liner system or the waste.
Many large metropolitan areas have records of
allowable design soil pressures that have been
successful and also those that have failed. These are
called presumptive bearing pressures because they
are based on past performance that the soil can
support such a pressure without experiencing a
bearing capacity failure or excessive settlement
(Reference 6, Chapter 9:5).
These values are not defendable for design work and
are not to be considered a performance standard. For
example, the sand pressures usually provided are
highly dependent on footing size; the soft-to-firm
clays need an analysis based on site-specific
conditions and soil properties; and the stiff-to-hard
clay assumes no fissures. However, the presumptive
values do provide a general guidance for the reviewer
of typical values to be expected. In short, they
provide a reference point, but not a hard and fast
design criterion. (Reference 7, Chapter 10:5)
2.4 Excavations
Most hazardous waste units are constructed below
existing grade. Therefore, most sites must be
excavated to final foundation grade. If the total depth
of excavation exceeds approximately ten feet, a slope
stability analysis of the excavated slopes should be
made. Slope stability analyses are discussed in.
Chapter 3.0, and generally the same procedures
apply to cut slopes, except that lower factors of safety'
are acceptable.
2.5 Quality Assurance
Once the design of the foundation has been
completed in accordance with acceptable standards,
the foundation's construction can begin, as designed
and specified, following strict quality assurance
procedures. The primary quality assurance issues for
foundations are to assure the adequacy of the,
subgrade and, if necessary, the compacted fill
through density testing. A brief discussion is provided
below on EPA's guidance for construction quality
assurance procedures, as provided in Reference 1.
This is followed by a discussion of field density
testing that can be performed to establish adequate
subgrade and compaction (Reference 2, p. 3-26).
2.5.1 General Quality Assurance Procedures
Reference 1 provides technical guidance regarding
quality assurance procedures during the
preconstruction, construction and post-construction
procedures. In addition, Reference 9 provides a
summary of items for inspection of old or new
concrete. During the preconstruction phase, it is
especially important for all construction quality
assurance personnel and the construction contractors
to review site investigation information to familiarize
themselves with the expected site conditions upon
which the facility designs were based. This will help
ensure their ability to identify any unexpected site
conditions encountered during construction (Re-
ference 1, pp. 12-15).
2.5.2 Materials
Soil and rock underlying the facility must possess
adequate strength to support the expected loading. If
tests on samples of the materials examined in the
laboratory and on-site bearing tests show
inadequate properties, the site design and
construction plans should include specifications that
provide for preparation of an adequate foundation. If
appropriate, samples of materials from potential
borrow areas to be used to construct the foundation
should be analyzed to determine their acceptability for
the specified design. This information is used to
identify desirable materials and reject undesirable
materials. The principal concern is to verify that the
specified materials of any load-bearing foundation
are specified in enough detail to compare with the
characteristics shown to be required by the engi-
neering analysis. (Reference 2,, pp. 1-4).
-------
2.5.3 Subgrade Requirements
Visual observation of the subgrade is necessary to
assure that the foundation is constructed as
designed. The site engineer needs to ensure that all
soft, organic and otherwise undesirable materials are
removed. This can be done by proof rolling with
heavy equipment to detect soft areas. As outlined in
Reference 1, various tests are available to verify the
condition of the foundation subgrade.
In addition, the site engineer should inspect soil and
rock surfaces for rock joints, clay fractures and
depressions. These features should be adequately
filled. If sand seams are encountered, they should be
removed and refilled with compacted material
(Reference 1, p. 13).
2.5.4 Compaction Requirements
If required, selection of backfill material that can be
compacted to tne required density and permeability
involves a series of laboratory tests of the engineering
properties of the candidate materials. One such
engineering characteristic is the water content/density
relationship which is established for the material by
compacting samples of the material at various water
contents with a set compactive effort (Reference 2;
pp. 3-21 to 3-25). A standard method has been
adopted and described in ASTM standard test method
D698 (Reference 8; ASTM D698).
Based on the compaction test data, compaction
specifications should indicate the minimum percent of
maximum density and the water content relative to
the optimum water content at which the soil should
be compacted. Soils have different characteristics at
water contents above, at, or below the optimum water
content. For instance, clays compacted on the wet
side of the optimum water content are less permeable
than those compacted on the dry side. On the other
hand, clays compacted dry of optimum are stronger
and have a higher stress-strain module than do
clays compacted wet of optimum (Reference 2, pp.
3-21 to 3-25).
2.5.5 Concrete Requirements
In some cases, a hazardous waste management unit
may be placed on a concrete surface. The concrete
might be new (poured recently with the intent of being
the FML supporting surface) or old (an older structure
that is being retrofitted with an FML) (Reference 9, p.
4-17). Old concrete must be checked very carefully,
because it is more likely to have cracks, surface
chipping, and a rougher surface. Old concrete is also
more likely to chip and crack when drilling is required
to set items like FML anchor bolts. The effects of
surface irregularities may be minimized by the use of
various coating materials or by covering with a
geotextile (Reference 9, pp. 4-17).
New concrete must be allowed to age in order to
obtain the strength needed to set items like FML
anchor bolts. In addition, any wax-type curing
compound used must be removed prior to FML
placement, since sealing compounds (adhesive,
cements, and caulks) will not adhere to this type of
surface. If surface voids exist, they must be
eliminated by sacking with cement grout (Reference
9, p. 4-19).
2.5.6 Placement
During placement of soil materials, the soil is spread
uniformly as specified. The loose lift thickness of the
soil should be measured systematically over the
entire site, with a marked staff or shovel blade, and
survey levels should be made every few lifts for
verification and documentation of fill thickness.
Following spreading, the backfill material is disked or
tilled to break up large soil aggregates and to
homogenize the material (Reference 2, pp. a-6 to
a-9).
2.5.7 Compaction Equipment
The principal types of compacting equipment are the
smooth wheel roller, the rubber-tired roller, the
sheepsfoot roller, and the vibratory compactor. The
latter would be the most effective piece of equipment
for compacting coarse-grain, cohesionless soils.
However, vibratory rollers are the least effective
compactors for cohesive soils. Rubber-tired rollers
with high tire pressures and sheepsfoot rollers are
effective for cohesive soils. Sheepsfoot rollers are
particularly effective at bonding of lifts during
compaction of cohesive soils. Reference 6 provides a
detailed discussion of compactive equipment and
methods.
2.5.8 Field Density Testing
Two traditional methods are used for measuring
density in the field. In one type of test, a small hole is
dug in the compacted fill and the excavated material
is saved and weighed. The volume of the hole is
measured by filling it with sand or liquid with a device
that measures the amount of material required to fill
the hole. The sand cone and rubber balloon methods
are examples of this type of test. Another technique is
to drive a hollow cylinder into the fill, remove a core,
trim it to a known volume, and then determine its
weight. This drive-cylinder method and the sand
cone and rubber balloon methods take time because
the sample must be oven dried before the dry density
can be determined (Reference 2, p. 3-26).
Nuclear probes (Reference 8, ASTM D-2922 and
D307) offer a faster and more convenient method for
-------
measuring field density and water content than the
traditional methods and are presently widely used for
earthwork compaction quality control. Nuclear gauges
are designed to give very rapid measurements of
density and moisture content. The operation of
nuclear gauges is discussed in Reference 2, p. 3-
29. In order to compensate for the soil compositions
that may affect the neutron response, it is customary
to calibrate the nuclear density gauge against oven
dried water content measurements by the appropriate
laboratory test method (Reference 8; ASTM D2216)."
2.6 References
1. Technical Guidance Document: Construction
Quality Assurance for Hazardous Waste Land
Disposal Facilities; Hazardous Waste
Engineering Research Laboratory, Office of
Research and Development, U. S.
Environmental Protection Agency, Cincinnati,
Ohio 45268; EPA Contract No. 68-02-3352,
Task 32; October 1986.
2. Draft Technical Resource Document: Design,
Construction, and Evaluation of Clay Liners for
Waste Management Facilities; Hazardous Waste
Engineering Research Laboratory, Office of
Research and Development, U.S. Environmental
Protection Agency; Cincinnati, Ohio 45268;
EPA/530 - SW-86-007, March, 1986.
3. Permit Applicant's Guidance Manual for
Hazardous Waste Land Treatment, Storage1 and
Disposal Facilities (Volumes 1 and 2), Office of
Solid Waste and Emergency Response, U.S.
Environmental Protection Agency, Washington,
D.C. 20460; SW-970, January 1984.
4. Soil Mechanics by T. William Lambe and Robert
V. Whitman, Massachusetts Institute of
Technology, John Wiley & Sons, Inc., New York,
New York, 1969.
5. Anderson, D.: 1982, In-place Closure of
Hazardous Waste Surface Impoundments •
(draft), Chapter 5, "Evaluating Stabilized Waste
Residuals," EPA-68-83-2943.
6. Technical Guidance Document: Pre-
diction/Mitigation of Subsidence Damage to
Hazardous Waste Landfill Covers by P.A.Gijbert
and W.L.Murphy of the U. S. Army Engineer
Waterways Experiment Station at Vicksburg,
MS, for the Hazardous Waste Engineering
Research Laboratory, Office of Research and
Development, U. S. Environmental Protection
Agency, Cincinnati, OH 45268; Interagency
Agreement No.DW21930680-01-0, July,
1987.
7. Introductory Soil Mechanics and Foundations by
G.B. Sowers & G.F. Sowers, Third, Edition,
MacMillan Publishing Co., Inc., New York, New
York, 1970.
8. ASTM 1988, The American Society for Testing
and Materials 1985 Annual Book of ASTM
Standards, Volume 4.08, "Soil and Rock;
Building Stones," Philadelphia, Pennsylvania.
9. Wright, T.D., W.M. Held, J.R. Marsh, and LR.
Hovater: 1987 Manual of Procedures and
Criteria for Inspecting the Installation of Flexible
Membrane Liners in Hazardous Waste Facilities
prepared for Hazardous Waste Engineering
Research Laboratory, Office of Research and
Development, U.S. Environmental Protection
Agency, Cincinnati, Ohio 45268, EPA-68-03-
3247.
10. Wahls, H.E., "Tolerable Settlement of
Buildings," Journal of Geotechnical Engineering,
ASCE, Vol. 107, No. GT11, 1981, pp.1489-
1504
11. Mesri, G., "Coefficient of Secondary
Compression," Journal of the Soil Mechanics
and Foundations Division, ASCE, Vol. 99, No.
SM1, 1973, pp. 123-137. ;
12. Foundation Engineering Handbook, by
H.F.Winterkorn and H.Y.Fang, 1975, Van
Nostrand Reinhold, New York, 751 pgs
13. "Subsurface Exploration and Sampling of Soils
for Civil Engineering Purposes" by M. Hvorslev,
Reprint of Research Project Report for the
Committee on Sampling and Testing, Soil
Mechanics and Foundation Division, American
Society of Civil Engineers; 1965; 521 pgs.
(Recently Updated).
10
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Chapter 3.0
Dike Integrity and Slope Stability
Most landfills and surface impoundments are
constructed above natural grade through the use of
earthen dikes, excavated below grade slopes
constructed around the unit, or some combination of
dikes and excavation, depending on site topography.
Surface impoundments are often designed to achieve
some balance of cut-and-fill, with the excavated
soils used to construct the dikes. Landfill cells are
excavated below grade in order to provide operating
cover materials and to allow for restoration of the site
after filling.
These excavated slopes and earthen dikes are
vulnerable to stability failures via several mechanisms
that will be discussed in this chapter. Slope and dike
failures at hazardous waste management units are
potentially very serious; a surface impoundment
failure can allow the sudden release of large amounts
of hazardous waste to ground water and surface
waters, and a landfill slope failure can seriously
damage the liner system, allowing releases of waste
and leachate to surrounding soils and ground water.
For these reasons, earthen dikes must be carefully
designed and excavated slopes must be carefully
evaluated to assure that they are sufficiently stable to
withstand the loading and hydraulic conditions to
which they will be subjected during the unit's
construction, operation and post-closure periods.
This chapter will discuss the regulatory requirements
that apply to slope stability issues and will describe
how to design and evaluate dikes and slopes for
stability.
One of the apparent differences between landfill and
a surface impoundment unit is that of solid vs. liquid
wastes. From the viewpoint of stability, however,
there is no real difference; the forces on a slope
exerted by liquids are modeled in a manner identical
to those of solids. Another issue related to the
impoundment of liquids is that of seepage through the
dikes, causing piping or hydrostatic uplift pressures;
however, this seepage condition is modeled in a
manner identical to the condition of ground-water
seepage at a cut slope. Since the failure mechanisms
are similar for dikes and excavated slopes, these two
configurations will be discussed concurrently.
3.1 The Regulations and Performance
Standards
The regulations for surface impoundments, 40 CFR
264.221 (g), (Reference 1), require simply that
massive failure of dikes be prevented through
adequate design, construction and maintenance. This
is a performance standard only; the regulations do not
contain design standards. For landfills, there are no
specific slope stability regulations; however, the
regulations at 40 CFR 264.301 require that a liner
system in a landfill be placed upon a foundation or
base that will prevent the failure of the liner.
In order to demonstrate that the entire liner system is
placed upon a stable base, the stability of the slopes
must be demonstrated.
The surface impoundment regulations (Reference 1,
40 CFR 270.17 and 264.226) also require that the
structural integrity of each dike be certified by a
qualified engineer. Exhibit 3-1 summarizes the types
of information and technical parameters commonly
used to demonstrate that the performance standard is
met.
3.2 Design and Materials Selection
Slope stability failures usually occur in one of three
major modes, depending upon the site soils, slope
configuration, and hydraulic conditions (Reference 2).
These three major failure modes are the following:
• rotation on a curved slip surface approximated by
a circular arc.
• translation on a planar surface whose length is
large compared to its depth below ground.
• displacement of a wedge-shaped mass along
one or more planes of weakness in the slope.
Exhibit 3-2 illustrates basic concepts of translational
and rotational failures, and Reference 2, Chapter 7
shows more examples of these potential slope
failures in natural and in cut and fill slopes.
Slope failures occur when sliding forces from the
weight of. the soil mass itself and external forces
11
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Exhibit 3-1. Information Typically Submitted to
Demonstrate Satisfaction of Performance
Standards for Dike Integrity
FS=
Information.
Typical Parameters
Sum of resisting momemts
Sum of sliding moments
Description of Dike Design
Demonstration of Stability
Demonstration of Erosion
and Piping Protection
Analysis of Subsurface
Conditions
Stability Analyses
Construction Specifications
Engineer Certification
Data and/or drawings specifying:
• Design layout of dikes
• Design layout of components
• Materials of construction
• Elevations of critical points
Capability to withstand expected
static and dynamic loadings and
the effects of erosion
Demonstration of minimization of
erosion and prevention of failure
considering the erosion potential
of:
• Rainfall
• Surface water runon and
runoff
• Contact between impounded
wastes and dikes
• Potential leakage and piping
through dikes
• Potential leakage and piping
along conduits or structures
through dikes
Engineering characteristics of
foundations and soil dike materials
through testing and subsurface
explorations, such as:
Test borings
Test pits or trenches
In situ tests
Geophysical methods
Strength and consolidation
tests on foundation materials
• Permeability
Description and results of stability
analyses for the following
conditions, as appropriate:
• Foundation soil bearing
failure or settlement
• Failure in dike slopes
• Failure of impoundment cut
slopes
• Build-up of hydrostatic
pressure due to failure of
drainage system, dike cover,
and liner
• Rapid drawdown
Procedures for dike construction
Certification of integrity'of dike
designs and construction
When a stability analysis is performed, a slope is
analyzed for one or more of several potential modes
of failure, including rotational, translations! and wedge,
as appropriate. A safety factor is obtained for each
mode, and the lowest FS is the most critical.
In addition to the three major failure modes, dikes and
excavated slopes are also vulnerable to failure due tq
differential settlement, seismic effects including
liquefaction, and seepage-inducted piping failure.
Safety factors are determined in a manner similar to
the three modes. These failure modes will be
discussed in greater detail in Section 3.2.3.
Exhibit 3-3 lists the EPA-recommended minimum
factors of safety for slope stability analyses. If a dike
or excavated slope design analysis yields lower safety
factors, then steps should be taken to reduce the
sliding forces or increase the resisting forces, or the
slope should be redesigned to produce a safer
structure.
In order to evaluate an existing, conceptual, or final
slope design, the designer or reviewer must consider
the following factors:
• the adequacy of the subsurface exploration
program
• the stability of the dike slopes and foundation
soils
• liquefaction potential of the soils in the dike and
the foundation
• the expected behavior of the dike when subjected
to seismic effects . .
• potential for seepage-induced piping failure
• differential settlements in the dike.
The following sections will discuss each of these
factors, including the use of an EPA-developed
computer model called GARDS, Geote.chnical
Analysis for the Review of Dike Stability (Reference
4).
including waste pressures exceed the resisting forces
from the strength of the soil and from any reinforcing
structures. Slope stability analysis consists of a
comparison of these resisting forces or moments to
the sliding forces or moments, to obtain a factor of
safety, (FS). Generally, the FS takes the following
form (Reference 3, Section 12-1):
3.2.7 Subsurface Exploration Program
Site investigations are conducted to delineate a site's
topography, subsurface geology and hydrogeology.
They are necessary to evaluate the foundation for a
constructed dike, to evaluate dike materials obtained
from a borrow area, and to evaluate a slope
excavated below ground. These investigations include
12
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Exhibit 3-2. Conceptual slope failure modes.
Active Wedges
Central Block
Passive Wedges
Firm Base
« c
' Elements of the Translational (Wedge) Slope Stability Analysis
(Reference 4, p. 42)
Water
TJT
a. Circular segment divided into slices b., Forces acting on slice 3
Method of Slices for Circular Arc Analysis of Slopes in Soils Whose Strength Depends on Stress (Reference 3, p. 578)
field testing performed during drilling programs and
laboratory testing performed on field samples. Of
particular importance in some circumstances are
laboratory strength tests performed on soil samples to
determine the strength of the foundation and
embankment soils under the expected conditions of
saturation and consolidation. Site investigations
include field exploration procedures such as remote
sensing techniques, geophysical methods, test pits
and trenches, and borings. The field exploration is
followed by laboratory analysis of soil samples
obtained during the field program. The field and
laboratory data is then used to obtain a detailed
characterization of the site with respect to the
engineering properties of the soils and rock. These
engineering properties provide the input data for
evaluation of the stability of slopes. (See Chapter 2 of
this guide for additional discussion on field
investigations).
13
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Exhibit 3-3.
Recommended Minimum Values of Factor of Safety for Slope Stability Analyses (Reference 4)
Uncertainty of Strength Measurements
Consequences of Slope Failure
Small-i
Largea
No imminent danger to human life or major environmental
impact if slope fails
Imminent danger to human life or major environmental impact if
slope fails !
1.25
(1.2)"
1.5
(1.3)
1.5
(1.3)
2.0 or greater
(1.7 or greater)
1. The uncertainty of the strength measurements is smallest when the soil conditions are uniform and high quality strength test
data provide a consistent, complete, and logical picture of the strength characteristics.
2. The uncertainty of the strength measurements is greatest when the soil conditions are complex and when available strength
data do not provide a consistent, complete, or logical picture of the strength characteristics.
" Numbers without parentheses apply for static conditions and those within parentheses apply to seismic cemditions.
The number of borings or test pits required to
characterize the subsurface is dependent on its
complexity; a site with fairly uniform geologic
conditions across the site can be sufficiently
characterized with fewer exploratory probes than a
more complex site. In any case, the test pits or
borings must be performed at locations within or very
near to the actual unit. Reference 2, Chapter 2
provides a discussion of field exploration, testing' and
instrumentation methods used to characterize a site.
For embankments constructed of on-site borrow
materials, the borrow area should also be investigated
to verify that it contains an adequate volume of
acceptable material. This investigation will be very
similar to that performed for excavated units, with the
notable exception that all laboratory testing, i.e.,
strength, hydraulic conductivity, should be performed
on remolded samples because the soils will be
excavated and recompacted.
Hydrogeologic investigations are also necessary to
determine the elevation of the water table at the site
(including seasonal variability) and to locate, identify,.
and delineate hydrologic pathways (e.g., fractures [and
sand seams) that can contribute to slope failure
(Reference 5). The significance of hydrogeolbgic
conditions concerning slope stability will be discussed
further in Section 3.2.3.
Laboratory testing is conducted using representative
soil samples. Testing, as appropriate to evaluate the
embankments, the foundation area, and those areas
under consideration as a source for borrow material,
include Atterburg Limits (Plasticity Index, Liquid Limit),
grain-size distributions, shrink/swell potential, shear
strength, compressibility, consolidation properties,
density and water content, moisture-density
relationships, and laboratory hydraulic conductivity
(Reference 5).
For slope stability analyses, the most critical soil
parameter is that of shear strength. The shear
strength of a soil is a measure of the amount of
stress that is required to produce failure in plane of a
cross section of the soil structure. The shear strength
of a soil must be known before an earthen structure
can be designed and built with assurance that the
slopes will not fail (Reference 5). To adequately
determine a soil's shear strength, the potential effect
of pore water pressures from the expected site
loading conditions must be considered during testing.
While laboratory soil strength testing data is highly
desirable, these tests are limited to small size
samples, and in many locations dikes are constructed
using material which contains large particle sizes.
Furthermore, in existing dikes, the type of material
may make the obtaining of undisturbed soil samples
near to, if not, impossible. Therefore, it is not
uncommon in standard engineering practice to
estimate or assume these parameters based on the
best data available. While it is acceptable to do this, it
must be done and evaluated by a qualified
geotechnical engineer.
Slope stability is also dependent on hydraulic
conditions in the slope. Potential hydrostatic or
seepage forces from large hydraulic gradients should
be identified and considered during the stability
analyses. Ground-water levels and hydraulic
analyses are used to determine the configuration of
the steady-state piezometric surface through
sections of the foundation and/or the dike structure.
For sections involving a steep piezometric surface or
an upstream static or flood pool, hydraulic analyses
also determine (Reference 5):
• seepage quantity
• critical (highest) exit gradient;
• potential for uplift of a clay liner due to excess
pore pressures produced by a confined seepage
condition
Hydraulic boundary conditions may reflect
unconfined, steady state seepage conditions or
confined seepage conditions involving an
impermeable barrier (soil liner) and excess pore
pressure on the barrier. The hydraulic conditions of a
slope are determined using seepage analysis, as
discussed in Reference 13, Chapter 10.
3.2.2 Design
The design plans for dikes arid cut slopes should
show the design layout, cross-sections showing the
14
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proposed grade and bearing elevations relative to the
existing grade, and details of the dikes or cut slopes,
including all slope angles and dimensions. Materials
present at the cut slope or to be used to construct
the dike must be adequately characterized or
specified (Reference 5). This design configuration
then must be evaluated for its stability under all
potential hydraulic and loading conditions. If the
stability analyses result in unacceptably low factors of
safety, then the design must be modified to stabilize
the slope. The revised design must then be evaluated
to verify that it is sufficiently stable.
In addition, in a landfill or surface impoundment, often
the cut slopes or dikes will not be identical around the
entire perimeter of the unit. For this reason, it is
important that the most critical slope or dike section
be identified for analysis. Generally, the most critical
section will be the steepest and/or the highest portion
of the slope or dike. However, particularly in a cut
slope, the in situ materials may vary enough that the
more critical slope may be shallower or flatter, but
may be composed of weaker soils or may be subject
to significant pore pressures or seepage from high
ground-water levels.
3.2.3 Stability Analyses
Slope stability analyses are performed for both
excavated side slopes and above-ground
embankments. Three analyses will typically be
performed as appropriate to verify the structural
integrity of a cut slope or dike; they are slope stability,
settlement and liquefaction. Exhibit 3-4 indicates the
minimum required soil parameter data usually needed
to perform these analyses. Slope stability analysis
requires the establishment of various site conditions
including:
1) The soil shear strength conditions that represent
actual site conditions (discussed in Section 3.2.1)
2) The steady state hydraulic boundary conditions
occurring through the site's section (discussed in
Section 3.2.1)
3) The seismic conditions established for the site
area.
The slope stability is typically, evaluated using either a
rotational (slip circle) analysis and/or a translational
(sliding block or wedge) analysis using a computer
model. These analyses are run for both static and
dynamic (seismic) conditions. The latter is typically
performed using a coefficient that approximates
seismic conditions established for the site area. For
large dikes in areas of major earthquakes, a more
rigorous method of dynamic analysis may be
warranted. When appropriate, the liquefaction
potential of the foundation or embankment is also
determined using seismic conditions established for
the site area.
Analyses to establish total and differential settlement
are also performed to ensure that the estimated
settlement will not adversely impact the integrity of
the unit and its components. The analysis of potential
settlement is discussed in Chapter 2.0.
The slope stability analysis uses data from the site
investigation and soil testing to perform either of two
conventional slope stability analyses. The first is a
rotational (circular slip surface) analysis and the
second is a translational (plane slip surface or wedge)
analysis. The translational wedge analysis applies
primarily to stratified sections, especially where
stratum boundaries are inclined or where a stratum
with low shear strength exists. Even so, a rotational
stability analyses may yield a lower Factor of Safety
for the section and should always be checked
(Reference 4).
3.2.3.1 Rotational Slope Stability Analysis
A rotational slope stability analysis is typically
performed using a method that divides the slope into
discrete slices and sums all driving and resisting
forces on each slice. For each trial arc, the section is
subdivided into vertical slices, each having its base
coincident with a portion of the trial are. Slices are
defined according to the section geometry such that
the base of each slice comprises only one soil type. .
The driving and resisting forces acting on each slice
are then used to compute driving and resisting
moments about the center of rotation of a circular
section of the slope. The overturning and resisting
moments for each slice are then summed and the
Factor of Safety is computed (Reference 4).
3.2.3.2 Translationa! Slope Stability Analysis
The major features of the translational analysis are
the same as those for the rotational case except that
the trial surface consists of straight line segments
which form the base of one or more active (thrusting)
wedges, a neutral or thrusting central block, and one
or more passive (restraining) wedges. This analysis is
based upon selection of a trial central block defined
by the surface and subsurface soil layer geometry,
followed by computation of the coordinates for the
associated active and passive wedges (Reference 4).
3.2.3.3 Settlement Analysis
Settlement analysis is used to determine the
compression of foundation soils due to stresses
caused by the weight of an overlying dike. Required
parameters for each soil include unit weight, initial
void ratio, compression and recompression indices,
and the overconsolidation ratio (Reference 4). A
15
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Exhibit 3-4. Minimum Data Requirements for Stability Analysis Options (Reference 5)
Stability Analysis Options
Soil Parameter
1. Cohesion" (UU, CU, CD cases)
2. Angle of internal friction' (UU, CU, C
cases)
3. Total (wet) unit weight
4. Clay content
5. OvorconsolkJation ratio
6. Initial void ratio
7. Compression index
8. Recompression index
9. Hydraulic conductivity"* (permeability, k)
10. Median grain size
11. Plasticity index (PI)
12. Liquid limit (LL)
13. Standard penetration number (N)
Units Rotational
pounds/sq.ft (psf) X
degrees X
pounds/cu. ft. (pcf) X
percent (6 to 100)
unitless (decimal)
unitless (decimal)
unitless (decimal)
unitless (decimal)
ft/yr
mm
percent (0 to 100)
percent (0 to 100)
unitless (integer)
Translational Settlement
X
X
X X
X
X
X
X
'
Liquefaction
XCD
X
X
X
X
X
X
• Required strength case dependent upon hydraulic boundary condition selected
w Used only in hydraulic analysis
detailed discussion of settlement analysis is provided
In Chapter 2 of this Guide.
Settlements are calculated at the toes, crest points,
and centerline of the dike. The consolidation of each
soil is calculated for each layer and summed up for all
soils to determine the total settlement at each point.
Differential settlements are calculated between each
toe and crest, toe and centerline, and crest land
centerline on both sides of the dike. Recommended
maximum differential settlements can be found in
Reference 4.
3.2.3.4 Liquefaction Analysis
The liquefaction analysis determines the potential for
liquefaction of the dike and foundation soils to occur
during seismic events.
Factors which most influence liquefaction potential
are: soil type, relative density, initial confining
pressure, and the intensity and duration of earthquake
motion (Reference 4). Reference 4 provides
information on seismic risk zones of the U.S. and on
the range of seismic parameters for source zones.
Methods for estimating the potential for liquefaction
are provided in the GARDS software package
described in Section 3.2.3.5. Additional methods and
charts for estimating the liquefaction potential can be
found in References 14, 15, and 16, Chapter 11.
3.2.3.5 Geotechnlcal Analysis for Review of Dike
Stability (GARDS)
A computer software package called Geotechnical
Analysis for Review of Dike Stability (GARDS) has
been developed by EPA's Risk Reduction
Engineering Laboratory (RREL) to assist permit
writers and designers in evaluating earth dike stability.
GARDS details the basic technical concepts and
operational procedures for the analysis of site
hydraulic conditions, dike slope and foundation
stability, dike settlement, and liquefaction potential of
dike and foundation soils. It is designed to meet the
expressed need for a geotechnical support tool to
facilitate evaluation of existing and proposed earth
dike structures at hazardous waste sites.
The GARDS software package is available from
RREL, and a technical manual explaining its
operation, Reference 4, is also, available. Both the
software and this support documentation contain text
explanations and graphic examples designed to guide
the user through the customary steps of earth dike
analysis. User-friendliness is accomplished through
the use of menu selection of available program
options, including data check and simplified editing
procedures, automatic internal check of input
parameter values, cautionary statements regarding
the recommended sequence of program options, and
error diagnostic statements with interactive
instructions for corrective action.
GARDS is designed to guide the reviewer through the
customary steps of earth dike analysis considering
slope stability, settlement, liquefaction, hydraulic flow
and pressure conditions. GARDS includes an internal
automatic search routine to determine the critical
failure surface for both rotational (slip circle) and
translational (wedge) stability analyses; an internal,
automatic search routine to locate zones of greatest
liquefaction potential and to compute total and
16
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differential settlements of foundation soils; and
internal finite element hydraulic analysis to determine
the steady state piezometric surface through the
section, including the case of an impermeable barrier
such as soil liner; the ability to model excess pore
pressure conditions produced by confined steady flow
and evaluate slope stability and any resulting uplift
conditions; and the ability to determine the maximum
exit gradient which defines the potential for piping
failure (Reference 4).
The GARDS user must identify the various site
conditions which need to be investigated and select
the appropriate combination of options which best
models those conditions. GARDS offers the user six
idealized hydraulic boundary conditions, three stability
options (slope stability, settlement, liquefaction), and
three soil shear strength xoptions: Unconsolidated
Undrained (UU); Consolidated Undrained (CU); and
Consolidated Drained (CD). A limited amount of
guidance logic has been built into GARDS to assist
the non-specialist user in making decisions
regarding the available analysis options (Reference
4).
GARDS incorporates summary output block which
allows the user to obtain a hard copy of the input data
and the results of all analyses run for the dike section
under study. The critical factors of safety, failure
circle center coordinates, radius, and plane failure line
segment coordinates are all highlighted in the output
listing, along with the computed differential
settlements, liquefaction potential, and critical exit
gradient. If an analysis was not run, this is indicated
in a summary table at the end of the output listing.
3.3 Materials/Specifications
Material and construction specifications should be
provided as appropriate for all load supporting
embankments.
3.3.1 Subgrade Requirements
The subgrade requirements for slope stability are the
same as these addressed in Section 2.4.1 for
foundation materials.
3.3.2 Borrow Materials
The native soil at the facility excavated during
foundation excavations is the ideal backfill material
from the standpoint of cost and convenience.
.However, if the native soil is not suitable, a suitable
soil from a nearby borrow source should be utilized
(Refere'nce 2). At a minimum, material specifications
should provide the range of acceptable materials. All
materials should then be required to meet the
minimum requirements of the national specifications
as verified through specified field testing.
3.3.2.1 Selection
Once potential borrow sources have been identified,
the site should be investigated (see Section 3.2.1) to
determine the amount of suitable materials present at
the site and the degree of spatial variability of material
properties in the soil deposits. The investigation
results can also be used to plan an efficient extraction
procedure for the materials (Reference 7).
As discussed in Section 3.4, representative samples
of the borrow material are subjected to laboratory
compaction and hydraulic conductivity tests to
establish the relationships among water content,
density, compaction effort and permeability
(Reference 7).
When suitable soils are not available at an economic
distance from the facility, the engineer may
recommend blending an additive, such as bentonite,
to the native soil in order to achieve the proper
material properties and performance (Reference 7).
3.3.2.2 Test Fill
Laboratory results and design assumptions need ,to
be verified in the field. This verification can be
accomplished through a test-fill program. The test-
fill program allows the engineer to establish the
material, equipment and construction procedures
required to meet the design requirements for the fill
materials that comprise the dike. The test-fill
program is also a convenient tool for evaluating
critical performance standards such as shear
strength, density, and permeability (Reference 7).
Test fills, if used, should be constructed for each
borrow source and whenever significant changes
occur in the material, equipment, or procedures used,»
to construct the soil liner (Reference 7). Samples of
the test fill should be obtained for testing to assure
that the materials meet the minimum specifications.
3.4 Embankment Construction
Embankment construction for landfills or surface
impoundments involves standard earthwork
construction practices. Dike construction activities
include fill placement and compaction, drainage
system construction, and implementation of erosion
control measures (Reference 7).
3.4.7 Compacted Fill Construction
Compacted fill may be part of the dike core, the dike
shell, or may constitute the entire dike. Critical
construction activities include emplacement,
conditioning, and compaction. To insure that these
activities are conducted properly, the following
measures must be taken (Reference 7, p. 16):
17
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• Placing loose lifts to the thickness established
during the test fill program
• Removing or reducing clod size material to a
maximum size as determined in the test fill
• Providing uniform compaction coverage using the
type of equipment and number of passes
specified in the test fill program
• Ensuring uniformity of backfill material
• Protecting the surface lifts from desiccation or
frost action
• Scarifying between compacted lifts
• Ensuring adequate connection between lifts
3.4.2 Drainage Systems Installation
Installation procedures and equipment for dike
drainage systems are similar to those for leachate
collection systems. The observations and tests that
are necessary to monitor the installation of drainage
system components are discussed in Section 4.4.3 of
this guide (Reference 7, p. 39).
3.4.3 Erosion Control Measures
Erosion control measures are applied to the outer
slopes of embankments and may include benches
and vegetative covers. The construction activities
necessary for ensuring the quality of erosion control
measures are the same as those for topsoil and
vegetation subcomponents of cover systems
discussed in Section 1. (Reference 7).
3.5 Quality Assurance/Quality Control
(QA/QC)
Observation of the construction process is the most
effective approach to QC, coupled with a well-
defined testing program. Beyond the minimum
specified test frequency and spacing, visual
observations are used to identify problem areas and
to call for more intensive testing to document and
delineate any substandard backfill areas. Typical
items to be on the lookout for include wet spots, large
clods in backfill material, effects of exposure to frost,
erosive effects of heavy rains and surface water
runoff, poor bonding between lifts due to lack of
scarification, and inclusion of undesirable foreign
objects. Remedial actions (e.g., removal and
reconstruction) are then ordered for the substandard
areas so delineated. A qualified inspector should be
on the site at all times during construction (Reference
5).
3.5.? Compaction
During compaction of each lift, compactive effort and
uniformity of compaction should be observed and
recorded. Compactive effort is estimated by the
number of passes or equipment of a known size and
weight that will achieve the design specifications for
the fill material (Reference 5). The compaction effect,
the testing program and the fill's engineering
properties are established by the test fill program.
Design specifications usually require achievement of
a minimum percentage of the maximum density at a
specified range of water contents (i.e. ASTM methods
D698 or 1557). The specified density/water content
corresponds to the density/water content at which the
minimum specified soil properties can be achieved.
This density/water content is then tested during
quality control of the backfilling (Reference 5).
Specific tests to ensure that compaction results
correspond to design standards include field density
tests (nuclear, sand-cone and others), field water
content measurements, laboratory compaction tests,
and both field and laboratory permeability tests. The
methods and QC measures for conducting these
tests may be found in several documents
(References 2, 8, 9). The main tools used for
controlling the quality of compaction are field density
and water content measurements, with supplementary
laboratory compaction tests to monitor changes in soil
material. Presently, nuclear probes are often used to
measure field density and water content because of
ease and quickness of testing. However, nuclear
devices must be calibrated for each soil that is to be
tested. In addition, if nuclear devices are used, other
field density and moisture content measurements,
such as sand cones and oven drying, should be
made periodically to confirm nuclear results. Again, it
is necessary to measure density, moisture, and
compactive effort in the field to ensure that the
required density and hydraulic conductivity is
achieved during compaction (Reference 5).
Minimum test frequency and test spacing should be
specified for all tests in the test plan (Reference 5).
Thin-walled tube or block samples may be taken for
laboratory hydraulic conductivity tests (ASTM D 1587;
Reference 9), or field hydraulic conductivity tests may
be performed using techniques such as a- sealed
double-ring infiltrometer. Several design engineers
recommend that sealed water content/density
measurements and thin-walled tube samples for
laboratory hydraulic conductivity tests be obtained
from the lift underlying the lift that has just been
compacted. Following thin-walled tube sampling or
nuclear density measurement, the resulting* hole is
filled with backfill material and hand-tamped or is
grouped with bentonite (Reference 5).
18
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Upon completion of the dike, QC personnel should
check that it is rolled smooth to seal the surface so
that precipitation and/or leachate can run freely to the
leachate collection sump. The completed dike should
be surveyed to ensure that thickness, slope, and
surface topography are as required by the design
specifications. Seals around objects penetrating the
slope and dikes (e.g., leak detection system stand
pipes) also should be checked for integrity
(Reference 5).
3.5.2 Backfill Material Inspection
Inspection of the backfill material can be largely
visual; however, QC personnel conducting this
inspection must be experienced with visual-manual
soil classification techniques (Reference 9; ASTM D
2488). Changes in color or texture may indicate a
change in soil type or soil water content. The soil also
may be inspected for roots, stumps, and large rocks.
In addition, as a check of visual observations,
samples of the soil usually are taken and tested to
ensure that the soil's index properties are within the
range stated in the specification. The number of index
tests to be conducted depends on site-specific
conditions (i.e., soil type and heterogeneity) and the
experience of the QC personnel. Usually a minimum
number of tests per cubic yard of material is
specified, with additional tests required by the
inspector if visual observations suggest a change in
soil type (Reference 5).
When bentonite/soil mixtures are specified, incoming
bentonite should be inspected to ensure that its
quality is as specified. For all bentonite shipments,
certification of compliance with material specification
should be obtained from the manufacturer or supplier.
In addition, the quality of the arriving bentonite should
be tested frequently for dry fineness, pH, and
viscosity and fluid loss of a slurry made from the
bentonite. Dry fineness is the percentage passing a
200-mesh sieve. It is necessary to control dry
fineness to ensure proper mixing of the bentonite
(Reference 5). Slurry viscosity, slurry fluid loss, and
pH are standard tests specified by the American
Petroleum Institute (Reference 10).
3.6 References
1. Code of Federal Regulations Title 40, Parts 264
and 270, July 1, 1987.
2. U.S. Department of the Navy, May 1982,
Engineering Design Manual NAVFAC DM-7-1,
Naval Facilities Command, Washington, D.C.
3. Sowers G.F. 1979, So/7 Mechanics and
Foundations: Geotechnical Engineering, The
MacMillan Company, New York.
4. Technical Manual: Geotechnical Analysis for
Review of Dike Stability (GARDS) developed by
R. M. McCandless, Dr. A. Bodoczi, and P. R.
Cluxton of the University of Cincinnati, for the
Hazardous Waste Engineering Laboratory, Office
of Research and Development, U. S.
Environmental Protection Agency; Cincinnati,
Ohio 45268; EPA Contract No. 68-03-3183,
Task 19, March 1986.
5. Draft Technical Resource Document: Design,
Construction, and Evaluation of Clay Liners for
Waste Management Facilities; .Hazardous Waste
Engineering Research Laboratory, Office of
Research and Development, U.S. Environmental
Protection Agency; Cincinnati, Ohio 45268;
EPA/530-SW-86-007, March 1986.
6 Permit Applicant's Guidance Manual for
Hazardous Waste Land Treatment, Storage and
Disposal Facilities (Volumes 1 and 2), Office of
Solid Waste and Emergency Response, U.S.
Environmental Protection Agency; Washington,
D.C. 20460; SW-970, January 1984.
7. Technical Guidance Document: Construction
Quality Assurance for Hazardous Waste Land
Disposal Facilities', Hazardous Waste Engineering
Research Laboratory, Office of Research and
Development, U. S. Environmental Protection
Agency, Cincinnati, Ohio 45268; EPA Contract
No. 68-02-3952, Task 32; October 1986.
8. U.S. Department of the Army, 1970, Laboratory
Soils Testing EM-1110-2-1906, Office of the
Chief of Engineers, Washington, D.C.
9. ASTM, 1985, The American Society for Testing
and Materials 1985 Annual Book of ASTM
Standards Volume 4.08, "Soil and Rock; Building
Stones," Philadelphia, Pennsylvania.
10. API 1982, American Petroleum Institute, API
Recommended Practice: Standard Procedure for
Testing Drilling Fluids API RP13B, American
Petroleum Institute, Dallas, Texas
11. Boutwell, G.P. Jr., R. B. Adams, and D.A. Brown,
1980. Hazardous Waste Disposal in Louisiana.
Geotechnical Aspects of Waste Disposal.
American Society of Chemical Engineers, A Two
Day Seminar.
12. An Engineering Manual for the Evaluation of
Stability of Dikes, Permit Writer's Training
Program, U.S. EPA, 1983.
13. Groundwater. Freeze and Cherry, Prentice-Hall,
Englewood Cliffs, New Jersey, 1979.
19
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14. Seed, H. Bolton, et. al., "Evaluation of
Liquefaction Potential Using Field Performance
Data"; In: Journal of Geotechnical Engineering;
Vol 109, No. 3, March 1983.
15. Seed and Idriss; "Simplified Procedure for
Evaluating Soil Liquefaction Potential"; In: Journal
of the Soil Mechanics and Foundations Division
ASCEVo\. 9, No. 7, September, 1971, p. 1,249-
1,273.
16. Das, Braja M.; Fundamentals of Soil Dynamics;
Elsevier Science Publishing Co., Inc., 1983.
20
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Chapter 4.0
Liner Systems
Current regulations (40 CFR Part 264) require that
hazardous waste land disposal facilities be
constructed with liner systems that prevent any
migration of wastes out of the unit. A single liner
system required for certain units identified in Section
4.1, consists of one liner and one leachate collection
system as shown in Exhibit 4-1. A double liner
system required for new units, also identified in
Section 4.1, includes two liners, primary and
secondary, with a primary leachate collection system,
above the primary (top) liner and a secondary leak
detectipn/leachate collection system between the two
liners, as shown on Exhibit 4-2. The term "liner
system" includes the liner(s), leak detection/leachate
collection system(s), and any special additional
structural components such as filter layers or
reinforcement. The major components of both single
and double liner systems are the following:
• • Low-permeability soil liners,
• Flexible membrane liners (FML), and
• Leachate Collection and Removal Systems
(LCRS).
This chapter will discuss the regulatory performance
requirements for liner systems and will provide criteria
for the design and construction of liner systems and
the review of liner system designs and construction
plans on a component-by-component basis. This
discussion will begin with low-permeability soil liners,
usually the lower-most system component, and end
with leachate collection and removal systems. Each
component will be discussed with respect to its
purpose, design configuration and design calculations,
material specifications, and construction
specifications, with an additional discussion of quality
control issues specific to that component.
4.1 The Regulations and Performance
Standards
There are two "sets" of regulations and standards
that apply to permitted landfills and surface
impoundments. The first is in 40 CFR 264.221 (a) and
264.301 (a) (Reference 1) and applies only to portions
of existing units that are not covered by waste at the
time of permit issuance. For landfills, this regulation
requires only a single liner and a leachate collection
and removal system above the liner. This system
would include only the synthetic -or composite liner
and the LCRS shown on Exhibit 4-1 (Reference 6).
For a surface impoundment, the corresponding
regulation in 40 CFR 264.221 (a) requires only a single
liner (synthetic, soil, or composite) and no LCRS.
Waste piles are required by 40 CFR 264.251 to have
only a single liner with a LCRS above it, similar to
landfills.
The other "set" of regulations and standards were
mandated by Section 3004(o) of HSWA, are codified
in 40 CFR 264.221(c) and 264.301(c) (Reference 1),
and apply to:
• Each new landfill or surface impoundment,
• Each new landfill or surface impoundment at an
existing facility,
« Each replacement of an existing landfill or surface
impoundment unit, and
• Each lateral expansion of an existing landfill or
surface impoundment unit.
These regulations, containing the minimum
technological requirements (MTR) mandated by
HSWA Section 3004 (o), require two or more liners
and a leachate collection system between the liners
in a surface impoundment, and two or more liners
and two leachate collection systems in a landfill. The
basic landfill design is that shown in Exhibit 4-2
(Reference 7). The basic surface impoundment
design is similar to the landfill, without the uppermost
LCRS. This chapter will discuss the design and
construction of the three major liner system
components identified earlier and will provide
references containing detailed information to design
and to evaluate these systems.
There is some inconsistency in the regulations and
the guidance concerning terminology of LCRS, with
the lower system occasionally referred to as a leak
detection or leachate detection system. This
document will use the term "primary LCRS" to refer
to the system above the top liner in a landfill
(leachate collection zone), and "secondary LCRS" to
designate the system between the liners (leak
detection/leachate collection zone).
21
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Exhibit 4.1. Schematic of a Single Clay Liner System for a Landfill
Protective filter Medium
Soil or Cover
(optional)
Leachate
Collection and
Removal System
Being Proposed as the
Leak Detection System
Native Soil Foundation
Lower Component
(compacted soil)
(iNot to Scale)
4.1.1 Low-Permeability Soil Liners
For soil liners, the regulations in 40 CFR 264.221,
264.251, and 264.301 (Reference 1) apply to surface
impoundments, waste piles, and landfills, respectively.
For surface impoundments and waste piles that will
be closed by removal, a soil liner must prevent
migration of wastes out of the unit, that is, through
the liner, during the active life of the facility. For
surface impoundments that will be closed in place
Exhibit 4-2. Schematic of a Double Liner and Leak Detection System for a Landfill
Protective Filter Medium
Soil or Cover
(optional)
Top Liner
(FML)
Bottom Composite
Liner
Primary Leachate
Collodion and
Removal System
Secondary Leachate
Collection and
Removal System
Being Proposed as the
Leak Detection System
Upper
Component
(FML)
Native Soil Foundation
Leachate
Collection
System
Sump
Lower Component
(compacted soil)
(Not to Scale)
22
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and for landfills, the regulations require that liners be
constructed of materials that prevent wastes from
migrating into the liner; this effectively requires a
synthetic primary liner (flexible membrane liner), since
soil liners cannot prevent migration into the soil. For
those units required to have double liners, the
secondary liner must meet the above performance
standard, that of preventing migration through the
liner. The criteria by which a soil liner is determined
to meet this standard are discussed in Section 4.2.1.
4.1.2 Flexible Membrane Liners
The requirements of 40 GFR 264.221 (a) for surface
impoundments, 264.251 (a) for waste piles, and
264.301 (a) for landfills state that "the liner must be
designed, constructed, and installed to prevent any
migration of wastes out of the impoundment to the
adjacent subsurface soil or ground water or surface
water at any time during the active life (including the
closure period)" of the surface impoundment, waste
pile, or landfill. The liner must be constructed of
materials that have appropriate chemical properties
and sufficient strength and thickness to prevent
failure due to pressure gradients (including static
head and external hydrogeologic forces), physical
contact with the waste or leachate to which they are
exposed, climatic conditions, the stress of installation,
and the stress of daily operation." (Reference J). In
short, the regulations require FMLs to have physical
strength, low permeability, and chemical compatibility
with the waste contained by the liner. EPA regulations
and guidance refer to synthetic liners as FMLs
(flexible membrane liners). The manufacturers of
these materials, as well as a large number of
engineers, refer to these materials as geomembranes.
On March 28, 1986, the EPA proposed a rule
implementing minimum technology requirements
(MTR) for double liner systems and leachate
collection systems. These regulations implement the
statutory requirements of HSWA (Reference 2).
Following proposal of the MTR regulations, the EPA
collected data characterizing and comparing the
performance of compacted soil bottom liners and
composite (soil/FML) bottom liners. This data was
evaluated with respect to leachate collection
efficiency, leak detection capability, and leakage, both
into and out of the bottom liner; and the data
indicated that the use of an FML improved the
performance of a composite bottom liner over that of
a compacted soil liner with respect to all three
parameters. (Reference 3). On April 17, 1987, the
EPA made available the background document
presenting the data on bottom liner performance
(Reference 4) and the draft minimum technology
guidance documents on single and double liner
systems [References 5, 6, 7 and 3].
4.1.3 Leachate Collection and Removal Systems
The leachate collection and removal system (LCRS)
regulations for single-lined waste piles and landfills
specifically require that the system be designed and
operated to ensure that the leachate depth over the
liner does not exceed 30 cm (one foot). The system
must also be chemically resistant to wastes and
leachate, sufficiently strong to withstand landfill
loadings, and protected from clogging through the
scheduled closure of the unit [Reference 1, 40 CFR
264.251 (a) and 264.301 (a) ].
The regulations concerning double liner systems for
new surface impoundments [40 CFR 264.221 (c)] and
landfills [40 CFR 264.301 (c)] require only that the
liner and leachate collection systems protect human
health and the environment. However, minimum
technology guidance for double liners (Reference 7,
pages 4-5 and 43-44) provides specific design
criteria for primary and secondary LCRS. These
criteria include the following:
• the primary system should be capable of
maintaining a leachate head of less than 30 cm
(one foot) (Reference 7, p. 1);
• each system should have at least a 12-inch
thick granular drainage layer that is chemically
resistant to the waste and leachate, with a
hydraulic conductivity not less than 1 x 10"2
cm/sec, or an equivalent synthetic drainage
material (geonet), and with a minimum bottom
slope of two percent;
• the primary system should have a granular or
synthetic fabric filter (geotextile) above the
drainage layer to prevent clogging;
• both systems should have a drainage system of
pipes to efficiently collect leachate; the pipes
should have sufficient strength and chemical
resistance to perform under landfill loadings.
Section 4.2.3 of this Guide will discuss how an LCRS
is designed to meet the regulations and guidance,
and how a design can be evaluated to determine if it
meets these requirements.
4.2 Design and Materials Selection
4.2.1 Low-Permeability Soil Liners
Low-permeability soil liner design is site- and
material-specific: Prior to design, many fundamental
yet important criteria should be considered, such as:
• in-place permeability of the liner;
• liner stability against slope failure, settlement and
bottom heave;
• liner - waste compatibility; and
• long-term integrity of the liner.
Important criteria to consider when reviewing a design
for a soil liner include liner site and material selection;
liner thickness; strength and bearing capacity; slope
stability and controls for liner failure. Exhibit 4-3
23
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Exhibit 4-3.
Typos of Information Used to Demonstrate that
the Performance Standard for Soil Liners is Met
Information
Typical Parameters
Description of Soil Liner
Material Testing Data
Liner Compatibility
Data
Liner Thickness
Construction
Specifications
Construction Quality
Assurance
Description of:
• Classification
• Thickness
• Hydraulic conductivity
• Source of soil
• Any recompacting and
amendments (in-place soil)
• Location of borrow area and any
amendments (borrow material)
Test results for:
• Index tests
• Hydraulic conductivity
• Strength
• Consolidation
• Shrink-swell properties
• Potential for dispersion/piping bf
soil due to liquid flow through liner
Results of hydraulic conductivity testing
of liner material with representative
leachate.
For units with only one FML,
demonstration that the soil liner's
thickness is sufficient to retard liquid
flow-through during operating life and
post-closure period.
Procedures for liner installation,
including:
• Method of compaction
• Degree of compaction, water
content to be achieved
• Lift thickness
• Methods to alter water content of
soil •,
• Scarification requirement between
lifts
• Method of amending soil, if
• applicable
Description of QA program and testing
procedures.
summarizes the types of design information and
technical parameters commonly used to demonstrate
that the performance standards are met, if low-
permeability soil liners are used at a facility. These
design considerations are important throughout the
installation and construction phases of a clay liner.
The following sections provide information on design
criteria.
4.2.1.1 Site and Material Selection
A site investigation should be conducted prior to
design, and the following factors should be
considered:
• Site geology,
• Topography (especially drainage patterns),
• Analyses of soil properties,
• Field and laboratory hydraulic conductivity,
• Bedrock characteristics,
• Hydrology, and
• Climate.
All these factors are important to the design of the
soil liner. The site will require a foundation designed
to control settlement and seepage and to provide
structural support for the liner. If satisfactory contact
between the liner and the natural foundation is
achieved, settlement and cracking will be minimized.
Engineering designs and test methods for site
foundation construction are discussed in Chapter 2 of
this Guide.
Soil liners must meet the following requirements:
• A field hydraulic conductivity of 1 x 10'7
cm/sec when compacted;
• Sufficient strength after compaction to support
itself and the overlying materials without failure;
and
• Compatibility with hazardous wastes or leachates
to be contained at the site.
Soil liner material may originate at the site or may be
hauled in from a nearby borrow site if the native soil
is not suitable. If suitable soils are not available on
site or from a borrow site, it may be necessary to
introduce a soil additive such as bentonite to increase
performance potential of the selected material.
Although additives such as bentonite are known to
decrease hydraulic conductivity, it is important to test
additives under actual field, conditions as with any
potential soil liner material (Reference 8, pp. 9-25).
Because physical properties diffeir from one soil to the
next, testing procedures are necessary to assist in
the selection of liner material. Once potential soil
sources have been identified, it is necessary to begin
testing to eliminate undesirable soils or to determine
whether the source requires an amendment. Many
procedures have been standardised for soil testing by
organizations such as the American Society of
Testing and Materials (ASTM) and by individuals
currently researching clay soils for use in soil liner
construction (Reference 9; Sec. 2.3.4 and Reference
10).
Representative samples of the proposed material
must be subjected to laboratory testing. This will
establish the properties of the material with respect to
water content, density, compactive effort and
hydraulic conductivity. As will be discussed in Section
4.3.1.2.3 of this chapter, clay soils exhibit
characteristic changes when compacted. Therefore,
24
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all analyses of a potential material must be performed
on a compacted sample. Exhibit 4-4 provides a
listing of pertinent soil tests and methods (Reference
9, p. 84).
Exhibit 4-4. Methods for Testing Low-Permeability Soil
Liners (Reference 9)
Parameter to be
Analyzed Methods Test Method Reference
Soil type
Moisture content
In-place density
Moisture-density
relations
Strength
Cohesive soil
consistency (field)
Hydraulic conductivity
(laboratory)
Visual-manual
procedure
Particle size analysis
Atterberg limits
Soil classification
Oven-dry method
Nuclear method
Calcium carbide
(speedy)
Nuclear methods
Sand cone
Rubber balloon
Drive cylinder
Standard effort
ASTM D2488
ASTM D422
ASTM D4318
ASTM D2487
ASTM D2216
ASTM D3017
AASHTO T217
ASTM D2922
ASTM D1556
ASTM D2167
ASTM D2937
ASTM D698
Modified effort ASTM D1557
Unconfined ASTM D2166
compressive strength
Triaxial compression ASTM D2850
Penetration tests
Field vane shear test
Hand penetrometer
Fixed-wall double ring
permeameter
Flexible wall
permeameter
Hydraulic Conductivity Sealed Double-Ring
(field) Infiltrometer
Sai-Anderson-Gill
double-ring
, Infiltrometer
ASTM D3441
ASTM D2573
Horslev, 1943
EPA, 1983 SW-870
Anderson et. al.,
1984
Daniel et al., 1985
SW-846 Method
9100 (EPA, 1984)
Day and Daniel,
1985
Anderson et al.,
1984
Compatibility testing of liner soils to wastes or waste
leachates should be conducted as part of the material
selection process. Compatibility testing procedures
and the problems associated with them will be
discussed in Section 4.3.1.5 of this chapter.
4.2.1.2 Thickness
Transit time prediction methods can be used to
determine the required thickness of a soil liner. These
methods are used to determine the rate at which
hazardous constituents will eventually pass through
the liner if the overlying FML is ever breached. This
transit time equation is instrumental in determining
liner thickness for new liners and in evaluating
performance of existing liner systems (Reference 11,
pp. 161-165 and Reference 8, pp. 9-25). Transit
time prediction methods are seriously flawed unless
they accurately account for channel or bypass flow
within the soil liner as a whole. For this reason,
laboratory and field hydraulic conductivity values must
be determined under careful quality control, so that
they reflect the actual soil liner performance. The
regulations (Reference 1, 40 CFR 264.301) state that
a soil layer 3-feet thick, with a hydraulic conductivity
of 1 x 10'7 is assumed to satisfy the regulatory
standard discussed in 4.1.1.
Liners are designed to be of uniform thickness over
the entire facility. Thicker areas may be encountered
wherever there may be recessed areas for leachate
collection pipes or collection sumps. Some engineers
suggest extra thickness and compactive effort for the
edges of the sidewalls to adequately tie them together
with the liner itself. In smaller facilities, a soil liner
may be designed for installation over the entire area,
but in larger or multi-cell facilities, liners are
designed in segments. If this is the case, it will be
necessary to specify in the design a beveled or
step-cut joint between segments to ensure the
segments properly adhere together (Reference 12,
pp. 5-27).
4.2.1.3 Hydraulic Conductivity
The coefficient of permeability or hydraulic
conductivity expresses the ease with which water
passes through a soil. Hydraulic conductivity is the
most critical criterion for a potential soil liner. The
hydraulic conductivity of a soil is dependent on the
viscosity and density of the permeant liquid and on
the size, shape and area of the conduits through
which the permeant liquid flows. Darcy's Law is the
basis for the equation of hydraulic conductivity. The
hydraulic conductivity of a partially saturated soil will
be less than the hydraulic conductivity of the same
soil when saturated. This is due to air in the voids
(pore space, cracks etc.) reducing the flow cross
section and blocking the smaller voids completely
(Reference 13, pp.59-63). Clay minerals also affect
the hydraulic conductivity of a soil because they are
small and, therefore, have smaller pores creating a
lower hydraulic conductivity value. (Reference 14, p.
155).-Most importantly, clod remnants and improper
scarification have the greater affect on soil hydraulic
conductivity. This is due to remnants creating
improper moisture conditions and large voids. Poor
scarification may cause shifting in lifts.
When designing a. soil liner, field hydraulic
conductivity is the most important factor to consider.
Therefore, conductivity tests must be conducted on
the proposed soil to evaluate liner performance. Test
fills are the most accurate method of determining
hydraulic conductivity because laboratory values
25
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generally are lower than those measured in test fills
or actual liners. Therefore, it is essential that field
hydraulic conductivity testing be conducted on a test
fill area or on the liner itself (Reference 9, pp. 19-
24).
4.2.1.4 Strength and Bearing Capacity
Another important criterion to consider when
designing a soil liner is the strength and bearing
capacity of the liner material. Analysis of these
parameters will determine the stability of the liner
material. Simply, the bearing capacity of a soil is its
ability to withstand overburden pressure without the
soil failing from within. More detailed discussions; of
bearing capacity and strength can be found in
Chapter 2 of this Guide; Reference 15, Chapter 4,
and in Reference 12, Chapter 3.
4.2.1.5 Slope Stability
The strength of a soil also controls its resistance! to
sliding. Failure of a liner slope can result in slippage
of the compacted soil liner along the excavated slope.
This would result in a breach of the liner and allow
potential pollutants to escape into the environment.
Therefore, analysis of slope stability must Ibe
considered in the design of a soil liner. More detail on
slope stability analysis is provided in Chapter 3 of this
Guide. In addition, a detailed discussion of slope
stability can be found in Reference 12, Chapter 5 and
Reference 13, Chapter 4.
4.2.2 Flexible Membrane Liners (FMLs)
The design of a lined hazardous waste management
unit requires consideration of more than the
performance requirements of the FML; it also requires
careful design of the foundation supporting the FML
(see Chapter 2 for a discussion of foundation design).
The foundation provides support for the liner system,
including the FMLs and the leachate collection and
removal systems. If the foundation is not structurally
stable, the liner system may deform, thus restricting
or preventing its proper performance. Severe
deformation of the liner system may result in failure in
any of its components (References 10 and 11).
Additional information on foundation design and slope
stability is found in Chapters 2 and 3 of this Guide, as
well as in References 16,17, and 18.
4.2,2.1 Performance Requirements of the FML
The performance requirements of an FML used in a
hazardous waste management unit include low
permeability, chemical compatibility, mechanical
compatibility, and durability. The land disposal unit
designer must specify the necessary criteria for each
of these properties based on engineering
requirements, performance requirements of the unit,
applicable regulatory requirements, and the specific
site conditions. Exhibit 4-5 summarizes the types:of
design information and technical parameters used to
demonstrate that the FML performance standards are
met. In addition, the FML design must be compatible
with the present technology used in the installation of
FMLs (Reference 19, Chapter 7).
Exhibit 4-5. Types of Information Used to
Demonstrate that the Performance
Standards for Flexible Membrane
Liners are Met
Information Typical Parameters
Description of Flexible
Membrane Liner (FML)
Liner Compatibility Data
Liner Strength
Adequacy of Liner Bedding
Construction Specifications -
Descriptions of:
Type of FML
Material of
construction
Thickness
Brand Name
Manufacturer
Detailed material
specifications
Results of liner/waste
compatibility testing
demonstrating strength and
performance adequate after
exposure to representative
waste and to both primary
and secondary leachates
Demonstration that liner and
seams will have sufficient
strength to support
expected loads/stresses
after exposure to waste and
leachates
Demonstration that
sufficient bedding will be
provided above and below
FMLs to prevent rupture
during installation and
operation.
Procedures for placement
of FMLs including:
« Inspection of liner bed
for protrusions
« Placement procedures
• Liner seam bonding
techniques
• Procedures of
protection of liner
before and during
covering
• Placement of
protective layers
4.2.2.2 Selection of the FML
The performance requirements determined by a
designer/engineer serve as the baisis for the selection
of an, FML for a given facility,, Based ^upon the
designed use of the unit, the designer must make
decisions on the composition, thickness, and
construction (fabric-reinforced or unreinforced) of an
FML. Composition of the liner is based primarily on
chemical compatibility. Mechanical compatibility and
sometimes permeability determine the thickness of
26
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the FML sheeting. It should be noted that liner
performance does not correlate directly with any one
property (e.g., tensile strength) and that specifications
that appear in specific technical resource documents
such as Reference 19 should not be used alone as
the basis for selection of an FML.
FMLs are based on polymeric materials, particularly
plastics and synthetic rubbers. There are four general
types of polymeric materials used in the manufacture
of FML sheeting (Reference 19, Chapter 4):
• Thermoplastics and resins, such as PVC and
EVA; and
• Semicrystalline plastics, such as polyethylenes.
The various polymers are used to make a variety of
liners that can be classified by production process
and reinforcement. Hazardous waste management
unit liners are normally constructed using factory-
manufactured sheeting.
4.2.2.2.1 Polymers Used in FMLs
A'variety of polymers have been used in FMLs to line
waste management facilities. This section will discuss
those polymers that are currently used in the
manufacture of FML sheeting. Exhibit 4-6 lists the
polymers currently used in lining materials (Reference
19, Table 4-4).
The polymers used in FMLs have different physical
and chemical properties, and they also differ in
methods of installation and seaming, costs, and
chemical compatibility with wastes. Composition and
properties of compounds within a generic polymeric
type can also differ considerably since polymers are
usually not used alone in a product. Polymers are
usually compounded with a variety of ingredients to
improve properties and to reduce compound cost.
The other ingredients mixed with the polymer include
fillers, plasticizers or oils, antidegradants, and
curatives. Some compounds used in FMLs use a
blend, or alloy, of two or more polymers to improve
specific properties (Reference 19, Chapter 4).
Reference 19, Chapter 4 provides detailed information
about the composition and properties of each of these
polymers.
4.2.2.2.2 Seaming of FML Sheeting
The construction of a continuous watertight FML is
critical to the containment of hazardous waste and is
heavily dependent on the construction of the seams
bonding the sheeting together. The seams are the
most likely source of failure in an FML. Seams are
manufactured both in the factory and the field.
Sheeting manufactured in relatively narrow widths
(less than 90 inches) is seamed together to fabricate
panels. These factory seams are made in a controlled
environment and are generally of high quality. Both
fabricated panels and sheeting of wider widths (21 to
64 feet) are seamed on site during the installation of
the FML. The quality of field seams is difficult to
maintain since the installer must deal with changing
weather conditions, including temperature, wind, and
precipitation, as well as construction site conditions,
which include unclean work areas and work on
slopes. Constant inspection under a construction
quality assurance plan is necessary to ensure the
integrity of field seams (see Section 4.5 of this
chapter) (Reference 19, Chapter 4).
Several bonding systems are available for the
construction of factory and field seams in FMLs.
Bonding systems include solvent methods, heat
seals, heat guns, dielectric seaming, extrusion
welding, and hot wedge techniques. The selection of
a bonding system for a particular FML is dependent
primarily on the polymer making up the sheeting.
To ensure the integrity of seams, a given FML should
be seamed using the bonding system recommended
by' the FML manufacturer (Reference 19, Chapter 4).
Additional information on the applicability and
performance of bonding systems is presented in
Reference 19, Chapter 4.
At the present time, none of the available bonding
systems can be used to repair leaks and damage in
FMLs that are covered by wastes. Repairs can be
made, however, in FMLs exposed to weather if the .
polymer has not degraded and the bonding surfaces
are clean and dry (Reference 19, Chapter 4).
4.2.2.2.3 Properties and Characteristics of FMLs
The principal characteristics of concern regarding
FML sheeting include:
• Low permeability to waste constituents,
• Strength or mechanical compatibility of the
sheeting,
• Chemical compatibility with the contained waste,
and
• Durability for the lifetime of the facility.
These characteristics are assessed through
laboratory and pilot-scale testing of the various
properties of FML sheeting (Reference 19, Chapter
4). The sheetings used for FMLs were developed by
the rubber, plastics, and textile industries, and,
consequently, three different groups of standard test
methods were developed for testing these materials,
since each industry developed inherently different
products. The analyses and tests that are performed
on FML sheeting measure its inherent analytical
properties, physical properties, permeability
characteristics, environmental and aging properties,
and performance properties (Reference 19, Chapter
4). Testing is essential to the designer/engineer who
uses the data to determine whether a specific FML
sheeting will meet the design requirements of the
waste facility. The selection of an FML should be
27
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Exhibit 4-6. Polymers Currently Used in FMLs for Waste Management Facilities
Type of compound used in liners
Polymer
Thermoplastic Cross-linked
Fabric reinforcement
With
Without
Chlorinated polyethylene (CPE)
Chlorosulfonated polyethylene (CSPE)
Elasticized polyvinyl chloride (PVC-E)
Polyester elastomer (PEL)
Polyethylene (LDPE. LLDPE, HOPE)
Polyvinyl chloride (PVC)
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
, Yes
Yes
Yes
Yes
No
Yes
Yes
No
No
Yes
Yes
Yes
based on actual test data characterizing the various
sheetings under consideration for lining a waste
facility (References 16 and 21). These tests are
discussed in detail in Reference 13, Chapter 4 and in
References 19, 30 and 31.
4.2.2.2.4 Permeability
The primary function of a liner system in a hazardous
waste management unit is to minimize and control the
flow of hazardous waste from the unit to the
environment, particularly the ground water. A properly
designed FML has a low permeability to the waste
contained within the liner, allowing it to perform its
primary function in accordance with the MTR
guidance. However, the permeability of an FML made
of a particular polymer may change upon exposure to
waste or leachate, depending on the composition of
the waste contained by the FML. This property is the
chemical resistance, or compatibility, of a particular
polymer to specific chemicals. Since different plastics
and rubbers exhibit various degrees of compatibility
with different chemicals, a number of materials are
used to manufacture FML sheeting. The material is
selected based on exposure during its intended use.
An FML that is compatible with a specific waste
displays a low permeability toward that waste and will
minimize its flow through the FMb to the environment.
Additional factors affecting the rate of transmission
through the FML are the thickness of the FML
sheeting and concentration of the chemical species
(Reference 19, Chapter 5, and Reference 21). ;
4.2.2.2.5 Mechanical Compatibility
An FML must be mechanically compatible with the
designed use of the lined facility in order to maintain
its integrity during and after exposure to short-term
and long-term mechanical stresses. Short-term
mechanical stresses can include equipment traffic
during the installation of a liner system, as well as
thermal expansion and shrinkage of the FML during
operation of the unit. Long-term mechanical stresses
usually result from the placement of waste on top of
the liner system or from differential settlement of the
subgrade (Reference 19, Chapter 6).
Mechanical compatibility requires adequate friction
between the components of a liner system,
particularly the soil subgrade and the FML, to ensure
that slippage or sloughing does not occur on the1
slopes of the unit. Specifically, the foundation slopes
and the subgrade materials must be considered in
design equations in order to evaluate:
• The ability of an FML to support its own weight on
the side slopes,
• The ability of an FML to withstand down-
dragging during and after filling,
• The best anchorage configuration for the FML,
and
• The stability of a soil cover on top of an FML.
Mechanical compatibility requirements may affect the
choice of FML material, including polymer type, fabric
reinforcement, and* thickness (Reference 13, Chapter
7 and Reference 21).
4.2.2.2.6 Durability
An FML must exhibit durability, that is, it must be able
to maintain its integrity and performance
characteristics over the operational life and the post-
closure care period of the unit. The service life of an
FML is dependent on the intrinsic durability of the
FML material and on the conditions under which it is
exposed. Exposure conditions can vary greatly within
a given facility and an FML must resist the combined
effects of chemical, physical, and biological stresses
(References 6, 7, and 13, Chapter 7).
4.2.2.2.7 Chemical Compatibility
Chemical compatibility of FMLs and waste liquids or
leachates is a critical factor in the service life of a
liner system. Chemical compatibility requires that the
mechanical properties of the FML remain essentially
unchanged after the FML is exposed to the waste. If
the seams between the sheets are made with
materials other than the sheet parent products, they
also must be .compatible with the waste liquid.
Incompatibility is due primarily to the absorption of
waste constituents by the FML, the extraction of
components of the FML compound by wastes or
leachates, or reactions between FML constituents and
wastes or leachates (Reference 13, Chapter 7).
Incompatibility may result in a failure of the FML
material or of seams and consequent leakage of
28
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waste or leachates to the ground water. Due to the
serious consequences resulting from incompatibility,
an evaluation is required prior to permitting to
determine the effects that waste will have on the FML
proposed for installation at a facility.
Elevated exposure temperatures are believed, in
general, to be an effective means of accelerating the
effects of immersion on polymeric products and serve
as .the basis for Method 9090 immersion testing
(Reference 29). However, elevated temperature
acceleration is effective only for specific conditions. In
some cases, elevated temperatures may change a
polymeric product in ways that do not correlate with
service at a lower temperature (Reference 13,
Chapter 5). Limited data on field-exposed FMLs are
available, and there has been an attempt to correlate
the available information with data on FMLs exposed
in simulated service environments (Reference 13,
Chapter 6).
Evaluation of data obtained from compatibility testing
is best performed by specialists knowledgeable in
flexible membrane liners, FML testing, and EPA
Method 9090. EPA has developed a computer
advisory system, named the Flexible Liner Evaluation
Expert (FLEX), that serves as a tool to assist in
interpretation of data from Method 9090 tests. The
following discussion provides details on both EPA
Method 9090 and FLEX (Reference 36).
4.2.2.2.7.1 EPA Method 9090, Compatibility Test for
Wastes and FMLs
EPA Method 9090 is used to assess the compatibility
of a candidate FML with the specific waste liquid or
leachate to be contained. This test simulates the
conditions that the FML may encounter in service and
assesses what effects, if any, the exposure to waste
liquid has on the FML. The test involves immersion of
a candidate FML for a minimum of 120 days at 23°C
and 50°C in a representative sample of waste liquid.
Physical and analytical tests are performed on
unexposed FML sheeting to establish baseline data
and on samples exposed to waste liquid for 30, 60,
90, and 120 days (Reference 29). The test procedure
involves several steps, including (Reference 13,
Chapter 5):
• Selection of representative samples of the waste
liquid or leachate and the FML;
• Preparation of the exposure cells for operation
during the 120-day exposure period;
• Exposure of the FML samples to the waste liquid
or leachate in the simulated service environment;
• Physical and analytical testing of the unexposed
and exposed FML samples; and
• Analysis of test data for trends during the 120-
day'exposure period.
The FML samples selected for exposure and testing
should be free of flaws and defects in order that the
test specimens prepared from them are, as nearly as
possible, influenced only by exposure to the waste
liquid in the simulated service environment.
Maintenance of constant exposure conditions is
important since variation in any of them may influence
the quality of the data collected from testing the
exposed samples. Factors critical to the performance
of Method 9090 include (Reference 29):
• Use of exposure cells made of materials that are
not reactive with the waste liquid (e.g., a stainless
steel exposure cell may not be reactive with an
organic leachate, but corrodes in the presence of
an inorganic salt brine);
• Constant temperature of the waste liquid;
• Stirring of the waste liquid to prevent stratification
of phases (unless the system being modeled
allows stratification);
• Exchange of the waste liquid in the exposure cells
monthly or more frequently, to maintain constant
concentrations of constituents that may be
reduced due to their uptake by the FML samples
or volatilization into a headspace in the exposure
cell; and
• Maintenance of zero headspace in sealed
exposure cells to reduce the potential loss of '
volatile organic constituents from the waste liquid.
Method 9090 suggests testing semjcrystalline FMLs
(e.g., HOPE) for environmental stress-cracking since
ethylene plastics are susceptible to this type of failure
(Reference 29). Additional testing, not prescribed by
Method 9090, may be appropriate in certain cases.
Analytical testing may be performed on the
unexposed FML samples to yield a fingerprint, which
is a body of data that describes and identifies a
specific FML (Reference 33). Fingerprinting is
discussed in Section 4.2.2.2.8.
The data collected from Method 9090 testing are
analyzed for any changes or trends over the four-
month exposure period. The data should be compiled
and analyzed as the testing proceeds, with a final
analysis following the completion of testing.
Interpretation of Method 9090 data involves a holistic
assessment of all changes and trends in the data.
The physical property values are assessed with
respect to volatiles and extractables data, as well as
dimensions and weight, to determine whether uptake
of waste constituents or leaching of FML constituents
occurred in the FML and how these two mechanisms
affect physical properties. The effects of exposing an
FML to a simulated or actual service environment
may be one or more of three basic types (Reference
13, Chapter 5):
• Degradation of the polymer making up the FML;
29
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• Extraction of plasticizers or other compounding
ingredients in the FML; and/or
• Swelling of the FML due to absorption of organics
and water.
Based on the evaluation of the total effect of the
waste liquid or leachate on the FML in a simulated
service environment, a judgment may'be made on the
long-term performance, serviceability, and durability
of the FML in actual service.
4.2.2.2.7.2 FLEX - Flexible Membrane Liner
Advisory Expert System
In an effort to simplify and standardize the analysis of
Method 9090 data, the EPA developed FLEX, an
acronym for Flexible Liner Evaluation Expert, which is
a computer program designed to assist the reviewer
in the data analysis process. The program is not a
substitute for review of Method 9090 test results by a
trained professional, but rather a screening tool to be
used by those familiar with flexible membrane liners
and their testing, especially Method 9090 compatibility
testing (Reference 36, pp. 1-2). FLEX checks the
Method 9090 test, results for compliance with
applicable National Sanitation Foundation (NSF)
standards and requirements suggested by FML
specialists. Limits have been set on the acceptable
Statistical variation of test values for a specific
parameter. These limits have been determined
through interviews with liner manufacturers and
testing experts. The rules which are used to check
the test results are listed in Appendix A of Reference
36.
FLEX Version 2.0, used to evaluate Method 9090 test
results for HOPE, CSPE, and PVC liners, is available
for use at EPA Regional offices. Version 2.0 is being
distributed on a limited basis for the purpose of field
testing and to obtain feedback concerning the
system's performance and content (Reference 36, p.
15).
Details on the hardware requirements, contents of the
FLEX System, procedures for installing the system,
menus and data sheets are provided in the FLEX
User's Guide (Reference 36). Operation of the
system is straightforward and relatively simple since
each screen in the program provides instructions to
the user. The data required by FLEX is presented in
Exhibit 4-7 alongside the types of data provided by
Method 9090.
As a screening tool, FLEX pinpoints inconsistencies in
the test data and test results according to a series of
programmed decision-making statements and then
recommends that the liner is substandard or
incompatible if inconsistencies are found. However,
the recommendations provided by FLEX should not
be considered absolute, especially if the system finds
no problems with the data. In this capacity as a
screening tool, FLEX can save time, reduce
oversights, and enhance the consistency of Method
9090 test reviews (Reference 36, p. 2).
4.2.2.2.8 Fingerprinting of FMLs ;
The fingerprint of an FML is the sum total of its
analytical properties, which establish a body of data
that can identify a particular FML. A fingerprint
performed on an FML at the time of its installation
can be used to (Reference 13, Chapter 4):
• Assess the quality of the specific sheeting being
installed,
• Assure the designer/owner/operator of the facility
that the sheeting being installed is equivalent to
the sheeting tested incompatibility studies,
o Establish a baseline for assessing the effects of
service exposure on the FML.
4.2.2.2.9 Effects of Exposure on FMLs
Until recently, information on the effects of exposure
on FMLs was limited to laboratory studies of
simulated service environments. Accelerated testing
data are now being correlated with data collected on
the actual field performance of FMLs. Case studies of
field investigations and discussions of failure
mechanisms are presented in Reference 13, Chapter
6.
Chemical compatibility plays the greatest role in the
durability and service life of an FML in a hazardous
waste management unit. Swelling of the FML, in
particular, has the most serious effect on FMLs at
hazardous waste facilities and can cause loss in
strength, elongation, creep and flow, and loss in
puncture resistance. Extraction of plasticizers from
FML sheeting can result in embrittlement and
shrinkage and possibly breakage of the FML.
Dissolved organic constituents in the waste liquid or
leachate, even in very low concentrations, can
preferentially combine with organic liner materials and
may cause the FML to degrade and possibly fail
(Reference 13, Chapter 6).
4.2.2.3 Effect of FML Selection! on Design
FMLs made from different polymers or rubbers have
different physical characteristics that can affect the
design of a liner system. The coefficient of friction
between the FML and the foundation slopes is
specific to a particular FML sheeting; this factor, as
well as adequate anchorage of the FML, can
determine whether the installed FML will slip down
the foundation slopes. The use of an FML with a
relatively low friction angle, such as HOPE, can affect
the design of the anchor trench, or other anchorage,
used to secure the FML at the top of the slope
(Reference 13, Chapter 7 and Reference 21).
The coefficient of thermal expansion of the FML sheet
can affect its installation and its service performance.
30
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Exhibit 4-7. Data Requirements of FLEX
Data provided
by Parameter
Width
Length
Thickness
Weight
Tensile strength at yield
Tensile strength at break
Elongation at yield
Elongation at break
Tear Resistance
Puncture
Resistance
Modulus of
Elasticity
- Data required
Method 9090
in.
in.
in.
Ib. '
psi
psi
percent
percent
Ib/in. of width
Ib.
psi
Data points
provided
by FLEX
mm
mm
mm
grams
psi
psi
percent
percent
psi
psi
psi
Minimum number
required by
Method 9090
3
3
3
1
3M, 3T-
3M, 3T
3M, 3T
3M, 3T
3M, 3T
2
2M.2T
FLEX
3
3
3
3
3
3
3
3
3
3
3
*M = machine direction of test; T = transverse direction of test.
The FML should lie flat on the underlying soil, but
shrinkage and expansion of the sheeting due to
changes in temperature may result in excessive
wrinkling or tautness in the FML. Wrinkles on the FML
surface may affect drainage in the leachate collection
and removal systems. Tautness of the FML may
affect its ability to resist puncture and localized
stresses on the seams. In addition to thermal
expansion and contraction of the FML, residual
stresses from manufacture remain in some FMLs and
can cause shrinkage when the FML is heated by
sunlight. The design of the FML may need to include
provisions to deal with the dimensional changes
resulting from thermal expansion and contraction
(Reference 13, Chapter 7).
4.2.2.4 FML Layout
Upon selection of an FML, the designer must create a
layout plan for the sheeting or panels used to
construct the FML. The layout plan is a scale drawing
showing where each sheet or prefabricated panel is
placed within the FML; it also indicates the location of
each seam in the FML. The FML layout must take
into account the site conditions, as well as the width
of the rolls in which the FML is manufactured or the
dimensions of any prefabricated panels used in the
FML. Since an FML is an engineered structure, a
well-designed layout avoids using horizontal seams
on slopes and seams at the toe of slopes, because
these seams may be subjected to excessive stresses
(Reference 13, Chapter 7).
4.2.2.5 Appurtenances and Penetrations
Various ancillary components are necessary for the
proper operation of a lined system. These
components can be categorized as penetrations or
appurtenances. Appurtenances include any adjoining
structures to the liner system, such as sumps; splash
pads at pipe outfalls; anchorage systems; inlet, outlet,
overflow, or underflow pipes; gas vents; level-
indicating devices; emergency spill systems; pipe
supports; aeration systems; and protective soil
covers. Penetrations are made through the FML to
accommodate pipes, vents, level-indicating devices,
and pipe racks. However, to reduce the potential for
leaks, penetrations should be avoided when possible,
since they create additional locations where leakage
may occur. When penetrations are necessary, the
seal between the appurtenance and the FML must be
liquid-tight (References 6, 7, and 13). Additional
information on appurtenances and penetrations may
be found in References 16, 17, and 18.
4.2.3 Leachate Collection and Removal Systems
(LCRS)
Each leachate collection and removal system,
whether above (primary) or between (secondary) the
liners, consists of the following components shown on
Exhibit 4-2 (Reference 7; Sections I - III):
• A low-permeability base which is either a soil
liner, composite liner, or flexible membrane liner
(FML);
• A high-permeability drainage layer constructed
of either natural granular materials (sand and
gravel) or synthetic drainage material (geonet),
which is placed directly on the primary and/or
secondary liner, or its protective bedding layer;
• Perforated leachate collection pipes within the
high-permeability drainage layer to collect
leachate and carry it rapidly to the sump;
• A protective filter material surrounding the pipes,
if necessary, to prevent physical clogging of the
pipes or perforations;
• A leachate collection sump or sumps, where
leachate can be removed;
31
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The high-permeability drainage layer is placed
directly over the liner or their protective bedding
layers. Often the selection of a drainage material is
based on the on-site availability of natural granular
materials. Since hauling costs are high for sand and
gravel, a facility may elect to use geonets or synthetic,
Materials of
Construction
Descriptions of LCRS
Operation and Design
Drainage Capacity of
Synthetic Materials
Demonstration of
Grading and Drainage
• A protective filter layer over the high- Exhibit 4-8.
permeability drainage material which prevents
physical clogging of the material; and information
• A final protective layer of material that provides a
wearing surface for traffic and landfill operations.
The design features of each of these components
and operation of the entire LCRS will be described in
the following sections. Exhibit 4-8 summarizes. the
types of design information and technical parameters
used to demonstrate that the LCRS performance
standards are met. The primary LCRS acts as a
leachate collection system to remove leachate from
the landfill ,or waste pile before it can leak through the
primary liner component. This system also aids in
reducing leakage by removing or reducing the
hydraulic head of leachate that exists within the
system. The secondary LCRS acts as a leak
detection system by collecting and removing iany
leachate which leaks through the top or primary liner
component of the liner system.
4.2.3.1 Layout of System Components
The design of an LCRS for a hazardous waste land
disposal unit begins with a layout of the system
components within the unit. This layout should be
presented in plan view, cross-section, and detail
drawings of the unit. The drawings should show
dimensions and slopes of the unit design features and
all the components of the LCRS.
The system components should be shown on the
plan and cross-section drawings and should clearly
show the lateral and vertical extent of the primary and
secondary liners. The drawings should show, the
elevations of the tops of the liner system components
at critical points, including the toes of the sidewalls,
the boundaries of any sub-areas of the unit that
drain to different sumps, and the inlet and low point
elevations of the sumps. This information is essential
to evaluate the ability of the system to drain leachate
toward the collection sumps. The recommended
bottom liner slope is two percent at all points in each
system (Reference 7, Section I.A). This slope is
necessary for effective leachate drainage through the
entire operating and post-closure period; therefore,
these slopes must be maintained under operational
and post-closure loadings. The settlement estimates
performed as discussed in Chapter 2 must be
evaluated to ensure that the slopes will be two
percent throughout the period of operation of the
LCRS. It may be necessary to initially design the
slopes steeper than two percent to allow for
settlement.
Calculation of Maximum
Leachate Head
Compatibility
of LCRS
Information Typically Submitted to
Demonstrate Satisfaction of LCRS
Requirements
Typical Parameters
Detailed material specifications for:
« Drainage layer material
• Filter fabric or filter layer
• Piping
• Sumps
Description of:
• Design of system and timely
removal of leachate
• Design of system and timely
detection of leakage through
liners
• Removal of liquid from
system
Demonstration that synthetic
drainage material has drainage
capacity equal to or greater than a
12-inch granular layer with
hydraulic conductivity of 1 x 10'2
cm/sec.
Design details including:
Slopes
Ccintour plan
Layout and spacing of piping
Sumps, pumps, etc.
Fate of collected leachate
For slopes of < 2%,
demonstration of equivalency
• Sizing of pipes and
perforations
• Separate primary and
secondary leachate collection
sumps
Demonstration that leachate depth
over top of primary liner will not
exceed one foot. Include:
• Calculations
• Justification of assumed
parameters
• Numerical techniques used
Test results demonstrating
compatibility with wastes and
leachates for all system components
and materials.
drainage materials as an alternative. These materials
are described in Section 4.3.3. Frequently, geonets
are substituted for granular materials on steep
sidewalls in order to provide a layer that is more
stable with respect to sliding than a granular layer.
Each leachate collection system, primary and
secondary, must have a separate sump. This
configuration allows for the measurement of any
leakage through the primary liner into the secondary
LCRS. Therefore, each landfill cell must have at least
two LCRS sumps, more if the cell is divided into
leachate collection sub-areas using several different
collection points for each system. The drawings
should clearly show separate Slumps for each of the
systems, with separate methods for removing
32
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leachate from each sump. Access to the sumps is
usually provided by either a solid pipe laid in a
shallow trench along the sidewalls or a vertical
manhole that is constructed as the cell is filled.
The following sections of this Chapter will discuss
how the system is designed using sound engineering
practices and how design is evaluated to assure that
it will meet the performance requirements.
4.2.3.2 Sidewalls
Most hazardous waste landfills are constructed by
excavation of a cell, pit, or trench, followed by filling
and final restoration of the site to a condition similar
to its original topography. For this reason, liner
systems are placed on excavated sidewalls. The
issue of slope stability is discussed in Chapter 3;
however, in addition to the stability of the slope itself,
the stability of the individual liner components on the
slope must also be considered.
Reference 16, Section 4.3.5.2, provides a method for
calculating the factor of safety (FS) against sliding for
soils placed on a sloped FML surface. It considers the
slope angle and the friction angle between the FML
and its cover soil. Generally the slope angle is known
and is specified on the design drawings. A minimum
factor of safety is then selected. For a landfill, the FS
can be as low as 1.1 to 1.2 because the slope will be
unsupported (i.e., no waste will be filled against it) for
only a short time, and any failures that do occur can
be repaired fairly easily. Surface impoundment slopes
should be designed with an FS above 1.25, because
the slope will be subject to loading and unloading
cycles as the level of waste in the impoundment
varies, and because a failure would be more likely to
cause immediate releases to the environment.
From the slope angle and the FS, a minimum
allowable friction angle is determined, and the various
components of the liner system are selected based
on this minimum friction angle. If the design
evaluation results in an unacceptably low FS, then
either the sidewall slope or the materials must be
changed to produce an adequate design.
4.2.3.3 Grading and Drainage
In order for leachate to be effectively collected and
removed, both the primary and secondary systems
must be sloped to drain toward their respective
collection sumps. Minimum technology guidance
(Reference 1, p. 6) requires that the leachate depth
over the top liner not exceed 30 cm (one foot). The
regulations, 40 CFR 264.301 (a), contain this same
requirement for a single lined system.
4.2.3.3.1 Granular Drainage Layers
EPA SW-869 (Reference 20, Section 3.2) provides
a method for calculating the maximum leachate depth
over a liner for granular drainage systems materials,
using an equation developed by the author. (It should
be noted, however, that this equation contains an
error; the variable "n," for drainage layer porosity,
should not be included in the equation.) The leachate
head in the layer is a function of the liquid
impingement rate, bottom slope, pipe spacing, and
drainage layer hydraulic conductivity. The
impingement rate is estimated using a complex liquid
routing procedure discussed in Reference 6-26;
care must be taken to use input data to this
procedure that will yield a conservative result. If the
maximum leachate depth exceeds 30 cm for the
system, except for short term occurrences, the
design should be modified to improve its efficiency.
4.2.3.3.2 Geosynthetic Drainage Layer-Geonets
Geonets may be substituted for the granular layers in
either of the LCRS on the bottoms and sidewalls of
a the landfill cells. Geonets may be used if they are
equivalent to the granular design, including chemical
compatibility, flow under load, clogging resistance,
and protection of the FML (Reference 21, p. III-2).
A geonet used to replace the granular drainage layer
must provide the minimum flow capacity equivalent to
12 inches of material, with a minimum hydraulic
conductivity of 1x10-2 cm/sec (transmissivity of 3 x
10-5 m2/sec) and the capacity required to maintain
the liquid levels over the liner at less than one foot.
Reference 4, pages III-2 through III-6 provides an
explanation of the calculation used to compute the
capacity of a geonet. This transmissivity in
geosynthetics is strongly affected by large
compressive loads on the system exerted by
overlying wastes. Current research has shown that
the transmissivity of a geonet can be reduced by as
much as an order of magnitude during the first 30
days of service under load if a soil is immediately
adjacent to it. Therefore, it is very important that the
laboratory transmissivity ' test be performed under
conditions, loads, and configurations that closely
replicate the actual field conditions and that the
transmissivity value used in the LCRS design
calculations be selected based upon those loaded
conditions (Reference 21; p. III-5).
4.2.3.3.3 Piping
The design of piping systems requires the
consideration of pipe flow capacity and structural
strength. As indicated above, the spacing of leachate
collection pipes can be determined based on the
maximum allowable leachate head on the liner. This
maximum head calculation assumes that liquids can
drain away freely through the piping systems;
therefore, the pipes must be sized to carry the
expected flow.
The leachate piping configuration shown on, the
facility design drawings, as discussed in Section
4.2.3.1, should be evaluated for its ability to maintain
the maximum leachate head and for its ability to carry
the expected flows. Appendix V of Reference 13
33
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provides an algorithm to estimate the required flow
per 1000 feet of collection pipe in gallons per minute
based on the impingement rate (percolation)
discussed in Section 4.2.3.3.1 and the pipe spacing
as shown on the facility design drawings. Figure V-2
of the same reference can then be used to verify that
the pipe sizes are adequate.
4.2.3.3.4 The HELP Model
EPA has developed a computer program called the
Hydrologic Evaluation of Landfill Performance (HELP)
which is a quasi-two-dimensional hydrologic model
of water movement across, into, through, and out of
landfills. The model uses climatologic, soil, and landfill
design data and incorporates a solution technique
which accounts for the effects of surface storage,
run-off, infiltration, percolation, evapotranspiration,
soil moisture storage, and lateral drainage. The
program estimates run-off drainage and leachate
expected to result from a wide variety of landfill
conditions, including open, partially open, and closed
landfill cells. Most importantly, in consideration of this
topic, the model can be used to estimate the buildup
of leachate above the bottom liner of the landfill. The
HELP program can be used to estimate the depth of
leachate above the bottom liner for a variety of landfill
designs, time averages, and storm events. The
results may be used to compare designs or to design
leachate drainage and collection systems. References
22 and 23, a users guide and model documentation,
should be obtained before attempting to run the HELP
model.
4.2.3.4 System Compatibility
The failure of any one of the LCRS components can
lead to failure of the entire system; therefore, it is
essential that every component be demonstrated to
be chemically compatible with the expected leachate
from the landfill or surface impoundment. Synthetic
components of LCRS may be subjected to
compatibility testing similar to that for FMLs.
Qeotextiles, geonets, and plastic pipe and fittings may
be exposed in accordance with Method 9090, but the
testing protocols are different for these products, due
to their different physical properties. References 34
and 35 suggest general procedures for compatibility
testing of pipe, geotextiles, geonets, and earthen
materials. EPA Technical Resource Document SW-
870 also provides some general information on these
products and appropriate test methods (Reference
13, Chapter 4).
4.2.3.5 System Strength
All components of the LCRS must have sufficient
strength to support the weight of the overlying waste,
cover system, and post-closure loadings, as well as
stresses from operating equipment and from the
weight of the components themselves. They are! also
vulnerable to sliding under their own weight and the
weight of equipment operating on the slopes. The
components that are most vulnerable to strength
failures are the drainage layers and piping. LCRS
piping can fail by excessive deflection leading to
buckling or collapsing.
4.2.3.5.1 Stability of Drainage Layers
If the drainage layer of the LCRS is constructed of
granular soil materials, i.e., sand and gravel, then it
must be demonstrated that this granular drainage
layer has sufficient bearing strength to support
expected loads. This demonstration will be very
similar to that required for the foundations and low-
permeability soil liner in Chapters 2.0 and 4.0, with
the exception that the test selected must be
appropriate for drained cohesionless soils rather than
clays (cohesive soils).
The landfill design should provide calculations
demonstrating that the selected granular drainage
materials will be stable on the steepest slope (i.e., the
most critical) in the design. The calculations and the
assumptions should be shown, especially the friction
angle between the geomembrane and soil, and if
possible, supported by laboratory and/or field testing
data.
The friction factors against sliding for geotextiles,
geonets, and FMLs can often be demonstrated using
manufacturers data, since these materials do not
exhibit the range of characteristics that soil materials
do. It is important that the sliding stability calculations
accurately represent the actual design configuration
and the specified materials.
4.2.3.5.2 Pipe Structural Strength
Pipes installed at the base of a landfill can be
subjected to high loading of waste. The evaluation of
a design should consider both the maximum depth of
fill over the piping and the loading exerted by landfill
equipment on a pipe with very little cover. The pipe
must be selected based upon the most critical of
these loadings.
Leachate collection pipes beneath landfills are
generally installed in one of two configurations:
• a trench condition, where the pipe is placed in a
shallow trench excavated into the underlying soil
liner or foundation soil, and does not project
above the top of the trench and/or
• a positive projecting condition, where the pipe is
placed directly upon a lower liner system
component and projects above it.
Loads on the pipe in the trench condition are caused
by both the waste fill and the trench backfill. These
two loads are computed separately and then added to
obtain the total vertical pressure acting on the top of
the pipe. The total vertical load on the pipe itself is
reduced somewhat by the trench backfill; the amount
34
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of the load reduction is determined by load
coefficients which are a function of the type of
backfill. A detailed discussion of these load
coefficients is found in Reference 13, Appendix V. For
the projecting condition the vertical pressure on the
pipe is assumed to be equal to the unit weight of the
refuse fill multiplied by the height of the fill above the
pipe.
In the early phases of landfilling the piping system is
subject to concentrated and impact loadings from
trucks and landfill equipment. Since the pipe at this
point may be covered with only a foot or so of
granular drainage material, wheel and impact loads
are transmitted directly to the pipes. These loads may
be calculated using a method found in Reference 24,
Chapter 9, Section C. This traffic load should be
compared to the static load from the waste, and the
pipe selected based upon the larger of the two loads.
These loadings can cause pipe failure either by
excessive deflection followed by collapse or by
buckling. The deflection under the landfill loadings is
calculated using the Iowa formula (Reference 24,
Chapter 9, Section E.1.; and Reference 13, Appendix
V). This deflection should be compared to
manufacturers data on allowable deflection for the
size and material of pipe specified. The capacity of a
buried flexible pipe to support the landfill loads may
be limited by buckling. Buckling is a function of the
flexibility of the pipe and the type of backfill soil.
Specific buckling information for the sizes and pipe
materials proposed for use in the collection system
should be obtained from the pipe manufacturer.
Pipes are slotted or perforated to allow flow of
leachate into the collection system. These
perforations reduce the effective length of the pipe
available to carry loads and to resist deflection. See
Reference 13, Appendix V for a discussion of how to
allow for perforations in pipe strength calculations.
The piping system design must be evaluated for its
ability to support all the loads to which it is subjected
under all of its potential failure modes.
4.2.3.6 Prevention of Clogging
The piping system must be protected from physical
clogging by the granular drainage materials. This is
most effectively accomplished by careful sizing of
pipe perforations and by surrounding the pipe with a
filter medium, either a graded granular filter or a
geotextile. In addition, clogging of the pipes and
drainage layers of the LCRS can occur through
several other mechanisms, including chemical and
biological clogging. A detailed discussion of these
mechanisms is found in Reference 25, Section
9.2.2.3.4.
To prevent physical clogging of leachate drainage
layers and piping by soil sediment deposits, filter and
drainage layer size gradations should be designed
using criteria established by the Army Corps of
Engineers (Reference 25; p. 9-90). Drainage layers
should be designed to have adequate hydraulic
conductivity; and granular drainage media should be
washed before installation to minimize fines. Drain
pipe should be slotted or perforated with a minimum
inside diameter of six inches to allow for cleaning.
Two criteria are suggested for use in design of
drainage and filter layers for drain systems. The first
criterion is for the control of clogging by piping of
small soil particles into the filter layer and the drain
pipe system, while the second criterion is meant to
guarantee sufficient permeability to prevent the
buildup of large seepage forces and hydrostatic
pressure in filters and drainage layers. Calculations
for these criteria are explained in Reference 25,
Section 9.
When geotextiles (filter fabrics) are used in place of
graded filters, the protective filter may be only about
one mm in thickness. Caution should be exercised to
ensure that no holes, tears, or gaps are permitted to
form in the fabric. The advantages to using
geotextiles in place of granular filters are cost,
uniformity, and ease of installation. With increases in
costs of graded aggregate and its installation,
geotextiles are competitive with graded filters. One of
the most important advantages to geotextiles is
quality control during construction. The properties of
geotextiles will remain practically constant
independent of construction practices, whereas
graded filters can become segregated during
placement. These geotextiles must be designed and
References 16 and 21 provide guidance on how to
design those systems.
When drainage pipe systems are embedded in filter
and drainage layers, no unplugged ends should be
allowed, and the filter materials in contact with the
pipes must be coarse enough to be excluded from
joints, holes, or slots. Specifications for the drainage
layer materials should be checked against pipe
specifications to be sure that the piping system will
not become clogged by the granular drainage layer
particles.
Chemical clogging can occur when ionic species are -
dissolved in the leachate and then precipitate in the
piping of the LCRS. This type of clogging can be
controlled by providing a sufficiently steep slope in
the system to allow flow velocities high enough for
self-cleansing. These flow velocities are dependent
on the diameter of the precipitate particles and on
their specific gravity in the relationship given in
Reference 24. Generally, flow velocities should be in
the range of one to two feet per second to allow for
self-cleansing of the piping.
Biological clogging due to algae and bacterial growth
is a serious problem in sanitary landfills, but is less
critical in most hazardous waste landfills because of
the types of waste received. There are no universally
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effective methods of preventing biological growth in
the systems. If organic materials will be present in the
landfill and there will be a potential for biological
clogging, then, .the system design should include
features to allow cleaning of the piping system, which
includes the following components:
• Minimum of six-inch diameter pipes to facilitate
cleaning;
• Manholes located at major pipe intersections or
bends to allow for access, inspections, and-
cleaning; and
• Valves, ports, or other appurtenances to
introduce biocides and/or cleaning solutions.
4.3 Materials/Specifications
Following design development, the designer/engineer
must prepare a materials and construction
specification document for the proposed facility.'The
specification document includes plans, technical
specifications, and drawings for the proposed facility
and is used in the bid package for construction cost
estimates, as well as for the construction of the
facility. The specification document should also be
submitted with the Part B application for the proposed
hazardous waste management unit.
4.3.1 Low-Permeability Soil Liners
Low-permeability soils often used to construct liners
include clay, silty clay and sandy clay soils, all of
which exhibit low permeability values under
compaction. The following sections provide
information about low-permeability soils.
4.3.1.1 Sources
In order to reduce costs, in situ soils are often utilized
as a material for containment of hazardous
constituents. However, in some cases this may not
be possible. In the event that a suitable soil cannot be
located at the site, borrow material may be brought in
from a location off site. If the hydraulic conductivity
value after compaction of in situ or borrow material is
too high to be acceptable, it may be necessary to
amend these soils to make them less permeable and
more suitable. A soil amendment such as bentonite
may be mixed with a more permeable soil to create a
suitable soil to use as liner material. Bentonite, when
added by as little as two or three percent, has
reduced permeability of some soils after compaction
by two to three orders of magnitude (Reference 12,
pp. 5-10); however, reduced permeability enhanced
by bentonite is dependent on the specific properties
of the soil it is added to.
4.3.1.2 Soil Properties
Generally, the more clay a soil contains the lower the
hydraulic conductivity. Clay particles are less i than
0.002 mm in diameter and clay soils consist of at
least 40 percent clay particles (Preference 21, p. 237).
Clay minerals are made of hydrous silicates
(aluminum, magnesium and iron), which have an
affinity for water. Properties of clay soils are
influenced by their clay mineral content. The greater
the clay mineral content in a soil the greater the
influence on the behavior of the soil. Detailed
information on clay mineralogy and its effect on
compacted soils is provided in Reference 12.
4.3.1.3 Test Methods
Many test methods have been developed to predict
the performance of a soil, most of which have been
standardized. Moisture/density relationships, shrink-
swell potential, effective porosity, and most
importantly, hydraulic conductivity, are important
parameters to be measured (Reference 12, pp. 1-4).
The most important test for soil liners, field hydraulic
conductivity, has not yet been standardized.
Moisture/density relationships must be established
when evaluating liner material that is compacted. In
clay soils this refers to the optimum moisture content
that.results in maximum dry density after compaction.
The optimum moisture content of a soil provides
enough water to permit soil grains to distort and
reposition themselves when compacted, but not so
much., water that the void spaces are filled. Dry
density simply describes the weight or mass per unit
volume of the compacted soil. This moisture/density
relationship is usually established through a curve
developed from a standard proctor analysis. A
detailed description of this analysis is provided in
Reference 10, Test Method, p. 698.
Volume changes in soils are caused by changes in
moisture content. Shrinkage is caused by capillary
tension which compresses soil structure. Swelling is
caused by an increase in moisture content. Swelling
is influenced by a number,of factors, such as soil
elasticity, clay mineral affinity, cation exchange
capacity of the clay particles, and expansion of air
trapped in soil voids (Reference 12; Section 2). The
shrink-swell potential of clays has been correlated to
Atterberg limits (Reference 10, Test Method, p.
4,318), another standardized test for evaluating soils.
Effective porosity is that fraction of the total volume
through^ which flow can occur. Therefore the higher
the effective porosity for a given hydraulic
conductivity, the longer time it will take waste
constituents to move through a soil liner. A detailed
discussion of effective porosity is provided in
Reference 2, pp. 3-5 and Reference 27, pp. 7-46.
4.3.1.4 Hydraulic Conductivity
Darcy's Law is the basis for the equation of
permeability. (Reference 27, pp. 7-35).
Various laboratory methods are available for
determining the hydraulic conductivity of a soil
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compacted at optimum water content. These are
listed in Exhibit 4-3. Permeability values determined
in the laboratory are generally poor indicators of field
performance; however, they may provide some
information on the liner material. Several studies have
been conducted which indicate field hydraulic
conductivity is by far a better indicator than laboratory
methods. Therefore, it is, essential that field
permeability testing be conducted on all potential liner
material. Extensive field studies have been conducted
to evaluate field permeability (Reference 28, Chapter
4 and Reference 14, Chapter 19). A detailed
discussion on hydraulic conductivity, as related to
compacted soil liners, is provided in Reference 12,
Section 3.8 and in Reference 32.
Other tests to conduct when evaluating soil liner
- material include:
• Particle Size Distribution,
• Bulk Density,
• Particle Density,
.• Liquid and Plastic Limit and Plasticity Index,
• Compactive Effort,
• Strength and Bearing Capacity, and
• Compressibility.
Further discussion of these tests and related methods
is provided in Chapter 2.0 of this Guide and
Reference 12, Chapter 3. Exhibit 4-4 provides
references for most pertinent tests used in evaluating
clay liner material.
4.3.1.5 Liner-Leachate Compatibility
Compatibility testing is essential to determine if
chemical components in permeant fluids may affect
the hydraulic conductivity of a soil. Several studies
have been conducted by researchers on theories to
predict soil liner-leachate interactions. These studies
a,re summarized in Reference 12, Section 4.2.
Hydraulic conductivity may be affected by chemicals
in the following manner (Reference 13; Section 4.2
and Reference 28; Section 5.5):
• Alterations in the double layer surrounding clay
particles in soil fabric,
• Dissolution of soil fabric by strong acids and
bases, and
• Precipitation of solids in soil pore space.
Soil pore blockage from microorganism activity is a
biological factor which often affects hydraulic
conductivity values of soils.
Currently there is no 'standard testing procedure for
soil liner-leachate compatibility; however, EPA
Method 9100 provided in Reference 29, and the
hydraulic conductivity test in Reference 27 (pp. 7-
105 to 7-124) provide a means for evaluating soil
liner-leachate compatibility under laboratory
conditions.
Soil liner-leachate compatibility testing provides a
basis for evaluating the performance of a soil liner
material after it is exposed to the leachate that it is
designed to contain. Of interest is the
representativeness of the permeant liquid to actual
leachate at the site, and most of all, the outcome of
the analysis. It is critically important that the permeant
liquid used in the compatibility test should be
representative of the worst-case leachate that is to
be contained at the facility.
4.3.1.6 Mechanisms of Soil Liner Failure
Liner failure may be caused by various mechanisms.
These include:
• Desiccation cracking,
• Slope instability,
• Settlement,
• Piping and dissolution,
• Penetration,
•' Erosion,
• Earthquakes,
• Hydraulic uplift (heaving),
• Design or construction errors,
• Exposure to organic chemicals,
• Design and construction errors,
• Incomplete remolding of clods, and
• Inadequate scarification between lifts.
A detailed discussion of these liner failure
mechanisms is provided in Reference 12, Chapter 7.
4.3.2 Flexible Membrane Liners
The materials specifications document must include
detailed standards to assure that adequate FMLs are
selected and that inferior FML materials are not
substituted at any time during liner system
construction.
These minimum standards are selected based upon
the conditions identified during the unit design,
including loads and stresses and chemical
compatibility. The specifications should be carefully
compared to the design calculations and the
liner/leachate compatibility data to verify that the
selected specified FML will not fail during installation
or during its service life.
The specifications should list minimum mechanical,
analytical, and environmental and aging properties,
and the FML selected should be of the type that
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possessed all these necessary minimum properties. A
detailed discussion of material standards for FMLs is
provided in Appendix VIII of Reference 13.
4.3.3 Leachate Collection and Removal
Systems (LCRS)
After the design calculations have demonstrated that
the LCRS will perform as required, then the material
specifications must be carefully prepared to ensure
that the system components will be constructed of
materials that are equivalent to those •assumed in the
calculations. For example, if the pipe strength
calculations discussed in Section 4.2.3.5.2, above,
evaluate a pipe of a specified material and Standard
Dimension Ratio (SDR), then the construction
specifications must require that same pipe.. The SDR
of a pipe is the ratio of pipe diameter to wall
thickness. The following sections wijl discuss; the
types of information necessary to prevent j the
substitution of inferior materials in the LCRS.
4.3.3.1 Granular Drainage Layer Materials
For a granular drainage layer the most. critical
specification is the particle size distribution. In order
for the materials to drain properly, they must have a
saturated hydraulic conductivity of 1 x 10-2 cm/sec
(Reference 7). Therefore, the material specifications
should specify the range of particle sizes that has
been demonstrated, as discussed in Section 4.2.3.3.1
to provide adequate flow. Generally, sands and
gravels with a group designation of GW, GP, SW, or
SP on the Unified Soil Classification Chart (Reference
13, Appendix I) will satisfy the permeability
requirements. These specifications should be called
out on the design drawings and in the material
specifications, usually under the heading "drainage
layer material" or "select fill." They must also match
the assumptions used in the leachate system
drainage calculations, Section 4.2.3.3.
4.3.3.2 Geonets
For geonets the most critical specifications are
concerned with the materials' ability to transmit fluids
under loadings; therefore, the specifications rnust
include a minimum transmissivity under the expected
landfill loads. This transmissivity will' have been
demonstrated by the testing and calculations
discussed in Section 4.2.3.3.2. The specifications for
thickness and type of material should be called out on
the drawings and in the materials section of the
specifications, and they must match each other and
the design calculations.
4.3.3.3 Granular Filter Layers
The specifications required for granular filter layers
surrounding perforated pipes and protecting the
primary (upper) drainage layer from clogging include a
detailed particle size distribution, selected as
discussed in Section 4.2.3.6. The layers and their
specifications should be shown on all drawings and
described, with ranges of particle sizes, in the
materials section of the specifications. Again, the
criteria on the drawings and in the specifications must
match each other and must match the assumptions of
the design calculations in Section 4.2.3.6.
4.3.3.4 Geotextiles
Since the primary function of a geotextile is to
prevent the migration of soil fines into the drainage
layer and of drainage layer fines into the leachate
pipes, while allowing the passage of leachate, the
most critical specifications are those of permeability
and retention. The permeability of the geotextile
should be at least ten times the permeability of the
soil it is retaining. The retention ability for loose soils
is evaluated based upon the average particle size of
the soil and the apparent opening size (AOS) of the
geotextile. The maximum apparent opening size,
sometimes called equivalent opening size (EOS), is
determined for the soil to be retained, and a
geotextile is then selected that meets the
specification. The material specifications should
contain a range of AOS values for the geotextile, and
these AOS values should match those used in the
design calculations (Reference 21; p. III-6).
4.3.3.5 Piping
The selected pipe materials should be indicated on
the plans and drawings as well as in the
specifications, and these should match each other as
well as the assumptions in the design calculations.
The specifications should include:
• Type of piping material,
• Diameter and wall thickness,
• Size and distribution of slots or perforations,
• Type of coatings used in the pipe manufacturing,
and
• Type of pipe bedding material used to support the
pipes.
4.3.3.6 Sumps and Pumps
Often sumps are constructed of concrete or other
materials, although they may be simply extensions of
the leachate collection system layer or pipe trenches.
The drawings should clearly show the dimensions and
materials of construction for each sump. If the sumps
are constructed of materials not used elsewhere, then
there must be specifications for all the materials.
Concrete strength is indicated by rits specified
compressive strength at a given time which is a
function of the ratio of water to cement; generally the
lower the water-to-cement ratio, the higher the
compressive strength. The specifications will require
a minimum strength. Detailed concrete standards are
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contained in ASTM specifications (Reference 37) and
in standards of the American Concrete Institute
(References 38, 39, 40).
If the sumps are constructed of reinforced concrete,
specifications for the reinforcing must be indicated.
These include the ASTM bar numbers (Reference 41)
. and their configuration within the concrete. If concrete
strength calculations are included in the design
calculations, the specifications should be compared to
the design assumptions to assure that the sump will
be constructed as designed. This is particularly
important if the sump is a large concrete structure
supporting a tall vertical access manhole through the
landfill, because the sump may be supporting a large
load over a small area.
Pumps used to remove leachate from the sumps
should be sized to ensure removal of leachate at the
expected rate of generation and must have a
sufficient operating head to lift the leachate the
required height, from the sump to the access port.
Often portable vacuum pumps that can be moved in
sequence from one leachate sump to another are
used. The type of pump specified should be small
enough Jo be lowered through the leachate sump
access pipes. The specifications may include pump
details if the.pumps will be installed at the time of
construction; otherwise this information will be
included in the landfill operating equipment.
4.4 Construction/Installation
4.4.1 Low-Permeability Soil Liners
The following discussion provides a summary of soil
liner construction and installation activities. Prior to
construction of the actual liner system, a pilot
construction test should be conducted. Construction
activities should commence upon completion and
approval of the test fill analysis. These activities
include compaction, scarification, placement on side
slopes and final preparation for FML placement. A
detailed discussion of construction and installation
activities is provided in Reference 12, Chapter 5.2
and Reference 13, Chapter 5.0.
4.4.1.1 Soil Liner Test Fill
A soil liner test fill is a small-scale study conducted
to determine whether the design specification, liner
material, equipment and construction procedures will
result in an acceptable low-permeability soil liner
(Reference 12, Chapter 5.2) a schematic of a test fill
is shown in Exhibit 4-9. A test fill will minimize
potential costs and dangers of construction of an
unacceptable soil liner, as well as provide a quality
assurance measure for design specifications.
Procedures and equipment which are intended for
use with the full-scale facility should be utilized
during the construction of the test fill. Field hydraulic
conductivity analysis should be conducted on the test
fill prior to construction of the liner to verify that
laboratory hydraulic conductivities have been
achieved (Reference 9, Section 2.3.4.1.2).
4.4.1.2 Compaction
Soil liners are installed in a series of compacted lifts
of specified thickness. Lift thickness is dependent on
the soil characteristics, compaction equipment, and
the required compactive effort. Soil liner lifts must be
thin enough to allow adequate compactive effort to
reach the lower portions of the lift. Thinner lifts
provide more assurance that even compaction is
occurring between lifts; however, thinner lifts require
more lifts to achieve the proper soil liner thickness.
Some engineers think that adequate compaction of
thicker lifts (5-10 inches) is possible if heavy enough
compaction equipment is used (Reference 12, pp
5-54 to 5-58).
Placement of the soil liner material and compaction
activities may begin once the foundation preparations
are completed. Clods (soil aggregates) in the soil liner
material, if kept to smaller sizes, will facilitate in a
more uniform water content (Reference 12, p. 5-48).
Opinions differ among design engineers on maximum
clod sizes for soil liners. Some engineers suggest
one to three inches in diameter, no larger than one-
half the lift thickness and no longer than the lift
thickness (Reference 12, p. 5-51). The main
objective is to remold all clods in the compaction
process to keep the permeability values consistent
throughout the soil liner. It has been demonstrated in
field studies that it is the macropores between clod
remnants that can result in an unacceptably high field
hydraulic conductivity.
It is essential that the optimum water content derived
for the proper compaction of the material be
maintained in order to obtain the specified hydraulic
conductivity. Several EPA studies have found that
compacting a soil at or below optimum water content
often results in an unacceptably high field hydraulic
conductivity. These studies and numerous reports
from test fills indicate that a lower hydraulic
conductivity is obtained by compacting the soil at 2-
4% above optimum water content. Environmental
conditions such as rainfall or extreme temperatures
should be considered when initiating construction
activities. Adverse weather conditions, such as dry
and hot spells can cause desiccation cracks. Freezing
can cause shrinkage cracks. (Reference 9, p. 25).
Standard compaction procedures are normally
employed when constructing soil liners. The following
factors affect the quality of the compaction:
• lift thickness and number,
• full scale or segmented placement,
• number of equipment passes,
• scarification between lifts, and
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Exhibit 4-9. Schematic of a Test Fill
At least three lifts of compacted soil
A drainage layer or underdrainage collection system
L = Distance required for construction equipment to reach normal running speed
W = Distance at least four times wider than the widest piece of construction equipment
E3= Area to be used for testing
• material water content.
It is necessary to control all of these factors to
achieve the desired compaction in the liner. A
detailed discussion of current construction and
installation procedures, as well as available equipment
is provided in Reference 12, pp. 5-54 to 5-65 and
Reference 13, Chapter 5.
Soil/bentonite mixtures generally require central plant
mixing by means of a pugmill, cement mixer, or other
mixing type equipment where water is added during
the process. Not only is water content monitored
constantly, but bentonite content and particle size
distribution are of concern and must be measured
during the mixing and placement of the material.
Spreading of the soil/bentonite mixture may be
accomplished similar to spreading for natural soil
liners by using tracks, scrapers, graders, dozers or a
continuous asphalt paving machine. After mixing and
spreading the soil/bentonite liner, compaction by
means of vibrating smooth-wheeled rollers or
vibratory-plate compactors may take place. A
discussion of specific procedures for construction of
soil/bentonite liners is provided in Reference 12, pp.
5-67 to 5-76 and Reference 13, p.238.
4.4.1.3 Placement on Side Slopes
Placement of soil liner material and compaction on
side slopes is dependent on the angle of the ?lope.
Gradual inclines from the bottom of the liner enable
continuous placement of the lifts. This provides better
continuity between the bottom and sidewalls of the
soil liner. However, when steep slopes are
encountered lifts may have to be placed and
compacted horizontally. This is due to operational
difficulties of the heavy compaction equipment on
steeper slopes. When sidewalls are compacted
horizontally, it is essential to tie in the edges with the
bottom of the soil liner to prevent areas of weakness
in the soil liner. Information concerning sidewall
placement and compaction is provided in Reference
13, p. 219 and Reference 12, p. 521.
4.4.1.4 Final Preparation for FML Placement
Post-installation activities begin upon completion of
placement and compaction of the soil liner material.
Quality control inspection should be conducted prior
to placing the FML. It is essential to inspect the soil
liner to ensure that design specifications (thickness,
water content, sidewall slope, etc.) have been met
and that the integrity of seals around leachate
collection sumps or pipes are secure. Once these
inspections are conducted and complete, the FML
may be installed on top of the soil liner. If any amount
of time will pass before the FML is installed, a plastic
cover should be placed on the soil liner to prevent
desiccation, erosion, or freezing (Reference 12, pp.
5-78).
4.4.2 Flexible Membrane Uners
4.4.2.1 Materials and Construction Specification
Document for FMLs
The specification document for a proposed hazardous
waste facility must address the FML components of
the unit, including the bottom liner, the top liner, and
the cover. The technical specification for FMLs must
include detailed information concerning material
properties, shipping and storage of the FML sheeting
or panels, installation of the FML, and quality
assurance/quality control by the manufacturer,
fabricator (if panels are constructed), and the installer.
Installation procedures addressed by the technical
40
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specification include FML layout, deployment of the
FML at the construction site, seaming of the FML
sheeting or panels, and sealing of the FML to
appurtenances, both adjoining and penetrating the
liner. The performance of inspection activities,
including both nondestructive and destructive testing
of the sheets and seams, during installation of the
FML should be addressed in the technical
specification or in a separate construction quality
assurance document (Reference 13, Chapter 8).
4.4.2.2 Construction Procedures
Construction procedures must be incorporated in the
design plans and specifications for a construction
quality assurance program must also be established.
The design specifications should describe the
requirements for labeling, shipping, and on-site
storage of the FML sheeting or panels (Reference 13,
Chapter 8). The FML sheeting is shipped in rolls or
panels from the suppliers, manufacturers, or
fabricators to the construction site. Each roll or panel
must be labeled according to its position on the FML
layout plan so that installation of the FML does not
turn into a jigsaw puzzle. The sheeting must be
inspected upon delivery to the construction site to
determine whether any damage has occurred during
shipping. Proper storage of the rolls or panels prior to
installation is critical to the final performance of the
FML. Some FML materials may be sensitive to
ultraviolet exposure and should not be stored in the
direct sunlight prior to installation and placement of
soil cover. Others, such as CSPE and CPE, are
sensitive to moisture and heat and can partially
crosslink or block (stick together) under improper
storage conditions before installation. Adhesives or
welding materials should also be. stored appropriately
(Reference 42).
Deployment, or placement, of the FML panels or rolls
should be described in the FML layout plan. Rolls of
sheeting can be deployed by manually unrolling the
sheet, or it can be unrolled using a front-end loader
or a tractor. Panels are usually folded on pallets and
require a large crew of workers to manually unfold
and place the FML material. Placement of the FML
goes hand-in-hand with the seaming process; only
the amount of sheeting that can be seamed during a
shift or work day should be deployed at any one time
(Reference 42). On windy days, sand bags may be
used to hold the sheeting in place before and after
seaming in order to prevent the wind from whipping
and damaging the FML.
The seaming process is critical to the success or
failure of an FML installation. For this reason, several
conditions that may affect seam integrity must be
monitored and controlled during the seaming process;
control includes postponement of seaming until
conditions improve. The factors affecting the seaming'
process include (Reference 42):
• Ambient temperature at which the seams are
made,
• Relative humidity,
• Amount of wind,
• Effect that clouds have on the FML temperature,
• Water content of'the subsurface beneath the
FML,
• Supporting surface on which the seam is bonded,
• Skill of the seaming crew,
• Quality and consistency of the adhesive or
welding material,
• Proper preparation of the liner surfaces to be
joined, and
• Cleanliness of the seam interface, i.e., the
amount of airborne dust and debris present.
The bonding system used to seam the FML is
dependent on the polymer making up the sheeting.
Thermal methods of seaming require cleanliness of
the bonding surfaces, heat, pressure, and dwell time
to produce high-quality seams. The requirements for
adhesive systems are. the same as thermal systems,
except that the adhesive takes the place of the heat.
Sealing the FML to appurtenances and penetrating
structures should be performed in accordance with
detailed drawings included in the design plans and
specifications. As with the FML, the materials used to
seal the FML to appurtenances must be mechanically
and chemically compatible for use at the facility
(Reference 13, Chapter 8).
The FML must be anchored at its perimeter to
prevent sloughing or slipping down the slopes of the
containment. An anchor trench is generally used to
secure the FML along the berm of an embankment;
other anchorage systems may be employed at
appurtenances. In any case, the FML must be
anchored according to the detailed drawings provided
in the design plans and specifications (Reference 13
Chapter 8).
FMLs that are subject to damage from exposure to
weather and work activities should be covered with a
layer of soil as soon as possible after quality
assurance activities associated with the FML are
completed. When possible, the soil should be placed
without driving construction vehicles directly on the
FML. Trucks carrying soil may back over the FML as
they dump the soil in front of the path of the rear
wheels. This or a similar procedure should be used to
avoid damage to the FML caused by stones
embedded in the tires.
The FML used in a final cover system is installed
following the construction and filling of the facility.
The design and construction of the cover liner system
is detailed in the facility closure plan. Installation of
this FML is subject to the same procedural and
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quality assurance requirements as the FMLs lining the
containment. Additional information on cover systems
is provided in Chapter 5 of this Guide.
4.4.3 Leachate Collection and Removal Systems
After the LCRS design is completed and
specifications prepared, the construction will proceed
in the following general sequence:
• Sump construction and.pump installation,
• Piping installation,
• Placement of granular materials or geonet, and
• Granular filter layer or geotextile placement.
4.4.3.1 Sump Construction and Pump
Installation
Since the sumps will be the lowest points in the
LCRS, they will be constructed first. Generally the
components of a double-lined system will be sloped
toward the sumps, and the sump areas will be formed
to receive the sump structures. The drawings should
clearly show the dimensions of the sump areas and
the elevations of the low points of the inlets and of
the sumps themselves.
If the sump is concrete, the form work should be
constructed, reinforcing bars or wire installed, and the
concrete poured and allowed to cure. If precast
concrete structures are used, such as manhole
sections, these are placed before the drainage layers
and piping are installed. Again, it is critical to maintain
careful control of all elevations to assure. that the
system will drain properly.
Pumps that will remain in the sump need not be
installed until after the LCRS is completed. As
indicated in Section 4.3.3.6., leachate generally is
removed using.pumps that are portable and are not
left in the sumps between uses.
4.4.3.2 Piping Installation
The leachate collection piping is generally installed
after each liner is placed, but before the drainage
layer is constructed. The construction drawings and
specifications should clearly indicate the type of
bedding to be used under the pipes and the
dimensions of the trenches, if the pipes are placed in
trenches. The specifications should indicate how the
pipe lengths are joined. The drawings should show
how the pipes are placed with respect to the
perforations; in order to maintain the lowest possible
leachate head, there should be perforations near the
pipe invert, but not directly at the invert. The pipe
invert itself should be solid, in order to allow for
efficient pipe flow at low volumes. Geotextiles or
granular filter layers should be placed around the
pipes in the same manner as on the top of the
drainage layer, as discussed in Section 4.4.3.
4.4.3.3 Placement of Granular Materials
Granular materials are generally placed using
conventional earthmoving equipment, including
trucks, scrapers, bulldozers, and front-end loaders.
If the materials are being placed over an FML or
geotextile, then they should be placed without driving
vehicles directly over the FML or geotextile.
Coarse granular drainage materials, unlike low-
permeability soils, can be placed dry and need not be
heavily compacted. However, in order to assure that
settlement following placement will be minimized, the
granular material may be compacted with a vibratory
roller after placement and rough grading. The
elevations of the top of the drainage layer should be
carefully surveyed after final grading to assure that
the layer is of adequate thickness.
4.4.3.4 Placement of Geonets
Geonets are often used on the sidewalls of hazardous
waste disposal facilities because of their stability and
ease of installation. They should be placed with the
top ends in a secure anchor trench and the strongest
longitudinal length extending down the slope. They
should be installed with a minimum of joints or seams
on the slope. The geonets neied not be seamed to
each other on the slopes, only carefully butted or
overlapped and tied. They should be placed in a
loose condition, not stretched or in a configuration
where they are bearing their own weight in tension.
The construction specifications should contain
instructions to the contractor to follow the procedures
described above and the special installation
instructions provided with the geonet by the
manufacturer. All geonets not covered by an FML
need to be protected by a filter geotextile to prevent
clogging.
As with all components of the LCRS, the design
drawings and specifications must not contradict one
another and must match the assumptions used in the
design calculations.
4.4.3.5 Granular Filter Layeir Placement
Granular filter layers as described in Sections 4.2.3.6.
and 4.3.3.1. are provided to allow passage of liquids
while preventing the passage of fine soil particles into
the drainage layer or into the LCRS piping, thereby
preventing physical clogging of the system.
The critical placement criterion for this component,
once the materials are selected, is that of thickness.
Generally the granular filter materials will be placed
around perforated pipes by hand, forming an
"envelope," the dimensions of which should be
clearly shown on the drawings. This envelope can be
placed at the same time as the granular drainage
layer, but it is critical that it be complete, that is; that
the filter envelope protect all areas of the pipe where
the clogging potential exists. The plans and
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specifications should indicate the extent of the
envelope and the construction inspector should
observe the placement of the envelope, to assure that
it is complete.
A granular filter layer placed above the primary LCRS
must also be placed to the required thickness and
extent shown on the drawings. It is generally placed
using the same earthmoving equipment as the
granular drainage layer. The final thickness should be
checked by surveying its elevations to assure that
they are the specified distance above the top of the
drainage layer.
This filter is the uppermost layer in the LCRS;
however, most landfill designs include a buffer layer,
12 inches thick or more, to protect the filter layer and
drainage layer from damage due to traffic. This final
layer can be fairly general fill, as long as it is no finer
than the soil used to design the filter layer discussed
in Section 4.2.3.6.
4.4.3.6 Geotextile Placement
One of the advantages of geotextiles is their light
weight and ease of placement. The geotextiles are
brought to the site, unrolled, and held down with
sandbags until they are covered with a protective
layer. They are often overlapped, not seamed,
however on slopes or in other configurations they
may be sewn.
As in granular filter layers, it is critical that the design
drawings be very clear in their designation of the
extent of geotextile placement, such that no potential
route of pipe or drainage layer clogging is left
unprotected. If geotextiles are used on a slope, they
should be secured in an anchor trench similar to
those for FMLs or geonets.
Because geotextiles are vulnerable to damage from
sunlight, wind, or traffic, they should also be covered
with a layer of general fill, as soon as possible, that is
no finer than that used to select the geotextile.
4.5 Quality Assurance
A specific Construction Quality Assurance plan
(CQAP) should be developed for each construction
project that specifies the type, amount, sequence,
and frequency of inspections and testing for each
component. The CQAP should include example
reporting sheets, inspection logs, photographic logs,
problem and corrective measures reports, and any
other standardized forms to be used during
construction. There should be a place in the CQAP
documentation to record each of the observations
discussed in the following sections.
4.5.1 Low-Permeability Soil Liners
Liner failures at some facilities have been attributed to
poor construction quality assurance and quality
control. Therefore, quality assurance has become an
essential step in design and construction of
hazardous waste facilities. Detailed information is
provided in Reference 9 for construction quality
assurance of low-permeability soils at pages 18-27.
EPA considers quality assurance the highest priority
for all facility components.
Sampling and testing of the soil liner during all phases
of construction is necessary to ensure quality control.
Testing provides verification of visual inspections.
Field density and water content are two critical
parameters which must be tested frequently during
construction activities. Field and laboratory
determinations should be made for these parameters
and for hydraulic conductivity. Specific tests and
methods are provided in Exhibit 4-4 and in
References 9, 12 and 13. A specific sampling and
test plan should be determined prior to all
construction activities (Reference 9, p. 18).
4.5.2 Flexible Membrane Liners
Preconstruction quality control activities for FMLs
include inspection of the, raw materials, manufacturing
operations, fabrication operations, and final product
quality; observations related to transportation,
handling, and storage of the membrane; inspection of
foundation preparation; and evaluation of the
personnel and equipment to be used to install the
FML. Construction activities include inspection of FML
placement, seaming of the FML, installation of
anchors and seals, and placement of an upper
bedding layer, or protective cover. Postconstruction
activity includes checking for leaks in the installed
FML (Reference 9).
The quality of the FML seams and the seaming
process must be estimated from the results of
inspecting representative samples of the total material
installed in a lined facility. The quality of all materials
is assessed under a 100-percent inspection
program.
Nondestructive tests on seams are performed in the
field on an in-place FML and retain the integrity of
the FML seams or sheet being tested. Non-«
destructive test methods are listed in Exhibit 4-10
(Reference 43).
Destructive tests on seams are performed in either
the field or laboratory. The intent is to determine the
strength characteristics of a seam sample by
stressing the bond until either the seam or the FML
sheeting fails^ Destructive testing of factory and field
seam samples involves determining searn strength in
both shear and peel modes, which is performed on a
tensile testing machine (References 13 and 43).
If the test results for a seam sample do not pass the
acceptance/rejection criteria, then samples must be
cut from the same field seam on both sides of the
rejected sample location. Samples are collected and
tested until the areal limits of the low quality seam are
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Exhibit 4-10. Non-Destructive Test Methods from FML
Seams
Test Method
Detects
Probe test
Air lance
Vacuum box
Ultrasonic pulse echo
Ultrasonic impedance plane
Electrical spark test
Pressurized dual seam
Electrical resistivity
Hydrostatic test
Leak paths and unbonded edges of
seams.
Leak paths and unbonded edges of
seams.
Leak paths in seams or pinholes in
sheets.
Major voids or defective areas in the
seam.
Leak paths and unbonded factory field
seams.
Voids, pinholes, or unbonded areas
primarily in HOPE welds; it can also be
used to test solvent bonds.
Leak paths and unbonded edges of
double-wedge, thermally welded
seams where an air chamber exists
between the parallel bonds of the dual
seam.
Holes, seam unbonds, and improper
penetration seals in FML installation.
Any leaks in the FML including
pinholes, tears, seam unbonds, and
faulty attachments to penetrations.
defined. Corrective measures must be undertaken to
repair the length of searn that has not passed the
acceptance/rejection criteria. In many cases, this
involves seaming a cap over the length of rejected
seam (References 9,13, and 43).
4.5.3 Leachate Collection and Removal
Systems
Construction quality assurance (CQA) guidance for
leachate collection systems is discussed in Section
2.3.6 of Reference 9. With the exception of granular
drainage and filter layer materials which should have
soils laboratory testing, most of the CQA activities
involve observations and field testing.
4.5.3.1 Inspections
Prior to construction, all materials should be
inspected to confirm that they conform to the design
criteria, plans, and specifications. These include, as
appropriate:
• Geonets;
• Geotextiles;
• Pipe size, materials, and perforations;
• Granular material gradation and quality;
• Prefabricated structures (sumps, manholes);
• Mechanical, electrical, and monitoring equipment;
and
• Concrete forms and reinforcement.
In addition, the LCRS foundation (FML or low-
' permeability soil liner) should have been inspected
and surveyed upon its completion to ensure that it
has proper grading and is free of debris and liquids.
During construction, the following activities, as
appropriate, should be observed and documented:
« Pipe bedding placement including quality,
thickness, and areal coverage;
• Pipe network installation including location,
configuration, grades, joints, filter layer
placement, and final flushing] if necessary;
• Granular drainage layer placement including
thickness, coverage, compaction, and protection
from clogging by runoff;
• Geonet placement including layout, overlap,
seaming, weather conditions, and protection from
clogging by runoff;
• Granular filter layer placement including material
quality and thickness;
• Geotextile layer placement including coverage,
overlap and seaming;
• Sumps and structure installation; and
• Mechanical and electrical equipment installation
including testing.
4.5.3.2 Testing
In addition to field observations, actual field and
laboratory testing should be performed to assure that
the materials meet the specifications and that they
will perform adequately after construction. These
activities should also be documented in a manner
similar to the field observations. They include the
following:
• Geonet and geotextile layer sampling and testing;
• Granular drainage and filter layer sampling and
testing for particle sizing; and
• Testing of pipes for leaks, obstructions and
alignments.
Upon completion of construction, each component
must be inspected to assure that it has not been
damaged during its installation or during construction
of another component, e.g., pipe crushing during
placement of granular drainage layer. Any damage
that does occur must be repaired, and these
corrective measures must be documented in the CQA
records.
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4.6 References
1. Code of Federal Regulations, Title 40, Parts 199
to 399, July 1, 1987.
2. Federal Register. 1986. Proposed Codification
Rule for Minimum Technology Requirements for
Double-Liner Systems. Vol. 51, No. 60, March
28, 1986.
3. Federal Register. 1987. Draft Minimum
, Technology Guidance Documents for Single and
Double Liner Systems. Vol. 52, No. 74, April 17,
1987.
*
4. EPA/OSW. 1987. Background Document on
Bottom Liner Performance in Double-Lined
Landfills and Surface Impoundments.
EPA/530-SW-87-013. U.S. Environmental
Protection Agency, Washington, D.C.
5. EPA/OSW. 1987. Guidance on Implementation
of the Minimum Technological Requirements of
HSWA of 1984, Respecting Liners and Leachate
Collection Systems. EPA/530-SW-87-012,
May 24, 1985. U.S. Environmental Protection
Agency, Washington, D.C.
6. EPA/OSW. 1987. Draft Minimum Technology
Guidance on Single Liner Systems for Landfills,
Surface Impoundments, and Waste Piles--
Design, Construction, and Operation.
EPA/530-SW-85-013, May 24, 1985. U.S.
Environmental Protection Agency, Washington,
D.C.
7. EPA/OSW. 1987. Draft Minimum Technology
Guidance on Double Liner Systems for Landfills
and Surface Impoundments-Design,
Construction, and Operation. EPA/530-SW-
87-014, May 24, 1985. U.S. Environmental
Protection Agency, Washington, D.C.
8. Grube, W.E., Jr., M.H. Roulier, and J.G.
Hermmann. Implications of Current Soil Liner
Permeability Research Results. In: The 13th
Annual Research Symposium, Land Disposal
Remedial Action, Incineration, and Treatment of
Hazardous Waste. EPA/600/9-87/015, pp. 9-
25. July 1987.
9. Technical Guidance Document: Construction
Quality Assurance for Hazardous Waste Land
Disposal Facilities; Hazardous Waste
Engineering Research Laboratory, Office of
Research and Development, U.S. Environmental
Protection Agency; Cincinnati, Ohio 45268; EPA
Contract No. 68-02-3952, Task 32; October
1986.
10. ASTM 1987, The American Society for Testing
and Materials 1987 Annual Book of ASTM
Standards, Volume 4.08, Soil and Rock;
Building Stones, Philadelphia, Pennsylvania.
11. Goode, D., Evaluation of Simplified Technique
for Predicting Moisture Breakthrough of: Soil
Liners, National Conference on Management of
Uncontrolled Hazardous Waste Sites,
Washington, D.C., October 31 - November 2,
1983.
12. Technical Resource Document: Design,
Construction, and Evaluation of Clay Liners for
Waste Management Facilities; Hazardous Waste
Engineering Research Laboratory, Office of
Research and Development, U.S. Environmental
Protection Agency; Cincinnati, Ohio 45268;
EPA/530-SW-86-007F, September 1988.
13. Technical Resource Document: Lining of Waste
Impoundment and Disposal Facility; Solid and
Hazardous Waste Research Division, Municipal
Environmental Research Laboratory, U.S.
Environmental Protection Agency, Cincinnati,
OH 45268; SW870; March 1983. (Revised
version Reference 19)
14. Anderson D. and S. Jones, Clay Barrier -
Leachate Interaction, National Conference on
Management of Uncontrolled Hazardous Waste
Sites, Washington, D.C., October 31 -
November 2, 1983.
15. Foundations and Earth Structures, Naval
Facilities' Engineering Command, Design Manual
7.02, September 1986.
16. Koerner, R. M. 1986. Designing With
Geosynthetics. Prentice-Hall, Englewood Cliffs,
NJ.
17. EPA. 1982. Landfill Design, Liner Systems, and
Final Cover. Draft RCRA Guidance Document,
July 1982.
18. EPA. 1982. Surface Impoundments, Liner
Systems, Final Cover, and Freeboard Control.
Draft RCRA Guidance Document, July 1982.
19. Lining of Waste Containment and Other
Impoundment Facilities. Draft Technical
Resource Document. EPA-600/2-88-052,
September 1988.
20. Moore, Charles A.; Landfill and Surface
Impoundment Performance Evaluation; USEPA
SW-869;1982.
21. Richardson and Koerner; Geosynthetic Design
Guidance for Hazardous Waste Landfill Cells
and Surface Impoundments; EPA-600/2-87-
097; NTISPB88-131263; December 1987.
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22. The Hydraulic Evaluation of Landfill Performance
(HELP) Model - Volume 1. User's Guide for
Version 1; USEPA; June 1984.
23. Hydraulic Evaluation of Landfill Performance
(HELP) Model - Volume 2. Documentation for
Version 1; USEPA, June 1984.
24. ASCE, WPCF; Design and Construction of
Sanitary and Storm Servers, ASCE Manual on
Engineering Practice No. 37, WPCF Manual of
Practice No. 9; 1969
25. GCA Corporation; Draft Permit Writers'
Guidance Manual for Hazardous Waste
Treatment, Storage, and Disposal Facility, 1983.
26. Perrier, E.R. and Gibson, A.C., Hydrologic
Simulation on Solid Waste Disposal Sites, U.S.
Environmental Protection Agency, Report SW-
868, Washington, D.C., 1980. PB 81-166-
332.
27. Technical Manual: Guide to the RCRA Land
Disposal Permit Writers' Training Program,
Volume 1, Prepared by GCA Corporation, U.S.
Environmental Protection Agency, Land Disposal
Branch, Washington, D.C. 20460, EPA Contract
No. 68-02-3168, September 1984.
28. Anderson, D.C., J.O. Sai, and A. Gill,
Permeability and Morphology of a Soil Liner
Permeated by Acid, and Field Permeability
Testing for Soil Liners, Division of Solid and
Hazardous Waste Research, Municipal
Environmental Research Laboratory, U.S.
Environmental Protection Agency; Cincinnati,
Ohio 45268; EPA Contract No. 68-03-2943.
29. EPA SW-846, Test Methods for Evaluating
Solid Waste, September 30, 1986. U.S. EPA,
Washington, D.C.
30. American Society For Testing and Materials.
Issued Annually. Annual Book of ASTM
Standards; Section 8, Vols. 8.01, 8.02, 3.03,
and 8.04; Section 9, Vols. 9.01 and 9.02.
American Society for Testing and Materials,
Philadelphia, PA.
31. U.S. General Services Administration. 1980.
Federal Test Method Standard 101C. U.S.
General Services Administration, Washington,
D.C.
32. Daniel, D.E. and K.W. Brown, Landfill Liners:
How Well Do They Work and What is Their
Future? Chapter 19, in: Remedial Action and
Case Histories.
33. Haxo, H. E. 1983. Analysis and Fingerprinting of
Unexposed and Exposed Polymeric Membrane
Liners. In: Proceedings of the Ninth Annual
Research Symposium: Land Disposal,
Incineration, and Treatment of Hazardous
Waste. EPA-600/9-83-018, pp. 157-171,
Sept. 1983. U.S. EPA, Cincinnati, OH.
34. Landreth, R. E. 1987. The EPA Testing Program
for Components of Treatment, Storage, and
Disposal Facilities. Presented at the
Geosynthetics '87 Conference, February 24-
26, 1987, New Orleans, LA
35. EPA. 1986. Supplementary Guidance on
Determining Liner/Leachate Collection System
Compatibility. U.S. EPA Office of Solid Waste
and Emergency Response, Policy Directive No.
9480.00-13, August 7, 1986.
36. EPA. 1987. FLEX: Flexible Membrane Liner
Advisory Expert System User's Guide, Version
2.0. U.S. EPA, Cincinnati, OH.
37. ASTM Annual Book of ASTM Standards, Volume
04.01 Cement; Lime; Gypsum, Volume 04.02
Concrete and Mineral Aggregates, Philadelphia,
PA.
38. American Concrete Institute Building Code
Requirements for Reinforced Concrete, AC I
318-83, ACI, Detroit, Ml.
39. American Concrete Institute Concrete Sanitary
Engineering Structures, ACI 350 R-83, ACI,
Detroit, Ml.
40. American Concrete Institute Building Code
Requirements for Structural Plain Concrete, ACI
318.1-83, ACI, Detroit, Ml.
41. ASTM; Annual Book of ASTM Standards;
Philadelphia, PA Volumes 01.04 and 01.05
Steel.
42. Haxo, H. E. 1986. Quality Assurance of
Geomembranes Used as Linings for Hazardous
Waste Containment. Geotextiles and
Geomembranes, No. 3 (1986), pp. 225-247.
Elsevier Applied Science Publishers Ltd.,
England.
43. SCS Engineers. 1987. Manual of Procedures
and Criteria for Inspecting the Installation of
Flexible Membrane Liners in Hazardous Waste
Facilities. U.S. EPA, Hazardous Waste
Engineering Research Laboratory, Cincinnati,
OH.
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CHAPTER 5.0
Cover Systems
Land disposal unit covers are the final component in
the design of a land disposal management system. As
the protective outer layer placed on a landfill or a
disposal impoundment after it has been filled, the
cover should isolate the wastes from the
environment. Specifically, the cover must be
designed to minimize the infiltration of surface water,
thus minimizing liquid migration and leachate
formation. To achieve this performance standard, the
owner/operator of a land disposal facility must design
and construct a multi-layered cover system that can
function with minimum maintenance. Generally, the
system will include:
• an uppermost vegetated layer to prevent erosion
and promote evapotranspiration;
• an underlying drainage layer to convey
percolation out of the cover; and
• a moisture barrier to prevent infiltration.
Each of these layers is constructed either of natural
soil materials or of synthetic materials including
geomembranes (FMLs), geonets, and geotextiles.
This chapter provides an overview of current
regulatory performance standards and procedures for
the design, materials selection, construction,
maintenance, and quality control of cover systems.
5.1 Regulations and Performance
Standards
The performance standards set forth in 40 CFR
264.228 and 264.310 state that disposal
impoundments and land disposal units must have
covers that provide long term minimization of liquid
migration. The covers must be able to function with
minimum maintenance, promote drainage to minimize
erosion and abrasion, accommodate settling and
subsidence without losing integrity, and have a
permeability no greater than the bottom lined system.
The permitting standards of 40 CFR 270.21 (e) require
the permit applicant to provide detailed plans and an
engineering report of the cover design as part of the
closure plan for the facility. These plans and report
must demonstrate through calculations and
specifications that the cover will function as required.
The multi-layer cover illustrated in Exhibit 5-1 has
been provided as an example of a design that would
meet minimal RCRA requirements as defined in
Exhibit 5-1. Landfill Cover System Components
60-cm vegetable layer
Fabric filter or soil layer (10 cm)
30-crn Drainage layer (ka10-3 cm/sec)
20-mil synthetic membrane
(with soft bedding above and
below
60-cm Low permeability layer
(k<10"7 cm/sec)
Source: USER A, 1987 (Ref 1), p. 3
current EPA guidance (Reference 1). The
performance objectives for each layer will be
discussed in Section 5.2.6 Cover System Bements.
Actual geological settings are complex and can differ
greatly; therefore, the minimal design shown cannot
be directly applied to any one site. A technical
analysis must be performed by a qualified design
engineer registered in the state to guarantee that the
performance standards for each cover system layer
have been achieved in a particular geological setting
5.2 Design
To design a cover system, the characteristics of the
site and the wastes in place must be accurately
assessed. With this information, site and waste
constraints, such as settlement and subsidence
potential, may be calculated. Following this evaluation,
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the requirements for cover composition and
configuration will be selected. Reference 1 provides
technical guidance on the design process. Reference
2 describes a recommended 39-step approach to
evaluating cover designs. The major design elements
to consider will be discussed in the following section.
5.2.1 Site Characterization
Site characterization is of primary importance
throughout the design process for a land disposal
facility beginning with the initial siting of the facility.
For cover design, site characterization directly
impacts the criteria chosen for material selection and
design to prevent erosion and to promote; the
establishment of hardy vegetation. The following
discussion will introduce several key aspects of site
characterization and their impacts on cover design.
5.2.1.1 Topography
Topography becomes a major factor in cover design
when the landfill is sited in areas with hilly terrain or
canyons and where surface impoundments may be
below ground. In these environments, a considerable
amount of surface water run-on should be expected,
and the designer must be prepared to manage or
prevent this surface run-on from traveling onto the
cover. The designer should address the potential
problem by performing the routine analyses of surface
water flow for the surface water management
requirements described in Chapter 6.0. It may be
determined after analysis that traditional perimeter
diversion systems will not be adequate and that other
designs need to be considered. One option is to
construct a central drainage system through the
center of the landfill. For a flat site without natural
positive drainage, the cover system must be designed
to provide positive drainage of precipitation off the
site to prevent ponding over the cover.
5.2.1.2 Precipitation
The intensity, duration, and frequency of storms,must
be determined to calculate the volume of surface
water run-on or run-off that must be managed. The
rate of infiltration (percolation) will directly impact the
design of the drainage layer. An analysis of local
precipitation patterns will also provide information on
whether a potential for flooding exists. If there is
flooding potential, the flood characteristics (e.g.
stagnant backwater or scour potential due to flow)
must be evaluated and measures designed to prevent
damage to the cover or ponding on the cover. Local,
site-specific precipitation data should be used
whenever available for design calculations. Average
annual precipitation maps developed by the U.S.
Department of Agriculture and the National Weather
Service are appropriate for use in review of designs.
Section 2 of Reference 2 provides a more in-depth
discussion of the review of precipitation data.
Precipitation data, particularly the annual distribution
of rainfall, is also critical to the selection of the types
of vegetation to be established on the cover. Since
closure performance standards require that
maintenance be minimized, vegetation should be
selected that would adapt to the environment with a
minimum amount of irrigation.
5.2.1.3 Other Cllmatological Data
In climates that experience freezing temperatures or
drought, the upper surface layers of a land disposal
unit cover may be damaged by the buckling or sliding
of layers after thawing or by cracking during extended
periods of drought. As a general rule, the
geomembrane and the top of the low permeability soil
layer, are the most susceptible to damage due to •
severe weather and should be placed below the
depth of freezing or severe drying. Freezing also
increases the amount of surface water run-off
expected during winter months, as percolation
through frozen ground is limited. This fact should be
considered in run-off dischairge calculations for
drainage channel design. Freezing indexes illustrated
as map contours have been developed by the U.S.
Weather Bureau. Indices showing the severe drought
regions of the country are available from regional Soil
Conservation Service offices. As was true for
precipitation data, the more site-specific data
available, the more accurate the design calculations.
Reference 1 discusses the influence climatology has
on cover design in Section 3. Reference 2 addresses
the review of climatological data in Section 3.
5.2.1.4 Soils
An assessment of the properties of the in-situ soils,
while not a constraint in the design of the cover
system, is important from a cost-effectiveness
standpoint. Considerable savings could be gained if
site material can be used as part of the intermediate
or final cover system. Soil tests run under the
direction of a qualified geotechnical engineer for land
disposal unit siting and design are useful sources of
information during the material selection process. A
more detailed discussion of soil properties is provided
in Section 5.3, Materials, of this chapter.
5.2.1.5 HELP Model
To assist the designer in determining the influence
that site characterization factors will have on the
performance of a cover system design, a computer
model was developed called Hydrologic Evaluation of
Landfill Performance (HELP). The HELP Model was
designed by the U.S. Army Corps of Engineers
Waterways Experiment Station (WES) for the U.S.
EPA Municipal Environmental Research Laboratory.
The Model is generally accepted for designing landfill
cover layer systems and for comparing alternative
cover and total landfill configuration designs.
(Reference 3, Section 1). Use of the Model in design
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of leachate collection systems is discussed in
Chapter 4.0.
The HELP program calculates daily, average and
peak estimates of water movement across, into,
through, and out of landfills. The input parameters for
the model include soil properties, precipitation and
other climatological data, vegetation type, and landfill
design information. Default climatologic and soil data
are available but should be verified as reasonable to
expect in the particular site setting. Outputs from the
model include precipitation, runoff, percolation
through the base of each cover layer subprofile,
evapotranspiration, and lateral drainage from each
profile. The Model also calculates the maximum head
on the barrier soil layer of each subprofile and the
maximum and minimum soil moisture content of the
evaporative zone. Data from the model are presented
in a tabular report format and include the input
parameters used and a summary of the simulation
results. Results are presented in several tables of
daily, monthly and annual totals for each year
specified. A summary of the outputs is also produced,
which includes average monthly totals, average
annual totals and peak daily values for various
simulation variables. (Reference 3, Sections 4 and 5)
Use of the HELP model should not be attempted
without reading the User's Guide, Reference 3, or the
Model Documentation, Reference 4, both prepared by
the designers of the program.
5.2.2 Waste Characterization
Cover settlement has been determined to be caused
by primary consolidation and secondary compression
of the waste mass, underlying natural soils, and from
collapse of voids or cavities in the fill and around
containers. Primary consolidation occurs when the
void ratio of a soil or waste is decreased due to the
expulsion of fluids from the voids under excess
hydrostatic pore pressure. Secondary compression
occurs by deformation of the skeletal structure of the
mass and compression of gases in the voids. The
collapse of voids or cavities is due to corrosion,
oxidation, combustion, or biochemical decay of the
landfilled materials. The designer should be aware of
the distribution of void spaces and other physical
conditions of the waste at the time of burial, the
waste placement operations (e.g. lift thickness,
compactive effort, etc.) and the chemically-related
changes due to the composition of the wastes that
may take place over a long period (Reference 1, p. 9,
Reference 5, p. 2).
Wastes which enter the land disposal unit are either
disposed of in bulk or in containers. Bulk wastes may
exhibit the settlement characteristics of soils in that
they continue to consolidate over time, but at a
steadily decreasing rate depending upon the physical
characteristics of the waste and the methods of waste
placement. To assist in the settlement analysis,
recent efforts have been made to determine the
engineering properties of several types of wastes
through laboratory testing. Results of these efforts are
presented in Reference 5 in Section 2. The laboratory
analyses, however, should never be used as more
than a general guide to expected properties. The
wastes reported to have been disposed of at a given
facility should be evaluated to the extent possible for
a site-specific determination.
Containerized wastes do not behave as predictably as
bulk wastes. Consolidation of drummed wastes
occurs at a considerable period of time after waste
placement when drum deterioration occurs. An
acceptable, accurate analytical method is not
currently available for prediction of the time and
extent of this later settlement due to container
deterioration. However, the designer should address
the potential for future subsidence due to the disposal
of containers and qualitatively approximate the
potential damage.
Another important characteristic of land disposal unit
wastes which directly effects settlement is the
percent of void space within the cell configuration of
wastes. An estimate of the effects these void spaces
will have on long term settlement is required. Often,
sufficient attention is not given to filling the void
spaces between containers within landfill cell lifts.
When the lifts have not been properly backfilled, void,
spaces several rows deep may be left as channels for
the downward migration, or piping, of backfill. Backfill
piping can cause differential settlement and damage
to the cover.
The chemical composition of the wastes must be
carefully reviewed to determine gas generation
potential. Diversions and vents may be required in the
design to provide a release pathway for gases
blocked by the cover from upward migration. If low
concentrations of toxic components are expected,
vents directly to the atmosphere may be adequate for
dispersion in the air at acceptable levels. It may be
necessary to provide on-line or contingency features
for absorption filters or other means of reducing
concentration of toxic components if the potential
exists for the gas or volatile component to reach
harmful concentrations (Reference 1, p. 13). Gases
evolve from the decay or biodegradation of buried
organic matter; thus, gas control (venting) is
principally a concern at municipal waste, not
hazardous waste, landfills.
5.2.3 Settlement/Subsidence
A potential threat to the integrity of the cover is
uneven settlement of the wastes and fill that comprise
the foundation of the cover. Recent guidance
(Reference 5) has been published regarding the
prediction/mitigation of subsidence damage to covers
and will be briefly summarized in the following
paragraphs.
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Long-term settlement of hazardous waste 'land
disposal units should be analyzed on the basis of the
deformation of the waste layers and the deterioration
of the waste containers. Settlement due to
deformation of the waste layers is most likely to occur
after closure of the land disposal unit and final
placement of the cover. Therefore, this type of
settlement has more potential to cause subsidence
damage to the cover than consolidation settlement,
much of which can occur or can be made to occur
prior to closure. (Reference 5, p. 19)
Several models have been developed to analyze the
process of differential settlement. Most equate the
layered cover to a beam or column undergoing
deflection due to various loading conditions. While
these models are useful to designers in
understanding the qualitative relationship between
various land disposal unit characteristics and in
identifying the constraining factors, accurate
quantitative analytical methods have not been
developed (Reference 5, Section 4).
If settlement is anticipated, several design options are
available. For example, the cover thickness can be
designed such that after displacement occurs,
drainage of run-off is still adequate. Exhibit 5-2
illustrates this design compensation method. Another
option is to increase the side slopes of the cover.
(References, p. 71).
In summary, although settlement has the potential to
seriously damage a land disposal unit cover, the
analytical methods available to estimate the effects
are still inexact and require additional experimentation
and field observation. For now, the designer of land
disposal unit covers should determine whether the
potential for settlement exists due to the type of
wastes and landfilling procedures used and design
the cover to provide a tolerance for settlement effects
(Reference 5, Section 5).
5.2.4 Slops Stability
One threat to the continued soundness of the cover is
displacement due to the slope instability. Slope
stability analyses should be performed to assess the
potential for slope failure by various failure modes
(e.g., rotational, sliding, wedge), as appropriate,
based upon the slope configuration. To adequately
perform the stability analyses, the strength properties
of the cover system components, the waste, and the
foundation soils must be known along with seepage
conditions. A detailed discussion of slope stability can
be found in Chapter 3 of this document.
5.2.5 Erosion Potential
In addition to ensuring embankment slope stability,
the designer should design the cover to minimize soil
erosion. To assist the designer in predicting erosion
potential of various design options, EPA recommends
use of an empirical, formula called the Universal Soil
Loss Equation (USLE) which is used to calculate the
average annual soil loss. The average annual soil loss
is predicted based upon a number of factors including
the geographical location, the length and steepness of
slopes, the texture of the cover soil, and the
vegetation established.
Exhibit 5-2. Thickened Cover for Tolerance of Settlement
5 percent slope
a. Before Settlement
Potential cracks
b. After Settlement
c. Thickening cowr before
and after settlement
Reprinted: USEPA, "Design of Cover Systems," 19S7 (Ref. 1)
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Reference 1 provides complete instructions for use of
the USLE. Also provided in Reference 1 on pages 37
through 42 are various tables and figures from which
input parameters can be selected. Cover design
features that are used to prevent erosion include the
establishment of vegetation and the construction of
terraces or benches. Current EPA guidance suggests
that the average annual soil loss not exceed 2.0
tons/acre in order to minimize the potential for gully
development and future maintenance. (Reference 6,
P- 29)
5.2.6 Cover Systems Elements
The requirements for the cover are performance-
based; therefore, the elements incorporated into the
design may vary depending on the type of materials
available, the environment in which it is placed, and
the type of wastes landfilled. However, the cover
components that are shown in Exhibit 5-1 are found
in most cover systems and will be described in the
following section. Reference 1 was used as the main
source of information for the following description of
the cover system components.
Each of the elements must be shown on the cover
design drawings. At a minimum, the drawings that
should be included are:
• plan view of the landfill or disposal impoundment;
• cross-sections at several locations;
• details where cover components key into the liner
system;
• details for cover connections between adjacent
cells; and
• details of penetrations such as leachate removal
manholes or gas vents (if required).
Drawings should clearly show the cover
configurations, the dimensions (thickness, slope) of
each element, and the finished elevations of the tops
of the soil layers at critical points. These drawings
should reflect the same dimensions and slopes used
in the design calculations, especially those involving
run-off controls, percolation estimates, and drainage
layer performance.
5.2.6.1 Foundation, Backfill
The foundation or foundation layer serves several
purposes. As the intermediate layer between the
wastes and the cover, the backfill brings the landfill
cell to final grade in preparation for cover placement
and it serves as protection to the cover from the
direct impact of settlement within the waste cell.
Accordingly, the backfill will distribute the load and
deformation imposed on the cover.
5.2.6.2 Low-Permeable Soil Layer
A two-foot thick low-permeable soil layer is
recommended by EPA guidance in the design of
cover systems. The low-permeable soil layer is
commonly referred to as the secondary hydraulic
barrier. (The* geomembrane is considered the primary
hydraulic barrier.) This soil layer is designed and
constructed in a manner similar to that used for soil
liners, with the following differences:
• Since the soil component of the cover is not
exposed to waste leachate, liner-leachate
compatibility is not considered.
• Since the cover is generally not subjected to
large overburden loads, the issue of compression
is less critical, unless post-closure land use will
exert large loads. '
• The soil cover, however, is subject to the
loadings of construction equipment that is used
during placement and compaction of the lifts, and
during placement of the overlying drainage layer
and topsoil. This type of loading is discussed in
Chapter 4.0.
• The soil cover is also subject to loadings from
settlement, as discussed in Section 5.2.5.1. The
settlement potential should be evaluated and the
cover and post-closure maintenance plan
designed to compensate for future settlement.
For a further discussion of soil liners, refer to Chapter
4.0.;
5.2.6.3 Geomembrane (Synthetic Membrane)
The primary hydraulic barrier is recommended by
EPA guidance to be a geomembrane of at least 20
mil. thick. When used in cover design, the material
properties of major concern include puncture
resistance, burst strength, tensile strength,
permeability, freezing effects, heating effects, and
temperature cycling. For a complete discussion of
geomembranes, refer to Chapter 4.0 (on FMLs).
5.2.6.4 Drainage Layer
The drainage layer provides a passageway for the
rapid removal of rainfall infiltration from the center and
for the horizontal migration of any gases that may
have permeated from the wastes through the
geomembrane and low-permeable soil layer. The
drainage layer should be constructed of materials
exhibiting a minimum hydraulic conductivity of 1 x
10-3 cm/sec and be at least 12 inches thick to
accommodate infiltration from major storms. A slope
of at least two percent after allowing for settlement is
recommended in order to provide positive drainage
through the layer. The most common materials used
are narrowly graded granular soils such as sand or
gravel. To enhance the effectiveness of the drainage
layer, perforated pipes or vents can be used.
Alternate drainage layer designs, e.g. use of geonets,
are commonplace and acceptable. However, the
permit applicant should demonstrate that an alternate
design will perform as effectively as the
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recommended design. Section 5 of Reference 1 and
Section E of Reference 6 provide guidance on the
design of the drainage layer.
5.2.6.5 Filters
EPA guidance recommends that the drainage layer be
overiain with a graduated granular soil or a geotextile,
to prevent clogging of the porous layer with the
overlying soil. The purpose of the filter is to block the
downward migration of particles with the percolating
water. The filter material, therefore, should have
intermediate pore sizes which can provide a
framework for particle bridging. A concern has been
raised that filters may be permanently damaged by
physical deterioration or biological or chemical
deposits; however, insufficient field data is available
on the service life of filters, fabric or soil. Detailed
information regarding the design of filter materials is
provided on pages 59 through 61 of Reference 1. In
addition, the design and construction of both the
drainage layer and the filters is very similar to the
LCRS discussed in Chapter 4.0 of this Guide.
5.2.6.6 Vegetated Topsoil
The final cover component, the vegetated topsoil
layer, protects the other cover layers from the effects
of wind and water erosion. The topsoil layer is
generally two feet thick to accommodate the roots of
the vegetation. Rnal thickness design should consider
the water needs of the vegetation and the water
holding capacity of the soil. In addition, the combined
thickness of the drainage layer and topsoil layer
should be greater than the local frost depth to prevent
frost damage to the geomembrane and low-
permeable soil layer.
The major criteria considered for selection of the
topsoil are soil type, nutrient and pH levels, climate,
species selection, mulching, and seeding time. The
most desirable soil to use is loam, a balanced mixture
of clay, silt and sand. Loam is preferred because it is
relatively easy to maintain in good condition, it
provides a conducive environment for seed
germination, and it is easily penetrated by roots.
The condition of a soil for purposes of topsoil
selection is determined by its pH level, nutrient
content, and other favorable physical properties such
as workability, either in the native or amended state.
Generally, it is recommended that the soils be
maintained at a pH of 6.5. (Reference 2, p.46). Lime
applications may be necessary to achieve this pH
level. The primary nutrients of concern are nitrogen,
phosphorus, potassium and organic matter. A table is
provided on page 47 of Reference 2 that indicates the
acceptable ranges of organic matter content in
different soil types and an approximate range of |0ther
nutrient levels. Reference 7 also provides general
guidelines for the application rates of the various
nutrients to soils that are determined to be in "poor
condition."
The major characteristics that should be considered
during species selection, as recommended by
Reference 2, include low growing and spreading from
rhizomes or stolons, rapid germination and
development, and resistance to fire, insects, and
disease. Grasses and legumes are usually selected.
Reference 2 provides a table on page 49 that lists the
important characteristics for species selection and
provides examples of grasses and legumes that
exhibit those characteristics.
Planting or seeding is generally recommended for the
fall or very early spring. It is important for the
seedlings to establish a root system during a cool,
moist period before enduring a winter freeze or
summer drought. Recommended planting times are
discussed in detail on page 50 of Reference 2.
Many good resources are available to assist the
designer in the selection of the plant species, the
seeding rates and seasons, and the fertilization
applications required. In addjtion to the EPA
references mentioned above, agronomists familiar
with the local soils and climate should be consulted.
These experts can be found at the local county
agricultural extension or soil conservation service
office.
Chapter 8 of Reference 1 provides a good discussion
of the design and maintenance of the final vegetative
layer. Reference 7 offers a more in-depth discussion
of the standardized procedures for planting vegetation
on completed disposal unit covers.
Sites in arid regions of the country tend to use a
granular cover surface instead of a topsoil layer. This
is done because conditions are such that the
vegetation is very difficult to establish and keep
growing due to low rainfall. In those cases, the
organic and chemical make-up of the topsoil layer is
not as important. The surface layer, usually
constructed of gravel or rock, must be designed to
prevent erosion of the surface.
5.2.6.7 Boundaries
Lateral boundaries of the outer edges of the cover
system are considered critical elements to ensure the
integrity of the entire cover. The cover boundary
serves to block infiltration from entering the outer
edges of the landfill, and to key the cover system into
the liner system or to the site geological structure. In
addition, the boundary of the cover system must
provide a path for free drainage of water from the
drainage layer. One simple edge feature could be a
vertical low-permeable soil barrier wall that connects
the hydraulic barrier in the cover to a low-permeable
soil stratum in the site media. In many landfills, an
acceptable boundary feature can be simply
overlapping or welding of the cover geomembrane
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across the FML. Another suggested alternative, if
practical, is to extend the outer edge of the cover
beyond the outer edge of the landfill liner system and
provide a separate anchor trench for the cover layers.
A detailed discussion of the alternatives to consider
for boundary design is included in Section 4 of
Reference 1 on pages 72-75.
5.3 Materials
As discussed in the design sections, the materials
generally used for covers include sand, gravel, soil,
geotextiles, and geomembranes. Other materials that
are used less frequently include geonets, fly ash,
pordand cement concrete, bituminous concrete, and
seal coats. Due to the high costs of importing
materials, the common practice is to use, modify or
amend native soils or other available site material.
Adequate characterization of the soils through proper
testing is essential. Material testing is required during
design to ensure that available soils can achieve the
cover layer performance criteria. Material testing is
also critical during construction to confirm that the
materials and modification procedures were sufficient
to achieve the design specifications for the various
cover materials. The testing done during construction
will be discussed in Section 5.4;
Current EPA guidance, Reference 1, recommends
the use of standard tests when specifying material
characteristics and properties. Preference is given to
tests sanctioned by the American Society for Testing
and Materials (ASTM). Each layer of the cover
system has specific performance goals, therefore,
different tests are required. A number of tests,
however, should be specified as a minimum for each
layer to determine soil characteristics and
performance properties. Soil characteristics tests
include particle-size distribution, Atterberg limits, soil
classification, and water content. Standard
performance properties tested include compaction
and permeability. Chapter 2.0 of this Guide and
Section 3 of Reference 1 contain a detailed
discussion and a table of the tests available for use in
materials selection.
Reference 1 (pp. 26-28) also provides an extensive
table ranking the suitability of various soils for some
of the specialty cover functions such as trafficability,
water percolation, gas migration, erosion control,
reduction of freeze action, crack resistance, slope
stability, and vegetation support. The table ranks soils
from I through XIII for a general indication of the
"best" through "poorest" material for each cover
function. Reference 2 (p. 20) suggests that soils rated
as IV or higher for a specific function require
modification prior to being selected for that use.
The cover foundation is a non-select soil present at
the site and is used to establish a uniform base layer
for the cover system. If this base layer will be directly
overlain by a geomembrane, rather than a low-
permeable soil layer, then the soil must be fine and
free of pebbles and clods that could puncture the
geomembrane.
The specifications required for the low permeability
soil layer and the geomembrane are identical to those
required for liner systems, and are described in
Chapter 4.0 of this Guide.
The drainage layer due to its function, requires a high
permeability which can be determined from the
standard tests described in Chapters 2.0 and 4.0 of
this Guide. The filter layer must be properly graded
based upon gradation of the drainage layer to prevent
its finer particles from migrating below and clogging
the drainage layer. Reference 2, p. 34 and Reference
1, p. 60 provide standard criteria to use to select
properly graded filter media.
If a geotextile is used, an equivalent opening size
(EOS) and percent open area are used as the criteria.
Specific geotextile criteria are outlined on page 61 of
Reference 1. Additional performance tests commonly
specified for geotextiles can include, as appropriate,
tensile strength, bursting strength, puncture strength,
abrasion resistance, seam breaking, permeability,
grab strength, and grab elongation. A table of the
recommended standard methods and requirements
for these tests is provided on page 167 of the
example contract specifications in Reference 1.
The soil for the vegetated cover layer requires
specific testing for pH and buffering capacity as well
as nutrient content. The nutrients that should be
tested for include, in order of importance, nitrogen,
organic matter, phosphorus, and potassium.
5.4 Construction
The major tasks of construction of a cover are:
• excavation of borrow for cover materials
• material preparation (blending, amendment)
• material placement
• compaction
• geomembrane placement
Standard construction procedures used in building
and roadway earthwork also are used for construction
of covers. Crews must be made aware that it is
critical that the integrity of the cover layer beneath the
working surface be protected from damage at all
times. This caution may preclude the use of certain
earthwork equipment such as sheepsfoot rollers.
Sheepsfoot rollers have steel feet designed to help
interlock lifts in foundation construction. This .feature
could, however, penetrate the relatively thin barrier
layers used for covers. Another limitation on selection
of equipment is the need to use equipment whose
weight is within the allowable load-bearing range of
the cover components. Detailed guidance on
construction techniques is available in Section 6 of
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Reference 1. An outline of construction techniques to
note during a review is shown on page 36 of
Reference 2. The following sub-sections will
highlight general construction issues.
5.4.1 Excavation
The primary concern during the excavation phase is
that the materials excavated are of the desired
quality. Sufficient test borings and laboratory tejsting
should be conducted to determine if the material
meets specification, and to determine the extent of
available borrow.
5.4.2 Soil Material Preparation
Material preparation can be accomplished by blending
or modification with additives. Soils can be modified
with additives such as lime, bentonite, cement or
asphalt. Soil amendment is typically performed to
enable the use of in-situ soils rather than imported
material. Prior to using the blended or modified soil
mixtures, tests should be run to verify that the mixture
will meet the design specifications.
5.4.3 Soil Placement
Soil or material placement is the most critical phase
of the construction process. Soils must be spread
evenly and, as discussed previously, specific
minimum thicknesses must be achieved. Soil
placement must be carefully supervised to avoid
disturbance of the underlying layer of cover. As an
extra precaution during construction over the top of
the geomembrane layer, Reference 1 recommends
the placement of a buffer layer of soil or a fine mesh
geogrid above the geomembrane.
If the cover will be placed over a disposal
impoundment, placement should not occur until waste
solidification has been completed. Because
solidification may take several days or weeks, this
period should be noted in any construction schedules
or accompanying specifications.
5.4.4 Soil Compaction
Compaction of the soil layers is performed to increase
the strength of the soil through the process of
densification. For low-permeable soil layers serving
as moisture barriers, compaction is also necessary to
achieve the desired low permeability, as discussed in
Chapter 4.0 of this Guide. Compaction is generally
required for all layers except the topsoil where
compaction would prevent proper root growth.
When cover layers exceed six inches in thickness, it
is recommended that placement occur in lifts. While
different lift thicknesses have been studied, six-inch
lifts are generally recommended as optimum.
5.4.5 Geomembrane Installation
Geomembrane installation involves several steps
including: placement of strip panels, seaming the
panels with appropriate bonding or heat treatment,
sealing the membrane around penetrations, and
covering the geomembrane with a bedding layer.
Reference 1 cites seam failures and punctures or
abrasions of the geomembrane during installation as
the most common problems encountered during
geomembrane installation. Refer to Chapter 4,0 of
this Guide and Chapter 6 of Reference 1 for more
complete discussions.
5.5 Maintenance
A maintenance program must be developed, per 40
CFR 264.310, to insure the continued integrity and
effectiveness of the cover. Preventative maintenance
work should be scheduled periodically for two to three
years after cover installation; to prevent loss of
vegetation and gully development. Maintenance
inspections should be regularly scheduled to provide
early warning of more serious problems developing
that would impact the cover's integrity such as cover
subsidence, slope failure, leachate or upward gas
migration, or deterioration of the drainage system.
Exhibit 5-3 provides a brief overview of the elements
of a typical maintenance program. Section 4 of
Reference 7 and Section 10 of Reference 1 provide
detailed guidance on development of a post-closure
maintenance program.
5.6 Quality Control .
The principal activities of. a Quality Control (QC)
program for the construction of the cover include:
• screening incoming materials
• construction and testing of test fills
• observation of construction procedures
• measurement of final cover layer thicknesses
• surveying of final grades
The QC inspector must become thoroughly familiar
with the specifications to ensure that materials and
installation procedures conform to contract standards.
More detailed discussions of QC testing for soil and
geomembranes are provided in Chapter 4.0 of this
Guide and Section 2 of Reference 8. One of the most
important tasks for the QC inspector is to make the
construction crews sensitive to the relative "fragile"
nature of the cover layer components. In many cases,
these crews are more experienced in roadway or
building excavation and are not aware that
carelessness such as driving equipment over or
leaving equipment on the georniembrane or filters may
compromise the overall integrity of the cover.
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Exhibit 5-3. Typical Elements of Maintenance Program
PREVENTATIVE MAINTENANCE (2 to 3 years)
Cover System Component
Vegetation
Topsoil
Frequency
twice per year
annual
as needed
Task
mowing (weed and brush)
fertilization
soil reconditioning (supplemental
fertilization, aeration)
PROBLEM IDENTIFICATION/CORRECTION
Cover System Component
Cover System
Run-off Control System
Problem
gully development
subsidence
•slope instability
gas migration that
causes cracking
erosion, siltation
Repair
backfill to original grade with stone of
narrow size range
regrade cover
replant vegetation
backfill with additional cover soil (care
should be taken to maintain continuity
of low permeable soil layer,
geomembrane and drainage layer)
reconstruct cover
flatten slopes
add toe berm along base of slope
upgrade or install gas venting system
install perimeter vents
placement of stone riprap or concrete
modify channel alignment and/or
gradients
5.7 References
1. U.S. EPA, "Design, Construction, and
Maintenance of Cover Systems for Hazardous
Waste, An Engineering Guidance Document,"
U.S. Army Engineer Waterways Experiment
Station, Vicksburg, MS. PB 87-191656. May
1987..
2. U.S EPA, "Evaluating Cover Systems for Solid
_ and Hazardous Waste," Office of Solid Waste and
- Emergency Response, Washington, D.C. SW
867, September 1982
3. U.S. EPA, "The Hydraulic Evaluation of Landfill
Performance (HELP) Model-Volume 1. User's
Guide for Version 1." June 1984.
4. U.S. EPA, "The Hydraulic Evaluation of Landfill
Performance (HELP) Model-Volume Z.
Documentation for Version 1." June 1984.
5. U.S. EPA, "Prediction/Mitigation of Subsidence
Damage to Hazardous Waste Landfill Covers,"
Hazardous Waste Engineering Research
Laboratory. EPA 600-2-87-025. PB 87-
175378.
6. U.S. EPA, "Landfill Design, Liner Systems and
Final Cover," draft RCRA Guidance Document
July 1982
7. U.S. EPA, "Standardized Procedures for Planting
Vegetation on Completed Sanitary Landfills,"
Municipal Environmental Research Laboratory,
Cincinnati, Ohio. Grant No. CR-807673. July
1982.
8. U.S. EPA, "Construction Quality Assurance for
Hazardous Waste Land Disposal Facilities,"
Office of Solid Waste and Emergency Response,
Washington, D.C. EPA/530-SW-86-031.
OSWER Policy Directive No. 9472.003.
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CHAPTER 6.0
Run-On/Run-Off Controls
Surface water management is necessary at
hazardous waste facilities to minimize erosion
damage to earthen hazardous waste containment
structures, and to prevent interference with the
natural processes involved in certain hazardous waste
treatment methods. Design of a surface water
management system requires a knowledge of local
precipitation patterns, surrounding topographic
features, geologic conditions, and facility design.
Surface water management systems do not have to
be expensive or complex to be effective. The
equipment and materials used for construction of the
surface water management system are the same as
those used for general earthwork and foundation
construction. Construction may include excavation of
a series of shallow channels to direct surface water
flow, or in some cases, installation of basins to retain
rainfall accumulation from sudden, intense storms.
Surface water management systems are required for
all hazardous waste treatment, storage and disposal
areas. Because these systems are integral with
hazardous waste facilities, the system components
should be constructed during facility construction.
This chapter introduces the regulatory requirements
for surface water management systems, provides a
general discussion of design criteria for those
systems, and describes the types of system
components, materials, and construction techniques
that may be employed to control run-on/run-off at
land disposal units.
6.1 The Regulations and Performance
Standards
The requirements of 40 CFR Parts 264 and 270
concerning surface water management are
comparable for most hazardous waste management
units. For landfills (and waste piles), the objective of
surface water management is to prevent increased
generation of hazardous wastes through the mixing of
run-on and precipitation with hazardous waste or
leachate. For surface impoundments and secondary
containment structures, control measures are
intended to prevent overtopping of the structures by
accumulated precipitation. (For land treatment
operations, the .prevention of run-on will prevent
reduced efficiency of biodegradation processes and
impaired tilling operations.)
The performance standards of 40 CFR Part 264
require the owner/operator' of landfills to "design,
construct, operate, and maintain run-off and run-on
management systems capable of collecting and
controlling at least the water volume resulting from a
24-hr, 25-yr storm". Surface impoundments must
be designed to prevent overtopping resulting from
run-on based on a 24-hour, 100-year storm.
Collection and holding structures that are part of the
system design must be expeditiously emptied or
otherwise managed after every storm to -maintain
design capacities. Inspection is required during
construction to ensure quality workmanship and
materials. During operation, continued inspection is
required to prevent erosion or other deterioration of
the primarily earthen structures. Run-on/run-off
control is required in both the closure and post-
closure phases of operation.
To demonstrate that the standards will be met, the
permit application requirements of 40 CFR Part 270
include for each unit, a provision for detailed plans
and an engineering report that show the design and
supporting calculations for run-on/run-off controls
including collection and holding units. (The only
exception to this requirement is for land treatment
units where a description, rather than actual design
plans, is specified.) Information commonly used in
demonstrating that standards for run-on/run-off
controls are met is provided in Exhibit 6-1.
6.2 Design and Operation
In accordance with the mixture rule [40 CFR
261.3(a)(2)(iii) and (b)(2)], precipitation run-off that
"mixes" with hazardous waste is also considered a
hazardous waste and must be treated, stored, or
disposed of as such. The first step in the design
process is to properly categorize the surface waters
that drain across the facility as either uncontaminated,
potentially contaminated, or contaminated. These
classifications are described as follows:
• Uncontaminated waters: run-off from areas not
used for treatment, storage, or disposal of
hazardous wastes and from closed sites
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Exhibit 6-1. Information Commonly Used in Demonstrating
That Performance Standards for Run-
On/Run-Off Controls are Met
Information Typical Parameters
Description of Run-on
Control System
Calculation of Peak
Run-on Flow
Description of Run-off
Control System
Calculation of Peak
Run-off Flow
Management of Collection
and Holding Units
Description of Construction
and Maintenance
Description of:
• System used to prevent
run-on
• Design of run-on control
system
• Include plan view, drawing
details, profiles, cross
sections, and calculations
used to size system
Calculate peak surface water
flow expected from design
storm. Include:
• Data sources
• Methods used to calculate
peak flow
Description of:
• System used to collect
and control run-off from
active portions
• Design of system
• Include plan view, drawing
details, cross sections, and
calculations demonstrating
that system has sufficient
capacity to collect total
run-off volume
• Procedures for
determination of whether
run-off is hazardous
waste ;
Calculate total run-off volume
expected from the design storm.
Include:
• Data sources
• Method.s used to calculate
peak run-off flow
Description of.
• Procedures for emptying
collection and holding units
to maintain capacity
• Fate and management of
discharged liquids
Provide:
• Detailed construction and
material specifications
• Construction quality
control program
• Description of required
maintenance activities
Potentially contaminated waters: run-off from all
on-site roadways over which hazardous waste
materials are transported. This classification could
also include processing areas, operations areas,
partially closed cells, and all equipment
storage/parking areas. Potentially contaminated
run-off must be collected in a hazardous waste
surface impoundment or tank that meets the
minimum technical requirements of the RCRA
regulations. The collected run-off must be
tested. If it is not contaminated, it can be
discharged to the storm water drainage system.
• Contaminated waters: run-off collection from
active landfill cells; sumps receiving or containing
materials resulting from the generation of
hazardous wastes; and leachate collected from
the secure cell leachate collection system.
Contaminated run-off must be collected in. a
hazardous waste surface impoundment or tank
that meets the minimum technology requirements
of the RCRA regulations. Within an active landfill
cell, the contaminated run-off should be
channeled into the leachate collection system.
Run-off within the cell must be treated as a
hazardous waste until an intermediate cover is
placed over the cell.
The following sections provide an introduction to the
basic concepts involved in the design of run-on and
run-off controls, with guidance to sources providing
more detailed information.
6.2.1 Design Overview
The two methods commonly recommended by EPA,
for use in designing surfacei water management
structures are the Soil Conservation Service (SCS)
method and the Rational method. Both will be
introduced here. Before discussing these methods, a
brief review of basic hydrologic concepts is helpful.'
In surface water management, the study of rainfall
effects is limited to the natural drainage basin within
which the facility lies. This basin is referred to as the
local watershed. The designer must understand the
watershed's responses to precipitation, such as how
much of the total volume of rainfall will infiltrate the
ground surface, how much will evaporate, and how
much rainfall will flow across the earth's surface as
run-off. Site characteristics that influence rainfall
infiltration rates include soil type, soil moisture,
antecedent rainfall, cover type, impervious surfaces,
and surface retention. Travel time for run-off from
the most distant boundary of the watershed to the
point of outfall must be determined. Calculations for
travel time are based on slope, length of flow path,
depth of flow, and roughness of flow surfaces. The
relationship of these parameters, along with the total
drainage area of the watershed, determines peak
run-off discharges and the total drainage area of the
watershed. Other factors which impact final run-off
discharge calculations include the effect of any flood
control works or other natural or manmade storage,
and the time distribution of rainfall during a storm.
(Reference 1, p.1-1). Chiapters 1 and 2 of
Reference 1 provide a good overview of surface
hydrology and the factors that impact run-off
calculations.
58
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Hydrologists graph the run-off accumulation, with
respect to time, in a hydrograph. Exhibit 6-2
provides an example of a hydrograph. For a complete
discussion of the development and use of
hydrographs, including elementary precipitation -
streamflow relationships, see Reference 2, Chapter 4.
Exhibit 6-2. Hydrograph
Time
Source: Viessman et al, 1977 p. 102
6.2.2 Design Approach
The standard design approach for a surface water
control system is to:
• Identify the intensity of the design storm;
• Determine the peak discharge rate;
• Calculate the run-off volume during peak
discharge;
• Determine the control system design criteria and
the required capacity for the control systems; and
• Design the control system.
Each step will be discussed briefly in the following
sections.
6.2.2.1 Identify Design Storm
The regulations require the owner/operator to use
data from a design storm of 24-hour, 25-year
frequency for land disposal units, waste piles, and
land treatment facilities. Surface impoundment
freeboard must be able to contain the rainfall
accumulation from a 24-hour, 100-yr storm.*
Designers should obtain the design storm information
from local planning agencies, civil works departments,
or zoning boards for use in designing housing
developments and roadway drainage systems. Actual
precipitation data for at least the past 25 years, if
The design storm requirement for surface impoundments is not
specifically stated in the regulations, but is specified in the preamble
to the surface impoundment regulations, which was published in the
Federal Register of July 26, 1982, 47 FR 32357.
available, should be reviewed to verify the intensity
curves developed.
Permit reviewers do not need to independently gather
local precipitation data for their initial review. The
Precipitation-Frequency Atlas of the Western United
States, the NOAA Atlas 2, is generally regarded as a
good reference to verify the acceptable range of
values for local data. If the local data used by the
designer does not fall within acceptable ranges, then
additional research by the permit reviewer into local
data is necessary. For a complete list of the most
current 24-hour rainfall data published by the
National Weather Service (NWS), refer to Reference
3, p. B-3.
6.2.2.2 Determining Peak Discharge
Rate/Calculating Run-off: SCS Method
A method that is most often appropriate for estimating
run-on/run-off and peak discharge rate from a
storm's rainfall is the Soil Conservation Service (SCS)
Method. The SCS Method was originally designed to
determine run-off volumes for small agricultural
watersheds where insufficient long-term stream flow
and precipitation data had been collected, but where
soil types, topography, vegetative cover, and
agricultural practices had been documented.
The model assumes that the rate and amount of
rainfall is uniform throughout the watershed over a
specified period of time. The calculated mass rainfall
is converted to mass run-off by using a run-off
Curve Number (CN). CNs were developed to account
for the effects of soils, plant cover, amount of
impervious areas, interception, and surface storage.
Run-off is plotted on a hydrograph and routing
procedures that depend on run-off travel time
through segments of the watershed. For a further
description of the development and'use of the SCS
Method, refer to Reference 1.
6.2.2.3 Rational Method
The Rational " Method can be applied when
determining peak discharge rates for significantly
urbanized areas with largely impervious surface
covers. The Rational Method is based on the premise
that maximum run-off resulting from steady,
uniformly intense precipitation will occur when the
entire watershed, upstream of the site location,
contributes to the discharge. The point in time at
which this condition occurs after precipitation begins
is called the Time of Concentration (TC). TC is
normally estimated from consideration of the hydraulic
characteristics of the watershed (Reference 4, p. 4-
17).
6.2.3 Control System Structures
To achieve the standards set forth in 40 CFR Part
264, the designer will incorporate several structures,
both temporary and permanent, into the system
59
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design. Exhibit 6-3 provides a list of tha most
frequently used structures.
Exhibit 6-3. Surface Water Diversion and
Collection Structures
Technology
Duration of Normal Use
Dikes and benms
Channels (earthen and CMP)
Waterways
Terraces and benches
Chutes
Downpipes
Seepage ditches and basins
Sedimentation basins
Temporary
Temporary
Permanent
Temporary and
permanent
Permanent
Temporary
Temporary
Temporary
Source: USEPA, Remedial Measures Handbook, 1985,
p.3-80.
6.2.3.1 Dlkes/Berms
Dikes and berms are well-compacted earthen ridges
or ledges constructed immediately upslope from, or
along the perimeter, of the intended area of
protection. A typical dike design is shown in Exhibit
6-4. Dikes are intended as short term protection of
critical areas by intercepting storm run-off and
diverting the flow to natural or manmade drainage
channels, manmade outlets, or sediment basins.
Typically, dikes and berms should be expected to
maintain their integrity for about one year, after which
they should be rebuilt. Dikes are generally classified
into two groups: interceptor dikes, designed to reduce
slope length, and diversion dikes, designed to divert
surface flow and to reduce slope length. Dikes can
also prevent mixing of incompatible wastes and can
reduce the amount of leachate produced in a landfill
cell by diverting the water available to infiltrate the soil
cover. Due to their temporary nature, dikes and
berms are designed for run-off from no larger than a
five-acre watershed (Reference 5, Chapter 3).
Exhibit 6-4 Typical temporary diversion dike.
Cut or fill slope
Flow
Existing ground
Source: USEPA, 1976
Adapted: USEPA, Remedial Measures Handbook, 1985 p. 3-39
A detailed design is not usually required for
construction of interceptor and diversion dikes.
Common design criteria are found in Reference 5, p.
3-38.
6.2.3.2 Swales, Channels and Waterways
Channels are excavated ditches that are generally ,
wide and shallow with trapezoidal, triangular, or
parabolic cross-sections. A typical channel design is
shown in Exhibit 6-5. Diversion channels are used
primarily to intercept run-off or reduce slope length.
Channels stabilized with vegetation or stone rip-rap
are used to collect and transfer diverted water off site
or to on-site storage or treatment. Applications and!
limitations of channels and waterways differ
depending upon their specific design. Reference 5,
Chapter 3 includes a good summary discussion of
their applications (Reference 5, pp. 3-40).
Swales are placed along the perimeter of a site to
keep off-site run-off from entering the site and to
carry surface run-off from a land disposal unit. They
are distinguished from earthen channels by side
slopes which are less steep and have vegetative
cover for erosion control (Reference 5, p. 3-42).
The specific design for channels, swales, and
waterways must consider local drainage patterns, soil
permeability, annual precipitation, area (arid use, and
other pertinent characteristics of the contributing
watershed. To comply with the permit regulations,
channels and waterways should accommodate the
maximum rainfall expected in a 25-year period,
Manning's formula for steady uniform flow in open
channels is used to design channels and waterways.
Refer to Reference 5, Chapter 3 for a detailed
discussion of the application of Manning's formula in
channel and waterway design.
6.2.3.3 Terraces
Terraces are embankments constructed along the
contour of very long or very steep slopes to intercept
and divert flow of surface water and to control erosion
of slopes by reducing slope length. A typical terrace
design is shown in Exhibit 6-6. Terraces may
function to hydrologically isolate sites, control erosion
of cover materials on sites which have been capped,
or collect contaminated sediments eroded from
disposal areas. For disposal sites undergoing final
grading, construction terraces may be included as
part of the site closure plan (Reference 5, pp. 3-52).
Refer to Reference 5 for a complete discussion of
design considerations.
6.2.3.4 Chutes and Downpipes
Chutes and downpipes are usually temporary
structures which can play an important role in
preventing erosion while landfill and surface
impoundment covers are "stabilizing" with vegetation.
A typical chute design is shown in Exhibit 6-7.
Chutes are excavated earthen channels lined with
non-erodible materials such as bituminous concrete
or grouted rip-rap. Downpipes ,are constructed of
rigid piping or flexible tubing and installed with
60
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Exhibit 6-5. Typical Channel Design
Parabolic cross-section
Source: USEPA, 1976
Reprinted: USEPA, Remedial Measures Handbook, 1985, p. 3-43
Exhibit 6-6. Typical Terrace Design
Ditch
Ditch
Source: USEPA, 1976
Reprinted: USEPA, Remedial Measures Handbook, 1985, p 3-43
prefabricated inlet sections. As a general rule, chutes
should not be used when hydraulic heads are
expected to be more than 18 feet. Downpipes should
not be used when the drainage basin is estimated to
be larger than five acres (Reference 5, Chapter 3).
6.2.3.5 Seepage Basins and Ditches
Seepage basins and ditches are used to discharge
water collected from surface water diversions,
ground-water pumping or leachate treatment. They
may also be used as part of an in-situ treatment
process to force treatment reagents into the
subsurface. A typical seepage basin design is shown
in Exhibit 6-8. They are most effective in highly
permeable soils where recharge can occur. They are
not applicable at sites where collected run-off or
ground water is contaminated. Typically, they are
used in areas with shallow ground-water tables.
Seepage ditches distribute water over a larger area
than achievable with basins. They can be used for all
soil where permeability exceeds about 0.9 inches per
day (Reference 5, Chapter 3).
A seepage basin typically consists of the actual basin,
a sediment trap, a by-pass for excess flow, and an
emergency overflow. A considerable amount of
recharge occurs through the sidewalls of the basin,
and therefore it is preferable that these be
constructed of pervious material such as packed
gravel. For general design parameters refer to
Reference 5, Chapter 3.
6.2.3.6 Sedimentation Basins
Sedimentation basins are used to retard surface
water flow such that suspended particulates can
settle. Sedimentation basins serve as the final step in
control of diverted, uncontaminated surface run-off,
prior to discharge. A typical basin design is shown in
Exhibit 6-9. Basins are especially useful in areas
where surface run-off has a high silt or sand
content. The major components include a principal
and emergency spillway, an anti-vortex device and
the basin. The principal spillway consists of a vertical
pipe or riser joined to a horizontal pipe that extends
through the dike and has an outlet beyond the
impoundment. The riser is topped by the anti-vortex
device and trash rack which improves the flow of
water into the spillway and prevents floating debris
from being carried out of the basin. For additional
design information, refer to Reference 5, Chapter 3.
6.3 Materials
The materials used for construction of surface water
management structures are generally local site
materials such as sand, gravel and soils. Common
techniques used to stabilize the structures include
seeding, mulching and the application of soil additives
or rip-rap. Other synthetic erosion control materials
routinely used include woven jute multifilament fiber
matting, polyester fiber matting, and three
dimensional polyethylene net. Channels and
downpipes may require the use of corrugated metal,
concrete pipe or flexible tubing made of heavy fabric.
Terraces and chutes are typically lined with
61
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Exhibit 6-7 Typical paved chute design.
^ ' •"• .^
Top of earth dike and top of lining
=\\\=\\\\
Slope varies, not steeper than 1.5:1
and not flatter than 20:1
Undisturbed soil
or compacted fill
Place layer of
sand for drainage
under outlet as
shown for full
width of structure
Source: USEPA, 1976
Adapted: USEPA, Remedial Measures Handbook, 1985 p. 3-58
Exhibit 6-8 Typical seepage basin design.
Bypass
Source: Tourbier and Westmacott, 1974
Adapted: USEPA, Remedial Measures Handbook, 1985 p. 3-65
Exhibit 6-9 Typical sedimentation basin design.
Anti-vortex Device
Water Surface (design)
Emergency Spillway Crest
Anti-seep Collars
Pipe Conduit or Barrel
Free Outlet
Principal Spillway
EMBANKMENT
Adapted: USEPA, Remedial Measures Handbook, 1985 p. 3-68
62
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bituminous concrete, concrete, grouted rip-rap or
similar non-erodible material. For a complete
discussion of available materials see Reference 5,
Chapter 3.
6.4 Construction/Equipment
The equipment used for construction of surface water
management structures is standard construction
excavation and grading equipment including dozers,
scrapers, and graders. Well established construction
techniques are used. Briefly, prior to construction, the
site should be cleared, grubbed, and stripped of
topsoil to eliminate trees, vegetation, roots, and other
debris. All material should be compacted to prevent
unequal settlement. Earthen dikes should be
compacted to reduce erosion. For a detailed
discussion of equipment and construction techniques
for each structure, see Reference 5, Chapter 3.
6.5 Quality Assurance (QA)/Quality
Control (QC)
Prior to inspecting any structure during construction,
the Quality Control inspector should thoroughly review
all design drawings and specifications to ensure the
constructability of the design. Factors to consider in
the review include: actual site conditions,
completeness of design, and consistency between
design drawings and specifications. For complete
guidance on QA inspections during construction, see
Reference 3.
6.5.1 Materials
All materials should be inspected prior to being
installed in the surface water management system.
The material submittals (catalog cuts) that were
submitted for review during the initial construction
phases should be checked to confirm that the
materials proposed are the same as those used in the
actual construction. Specific, material details that
should be field-verified include pipe sizes, soil
gradation and strength, concrete slump, and specified
pipe perforations.
6.5.2 Erosion Control Systems
Inspection activities for erosion control systems
include visual observations and surveys to ensure
that the dimensions of the completed structures are
as specified. Slopes are the most important items to
check; if slopes are too steep, they may be unstable
and eventually could fail. Other items to be checked
include berm width, crest width, overall height,
thickness and actual dimensions. Vegetative cover
should be inspected at regular intervals to ensure that
vegetation is properly established (Reference 3, pp.
18 and 19).
6.5.3 Operations/Maintenance QA
During operations, inspections are required
periodically and after each storm to ensure that
erosion has not damaged the integrity of the berms,
dikes, or channels. Inspections are also required to
ensure that collection basins and sumps are kept free
of standing water to continue to satisfy the capacity
requirements cited in the permit.
6.6 References
1. U.S. Department of Agriculture, Soil Conservation
Service. 6/1986. Urban Hydrology for Small
Watersheds. PB87-101580
2. Viessman, Knapp, et al. Introduction to Hydrology,
Harper and Row, Publishers, 1977 (Second
Edition)
3. U.S. Environmental Protection Agency. 10/1986.
Technical Guidance Document: Construction
Quality Assurance for Hazardous Waste Land
Disposal Facilities, OSWER Policy Directive No.
9472.003. EPA/530-SW-86-031
4. U.S. Environmental Protection Agency. 9/1985.
Covers for Uncontrolled Hazardous Waste Sites.
EPA/540/2-85/002. Hazardous Waste
Engineering Research Laboratory, Cincinnati,
Ohio
5. U.S. Environmental Protection Agency. 10/1985.
Handbook; Remedial Action at Waste Disposal
Sites (revised). EPA/625/6-85/006. Hazardous
Waste Engineering Research Laboratory,
Cincinnati, Ohio
63
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