&EPA
United States
Environmental Protection
Agency
Great Lakes
National Program Office
77 West Jackson Boulevard
Chicago, Illinois 60604
EPA 905-B96-004
September 1998
Assessment and
Remediation
Of Contaminated Sediments
(ARCS) Program
GUIDANCE FOR IN-SITU SUBAQUEOUS
CAPPING OF CONTAMINATED SEDIMENTS
Michael R Palermo and Steve Maynord
U S. Army Engineer Waterways Experiment Station
Vicksburg, Mississippi
Jan Miller
U.S Army Engineer Division, Great Lakes and Ohio River
Chicago, Illinois
Danny D. Reible
Louisiana State University
Baton Rouge, Louisiana
(?) United States Areas of Concern
0 ARCS Priority Areas of Concern
printed on recycled paper
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ASSESSMENT AND REMEDIATION OF CONTAMINATED SEDIMENTS
(ARCS) PROGRAM
GUIDANCE FOR IN-SITU SUBAQUEOUS CAPPING OF
CONTAMINATED SEDIMENTS
Great Lakes National Program Office
U. S. Environmental Protection Agency
77 West Jackson Boulevard
Chicago, Illinois 60604-3590
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
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DISCLAIMER
This document has been subject to the U. S. Environmental
Protection Agency's (USEPA) peer and administrative review,
and it has been approved for publication as a USEPA document.
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use by USEPA or
any of the contributing authors.
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ACKNOWLEDGMENTS
This document provides technical guidance for subaqueous, in-situ capping as a
remediation technique for contaminated sediments. The document was prepared as a part
of studies conducted for the U.S. Environmental Protection Agency (USEPA) under the
Assessment and Remediation of Contaminated Sediments (ARCS) Program, administered
by USEPA's Great Lakes National Program Office (GLNPO), in Chicago, Illinois. This is
one of a series of guidance documents developed by the ARCS Engineering/ Technology
Work Group (ETWG) to evaluate the feasibility of remediation alternatives and
technologies.
This report was written by Dr. Michael R. Palermo, Research Civil Engineer, U.S. Army
Engineer Waterways Experiment Station (WES); Dr. Steve Maynord, Research Hydraulic
Engineer, WES; Mr. Jan Miller, Environmental Engineer, U.S. Army Engineer Division,
Great Lakes and Ohio River; and Dr. Danny D. Reible, Louisiana State University.
Contributions of Mr. Tommy Myers, Mr. James Clausner, and Mr. Paul Gilbert, all of WES,
and Mr. David Petrovski, USEPA Region 5, are also acknowledged. Appendix B was
prepared with the partial support of the US Environmental Protection Agency through the
Hazardous Substance Research Center/South and Southwest and through cooperative
agreement CR 823029-01 -0 from the National Risk Management and Research Laboratory,
Cincinnati, Ohio. Appendix C was prepared by Dr. Hoe Peter Ling and Dr. Dov
Leshchinsky, both of the University of Delaware, under contract with WES.
The first draft of this document was prepared for EPA technical review in 1995. A second
draft was prepared for an in-situ capping seminar held by EPA Region 5 in Chicago in
November 1996. This final report addresses comments received from the seminar
participants. The report number EPA 905-B96-004 was assigned in 1996, and this number
was retained for final publication.
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Abstract
In-Situ Capping is defined as the placement of a subaqueous covering or cap of clean
isolating material over an in-situ deposit of contaminated sediment. ISC is a potentially
economical and effective approach for remediation of contaminated sediment. A number of
sites have been remediated by in-situ capping operations worldwide. This document
provides technical guidance for subaqueous, in-situ capping as a remediation technique for
contaminated sediments. The document was prepared as a part of the studies conducted
for the U.S. Environmental Protection Agency (USEPA) under the Assessment and
Remediation of Contaminated Sediments (ARCS) Program, administered by USEPA's
Great Lakes National Program Office (GLNPO), in Chicago, Illinois.
Caps for in-situ sediment remediation may be constructed of clean sediments, sand,
gravel, or may involve a more complex design with geotextiles, liners and multiple layers.
In-situ capping can provide several primary functions: physical isolation of the contaminated
sediment from the benthic environment; stabilization of contaminated sediments,
preventing resuspension and transport to other sites; and, reduction of the flux of dissolved
contaminants into the water column. To achieve these results, an in-situ capping project
must be treated as an engineered project with carefully considered design, construction,
and monitoring. The basic criterion for a successful capping project is simply that the cap
required to perform some or all of these functions be successfully designed, placed, and
maintained.
This document provides descriptions of the processes involved with in-situ capping,
identification of the design requirements of an in-situ capping project, and a recommended
sequence for design. Detailed guidance is provided on site and sediment characterization,
cap design, equipment and placement techniques, and monitoring and management
considerations.
This report should be cited as follows:
Palermo, M., Maynord, S., Miller, J., and Reible, D. 1998. "Guidance for In-Situ
Subaqueous Capping of Contaminated Sediments," EPA 905-B96-004. Great Lakes
National Program Office, Chicago, IL.
IV
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Contents
Acknowledgments iii
Abstract iv
1-Introduction 1
Background 2
ARCS Guidance 3
Document Purpose and Scope 3
In-Situ Capping Overview 6
Design Sequence for In-Situ Capping 9
2-Site Evaluation 12
Remediation Objectives 12
Remediation Scope 14
Site Conditions 15
Regulatory and Legal Considerations 21
Preliminary Feasibility Determination 22
3-ln-Situ Cap Design 23
General Considerations 23
Identification of Capping Materials 24
Physical Isolation Component 29
Stabilization/Erosion Protection Component 31
Chemical Isolation Component 35
Component Interactions 40
Geotechnical Considerations 41
Operational Considerations 45
4-Equipment and Placement Techniques 47
General Considerations 47
Equipment and Placement Techniques for Granular Cap Materials 49
Equipment and Placement Techniques for Armoring Layers 56
Placement of Geosynthetic Fabrics 57
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Positioning Requirements 58
5-Monitoring and Management 59
Monitoring Requirements 59
Design of Monitoring Programs and Plans 59
Construction Monitoring 62
Cap Performance Monitoring 65
Management Actions 70
6-Summary 72
Recommendations 73
References 75
Appendix A: Design of Armor Layers
Appendix B: Model for Evaluation of Long Term Flux of Contaminants . .
Appendix C: Case Studies on Geotechnical Aspects of In-Situ Sand Capping
VI
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List of Tables
Table 1. Summary of in-situ capping projects 7
Table 2-1. Standard Geotechnical Laboratory Test Procedures 19
Table 2. Sample of Tiered Monitoring Program for Dredged Material Capping 61
VII
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List of Figures
Figure 1. Conceptual Illustration of dredged material capping and in-situ
capping options 5
Figure 2. Flowchart showing sequence of steps involved with the design of an in-situ
capping project 11
Figure 3. Flowchart showing steps involved in design and evaluation of various in-situ cap
components 26
Figure 4. Illustrations of alternative combinations of cap components 27
Figure 5. Laboratory methods to evaluate chemical isolation by caps 38
Figure 6. Relationship between relative density and effective friction angle for clean
sands 42
Figure 7. Recommended cap edge overlap 43
Figure 8. Conceptual illustrations of equipment which can be considered for capping. . 48
Figure 9. Land-based cap placement at Sheboygan River 49
Figure 10. Spreading technique for capping by barge movement at Denny Way, Puget
Sound 51
Figure 11. Hydraulic washing of coarse sand, Eagle Harbor, Puget Sound 51
Figure 12. Spreader plate for hydraulic pipeline discharge 52
Figure 13. Spreader box or "sand box" for hydraulic pipeline discharge, Simpson Kraft
Tacoma, Puget Sound 53
Figure 14. Submerged diffuser system, including the diffuser and discharge barge 54
Figure 15. Hydraulic barge unloader and sand spreader barge (from Kikegawa 1983). . 54
Figure 16. Conveyor unloading barge with tremie (from Togashi 1983) 55
Figure 17. Tremie system employed at Hamilton Harbor 55
Figure 18. Stone placement at Sheboygan Harbor 57
Figure 19. Schematic of a settling plate used for monitoring cap consolidation 64
Figure 20. Illustration of Sediment Profiling Camera 65
Figure 21. Semi-permeable bags or "peepers" filled with an organic solvent used for
monitoring the levels of hydrophobic contaminants in sediment pore water 68
Figure 22. Seepage meter used to measure groundwater flow 69
VIM
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1 Introduction
This document provides technical guidance for subaqueous, in-situ capping as a remediation
technique for contaminated sediments. The document was prepared as a part of the studies
conducted for the U.S. Environmental Protection Agency (USEPA) under the Assessment and
Remediation of Contaminated Sediments (ARCS) Program, administered by USEPA's Great
Lakes National Program Office (GLNPO), in Chicago, Illinois.
Background
Although toxic discharges in the Great Lakes and elsewhere have been reduced in the last
25 years, persistent contaminants in sediments continue to pose a potential risk to human
health and the environment. High concentrations of contaminants in bottom sediments and
associated adverse effects have been well documented throughout the Great Lakes and
associated connecting channels. The extent of sediment contamination and its associated
adverse effects have been the subject of considerable concern and study in the Great Lakes
community and elsewhere. Contaminated sediments can have direct toxic effects on aquatic
life, such as the development of cancerous tumors in fish exposed to polycyclic aromatic
hydrocarbons (PAHs) in sediments. The bioaccumulation of toxic contaminants in the food
chain can also pose a risk to humans, wildlife, and aquatic organisms. As a result, advisories
against consumption of fish are in place in many areas of the Great Lakes. These advisories
have also had a negative economic impact on the affected areas.
To address concerns about the deleterious effects of contaminated sediments in the Great
Lakes, Annex 14 of the Great Lakes Water Quality Agreement between the United States and
Canada stipulates that the cooperating parties will identify the nature and extent of sediment
contamination in the Great Lakes, develop methods to assess impacts, and evaluate the
technological capability of programs to remedy such contamination.
The 1987 amendments to the Clean Water Act, in § 118(c)(3), authorized GLNPO to coordinate
and conduct a 5-year study and demonstration project relating to the appropriate treatment of
toxic contaminants in bottom sediments. Five areas were specified in the Act as requiring
priority consideration in conducting demonstration projects: Saginaw Bay, Michigan;
Sheboygan Harbor, Wisconsin; Grand Calumet River, Indiana; Ashtabula River, Ohio; and
Buffalo River, New York. To fulfill the requirements of the Act, GLNPO initiated the ARCS
Program. In addition, the Great Lakes Critical Programs Act of 1990 amended the section, now
§ 118(c)(7), by extending the program by 1 year and specifying completion dates for certain
interim activities.
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Chapter 1. Introduction
ARCS was an integrated program for the development and testing of assessment techniques
and remedial action alternatives for contaminated sediments. Information from ARCS Program
activities is being used to guide the development of Remedial Action Plans (RAPs) for all 43
Great Lakes Areas of Concern (AOCs, as identified by the United States and Canadian govern-
ments), as well as Lakewide Management Plans (LaMPs).
ARCS Guidance
The decision to remediate contaminated sediments in a waterway and the selection of the
appropriate remediation technology(s) are part of a step-wise process using the guidance
developed by the three ARCS technical work groups. The guidance developed by the Toxici-
ty/Chemistry Work Group (USEPA 1994a) is used to characterize the chemical and
toxicological properties of bottom sediments. The guidance developed by the Engineering/
Technology Work Group (ETWG) is used to evaluate the feasibility of remediation alternatives
and technologies (USEPA 1994b). The guidance developed by the Risk Assessment/ Modeling
Work Group (USEPA 1993) provides a framework for integrating the information developed in
the other two steps and evaluating the ecological and human health risks and benefits of
remedial alternatives, including no action.
This document is one of a series developed by the ETWG for evaluation of remediation
alternatives and technologies. The ETWG followed a systematic approach to evaluating
remediation technologies, beginning with a literature review of available technologies (Averett
et al. 1990), followed by laboratory or bench-scale testing of selected technologies (Fleming
et al. 1991; USEPA 1994c; Allen 1994), and culminating with field- or pilot-scale demonstrations
of at least one technology at each of the five priority AOCs (USAGE Buffalo District 1993,1994;
USAGE Chicago District 1994; USAGE Detroit District 1994). In addition to the technology
evaluations, the ETWG developed a series of conceptual plans for full-scale sediment
remediation projects (USEPA in prep).
The ARCS Remediation Guidance Document (USEPA 1994b) (RGD) was also developed by
the ETWG and describes procedures for evaluating the feasibility of remediation technologies,
performing bench- and pilot-scale tests, identifying the components of remedial design,
developing cost estimates for full-scale application, and estimating contaminant losses during
implementation. Detailed information on specific technologies (Averett et al. 1990) and
contaminant loss estimating procedures (Myers et al. 1996) are provided in other reports devel-
oped by the ETWG, which should be used as companion documents.
The consideration of in-situ capping as a remedial option should always be preceded by a
complete and detailed evaluation of the environmental need for remedial action, assessment
of the risks associated with remedial options, and consideration of feasible remedial techniques
available. It is not the intention of the authors that in-situ capping be perceived as universally
applicable to sediment remediation, or that capping be promoted as the recommended option
without careful consideration of the alternatives and consequences, as outlined in the complete
set of ARCS guidance documents.
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Chapter 1. Introduction
Document Purpose and Scope
The purpose of this document is to provide guidance for planning and design of in-situ capping
projects. Descriptions of the processes involved with in-situ capping, identification of the design
requirements of an in-situ capping project, and a recommended sequence for design are
discussed in this chapter. Detailed guidance is provided on site and sediment characterization
(Chapter 2), cap design (Chapter 3), equipment and placement techniques (Chapter 4), and
monitoring and management considerations (Chapter 5). The use of this document presumes
that a decision to remediate has been made, that remediation objectives have been defined,
and that a screening and evaluation of remediation alternatives has indicated that a more
detailed evaluation of the in-situ capping alternative is warranted.
In-Situ Capping
Four basic options for remediation of contaminated sediments exist: 1) Containment in-place,
2) Treatment in-place, 3) Removal and containment, and 4) Removal and treatment. In-situ
capping is a form of containment in-place.
In-Situ Capping (ISC) refers to placement of a covering or cap over an in-situ deposit of
contaminated sediment. The cap may be constructed of clean sediments, sand, gravel, or
may involve a more complex design with geotextiles, liners and multiple layers. A variation on
ISC could involve the removal of contaminated sediments to some depth, followed by capping
the remaining sediments in-place. This is suitable where capping alone is not feasible because
of hydraulic or navigation restrictions on the waterway depth. It may also be used where it is
desirable to leave the deeper, more contaminated sediments capped in-place (vertical
stratification of sediment contaminants is common in many Great Lakes tributaries).
Important distinctions should be made between ISC and dredged material capping which
involves removal of sediments, placement at a subaqueous site, followed by placement of a
cap. Dredged material capping is a disposal alternative which has been used for sediments
dredged from navigation projects, and may also be suitable for disposal of sediments and
treatment residues from remediation projects. Two forms of dredged material capping are
level bottom capping in which a mound of dredged material is capped, and contained aquatic
disposal (CAD) in which dredged material is placed in a depression or other provisions for
lateral confinement are made prior to placement of the cap. Examples of in-situ and dredged
material capping are illustrated in Figure 1.
Even though the technical aspects of cap design and placement and effectiveness for ISC and
dredged material capping are similar, dredged material capping is more likely done for
navigation, rather than remediation purposes, and involves the removal and placement of a
contaminated sediment prior to capping, while ISC does not involve such removal.
Considerations related to the site also differ. For dredged material capping, contaminated
sediments are removed from their in-situ location, and site evaluation issues are framed
around the selection of an acceptable site for placement and capping. For ISC, the site is a
given, and the site evaluation is framed around defining the acceptability of capping for that
given site.
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Chapter 1. Introduction
A considerable body of literature exists on the subject of subaqueous capping. Much of the
work in this area is associated with the handling of contaminated dredged material removed
from navigation channels performed by or in cooperation with the U.S. Army Corps of
Engineers (USAGE). Technical guidelines for dredged material capping have been developed
including guidelines for planning capping projects (Truitt 1987a and 1987b and Truitt et al
1989), determining the required capping thickness (Sturgis and Gunnison 1988), overall
design requirements (Palermo 1991 a), site selection considerations (Palermo 1991b),
equipment and placement techniques (Palermo 1991c), and monitoring considerations
(Palermo Fredette, and Randall 1992) for capping projects. A comprehensive dredged
material capping guidance document is also in preparation (Palermo et al in preparation).
An annotated bibliography prepared for the Canadian Cleanup Fund summarizes most of the
capping projects (both in-situ and dredged material) and studies completed through 1992
(Zemanetal., 1992).
The technical guidance on ISC provided in this document is based on experiences with both
dredged material and ISC projects. While the focus of this document is ISC of contaminated
sediment in riverine and sheltered harbor environments commonly found on the Great Lakes,
the guidance provided herein is generally applicable to contaminated sediments in deeper or
more open water situations such as estuaries, lake bottoms, or ocean shelf environments.
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Chapter 1. Introduction
CONTAINED
AQUATIC DISPOSAL
(CAD)
IN-SITU
CAPPING
(ISC)
PLACED CONTAMINATED SEDIMENT
Figure 1. Conceptual Illustration of dredged material capping and in-situ capping
options.
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Chapter 1. Introduction
In-Situ Capping Functions
Many processes influence the fate of contaminants in bottom sediments. Contaminants can
be transported into the overlying water column by advective and diffusive mechanisms. Mixing
and reworking of the upper layer of contaminated sediment by benthic organisms continually
exposes contaminated sediment to the sediment-water interface where it can be released to
the water column (Reible et al., 1993). Bioaccumulation of contaminants by benthic organisms
in direct contact with contaminated sediments may result in movement of contaminants into the
food chain. Sediment resuspension, caused by natural and man-made erosive forces, can
greatly increase the exposure of contaminants to the water column and result in the trans-
portation of large quantities of sediment contaminants downstream (Brannon et. al. 1985).
In-situ capping can remedy some or all of these adverse impacts through three primary
functions:
a) physical isolation of the contaminated sediment from the benthic environment,
b) stabilization of contaminated sediments, preventing resuspension and transport to
other sites, and
c) reduction of the flux of dissolved contaminants into the water column.
To achieve these results, an in-situ capping project must be treated as an engineered project
with carefully considered design, construction, and monitoring. The basic criterion for a
successful capping project is simply that the cap required to perform some or all of these
functions be successfully designed, placed, and maintained.
Synopsis of Field Experience
A limited number of ISC operations have been performed under varying site conditions, and are
summarized in Table 1. ISC has been applied to riverine, nearshore, and estuarine settings.
Conventional dredging and construction equipment and techniques have been used for ISC
projects, but these practices were precisely controlled. The success of projects to date and
available monitoring data at several sites indicates that ISC can be an effective technique for
long-term containment of contaminants.
In-situ capping of nutrient-laden sediments with sand has been demonstrated at a number of
sites in Japan, including embayments and interior lakes (Zeman et al, 1992). The primary
objective of the capping was to reduce the release of nutrients (nitrogen and phosphorous) and
oxygen depletion by bottom sediments, which were contributing to degraded water quality
conditions. Studies have included measurements of nutrients in interstitial and overlying water
at capped sites, development of a numerical model for predicting water quality improvements
from capping, and monitoring benthos recovery at capped sites. A number of Japanese studies
examining cap placement equipment are discussed in Chapter 4.
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Table 1 . Summary of Selected In-Situ Capping Projects
Project Location
Kihama Inner Lake, Japan
Akanoi Bay, Japan
Denny Way, Washington
Simpson-Tacoma,
Washington
Eagle Harbor, Washington
Sheboygan River, Wisconsin
Manistique River, Michigan
Hamilton Harbor, Ontario
Eftrheim Bay, Norway
St. Lawrence River, Massena,
New York
Contaminants
Nutrients
Nutrients
PAHs, RGBs
cresote, PAHs, dioxins
cresote
PCBs
PCBs
PAHs, metals,
nutrients
metals
PCBs
Site Conditions
3,700 m2
20,000 m2
3 acres nearshore with
depths from 20 to 60 ft.
1 7 acres nearshore with
varying depth
54 acres within
empayment
several small areas of
shallow river/floodplain
20,000 ft2 shoal in river
with depths of 10-1 5 ft
10,000 m2 portion of large,
industrial harbor
1 00,000 M2
75,000 ft2
Cap Design
Fine sand, 5 and 20cm
Fine sand, 20 cm
Avg 2.6 of sandy
sediment
4 to 20 feet of sandy
sediment
3 ft of sandy sediment
sand layer with armor
stone
40 mil plastic liner
0.5 m sand
geotextile and gabions
6 in sand/6 in gravel/ 6
in stone
Construction
Methods
Barge spreading
hydraulic pipeline with
"sandbox"
barge spreading and
hydraulic jet
direct mechanical
placement
placement by crane from
barge
Tremie Tube
deployed from barge
placed by bucket from
barge
Reference
Sumerietal 1995
Sumerietal 1995
Sumeriatal 1995
Eleder
Hahnenberg, pers
com
Zeman &
Patterson 1 996a
Instanes 1994
Kenna, pers com
I
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Chapter 1. Introduction
A variety of ISC projects have been conducted in the Puget Sound area. At the Denny Way
project, a layer of sandy capping sediment was spread over a three-acre contaminated
nearshore area with water depths of 20 to 60 feet. A combination of a sewer outfall discharge
and combined sewer overflow (CSO) had contaminated the site with lead, mercury, zinc, PAHs
and PCBs. The capping was a cooperative effort between the Municipality of Metropolitan
Seattle (METRO) and the Seattle District, USAGE (Sumeri 1989, 1995). At the Simpson-
Tacoma Kraft paper mill, ISC was conducted as part of a Superfund project. Discharges of
paper and pulp mill waste had contaminated the site with PAHs, naphthalene, phenol, dioxins,
and other contaminants. A 17 acre area was capped with material from a sand bar in the
adjacent Puyallup River. An in-situ capping project at the Eagle Harbor Superfund site at
Brainbridge Island placed a 3 to 6 foot layer of sand over creosote contaminated sediments in
water depths of 40-60 feet. Sediments dredged from the Snohomish River navigation project
were transported to Eagle Harbor and placed over a capped area of about 54 acres (Sumeri
1995). Other ISC projects in the Puget Sound area include those at the West Waterway and
Piers 51, 53, and 54.
ISC, with an armoring layer, has also been demonstrated at a Superfund site in Sheboygan
Falls, Wisconsin. This project involved placement of a composite cap, with layers of gravel and
geotextile to cover several small areas of PCB-contaminated sediments in a shallow (<5 feet)
river and floodway. A total area of about one acre of cap was placed with land-based
construction equipment and manual labor (Eleder, 1992).
At Eitrheim Bay in Norway, a composite cap of geotextile and gabions was constructed as a
remediation project in a fjord at an area contaminated with heavy metals (Instanes 1994). A
total area of 100,000 square meters was capped, in water depths of up to 10 meters.
At Manistique, Michigan, an interim cap of 40-mil thick plastic liner was placed over a small (0.5
acre) deposit of PCB-contaminated sediments in order to prevent the resuspension and
transport of sediments until a final remediation was implemented.
At Hamilton Harbor, in Burlington, Ontario, a 0.5 m thick sand cap was placed over a 10,000
m2 area of PAH-contaminated sediments as a technology demonstration conducted by
Environment Canada (Zeman and Patterson 1996a and 1996b).
PCB-contaminated sediments at the General Motors Superfund site in Massena, New York
were removed from the St. Lawrence River by dredging. The remedial objective for the site
was 1 ppm, but areas remaining at concentrations greater than 10 ppm after repeated dredging
attempts were capped. An area of approximately 75,000 square feet was capped with a three-
layer ISC composed of 6 inches of sand, 6 inches of gravel and 6 inches of armor stone
(Kenna, pers com, 1995).
Some field studies have been conducted on long term effectiveness of caps. Sequences of
cores have been taken at capped dredged material sites in which contaminant concentrations
were measured over time periods of up to 15 years (Fredette et al. 1992, Brannon and
Poindexter-Rollings 1990, Sumeri et al. 1994). Core samples taken from capped sites in Long
Island Sound, the New York Bight, and Puget Sound exhibit sharp concentration shifts at the
cap/contaminated layer interface. For the Puget Sound sites, these results showed no change
in vertical contaminant distribution in five years of monitoring with 18 mo and 5 yr vibracore
samples taken in close proximity to each other. In the New York Bight and Long Island Sound
sites, respectively, cores were taken from capped disposal mounds created approximately 3
8
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Chapter 1. Introduction
and 11 years prior to sampling. Visual observations of the transition from cap to contaminated
sediment closely correlated with the sharp changes in the sediment chemistry profiles. The
lack of diminishing concentration gradients away from the contaminated sediments strongly
suggests that there has been minimal long-term transport of contaminants up into the caps.
Additional sampling for longer time intervals is planned.
Design Sequence for In-Situ Capping
A recommended sequence of steps involved with the design of an in-situ capping project is
illustrated in the flowchart in Figure 2. The sequence involves the following general steps:
1. Set a cleanup objective, i.e. a contaminant concentration or other benchmark. The cleanup
objective will be developed as a prerequisite to the evaluation of all remediation alternatives.
[Refer to the logical framework in the ARCS Remediation Guidance Document, (USEPA
1994b)].
2. Characterize the contaminated sediment site under consideration for remediation. This
includes gathering data on waterway features (water depths, bathymetry, currents, wave
energies, etc), waterway uses (navigation, recreation, water supply, wastewater discharge, etc),
and information on geotechnical conditions (stratification of underlying sediment layers, depth
to bedrock, physical properties of foundation layers, potential for groundwater flow, etc). Deter-
mine if advective processes are present and the ability of the cap to control advective
contaminant losses. Determine any institutional constraints associated with placement of a cap
at the site.
3. Characterize the contaminated sediments under consideration. This includes the physical,
chemical, and biological characteristics of the sediments. These characteristics should be
determined both horizontally and vertically. The results of the characterization, in concert with
the cleanup objective, will determine the areal extent or boundaries of the area to be capped.
4. Make a preliminary determination on the feasibility of ISC based on information obtained
about the site and sediments. If site conditions or institutional constraints indicate that ISC is
not feasible, other remediation options must be considered.
5. Identify potential sources of capping materials, including clean sediments that might be
dredged and upland sites or commercial sources for soil, gravel and stone.
6. Design the cap composition and thickness. Caps will normally be composed of clean
sediments, however, other materials such as armor stone or geotextiles may be considered.
The cap design must consider the need for effective short and long-term chemical isolation of
contaminants, bioturbation, consolidation, erosion, and other pertinent processes. If the
potential for erosion of the cap is significant, the cap thickness can be increased, provisions
can be made for placement of additional cap material following erosion, other capping materials
could be considered, or an armor layer could be incorporated into the design.
7. Select appropriate equipment and placement techniques for the capping materials. The
potential for short term contaminant losses associated with cap placement should be
considered in selecting a placement approach.
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Chapter 1. Introduction
8. Evaluate if the capping design meets the cleanup objectives. If not, either reevaluate cap
design or consider other alternatives.
9. Develop an appropriate monitoring and management program to include construction
monitoring during cap placement and long-term monitoring following cap placement. The site
management program should include actions to be taken based on the results of monitoring
and provisions for future maintenance.
10. Develop cost estimates for the project to include construction, monitoring and maintenance
costs. If costs are acceptable, implement. If costs are unacceptable reevaluate design or
consider other alternatives.
More detailed descriptions of the design aspects related to each step are given in the remaining
chapters of this report.
10
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Chapter 1. Introduction
DESIGN SEQUENCE FOR
IN-SITU CAPPING PROJECTS
NOTE ALL BRANCHES
OFTHEaOWCHART
MUST BE FOLLOWED
Figure 2. Flowchart showing sequence of steps involved with the design of an in-situ capping
project.
11
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2 Site Evaluation
This chapter briefly discusses the types of considerations needed to determine if in-situ capping
will meet the objectives and scope of a sediment remediation project. The chapter also
describes considerations in characterizing the in-situ contaminated sediments and the site
conditions from the standpoint of determining in-situ capping feasibility. The types of data
which should be collected and where they enter into an in-situ capping design are also
discussed.
Remediation Objectives
Other documents developed by the ARCS Program provide information and guidance regarding
techniques for the assessment of contaminated sediments to determine their impacts on the
aquatic ecosystem and approaches for determining if some form of remedial action is
warranted (USEPA 1994a, 1994c). This document assumes that a decision to remediate some
contaminated sediments has been made. Although detailed discussion of the methods to reach
a decision to remediate (USEPA 1994a, 1994c) are not included in this document, the ability
of ISC to meet the objectives of a sediment remediation project will be discussed.
The objectives of contaminated sediment remediation may be quite site-specific. ISC is
compatible with some remedial objectives and not others. For example, ISC would not meet
an objective to destroy or remove some particular sediment contaminant from the aquatic
environment. On the other hand, ISC might be able to reduce exposure of aquatic organisms
to sediment contaminants thereby reducing contaminant uptake.
In-situ capping can be evaluated in two ways. The first is to determine if ISC will functionally
satisfy specific remedial objectives. Where remedial objectives are vague or poorly quantified,
a comparison can be made of ISC with other remedial alternatives.
Functional Analysis
In order to determine if ISC will achieve the remedial objectives at a site, one needs to consider
the three primary functions of a cap discussed in Chapter 1. In some cases, the remedial
objectives may be satisfied by a single ISC function. In other cases, two or all functions may
be needed to satisfy the remedial objectives.
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Chapter 2. Site Evaluation
If the remedial objectives are defined in terms of a reduction in risk associated with exposure
of the contaminants to benthic organisms, potential bioaccumulation, and potential movement
of contaminants up the food chain, the physical isolation of the contaminated sediment from
aquatic organisms may be the basic function and design requirement for the in-situ cap. The
physical isolation effects of the cap may be localized at the capped site, or may be more
widespread as a result of the stabilization function (as discussed below).
The ability of an ISC to isolate aquatic organisms from sediment contaminants is, in part,
dependant upon the character of any "new" sediments, i.e. those that could potentially be
transported from other contaminated areas and be deposited on the cap. If external sources
of contamination have not been sufficiently controlled, an in-situ cap may simply be a barrier
between two layers of contaminated sediments. Therefore, where physical isolation of
sediment contaminants is required to meet remedial objectives, ISC should only be considered
if source control has been implemented.
Stabilization of sediments in-place may be a basic design function where the remedial objective
is to prevent impacts caused by the resuspension, transport and redeposition of contaminated
sediments at remote areas. For example, a waterway where conditions are expected to remain
degraded may be considered for capping in order to keep sediments from contaminating higher
quality areas downstream. In such a case, a cap, designed solely to keep contaminated
sediments in-place might meet the short-term objectives of a remediation plan. An example
is the temporary cap used to stabilize contaminated sediments at Manistique River, Michigan
until a permanent remedial action could be implemented.
If a remedial objective is tied to the quality of the overlying water column, the design function
for the cap may be chemical isolation from the sediments. Such was the case for several of
the capping applications in Japan, where the primary objective was to reduce the loadings of
nutrients from sediments to the water column in order to improve the eutrophic conditions. The
control of the flux of dissolved contaminants should consider diffusive and advective transport
processes.
Comparative Analysis
Remediation objectives are often framed in generalities that make it difficult to eliminate
remedial technologies from further consideration. Where more than one technology is feasible
and capable of meeting remedial objectives, a comparative approach is needed. Because it
is one of the least costly sediment remediation alternatives, in-situ capping is likely to be
evaluated fully. In performing comparative analyses of ISC and other sediment remediation
alternatives there are a number of issues, both technical and policy, to be addressed.
In-situ capping has some fundamental differences from other sediment remediation alternatives
that may complicate a comparative analysis. Alternatives that remove contaminated sediments
from the waterway generally release some contaminants to the waterway during removal
(dredging) with short and, in some alternatives, long-term releases at a site (CDF or treatment
site) away from the water. ISC has both short- and long-term releases to the waterway.
Losses occurring at a terrestrial site may not be directly comparable to losses to the waterway
especially since the rationale for sediment remediation was based on aquatic impacts.
13
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Chapter 2. Site Evaluation
The duration or timeframe for such comparative analysis of impacts or contaminant loss is an
issue that can greatly alter the results. Most alternatives involving removal will have the
majority of losses occurring during removal and placement in a disposal facility or treatment.
ISC is expected to have some minor releases during construction. Following construction, any
releases will largely depend on the nature of any advective processes. There may be a higher
initial rate of release due to compression of pore water during and for some time following cap
installation. If a groundwater flow condition exists at the site, there will be a continuous release
due to advection. If no advection is present, there will be a very slow, but continuous diffusive
release occurring after a lag time. Remedial actions under Superfund are typically evaluated
for timeframes of 30 years. The differences between ISC and more "conventional" remedial
alternatives have raised questions about the adequacy of such a limited temporal analysis. A
comparative analysis performed for proposed remedial alternatives at Manistique Harbor,
Michigan considered timeframes of hundreds of years, based on calculations of flux and
assuming sorption of RGBs onto the capping materials (Blasland Bouck and Lee Inc. 1994,
1995).
Finally, when comparing ISC with other remedial alternatives, there is an element of cap design
that should be considered. The part of ISC design that addresses the susceptibility of the cap
to erosion must consider forces that are highly dynamic (i.e. river flows, propeller wash, wave
heights, etc.). ISC design analyses contain probabilistic factors that are not commonly present
in the design of treatment or confined disposal alternatives. The ability to predict these forces,
and the acceptability of risk associated with failure are concerns that are especially relevant for
in-situ capping.
Uncertainties will be encountered in evaluating the expected performance of any remedial
alternative. Direct comparison of alternatives to meet remedial objectives and the relative
performances of alternatives will be complicated by these uncertainties. Typically best profes-
sional judgement and sensitivity analyses of the effects of input variables on predictive models
is the best approach to weighing the benefits of remedial alternatives.
Remediation Scope
The scope of a remediation project defines the physical extent of the remediation in terms of
both space and time. Scope may be defined in terms of site or funding constraints, through a
negotiated or adjudicated settlement, or by a detailed risk assessment or modeling effort.
While the scope of a removal-based remedial action is typically expressed as a volume (e.g.,
50,000 cubic yards of sediment to be removed and treated or disposed), the scope of an ISC
alternative is more appropriately considered in spatial or areal terms (e.g.,14 acres of bottom
surface area to be capped). The volume of sediments under the cap may not effect the
decisionmaking, although the total mass of contaminants remaining may be a consideration.
The thickness and vertical distribution of contaminated sediments will enter into decisions
regarding cap design or selection of capping materials.
It should be recognized that there may be other differences between the scope of a removal-
based alternative and an ISC alternative. For instance, a set of remedial objectives that would
require sediments to be dredged from an area or reach of a waterway may require capping
over an entirely different "footprint".
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Chapter 2. Site Evaluation
Site Conditions
Site conditions, more than any other consideration, will dictate the feasibility of in-situ capping.
Site characteristics affect all aspects of a capping project, including design, construction
equipment, monitoring and management programs. Only some limitations in site conditions
can be accommodated in the ISC design. A thorough examination of site conditions should
generally determine if further consideration of ISC is appropriate. Site conditions that must be
considered include the physical environment, hydrodynamic conditions, sediment characteris-
tics, and existing or potential uses of the waterway.
Because in-situ caps are intended to function for extended periods of time, if not in perpetuity,
it is not sufficient to just examine the existing conditions of the site. The evaluator must also
consider future conditions that might significantly alter cap integrity or function. Examples
might include the removal of a dam or controlling structure on a river, decay or removal of
breakwaters or other protective structures, changes in the type or draft of vessels navigating
the waterway, or long-term trends in land or groundwater use. The permanence or stability
of site conditions for the long-term future should be factored into the evaluation of site
conditions.
Physical Environment
The physical environment of a proposed ISC site to be considered includes waterway
dimensions, water depths (bathymetry), tidal patterns, ice formation, aquatic vegetation, bridge
crossings and proximity of lands or marine structures (i.e., docks, piers, breakwaters). The
bathymetry of the site has an influence on the degree of spread during placement and stability
of capping material. The flatter the bottom slope the more desirable, especially if capping
material is to be placed hydraulically. It is difficult to estimate the effects of slope alone, since
bottom roughness plays an equally important role in the mechanics of the spreading process.
Water depths and tidal patterns may limit cap construction options, and cause effects on cap
design and waterway uses discussed later. The potential for ice jams and scour at riverine
sites in northern climates should be considered. The proximity of the ISC site to land areas or
marine structures may impact construction options and present legal issues concerning riparian
owners.
Some types of physical information are available from nautical charts and the "U.S. Coast Pilot"
(published by the National Ocean Service) and topographic maps (developed by the U.S.
Geological Survey). More detailed bathymetric surveys are maintained by the USAGE for
authorized federal navigation channels. Local governments (i.e., port authorities, planning
commissions, sewer and drainage districts) may also have detailed maps of waterways.
Hydrodynamic Conditions
Capping should be used in environments where the long term physical integrity of the cap can
be maintained. Low energy environments in protected harbors, low flow streams, or estuarine
systems are more appropriate for in-situ capping projects than waterways with high flows since
the long-term integrity of the cap will be of less concern and less extensive armoring (or none)
15
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Chapter 2. Site Evaluation
\l
will be required. In open water, deeper sites will be less influenced by wind or wave generated
currents and are generally less prone to erosion than shallow, nearshore environments.
However, armoring techniques or selection of erosion resistant capping materials may make
capping tecRnicaily feasible in some higher energy environments aswe'Crecog'hlzing that risks
increase.
Water column currents affect the degree of dispersion during cap placement and may influence
the selection of equipment for cap placement. Of more importance are bottom currents which
could potentially cause resuspension and erosion of the cap. In addition to ambient currents
due to normal channel flows, tidal fluctuations, etc., the effects of storm-induced waves or other
episodic events such as flood flows on bottom current velocities must also be considered.
Capping operations should not be conducted during storms, flood flows, or other extreme
events, so the designer doesn't need to consider such events in selection of equipment or
placement technique for the cap. However, ambient currents, waves and water levels may limit
construction techniques and hamper monitoring or maintenance activities.
The presence of an in-situ cap can alter existing hydrodynamic conditions. In harbor areas, or
estuaries, the decrease in depth or change in bottom geometry may affect current patterns.
In a riverine environment, the placement of a cap, by reducing depths and restricting flows may
significantly alter the flow carrying capacity of the channel. Changes in channel geometry may
also affect flow velocities, increasing shear stresses on a cap or to opposite or downstream
streambanks. Historic flow data may therefore not be adequate to characterize velocities at
the capping site. Modeling studies may be required to assess such changes in site conditions
due to placement of an in-situ cap.
The types of information needed to evaluate hydrodynamic conditions at a proposed ISC site
include currents, waves and flood flows. These phenomena are not static, but will vary with
meteorologic conditions. Information on recorded ranges (i.e., max and min water levels or
river flows) may be available from: National Ocean Service navigation/mariners guides; USAGE
records of Great Lakes water levels; U.S. Geological Survey publications of water level/flow
monitoring stations, and; flood insurance studies. Some states also collect river flow data.
Additional sources of information include studies conducted by the USAGE or local
governments in relation to flood protection and shoreline or streambank erosion.
Where published information is not available, or where projections of maximum levels are
needed, standard predictive methods and models may be used (Hydrologic Engineering Center
1995; Coastal Engineering Research Center 1984). The consideration of hydrodynamic
conditions in the assessment of cap thickness or need for armoring are described in Chapter
3, Cap Design.
Geotechnical/ Geological Conditions
The geotechnical conditions at the site must be assessed to include stratification of underlying
sediment layers, depth to bedrock and physical properties of foundation layers. This
information will be needed to evaluate the potential for consolidation of the underlying sediment
layers after cap placement. This evaluation is needed to properly interpret information on layer
16
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Chapter 2. Site Evaluation
thickness during placement and any observed movement of the bottom surface following cap
placement.
Hydrogeological Conditions
The environmental importance of ground water/surface water interactions is well documented
(USEPA, 1991). The significance of the ground water/surface interactions are determined by
the hydrogeologic characteristics of the site. A detailed evaluation and understanding of the
site's hydrogeology is a critical component in evaluating the acceptability of a capping proposal
at a proposed capping site and a prerequisite to proper cap design.
Groundwater flows from locations associated with high hydraulic head to locations of low
hydraulic head, moving from recharge areas along the path of flow to discharge areas.
Discharge areas can be defined as locations where the groundwater flow path has an upward
component (Freeze and Cherry 1979). The near shore portions of lakes and streams in the
midwestern portions of the United States commonly function as ground water discharge areas.
These are areas where ground water exits the ground water regime and enters the surface
water regime. Sediments reside at the interface of the ground water and surface water
regimes.
From a hydrogeologic perspective, most cap designs can be viewed as a thin granular layer
at the sediment-water interface. Such a cap would not differ in most ways from the sediment
which accumulates naturally at the bottom of the body of surface water under consideration.
Consequently, capping contaminated sediments with porous granular materials should not
significantly alter the groundwater flow characteristics of the site in most hydrogeologic settings.
In the presence of contaminated sediments, upward hydraulic gradients would sequentially
drive ground water from the underlying geologic materials through the layer of contaminated
sediments and the overlying porous cap into the surface water. Depending on the properties
and thickness of the capping materials, a fraction of the contaminants will be transported to the
overlying surface water. A knowledge of the groundwater flow is therefore needed to evaluate
the significance of this contaminant flux.
The development of instruments for the measurement of ground water surface water
interactions dates from the mid 1940's (USGS, 1980). Presently, methodologies for the
measurement of the quantity and quality of ground water being discharged to surface water are
also well documented (USEPA, 1991; USGS, 1980) and have been applied in the field (USGS,
1993; USGS 1994; Lee and Cherry, 1978; Taniguchi and Fukuo, 1993). Piezometers have
been used to quantify the magnitude of the upward hydraulic gradient and the hydraulic
conductivity (Lee and Cherry, 1978; USGS, 1993). Seepage meters can provide a direct
measure of the quantity of ground water being discharged to surface water and have been
used to determine the volume of flow per unit area per unit time at the sediment/water interface
(termed the specific discharge or flux) (USGS, 1993). If properly used, seepage meters can
also be used to determine the quality of the water being discharged to surface water. This is
done through the use of seepage meters as water sample collection devices. The samples are
later analyzed for the water quality parameters of concern (USGS, 1994).
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Chapter 2. Site Evaluation
Sediment Characterization
The physical characteristics of the contaminated sediment are of importance in developing the
cap design, selecting appropriate equipment for cap placement, modeling and monitoring cap
performance. Physical tests and evaluations on sediment should include: visual classification,
natural (in-situ) water content/solids concentration, plasticity indices (Atterberg limits), organic
content (specifically total organic carbon (TOC), grain size distribution, specific gravity, and
Unified Soil Classification System (USCS) classification. Standard geotechnical laboratory test
procedures, such as those of the American Society for Testing and Materials (ASTM) or the
USAGE, should be used for each test. Table 2-1 gives the standard ASTM and USAGE
designations for the needed tests, and also cross-references these procedures to those of
several other organizations that have standardized test methods.
The thickness of the contaminated sediment layer and the physical properties of the soil
underlying this layer need to be determined in order to evaluate the consolidation of the cap.
The thickness of contaminated sediment layers can be obtained by probings, remote sensing
techniques, or core sampling. The same type of physical data are needed for the underlying
material as obtained for the contaminated sediments. If the contaminated sediment or
underlying sediment layers are compressible, consolidation will occur due to cap placement.
The degree of potential consolidation should be evaluated based on standard consolidation
testing procedures (USAGE 1970), modified to account for the high water content of sediment
samples (USAGE 1987).
Shear strength of the contaminated sediment layer should be considered for evaluation of the
stability of the cap during placement. However, data and design guidance on bearing capacity
and slope stability considerations for subaqueous caps are presently limited (see Chapter 3).
Physical analysis of site water may also be required, e.g. suspended solids concentration and
salinity. These data must be developed using standard techniques.
The in-situ sediment will typically be characterized for chemical concentrations of contaminants
of interest in terms of both areal extent and vertical distribution. Chemical characterization data
is needed for modeling contaminant migration as well as for interpretation of monitoring data
during and following capping.
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Chapter 2. Site Evaluation
Table 2-1 . Standard Geotechnical Laboratory Test Procedures
Test
Soils
Water Content
Grain Size
Atterberg limits
Classification
Specific gravity
Organic content
Consolidation*
Permeability"
Shear Tests
Designation
ASTM1
AASHTO2
USAGE3
DOD4'5
Comments
D2216
D422
D4318
D2487
D854
D2974
D2435
D2434
D2573
T265
T88
T89
T90
T100
T216
T215
i
V
in
in
IV
VIII
VII
Method 105,
2-VII
2-III, 2-V, 2-VI
Method 103,
2-VI 1 1
2-IV
Use Method C
Field Test
1 American Society for Testing and Materials
2 American Society of State Highway and Transportation Officials
3 Dept. of the Army Laboratory Soils Manual EM 1110-2-1906 (USAGE 1970)
4 Dept. of Defense Military Standard MIL-STD-621A (Method 100, etc.)
5 Dept. of the Army Materials Testing Field Manual FM (50530 (2-III, etc.)
*Do not use the standard laboratory test for determining consolidation. Instead, use the modified standard consolidation
test and the self-weight consolidation test as described in USAGE 1987.
**One value of permeability must be calculated from the self-weight consolidation test.
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Chapter 2. Site Evaluation
Waterway Uses
The technical and socioeconomic feasibility of ISC at a particular site is, in part, dependant on
how the capping would impact or be impacted by existing or planned uses of the waterway.
Waterway uses that may conflict with a proposed in-situ cap include:
- navigation (commercial and recreational);
- flood control
- recreation (swimming, fishing, etc.);
- water supply and withdrawal (presence of intakes, etc.);
- stormwater or effluent discharge;
- sensitive or important aquatic habitats
- waterfront development; and,
- utility crossings.
The construction of an ISC may limit or eliminate some of the above waterway uses. Potential
sources of information on waterway uses include: local waterfront development plans;
wastewater discharge permits; remedial action plans; harbor authority masterplans; and,
municipal street/sewer improvement plans.
If the site under consideration is adjacent to or within a navigation or flood control channel, the
effects of cap placement on those functions of the channel must be evaluated. Placement of
a cap decreases the water depth and cross-sectional area, reducing the flow carrying capacity
of a channel and the navigable depth. By reducing water depths in a harbor or river channel,
commercial and recreational vessels may have to be restricted or banned entirely depending
on their draft. The acceptable draft of vessels allowed to navigate over a capped area must
consider water level fluctuations (seasonal, tidal and wave) and the potential effects of
groundings on the cap. Because of the potential erosion caused by propeller wash, restrictions
may also be needed on vessels based on engine size. Anchoring must not be allowed at
locations on or near the ISC site. Fishing and swimming may have to be restricted to avoid
vessels from dragging anchors across the cap.
If the area being considered for ISC is within a Federally authorized channel, the process
involved with the modification of that authorization or de-authorization should be considered.
The effects of de-authorization or a change in authorization on the project purposes and on
uses of the channel, the value of those uses, and any secondary impact should be considered
fully.
The presence of an in-situ cap may limit future uses of the waterway. For instance, the
locations of water supply intakes, stormwater or effluent discharge outfalls, utility crossings, and
the construction of bulkheads, piers, docks and other waterfront structures would have to be
evaluated with consideration of their potential impacts on cap integrity and maintenance.
Utility crossings (water, sewer, gas, oil, telephone, cable, and electrical) are commonly located
in urban waterways. Existing utility crossings under portions of waterways to be capped may
have to be relocated if their deterioration or failure might impact cap integrity or because they
could not be repaired without disturbing the cap. Future utility crossing my be prohibited in the
cap area with resulting social/economic considerations.
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Chapter 2. Site Evaluation
The ability to enforce use restrictions necessary to protect the integrity of an in-situ cap (e.g.,
vessel size limits, bans on anchoring, etc.) is an area with little or no operating experience.
Voluntary restrictions on uses of public lands and waters are often ineffective. Compliance,
enforcement, and the effectiveness of these measures as well as the consequences of non-
compliance on ISC should be considered.
Regulatory and Legal Considerations
Any sediment remediation alternative must address, and comply with a number of legal and
regulatory requirements. The ability of ISC to comply with some environmental laws and
regulations has been questioned, and full-scale applications of this technology are so limited
that some legal issues have not yet been resolved. Because of the potential effect of compli-
ance on the feasibility of this remedial alternative, legal and regulatory considerations should
be closely examined at the earliest possible time. An overview of legal and regulatory consid-
erations for sediment remediation is provided in the "ARCS Remediation Guidance Document"
(USEPA 1994b). This section will not detail all of the regulatory requirements for ISC, but will
discuss those that are unique or especially significant for the construction of an in-situ cap.
Construction in Waterways
Any structures or work that impact the course, capacity, or condition of a navigable water of the
United States must be permitted under Section 10 of the Rivers & Harbors Act of 1899 (33
CFR 403). The permit program for Section 10 permits is managed by the USAGE. Federal
regulations on the USAGE permit program are contained in 33 CFR Parts 320-330 (Regulatory
Programs of the Corps of Engineers).
For an ISC project, Section 10 permitting will require consideration of the cap as an obstruction
to navigation. The Coast Guard and local and regional navigation users are contacted. In
addition, the potential for the cap to obstruct flows, cause flooding or erosion are considered.
If the ISC is within an authorized Federal navigation project, Congressional action is needed
to deauthorize the project or modify the authority.
Discharge of Dredged or Fill Materials
The disposal of dredged or fill materials to waters of the United States is regulated under
sections of the Clean Water Act of 1972 (PL 92-500), as amended. Section 404 designates
the USAGE as the lead Federal agency in the regulation of dredge and fill discharges, using
guidelines developed by the USEPA in conjunction with the USAGE. Federal regulations on
the Corps permit program are contained in 33 CFR Parts 320-330 (Regulatory Programs of the
Corps of Engineers).
Cap material is a dredged or fill material (depending on its origin), and its placement in "waters
of the U.S.", which includes wetlands, requires a permit under Section 404 and a certification
of water quality compliance from the state under Section 401. Permits are not required for
superfund projects, but the technical evaluations required for a permit must be made. Capping
material that is dredged may require testing to determine it is not contaminated. The "Inland
Testing Manual" (USEPA/USACE in preparation) and "Great Lakes Dredged Material Testing
& Evaluation Manual" (USEPA/NCD 1994).
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Chapter 2. Site Evaluation
RCRA and TSCA
The ability of ISC to comply with the requirements of the Resource Conservation and Recovery
Act (RCRA) or the Toxic Substances Control Act (TSCA) has not been fully established. In-situ
capping of sediments with PCB contamination subject to regulation (> 50 ppm) was approved
by the USEPA at the Superfund project in Manistique Harbor, Michigan. In this case, it was
reasoned that since the sediments were not removed, TSCA was not invoked.
Preliminary Feasibility Determination
Following the assessment of remediation objectives, scope, sediment characteristics, and site
conditions, a preliminary determination of the overall feasibility of ISC at the site under
consideration should be made (as shown on figure 2). Since there are no specific criteria for
site suitability for ISC, such a determination must be largely qualitative in nature.
The ability of ISC to meet remedial objectives may be determined at this stage, or may require
contaminant migration modeling, as discussed in Chapter 3. It may be easier to determine that
ISC will not meet to specific objectives than concluding that it will.
The incompatibility of ISC with existing or planned waterway uses may be a direct indication
of infeasibility, especially where the use is of high value to the local community or represents
a significant economic benefit. Any consideration of limiting or eliminating waterway uses
represents a potentially controversial matter. All levels of government (Federal, state and local)
share the responsibility for the management of most waterways, and the interests of all users
must be considered.
Where there are incompatibilities between ISC and waterway uses, alternatives to an infeasi-
bility determination include a modified project design. For example, where an in-situ cap would
create water depths too shallow for essential navigation traffic, an alternative might be to
dredge just enough of the contaminated sediments to allow the cap to be constructed without
limiting navigation. Where a project modification can't be developed to alleviate
incompatibilities, users might be mitigated for lost use or revenue. At the Waukegan Harbor
Superfund project, a marina operation was relocated so that a slip with contaminated sediments
could be remediated in-place.
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3 In-Situ Cap Design
General Considerations
The composition and dimensions (thickness) of the components of a cap can be referred to as
the cap design. This design must perform one or more of the three functions discussed in
Chapter 1 (physical isolation, stabilize sediment, and reduce flux of dissolved contaminants).
The design must also be compatible with available construction and placement techniques.
Dredged material caps are typically constructed with a single layer of "clean" sediments
because: relatively large volumes are usually involved; "clean" sediments from other dredging
projects are often available as cap materials; and, a disposal/capping site with low potential for
erosion can ususally be selected. Guidance on dredged material cap design (Palermo et al
in preparation) focuses on the thickness of the cap as the major design criterion.
In contrast, in-situ capping projects usually involve smaller volumes or areas, clean sediments
are not always readily available as capping material, and site conditions are a given. For these
reasons, caps composed of multiple layers of granular materials as well as other materials such
as armor stone or geotextiles are often considered, and the in-situ cap design cannot always
be developed in terms of cap material thickness alone.
This chapter describes the considerations and procedures used to determine the necessary
cap components for the three basic functions discussed in Chapter 1. At present, the design
of in-situ caps is based on a combination of laboratory tests and models of the various pro-
cesses involved: (advective/diffusive contaminant flux, bioturbation, consolidation, and erosion),
field experience, and monitoring data. Since the number of carefully designed, constructed,
and monitored capping projects is limited, the design approach is presently based on the
conservative premise that the cap components are additive. No dual function performed by cap
components is considered. As more data become available on the interaction of the processes
affecting cap effectiveness, this additive design approach can be refined.
The general steps for in-situ cap design include:
a. Identify candidate capping materials and compatibility with contaminated sediment at
the site.
23
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Chapter 3. In-Situ Cap Design
b. Assess the bioturbation potential of indigenous benthos and design a cap component
to physically isolate sediment contaminants from the benthic environment.
c. Evaluate potential erosion at the capping site due to currents, waves, propeller wash,
and design a cap component to stabilize the contaminated sediments and other cap com-
ponents.
d. Evaluate the potential flux of sediment contaminants and design a cap component to
reduce the flux of dissolved contaminants into the water column.
e. Evaluate potential interactions and compatibility among cap components, including
consolidation of compressible materials.
f. Evaluate operational considerations and determine restrictions or additional protective
measures (e.g., institutional controls) needed to assure cap integrity.
A flowchart illustrating these steps is shown in Figure 3. More detailed discussion of these
design steps are discussed in the following paragraphs. If the objective of the cap does not
require all three basic functions (e.g., a temporary cap whose sole function is to stabilize the
sediments), a simpler design sequence could be followed.
Identification of Capping Materials
In the beginning of an ISC cap design, all potential cap material sources should be identified.
Sources of cap materials should be identified at the beginning of the design process because
these materials will generally represent the largest single item in the overall project cost, and
the utilization of locally available sediments, soils or other granular capping materials can have
a significant impact on ISC feasibility and implementation. The selection among cap materials
(or use of more than one) will be determined by subsequent analysis.
Most in-situ capping projects conducted to date have used sediment or soil materials, either
dredged from nearby waterways or obtained from upland sources, including commercial quar-
ries. At some locations, a simple layer of granular material can effectively perform all three
cap functions. In other cases, more complex cap designs may be required. Capping materials
such as geotextiles and plastic liners may be able to perform one or more of the basic cap
functions. These materials may also be used in conjunction with granular materials for const-
ructability or stabilization purposes. Examples of multi-layer cap designs are illustrated in
Figure 4.
Granular Materials
In most cases, granular materials such as quarry sand, natural sediments or soil materials
should be considered as a necessary part of the cap design to physically isolate the sediments
from the benthos and water column, prevent sediment resuspension and transport, and reduce
the flux of dissolved contaminants.
Previous studies have shown that both fine-grained materials and sandy materials can be
effective capping materials (Brannon et al 1985). Fine grained materials (clays) have been
used in Europe in connection with control of eutrophication (Klapper 1991, 1992). Suszkowski
(1983) found fine grain material to be a better chemical barrier than a sand cap. The chemical
24
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Chapter 3. In-Situ Cap Design
containment afforded by a granular cap material is dependent on the sorption capacity of the
material, and sandy (non-cohesive) materials usually have low sorption capacity compared to
silt or clay materials. For this reason, a naturally occuring sandy soil or sediment, containing
a fraction of finer grain sizes and organic carbon, is a more desirable capping material from the
standpoint of isolation than a clean, quarry-run or washed sand.
Hydrophobic organic pollutants of concern are typically strongly bound to the organic fraction
of the contaminated sediment which is largely found in the silty and smaller particle fraction of
the sediment. Fresh sorption sites in the cap will greatly reduce the rate at which the chemicals
move through the cap both during consolidation and long-term diffusive processes.
The migration of metals is more complex than that for hydrophobic organic chemicals because
several additional factors affect the chemistry of metals. Most importantly, the oxidation state
influences the solubility of the metal and thus its affinity for the stationary sediment matrix.
Thus the Eh, pH, bacterial activity, and presence sulfides, chlorides, carbonates, etc., all
influence metal migration. Due to the complexity of sediment chemistry with regard to metal
migration, the design presented in this document focuses primarily on the containment of
neutral hydrophobic organic chemicals which is enhanced by finer, higher organic carbon
content material.
Although fine grained sediments, especially those with significant amounts of organic carbon
would be an optimal cap material for reducing the flux of organic contaminants by advection/-
diffusion, there are several other considerations in favor of sandy materials. The placement
of non-cohesive materials is generally far easier than with fine grained materials. Silty materi-
als are more readily resuspended and therefore difficult to place in conditions with even low
currents or water velocities and more likely to require armoring. Sandy materials are stable at
steeper slopes than fine grained materials. As a result, the footprint of a silty cap will be larger
than a sand cap, and more fine grained material needed to cap the same deposit as a sandy
material.
Another potentially significant advantage of sandy cap material, is related to potential benthic
recolonization and bioturbation. As discussed below, the potential for penetration into the cap
by burrowing animals is far greater for unconsolidated, fine grained sediments than it is for
sandy sediments with little organic matter.
Information about potential upland sources of granular cap materials can be obtained from
organizations that design or perform all types of construction, such as state highway depart-
ments, county or city departments of engineering, roads, parks, and sewers, general contrac-
tors, and local quarry operators. Potential sources of sediments that are scheduled for dredg-
ing and might be used for cap material can be obtained from the Corps of Engineers, local
harbor authorities, and private marina operators.
The physical and chemical characteristics of materials under consideration for the cap should
be determined. Physical characteristics of importance include densities, plasticity indices (for
fine-grained materials), organic content, grain size distribution, and specific gravity (methods
cited in Table 2-1). These characteristics can be used to develop a Unified Soil Classification
System (USCS) classification for the material.
25
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Chapter 3. In-Situ Cap Design
IN-SITU
CAP DESIGN FLOWCHART
BIOTURBATION
COMPONENT
OPERATIONAL
COMPONENT
CONSOLIDATION
COMPONENT
CHEMICAL
ISOLATION
COMPONENT
CONTINUED
Figure 3. Flowchart showing steps involved in design evaluation of various insitu
cap components.
26
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Chapter 3. In-Situ Cap Design
3St^J3S&y»P»;£So/A^
Source Blasland and Bouck Engineers (1990)
GEOTEXTLE FABRIC -
12" GRADED AfiUOR STONE-
20" SAND MATERIAL
SEWUENT -
BEDROCK -
CCOTDOBX FABRIC
12" GRADED ARUOR STONE
20' ENHANCED SAND MATERIAL
(AUCMENIED WTH ACTIVATED
CARQON)
Figure 4. Illustrations of alternative combinations of cap components.
27
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Chapter 3. In-Situ Cap Design
From the standpoint of contaminants, the capping material must be one which is acceptable
for unrestricted open-water placement (that is a clean material). For sediments or soils,
procedures normally used to assess the acceptability of dredged material for open water
disposal should be used for the assessment of the suitability of a material for capping (USEPA/
USAGE 1991, USEPA/ USAGE in preparation, USEPA/ NCD 1995). Acceptability of such a
material from the standpoint of both potential water column and potential benthic effects must
be determined and some chemical and biological characterization of the material may be
required.
Geosynthetic Fabrics and Membranes
Geomembranes are impermeable, synthetic materials, commonly used as landfill liners and
other applications. Geotextiles are porous, synthetic fabrics, and have been used in many
construction applications in recent years. A common example is the use of a geotextile for
increased stability of a constructed earth embankment such as a dredged material disposal
dike. Tubes or containers composed of geotextile material have also been used for
containment applications, where the tubes are filled with sandy or fine-grained dredged material
(Fowler and Sprague 1995).
Geosynthetics (geomembranes and geotextiles) have also been used for subaqueous capping
applications, but field experience is limited. Potential functions of geosynthetics in ISC designs
include: provide a bioturbation barrier; stabilize the cap; reduce contaminant flux; prevent
mixing of cap materials with underlying sediments; promote uniform consolidation, and; reduce
erosion of the capping materials.
Geotextiles have been used in conjunction with granular material for the in-situ cap constructed
at Sheboygan River (Figure 4a) and at an ISC constructed in Eitrheim Bay, Norway (Instanes
1994). The design function of the geotextiles in these applications was not specified, although
it is believed to have been primarily for stabilization of sediments and constructability. The cap
design which had been proposed for Manistique River/Harbor (Figure 4b) included a geotextile
for stability and constructability purposes.
Geomembranes have been installed under water in association with the construction of a
dredged material confined disposal facility (Savage 1986). There is also field experience with
use of membranes for controlling plant growth in lakes (Cooke et al 1993). In
principle.geomembranes should be able to provide effective chemical isolation. However, there
are unresolved issues of constructability and long-term integrity. One such issue is the impact
of gas generation by contaminated sediments, and the potential lifting of the geomembrane.
This problem has occurred with lake applications (Cooke et al 1993).
A 40-mil HOPE membrane was placed over a 26,400 square foot area at Manistique River as
an interim control to temporarily prevent the erosion of contaminated sediments until a per-
manent remediation was implemented (Hahnenberg, pers com). This membrane was fitted
with stop valves to allow gas venting and was weighted with concrete block anchors. Following
installation, the membrane was observed to have billowed (ballooned), although it was not
determined if this was due to gas generation or water entry under the cap (Hahnenberg, pers
com; Blasland, Bouck & Lee 1994).
No data are available on the performance of geomembranes for chemical isolation in an in-situ
cap. Geosynthetics are available from many commercial sources, and are available in a
28
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Chapter 3. In-Situ Cap Design
variety of composition materials with specific characteristics, including woven/non woven,
thicknesses, weight/density, fitted with weights, vent holes, etc. It is conceivable that a
composite geosynthetic could be manufactured to perform multiple cap functions.
Armor Stone
An armoring layer for resistance to erosion can also be considered in the cap design (Envi-
ronmental Laboratory 1987; Maynord and Oswalt 1993). The caps constructed at Sheboygan
River and Massena, and the design which had been proposed for Manistique River/Harbor
(Figure 4) represent cases where a cap component has been included solely for erosion protec-
tion. In other cap designs, the exterior cap material has generally performed other functions
besides erosion protection. Armor stone are available from commercial quarries in a variety
of size gradations and stone types. Details on use of armor stone as a cap component are
found in Appendix A.
Physical Isolation Component
In many cases, sediment remediation is driven by concerns about the uptake of bioaccum-
ulative contaminants by aquatic organisms either directly from the sediments or by foraging on
benthos. In order to eliminate this pathway for contaminant uptake, an in-situ cap must
physically isolate the sediments from benthic or epibenthic organisms. To design a cap com-
ponent for this function, the bioturbation potential of indigenous benthic infauna must be
evaluated. The physical isolation component of the cap may include separate sub-components
for isolation, bioturbation and consolidation.
Isolation Component
The basic function of the required sediment cap is that associated with physical and/or chem-
ical isolation. For granular cap materials, the thickness which provides an effective physi-
cal/chemical barrier may be defined as T,. If the desired function of the cap is physical isolation
from benthic organisms, the isolation component provides a buffer between the organisms at
their burrowing depth and the contaminated materials. A thickness of one foot for the granular
capping material for this purpose is considered conservative. This approach to design of the
isolation cap component is satisfactory if the cap is intended to physically isolate the
contaminated sediments from benthic organisms or to physically isolate nutrient-rich sediments
or sediments with relatively low levels of contamination.
If the desired function of the cap is reduction of contaminant flux, a more involved analysis to
include capping effectiveness testing and modeling would be required as discussed below for
design of a chemical isolation cap component. In this case, a value of one foot for the
thickness of granular capping material may be considered as a trial value for the isolation
component for purposes of the modeling effort.
Bioturbation Component
In the context of capping, bioturbation may be defined as the disturbance and mixing of
sediments by benthic organisms. Aquatic organisms that live on or in bottom sediments can
greatly increase the migration of sediment contaminants through the direct movement of
sediment particles, increasing the surface area of sediments exposed to the water column, and
29
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Chapter 3. In-Situ Cap Design
as a food for epibenthic or pelagic organisms grazing on the benthos. The specific assemblage
of benthic species which recolonizes the site, the bioturbation depth profile, and the
abundances of dominant organisms are key factors in determining the degree to which
bioturbation will influence cap performance.
The depth to which organisms will bioturbate is dependent on the organism's behavior and the
characteristics of the substrate (i.e., grain-size, compaction, organic content, pore water
geochemistry, etc.). In general, the depth of bioturbation by marine benthos is greater than that
of freshwater benthos. The recolonization by the benthic infauna at marine dredged material
caps is primarily suspension feeders as opposed to burrowing organisms (Cullinane et at.,
1990; Morton, 1989; Myers, 1979).
The intensity of bioturbation is greatest at the sediment surface and generally decreases with
depth. A surficial layer thickness of sediment will be effectively overturned by shallow
bioturbating organisms, and can be assumed to be a continually and completely mixed
sediment layer for purposes of cap design. This layer is generally a few centimeters in
thickness. Depending on the site characteristics, a number of mid-depth burrowing organisms
overtime recolonize the site. The level of bioturbating activity for these organisms will decrease
with depth. The species and associated behaviors of organisms which occupy these surface
and mid-depth zones are generally well known on a regional basis. There may also be
potential for colonization by deep burrowing organisms (such as certain species of mud shrimp)
which may borrow to depths of 1 meter or more. However, knowledge of these organisms is
very limited.
In preparation for this document, a survey was made of noted aquatic biologists from several
research facilities around the Great Lakes. The survey described two hypothetical cap designs
under shallow water conditions typical of the Great Lakes; one with a cap surface of medium
to fine sand, and the other with a sand cap armored with gravel-sized stone.
The surveyed researchers generally agreed that the most likely benthic organisms to colonize
a sand cap in the Great Lakes would be Chironomids (midges) and Oligochaetes (worms).
One researcher indicated that Spaerids (fingernail clams), Trichopteran larvae and nematodes
might also colonize the sand cap. The armored cap would attract a greater diversity of
macroinvertebrates than the sand cap, including those that attach to surfaces (including Zebra
mussels) or inhabit the larger interstitial spaces. As the interstices of the gravel are filled with
"new" sediments, the benthos would likely become dominated by Oligochaetes and Chirono-
mids.
While some organisms indigenous to the Great Lakes can burrow 10-40 cm in soft silt or clay
sediments, most of the researchers surveyed felt that bioturbation in a sand cap would be
limited to the top 5-10 cm. The presence of armor stone should inhibit colonization by deep-
burrowing benthic organisms. The researchers indicated that the colonization of a sand or
armored cap would be sparse until "new" sediments with sufficient organic matter deposited
on the cap. If the "new" sediments are contaminated, the diversity of benthos colonizing the
cap would remain limited.
Based on these opinions, a minimal component (or thickness) of an in-situ cap constructed
with sand or one having an armored surface appears to be needed to accommodate
bioturbation at Great Lakes sites. Benthos at such a capped site is likely to be limited to the
fine-grained, organic-rich sediments which may deposit on top of the cap or settle in the
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Chapter 3. In-Situ Cap Design
interstices of armor stone. However, if a cap is constructed with a fine-grained material, the
potential for bioturbation penetration is more significant. Designers should always consult
with aquatic biologists about the bioturbation habits of benthic organisms native to the capping
area.
Where a cap component to accommodate bioturbation must be designed, there are several
options. If the cap contains granular material for chemical isolation or other functions, an
additional thickness of the same granular material (Tb), equivalent to the depth to which the
deepest burrowing organism can reach, may be added as a component for physical isolation.
Another option is to select a different granular material with properties that are less "attractive"
as a substrate for benthic infauna. A relatively thin layer of sand or gravel with little organic
matter may be as effective as a thick layer of silt in limiting bioturbation into the cap.
Geotextiles might also be used as bioturbation barriers in an in-situ cap design, although there
is no experience with their use for this purpose.
Consolidation Component
If the selected material for the cap is fine-grained granular material (defined as material with
less than 50% by weight passing a #200 sieve), the change in thickness of the material due to
its own self weight or due to other cap components should be considered in the overall design
of the isolation cap component. An evaluation of cap consolidation should be made in this
case, and an additional cap thickness component for consolidation, Tc, should be added to the
granular thickness for isolation so that the appropriate granular cap thickness is maintained.
Such consolidation occurs over a period of time following cap placement, but does not occur
more than once.
If the cap material is not a fine grained granular material, no consolidation of the cap may be
assumed, and no additional increase in the isolation thickness is necessary. However, con-
solidation of the underlying contaminated sediments may occur, and a consolidation analysis
may be necessary to properly interpret monitoring data. Procedures for evaluation of consoli-
dation are given below under the discussion of geotechnical considerations.
Consolidation of underlying sediments due to placement of a cap may also result in advection
of pore water upward into the cap. This is an important process in evaluation of potential
advective flux of contaminants. A consolidation evaluation is therefore necessary for an
evaluation of potential advective flux.
Stabilization/Erosion Protection Component
General Considerations
The cap component for stabilization/erosion protection has a dual function. On the one hand,
this component of the cap is intended to stabilize the contaminated sediments being capped,
and prevent them from being resuspended and transported offsite. The other function of this
component is to make the cap itself resistant to erosion. These functions may be accomplished
by a single component, or may require two separate components in an in-situ cap.
For example, a cap might be constructed to prevent erosion of contaminated sediments, using
a geotextile. The dimensions and opening size of the geotextile fabric might be selected to
31
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Chapter 3. In-Situ Cap Design
cover the area and not allow sediment particles to pass through. This geotextile is performing
the first function, that of stabilizing the sediments. However, a separate component, perhaps
a layer of sand or gravel, is likely needed to keep the geotextile in place.
The potential for erosion at the capping site highlights one of the most significant differences
between ISC and other sediment remediation alternatives; the role of dynamic conditions and
probability in the design. Most treatment and confined disposal alternatives are designed
assuming a relatively static physical environment at the remediation site. Topographic and
geologic conditions are assumed to be static for most upland sites outside the floodplain. In
contrast, the physical conditions at an ISC site are quite dynamic. Water levels, river currents,
ice and debris scouring, or wave conditions can create erosive forces at the cap-water interface
which are highly variable. The design of the ISC must account for these dynamic forces.
In the design of conventional marine or flood protection structures (i.e., breakwaters, dams or
levees), probability is used to make key design decisions. Such structures are typically de-
signed to withstand an event of a specific recurrence interval (e.g., 100-year flood), which may
be dictated by policy, legislation or funding constraints. The design of erosion protection
features of an ISC (i.e., armor layers) may also be based on the magnitude of erosive forces
projected at the capping site. There is no existing guidance in Superfund regarding the
selection of a recurrence interval or acceptable probability of failure for such applications. As
such, design criteria may have to be established on a case-by-case basis.
Sediment Stabilization
In most ISC applications to date, the stabilization of contaminated sediments has not been a
driving function of the cap design. In these cases, stabilization is generally accomplished by
the granular cap component for chemical isolation. Immobilization of contaminated sediments
is most likely to be the primary cap function where the potential for resuspension and transport
of in-place sediments is a concern. Conventional methods for analysis of sediment transport
are available to evaluate erosion potential can range from simple analytical techniques to
numerical modeling.
The design of a cap component to stabilize in-situ sediments must consider the ability of the
sediments to migrate vertically. A layer of coarse gravel, with interstitial voids many times
larger than the contaminated sediments, would not be an efficient stabilization component. The
grain size of granular cap material suitable for stabilizing contaminated sediments can be
determined using guidance developed for the design of sand and gravel filters (USAGE 1986;
SCS 1994). These filter design methods are discussed further in Chapter 4.
Evaluation of Erosion Potential
The potential for erosion of the cap should be carefully considered. As discussed in Chapter
2, capping should be used in environments where the long term physical integrity of the cap
can be maintained, and low energy environments are generally more appropriate for in-situ
capping projects. However, higher energy environments may be considered for capping,
recognizing that risks increase. The potential severity of the environmental impacts associated
with cap erosion and potential dispersion of the sediment contaminants in an extreme event
should determine the level of protection against erosion.
32
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Chapter 3. In-Situ Cap Design
The potential for erosion depends on streamflow or tidal velocity forces, depth, turbulence,
wave-induced currents, ship/vessel drafts, engine and propeller types, maneuvering patterns,
sediment particle size, and sediment cohesion. Therefore, detailed evaluations of erosion must
be based on analysis of the frequency of erosion of a specific capping material (grain size and
cohesion) for expected wave and current conditions over time (to include storms) predicted in
the area. The results from such an analysis will provide data that can be used to predict the
expected cumulative amount of erosion over time along with confidence intervals on the
answers. These numbers can then be used to define the need for, and design of an ISC
erosion component.
Knowledge of the frequency of occurrence of scour or degradation (i.e., how often a given
amount of vertical erosion will occur) is a critical component of a probabilistic cap design. An
underdesigned erosion component will compromise the cap, potentially allowing the
contaminants to be dispersed over the site and surrounding area. Conversely, an overdesigned
erosion component will have an unnecessarily high cost and also may result in unacceptable
site use constraints.
In most dredged material capping applications to date, granular materials used for chemical
and physical isolation were determined to be generally resistant to erosion under local site
conditions. In these cases, allowance was made for the gradual loss of small amounts of cap
material by erosion either with an additional thickness of granular material or through planned
periodic replenishment of cap material. The potential for granular cap materials used for other
functions (physical isolation, chemical isolation or sediment stabilization) to be eroded should
be evaluated to determine if a specific cap component for erosion protection is needed.
The hydrodynamic conditions driving potential erosion may include bottom velocity forces due
to stream flow or tidal fluctuations, wave-induced currents, or propeller-induced current
velocities. At an ISC site, each of these need to be considered to determine which represents
the greatest erosion potential. An examination of five Great Lakes sites for ISC feasibility found
that propellor-wash was the dominant factor influencing armor layer design in four of the sites,
and river currents in the other (Maynord and Oswalt 1993). In contrast, the armor layer of the
cap design which had been proposed for Mannistique River/Harbor was dominated by wave
conditions (BBL 1995).
The following sections describe methods to design the erosion component for sites where
erosion is expected to be a problem, based on which erosive force is dominant.
River/Tidal Current-Induced Erosion
The investigation of erosion potential at selected Great Lakes sites (Maynord and Oswalt 1993)
suggests that currents and flood flows are most likely to be the dominant erosive factor in
unnavigable portions of rivers, or areas where navigation has ceased. In shallow rivers, like
the Sheboygan River, in-situ caps may extend onto the bank and flood plain and resemble
streambank erosion structures. In deeper rivers and estuaries, like Puget Sound, tidal currents
may be the dominant erosive force, although no special erosion component may be required.
Several screening approaches could be used to evaluate potential for erosion of a granular
material of given grain size due to given unidirectional current and/or wave conditions (Teeter
1988, Dortch et al. 1990, Hands and Resio 1994, Scheffner 1991 a and b, ASCE 1975). A
33
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Chapter 3. In-Situ Cap Design
simple Shields diagram can be used to compare the stability of given materials against
unidirectional currents. Procedures for using this approach are found in Dortch et al. (1990).
The site evaluation and associated investigations described in Chapter 2 should provide the
current velocities and frequencies associated with episodic events which are needed for the
evaluation of erosion potential. The return interval or frequency of events such as storms or
flood flows which should be used for the design would depend on several factors, such as the
degree of risk if the contaminants were re-exposed and the possible degree of self-armoring
which may occur during erosion.
The selection of a design interval should be based on reasonable assumptions. The design
life of most civil works projects such as bridges or dams is 50 years. The confidence in ability
to predict the forces due to a 50 or 100 year event is high, because of the available data from
historic records usually includes events with comparable return intervals. Consideration of
events with return intervals in the range of 100 years is therefore appropriate for these types
of projects. In contrast, an in-situ cap is conceptually built to last forever. However,
consideration of extreme low probability, high impact events (e.g., a 500 year storm) may not
always be appropriate because the confidence in accurately describing the forces resulting
from such an extreme event is low. Further, the impact due to erosion of the cap from such an
event should be placed in context with other environmental effects including the loss of life and
property in the surrounding area.
Design procedures for armor stone as a cap component are found in Appendix A.
Wave-Induced Erosion
Wave-induced erosion is the dominant factor at virtually all dredged material capping sites, and
is likely to be dominant at open water ISC sites, including lakes, estuaries and harbors. Most
in-situ caps constructed in open water environments resemble a mounded dredged material
cap.
An extensive analysis of combined flood flow and wave induced erosion was performed for the
proposed capping option at Manistique Harbor (BBL 1994 and 1995). This analysis relied on
several computer models and design approaches including flood flow models developed by the
U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC xx) and wave models
developed by the U.S. Army Corps of Engineers Coastal Engineering Research Center (ACES
reference).
Palermo et al.(in preparation) describes detailed procedures for erosion screening for open-
water sites dominated by wave conditions and computing frequency of erosion studies for open
water sites.
The USAGE has developed a model to evaluate the long-term fate of a sediment or cap de-
posit (mound), i.e., mound stability over periods ranging from months to years, and this model
can be applied to predict cap erosion rates for open water sites such as estuaries, lakes, etc.
This model is called the Long Term FATE of dredge material (LTFATE) model (Scheffner,
Thevenot, and Mason, 1995). In LTFATE, hydrodynamic conditions at a site are considered
using simulated databases of wave and current time series or actual wave and current data
as driving forces. These boundary conditions are used to drive coupled hydrodynamic,
sediment transport, and bathymetry change models which predict erosion of dredged material
34
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Chapter 3. In-Situ Cap Design
mounds (of specific dimensions, grain size, and water depth) over time. Results from this
model indicate whether a given site is predominantly dispersive or non-dispersive and predict
potential erosion and migration of a mound for the given current and wave conditions, mound
geometry and sediment characteristics. Because this model was developed for open water
conditions, it may have only limited utility for some ISC applications such as riverine sites.
Propellor-lnduced Erosion
Contaminated sediments are generally associated with urban/industrial waterways, most of
which are active channels for commercial and recreational vessels. The ability of propeller jet
(or wash) from ships, towboats and even recreational watercraft to resuspend bottom
sediments is well documented. The ISC placed at Eagle Harbor, Washington has experienced
some erosion at the areas nearest a car ferry dock. The only case of an erosion component
specifically designed for navigation-effects was associated with a dredged material cap
considered for Indiana Harbor (Environmental Laboratory 1987). This design included armor
stone, and was ultimately rejected as infeasible.
Methods for predicting navigation-induced erosive forces were developed for design of river
bank protective and navigation structures. Erosive forces are calculated from information on
the propeller type, diameter, engine horsepower, and vertical distance from the propeller to the
cap. These methods are described in Appendix A. The uncertainty in the the design of a cap
for conditions dominated by river/tidal currents or waves is based on the predictability of future
meteorological events. The uncertainty in the design of an erosion component for a cap at a
site where navigation-impacts dominate is based on the predictability of navigation use and
traffic patterns. This requires foresight into the types of vessels that will be using a waterway
and where and how they will maneuver. It also requires knowledge of any short- or long-term
fluctuations in water surface elevations.
Chemical Isolation Component
Chemical Flux Processes
If a cap has a properly designed physical isolation component, contaminant migration
associated with the movement of sediment particulates should be controlled. Most contami-
nants of concern also tend to remain tightly bound to sediment particles. However, the
movement of contaminants by advection (movement of porewater) upward into the cap is
possible, while movement by molecular diffusion over long time periods is inevitable.
Advection refers to the movement of porewater. Advection can occur as a result of
compression or consolidation of the contaminated sediment layer or other layers of underlying
sediment. Movement of porewater due to consolidation would be a finite, short-term
phenomena, in that the consolidation process slows as time progresses and the magnitude of
consolidation is a function of the loading placed on the compressible layer. The weight of the
cap will "squeeze" the sediments, and as the porewater from the sediments moves upward, it
displaces porewater in the cap. The result is that contaminants can move part or all the way
through the cap in a short period of time. This advective movement can cause a short-term
loss, or it can reduce the breakthrough time for long-term diffusive loss.
35
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Chapter 3. In-Situ Cap Design
Through-cap transport due to consolidation can be minimized by using a cap that has sufficient
thickness to contain the entire volume of pore water that leaves the contaminated deposit
during consolidation. For example, Bokuniewicz (1989) has estimated that the pore water front
emanating from a consolidating two-meter-thick mud layer would only advance 24 cm into an
overlying sand cap (Sumeri et al. 1991).
Advection can also occur as an essentially continuous process if there is an upward hydraulic
gradient due to groundwater flow. In most upland hydrogeologic settings, advection due to
groundwater flow is thought to be the most significant mechanism of mass transport (Bear and
Verruijt 1987; Fetter 1993; Domenico and Schwartz 1990). In ground water, advection is
generally described in terms of Darcy's law. Darcy's law defines a linear relationship between
the groundwater flux (volume/unit area/unit time) and the hydraulic gradient (Domenico and
Schwartz 1990), and, with a slight modification, Darcy's law can be used to determine the
average rate of ground water flow (Freeze and Cherry 1979).
An estimation of the rate of groundwater discharge can either be obtained empirically through
the use of seepage meters, or calculated through the use of Darcy's law and a knowledge of
the site hydrogeology (see Chapter 2). In addition, seepage meters can also be used to
evaluate the quality of the ground water discharging to surface water through the collection of
ground water samples for chemical analysis.
Diffusion is the process whereby ionic and molecular species in water are transported by
random molecular motion from an area associated with high concentrations to an adjacent area
associated with a low concentration (Fetter 1994). Diffusional mass transport assumes that the
rate of transport is directly proportional to the concentration gradient. In an isotropic medium,
this occurs in a direction perpendicular to the plane of constant concentration at all points in the
medium. If the diffusional flux is steady-state, mass transport by diffusion is described by
Flick's first law (Fetter 1993). Fick's second law is used to describe systems in which the
contaminant concentrations are dependent upon time.
From an environmental perspective, diffusion is as slow as contaminant transport processes
can become in a porous medium. However, although diffusion is notoriously slow, diffusional
driven mass transport will always occur if concentration gradients are present. Consequently,
diffusion can transport contaminants through a saturated porous media in the absence of
advection.
Advection and/or diffusion transport processes can be viewed as end-members of a
continuum. Based upon random molecular motion attempting to equalize contaminant
concentrations, diffusion is commonly the slower of these two processes (Fetter 1993). In
contrast, advection as the bulk movement of ground water due to differences in hydraulic head
is generally a much more rapid transport process. In many/most geologic settings, mass
transport is driven by advection (Fetter 1993; Bear and Verruijt 1987). Generally, predictions
of contaminant transport based upon diffusion alone would only become appropriate for
geologic settings and/or cap designs which incorporate a porous layer associated with a very
low hydraulic conductivity value, or in the absence of hydraulic gradients (the hydrostatic case)
(Fetter 1993).
Even if contaminant concentrations are high in the pore water, a granular cap component would
act as both a filter and buffer during advection and diffusion. As pore waters move up into the
relatively uncontaminated granular cap material, these cap materials can be expected
36
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Chapter 3. In-Situ Cap Design
to remove contaminants (through sorption, ion exchange, surface complexation, and redox
mediated flocculation) so that pore water that traveled completely through the cap would
theoretically have a reduced contaminant concentration. The extent of the contaminant
removal in the cap is very much dependent upon the nature of the cap materials. For example,
a cap composed of quarry run sand would not be as effective as a naturally occurring sand with
an associated fine fraction and organic content.
Consideration of Advective/ Diffusive Flux
in Cap Design
If the desired function of the cap is to chemically isolate the contaminants in the long term or
reduce long term flux of contaminants such that a water quality standard or sediment cleanup
level can be maintained, both advective and diffusive processes should be considered in
determining the necessary design for isolation.
For example, if a ground water/surface water interaction study indicated that advection is not
significant at a given location, the cap design may only need to address diffusion and the
physical isolation of the contaminated sediments, ignoring dissolved and/or colloidially facili-
tated transport due to advection. In contrast, should ground water/surface water contaminant
release routes be significant, the hydraulic properties of the cap should also be determined
and factored into the cap design. These properties should include the hydraulic conductivities
of the cap materials, the contaminated sediments, and underlying sediments or geologic
deposits.
Laboratory Tests for Capping Effectiveness
Laboratory tests were first developed to evaluate cap thicknesses required for physical isolation
of dredged material. However, several testing approaches have been applied to define cap
thicknesses and the sediment parameters necessary to model their effectiveness in chemical
isolation. Laboratory tests may be used to define sediment specific and capping material
specific values of diffusion coefficients and partitioning coefficients. But, no standardized
laboratory test or procedure has yet been developed to fully account for advective and diffusive
processes and their interaction.
The USAGE developed a first generation capping effectiveness test in the mid 1980s as part
of the initial examination of capping as a dredged material disposal alternative. The test was
developed based on the work of Brannon et al. (1985, 1986), Gunnison et al. (1987), and
Palermo et al. (1989). This test (Sturgis and Gunnison 1988) has been used to determine the
thickness, T|, of a capping sediment required to isolate a contaminated sediment. The tests
basically involve layering contaminated and capping sediments in columns and experimentally
determining the cap sediment thickness necessary to chemically isolate a contaminated
sediment by monitoring the changes in dissolved oxygen, ammonium-nitrate, orthophosphate-
phosphorous, or other tracers in the overlying water column (Figure 5-2). The thickness of
granular cap material for chemical isolation determined using this procedure is on the order of
one foot for most sediments tested to date.
In retrospect, this testing procedure may be suitable for evaluating the short-term advective
movement of sediment pore water associated with consolidation. However, this column testing
procedure does not account for ground water induced advection of pore water or the long term
37
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Chapter 3. In-Situ Cap Design
flux of contaminants due to diffusion which may involve time scales of tens to hundreds of
years.
Louisiana State University has conducted laboratory tests to assess diffusion rates for specific
contaminated sediments to be capped and materials proposed for caps. A capping simulator
cell was used in which a cap material layer is placed over a contaminated sediment, and flux
due to diffusion is measured in water which was allowed to flow over the cap surface (see
Figure 5a). Initial tests measured flux of 2,4,6-trichlorophenol (TCP) through various cap
materials. These tests showed that the breakthrough time and time to steady state were
directly dependent on the partitioning coefficient and that cap porosity and thickness were the
dominant parameters at steady state (Wang, Thibodeaux, Valsaraj, and Reible 1991).
'/i" STAINLESS STEEL TUBING lOUTLETI
'/!" x '/*"COMPRESSION FITTING
%-NUT
%"ALL THREAD ftOD
o
I
r
TOP VIEW INTO CSC SHOWING LOCATION OF BOLTS
5a. LSU experimental cell.
SINTERED STAINLESS
STEEL DISTRIBtfTION
DISK IQJ875" X KTDIAtl)
SEDIUEHT CUMBER
SINTERED STAINLESS
STEEL DISTRIBUTION
DISK IQJ8?5"x KTOIAU)
5b. WES leach test.
Figure 5. Laboratory methods to evaluate chemical isolation by caps.
Environment Canada has performed tank tests on sediments from Lake Ontario to qualitatively
investigate the interaction of capping sand and compressible sediments. The tests were
carried out in 3.6 x 3.6 x 3.7 meter observation tanks in which the compressible sediments were
placed and allowed to consolidate and sand was placed through the water column onto the
sediment surface. In the initial tests, physical layering and consolidation behavior were
observed. Additional tests are planned in which migration of contaminants due to
consolidation-induced advective flow will be evaluated (Zeman 1994).
Diffusion coefficents for long-term modeling of diffusive transport of contaminants from con-
taminated sediment into cap material have also been measured using diffusion tubes (DiToro,
Jeris, and Clarcia 1985). In this method, sediment is spiked with radiolabled contaminant,
placed in small tubes, and covered with capping material. At times extending up to 3 years,
selected tubes are sliced (100-250um) using a microtome, and the thin slices are analyzed for
radioactivity. The results are used to develop contaminant profiles from which diffusion coef-
ficients that account for the sorptive properties of the cap materials can be calculated. The
diffusion tube approach is being used in a capping study for the U.S. Army Engineer District,
New York (Myers 1995, Personal Communication).
38
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Chapter 3. In-Situ Cap Design
The USAGE has also developed leach tests to assess the quality of water moving through a
contaminated sediment layer into groundwater in a confined disposal facility environment
(Myers and Brannon 1991) (see Figure 5b). This test is being applied to similarly assess the
quality of water potentially moving upward into a cap due to advective forces (Myers 1995,
Personal Communication).
Results of laboratory tests such as those described above should yield sediment specific and
capping material specific values of diffusion coefficients, partitioning coefficients. In addition,
other parameters such as the magnitude and rate of consolidation, changes in sediment
permeability/ porosity, and any advective flow conditions are needed to model long term cap
effectiveness. Model predictions of long term effectiveness using the laboratory derived
parameters should be more reliable than predictions based on so called default parameters.
Modeling Applications for Cap Effectiveness
A model has been developed by EPA to predict long-term movement of contaminants into or
through caps due to advection and diffusion processes. This model has been developed based
on accepted scientific principles and observed diffusion behavior in laboratory studies
(Bosworth and Thibodeaux 1990; Thoma et al 1993; Myers et al 1996). The model considers
both diffusive and advective fluxes, the thickness of sediment layers, physical properties of the
sediments, concentrations of contaminants in the sediments, and other parameters. This
model is described along with example calculations in Appendix B.
The results generated by the model include flux rates, breakthrough times, and pore water
concentrations at breakthrough. Such results can be compared to applicable water quality
criteria, or interpreted in terms of a mass loss of contaminants as a function of time which could
be compared to similar calculations for other remediation alternatives. The model in Appendix
B is applicable to the case of a single contaminated material layer and a single cap material
layer, each with a homogenous distribution of material properties. The diffusion relationships
used in the model have been verified against laboratory data. However, no field verification
studies for the model have been conducted.
There is a need for a comprehensive and field verified predictive tool for capping effectiveness
and additional research on this topic is planned. The USAGE has applied a refined version of
an existing sediment flux model (Boyer et al 1994) for capping evaluations, and more
refinements to the model are planned to account for a comprehensive treatment of all pertinent
processes. But in absence of such a tool, analytical models such as that in Appendix B should
be used in calculating long term contaminant loss for capped deposits as long as conservative
assumptions are used in the calculations.
Chemical Isolation Component Design
for Granular Cap Materials
In most in-situ caps constructed to date, granular material, including gravel, sand, and silt and
clay, has been used for chemical isolation.
39
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Chapter 3. In-Situ Cap Design
Modeling can be used to obtain an estimate of the required thickness of granular cap material
for chemical isolation. However, the thickness and properties (grain size distribution and total
organic carbon (K)C) content) of the granular cap material are necessary input parameters for
the models. Therefore, an efficient approach for design of the chemical component is to
determine the representative grain size and TOG of candidate capping materials, account for
other requirements such as bioturbation, consolidation and erosion in the cap design, then
evaluate long term effectiveness using the model provided in Appendix B.
When evaluating potential chemical isolation component designs, the properties of granular cap
materials should represent those that would be present in the materials after construction. The
method of placement and site conditions can alter the properties of capping material. For
example, the distribution of organic matter in some sandy materials may not be uniform, with
a high percentage of the TOC in a small fraction of fines. During cap placement, the loss of
these fines could result in a significant reduction to the ultimate TOC in the cap material after
placement.
If the modeling results indicate the design objectives are not met, additional cap thickness can
be added or granular cap materials with differing properties (grain size and TOC) can be
considered to further -decrease the contaminant flux. The evaluation process could then be
run in an iterative fashion if necessary to determine the chemical isolation component design
needed to meet the remedial objectives. Of course, if no reasonable combination of cap
thickness and cap material properties can meet the objectives, other remediation alternatives
or control measures must be considered or the remedial objectives reconsidered.
Chemical Isolation Component Design
for Membranes and Fabrics
Geosynthetic membrane materials (essentially impermeable) may be incorporated in a cap
design to reduce contaminant flux. However, the use of impermeable plastic liners as a
chemical isolation component is limited by concerns regarding gas generation in the underlying
sediments, and the need to vent this gas. Membranes have been placed with vents for release
of generated gas.
Geotechnical fabrics (permeable) have been incorporated in cap designs to prevent the mixing
of cap material with underlying contaminated sediments and to prevent potential migration of
contaminated sediment particles into the cap. Permeable fabrics would have little effect with
regard to reduction in flux due to advection of pore water or diffusion. Conceptually,
geotextiles or geotextile blankets may be fabricated to allow placement of materials with high
TOC (e.g., activated carbon) which would otherwise be difficult to place due to low density or
potential for resuspension.
Component Interactions
The most conservative design approach for an in-situ cap is to consider components necessary
for the three basic cap functions independently (as done above). Using this approach,
components are additive. This approach is most appropriate for caps designed with a single
type of granular material, where the total thickness of cap material is the sum of the thick-
nesses for physical isolation, chemical isolation and stabilization/erosion protection. Additional
amounts of granular material might be added to account for consolidation (discussed below),
or for other construction or operational considerations.
40
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Chapter 3. In-Situ Cap Design
The design of cap components for multiple functions will generally not be as conservative as
the additive approach. For example, say a 2-foot layer of sand is considered adequate for
chemical isolation and a 1-foot layer of the same material is considered adequate for physical
isolation. It might be reasoned that a 2-foot layer of sand could perform both functions. Howev-
er, the bioturbation of benthic infauna into the top foot of such a cap could result in their
exposure to contaminants migrating through the cap, or might alter the permeability of the cap,
increasing the contaminant flux.
The cap components for physical isolation and erosion protection would seem to have the
greatest potential for dual function. If a granular cap has a thickness at the surface that is
"sacrificial" for erosion, this layer might be lost during a storm event and would have to be
replenished afterward. Such an erosion component could not be relied on to perform other
functions. However, if an armored layer was placed on top of a cap, and designed to be stable
under all but very extreme events, the ability of such a layer as a deterrant to bioturbation might
be considered in addition to its erosion protection function.
Geotechnical Considerations
In-situ contaminated sediments to be capped will usually be predominately fine grained, and
may have high water contents and low shear strengths. Such materials are generally com-
pressible, and may be easily displaced or resuspended during placement of capping materials
unless appropriate controls are implemented. The cap stability against displacement or sliding
and settlement due to consolidation are two main geotechnical issues.
Bearing Capacity/ Slope Stability Considerations
As with any geotechnical problem of this nature, the shear strength of the sediments will
influence their resistance to localized bearing capacity or sliding failures which may cause
localized mixing of capping and contaminated materials. Stability immediately after placement
is most critical, before any excess pore water pressure due to the weight of the cap layer has
dissipated. Gradual placement of capping materials over a large area will reduce the potential
for such localized failures in most cases. For example, the sand cap placed in Hamilton
Harbor, Ontario was placed in three separate passes (Zeman and Patterson 1996a).
Settlement of the cap occurs as the sediments consolidate simultaneously with the dissipation
of excess pore water pressure while gaining additional strength.
A review of case studies on geotechnical aspects of capping projects where shear strengths
of the in-situ sediments were measured was conducted for the ARCS program (Ling et al
1996), and is provided as Appendix C. Conventional bearing capacity and slope stability
analysis using the measured shear strengths indicated stable conditions for most of the
capping projects evaluated (all of which used a sand cap).
Field monitoring data has definitively shown that contaminated sediments with low strength
have been successfully covered with sand caps. However, engineering data on the behavior
of soft deposits during placement of materials in the form of a cap is limited. Conventional
geotechnical design approaches should therefore be applied with caution to subaqueous cap
design, since such design approaches would likely be conservative for conditions normally
encountered in cap design. For example, a cap placed over an area of several acres at a
thickness of several feet would not be subject to a "punching" failure mode normally evaluated
by conventional bearing capacity analysis. Similarly, caps with flat transition slopes at the
41
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Chapter 3. In-Situ Cap Design
DENSITY, %
O) OO
0 0
111
b 40
LD
20
i
0
2
i PERLOFF AND BARON, 1976
D GILBERT, 1984
-
£ / •
/ D
-
a
/ /
Jy
\
a / D
/u
/ X-
/x
s
I
3 32 36 40 44
EFFECTIVE FRICTION ANGLE. DEGREES
Figure 6. Relationship between relative density and effective friction angle for clean sands.
edges would not be subject to a sliding failure normally evaluated by conventional slope stability
analysis.
The capping material should be applied slowly and uniformly to avoid problems with bearing
capacity or slope failures if the contaminated sediment deposit is soft. Uncontrolled release
of a large amount of material or the buildup of a localized mound can cause a bearing capacity
failure. If this occurs, cap material penetrates into the contaminated deposit and could cause
contaminated material to resuspend and disperse into the water column.
It is likely that contaminated sediments are subject to pore pressure buildup as cap material is
deposited on the surface. The buildup of excess pore water pressure reduces the shear
strength of the contaminated soil and increases the susceptibility to bearing capacity failure.
Therefore it is important to allow sufficient time for excess pore water pressure dissipation in
materials with low permeability. In materials susceptible to induced excess pore water pres-
sure, sand deposition and cap construction must proceed more slowly and deliberately. The
geotechnical engineering parameters associated with bearing capacity and their connection
with soil strength are discussed in more detail in Appendix C.
42
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Chapter 3. In-Situ Cap Design
Edge Effects/ Overlap Requirements
Accommodations in the cap design for bearing capacity and slope stability may only be appli-
cable in the case of a cap several feet in thickness which must be placed over a small area or
within a constricted site with little opportunity for transitions. In such cases, potential slope
failures at the edges of the cap can be accommodated by overlapping the cap beyond the edge
of the contaminated sediment deposit. Therefore, an important consideration becomes the
distance beyond the edges that the cap must cover.
Data relating the effective friction angle of sand with the relative density is shown in Figure 6
(Gilbert, 1984, and Perloff and Baron, 1976). If the cap materials are typically clean sands
that are loosely deposited by pluviation (settling of material through water), the relative density
is zero and, using Figure 6, the limiting effective friction angle is about 28°. If the angle of
Figure 7. Recommended cap edge overlap.
repose of the sand is equal to the effective friction angle as suggested by Taylor (1948) and
Hough (1957), then the slope at the edge for a clean sand of low density becomes 1 V : 1.88
H. A safety factor of 2 to 3 is recommended for these conditions, therefore, the end slope
becomes 1 V : 3.8 H to 5.6 H. The recommended cap overlap distance is therefore 3.8 to 5.6
times the thickness of the cap as shown in Figure 7.
Liquefaction
Liquefaction is a phenomenon in which a deposit of loose, saturated, cohesionless material
(such as sand) develops high pore water pressure as the result of a disturbance, progressively
loses a large portion of its shear strength, and flows like a frictional fluid. Liquefaction may be
triggered by seismic activity, wave action, blasting, or propwash from a vessel on the surface.
Submarine deposits have been documented to have experienced liquefaction and
moved/flowed thousands of feet before coming to rest (Terzaghi, 1956). Contaminated
deposits of sand or caps constructed of sand may be susceptible to liquefaction because sand
that has settled through water typically forms deposits of low density (Terzaghi, 1956, Gilbert,
1984). Gilbert (1984) showed in laboratory experiments that deposits of sand that are formed
by particles settling through water can have negative relative density (meaning that the deposit
achieved a lower density under water than is possible in air). Sands that are fine and uniform
are most susceptible to liquefaction. Depending on a number of factors such as the size of the
contaminated deposit, the engineering properties of the capping and contaminated sediments,
bottom slope, and probability of seismic activity, a full scale investigation for liquefaction
susceptibility may be warranted.
43
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Chapter 3. In-Situ Cap Design
Consolidation Analysis
Fine-grained granular capping materials may undergo consolidation due to self-weight. Un-
derlying contaminated sediment will almost always undergo consolidation due to the added
weight of capping material or armor stone. The cap design should therefore consider
consolidation from the standpoint of cap material thickness and interpretation of monitoring
data. The thickness of granular cap material should have an allowance for consolidation so
that the minimum required cap thickness is maintained following consolidation. Evaluation of
the consolidation expected will allow proper interpretation of any observed decreases in cap
surface elevation during monitoring.
If the granular capping material selected for physical or chemical isolation can be classified as
a sand based on its physical properties (i.e., the material has a distribution of grain sizes with
less than 50% passing the #200 sieve) no cap thickness component to offset cap consolidation
is necessary. If the material is classified as a silt or clay, i.e. has a distribution of grain sizes
with more than 50% passing the #200 sieve, an evaluation of cap consolidation should be
made, and an additional cap thickness component for cap consolidation, Tc, should be added
to the granular thickness for each component so that the appropriate granular cap thickness
is maintained.
Even if the cap material is not compressible, a consolidation analysis of the underlying con-
taminated sediment is usually necessary. Most contaminated sediments are highly compress-
ible, and an evaluation of consolidation is important in interpreting monitoring data to differ-
entiate between changes in cap surface elevation or cap thickness due to consolidation as
opposed to those potentially due to erosion. Also, the degree of consolidation will provide an
indication of the volume of water expelled by the contaminated layer and capping layer due to
consolidation. This can be used to estimate the movement of a "front" of porewater upward
into the cap. Such an estimation of the consolidation driven advection of pore water could be
considered in the evaluation of contaminant flux.
Potential strains due to consolidation are large, and therefore a finite strain approach which
accounts for large strains should be used to evaluate consolidation. Coarse-grained materials
will not consolidate appreciably. In evaluating consolidation, the magnitude of contaminated
sediment and capping material consolidation should be separately determined.
The finite strain approach for consolidation evaluation (Brandes et al. 1991) has been coded
for computer solution in a model called MOUNDS (Poindexter-Rollings 1990). This model
provides information on the magnitude and rate of consolidation of a mound and on gains in
shear strength as consolidation progresses. Consolidation test data from self-weight consoli-
dation tests and/or standard oedometer tests (USAGE 1970 and USAGE 1987) are required
to run the model.
The MOUNDS model and a second consolidation model, CONSOL (Gibson, Schiffman, and
Cargill 1981 and Wong and Duncan 1984), were used to predict consolidation of three capped
dredged material mounds in Long Island sound (Silva et al. 1994). Bathymetry of these sites
showed reductions in mound elevations of up to 3.5 m over time periods of 10 to 13 years after
cap placement. Comparisons between consolidation and bathymetry estimates were made to
show that the reductions in mound elevation could be attributed to consolidation rather than cap
erosion. Results showed the two models were reasonably accurate in predicting consolidation.
The work also pointed out the need to obtain more accurate geotechnical information on the
void ratios and initial effective stress of the contaminated materials.
44
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Chapter 3. In-Situ Cap Design
Filter Design Analysis
As part of the design of an in-situ cap component for sediment stabilization, or where the cap
design has more than one layer of granular material, one must consider the ability of the
sediments and cap materials to migrate vertically. The initial design for the proposed cap at
Manistique River/Harbor included an armor layer with stone of 7-10 inches in diameter for
erosion protection on top of a 20-inch thick layer of sand for chemical and physical isolation
(Blasland, Bouck & Lee 1994). Because of concerns about the movement of sand through the
voids in the armor stone, the initial armor layer design was modified to a more well-sorted
gradation of stone (Blasland, Bouck & Lee 1995).
Where one granular material is placed on top of another, the potential for vertical migration can
be determined using guidance developed for the design of sand and gravel filters (USAGE
1986; SCSI 994).
Operational Considerations
A detailed discussion of equipment and procedures that might be used for the placement of an
in-situ cap is provided in Chapter 4. Operational considerations discussed here are practices
and controls that may need to be implemented in order to assure that the in-situ cap functions
as designed and remains intact. These considerations may include planned maintenance of
the cap, restrictions on uses of the waterway at the capping site and other institutional controls.
Routine cap maintenance generally is limited to the repair or replenishment of erosion protec-
tion component material. The design of some dredged material caps includes a thickness of
granular material that is expected to be eroded during storm events of a known magnitude or
recurrence interval. For such a design, maintenance can be scheduled or planned for in
advance. This type of erosion control is not appropriate unless there is a dependable source
of capping material readily available. For an ISC, the ability to detect and quickly respond to
a loss of the erosion protection layer should also be taken into consideration. On the Great
Lakes, seasonal limitations, such as ice formation or closure of navigation structures (locks),
can limit the ability to monitor in-situ caps after a significant erosion event and respond with
maintenance if needed.
Aside from erosion caused by natural phenomena, the greatest threat to the integrity of an ISC
is from navigational activity. As discussed above, and in Appendix A, the erosive forces
created by propellers of ships, tug boats, and even recreational watercraft can be quite pow-
erful, especially where water depths are reduced by the presence of an in-situ cap. Other
activities, such as bottom drag fishing, direct hull contact, and anchoring create bottom stresses
that can damage a cap (Truitt 1987a). An in-situ cap, particularly one with an armor layer, may
be attractive to some fish, and consequently may be attractive to fisherman.
In order to inform navigation users of the presence of the ISC, navigation maps, mariners
guides, and local land-use documents should be updated to show the presence of the cap and
any use restrictions. Information about the cap and restrictions might also be posted at boat
launch areas, bait shops, and provided with fishing licenses. Signs should be posted at prom-
inent locations near the cap, and marker buoys deployed where appropriate. Active local public
education programs on the presence and purpose of the ISC may improve voluntary
compliance.
45
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Chapter 3. In-Situ Cap Design
The ability to enforce restrictions on navigation activities in and around ISC sites should be
weighed in considering the overall feasibility of capping. Restrictions that are codified as local
or state statutes are more likely to be adhered to than voluntary ones. However, enforcement
may require considerable resources. The cost of enforcement, posting, and education should
be considered in the evaluation of the feasibility of ISC.
46
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4 Equipment and Placement Techniques
This chapter describes considerations in selecting equipment and placement techniques for in-
situ cap placement. Considerations for both granular capping materials such as sediments and
soils and geosynthetic fabrics and armoring materials are provided.
A variety of equipment types and placement techniques have been used for capping projects.
Conceptual illustrations of equipment which can be considered for capping are shown in
Figure 8.
General Considerations
For granular cap components, the major consideration in selection of equipment and placement
of the cap is the need for controlled, accurate placement and the resulting density and rate of
application of capping material. In general, the cap material should be placed so that it
accumulates in a layer covering the contaminated material. The use of equipment or place-
ment rates which might result in the capping material displacing or mixing with the contami-
nated material should be avoided. Sand caps have been successfully placed over fine-grained
contaminated material with minimal mixing of the cap with the contaminated sediment (Man-
sky, 1984a, 1984b; Bokuniewicz, 1989; Bruin, Hattem and Wijnen, 1985, Zeman and Patterson
1996 a and 1996b). Since the surface area to be capped may be several hundred feet or more
in diameter, placement of a cap of required thickness over such an area may require placement
techniques to spread the material to some degree to achieve coverage.
Site considerations that can influence equipment selection include water depths and
wave/current conditions. Other site conditions such as bottom topography, other vessel traffic,
thermal/salinity stratification of the water columns (for deep water sites), etc. may also have an
influence. Pipeline and barge placement of dredged material for ISC projects is appropriate
in more open areas such as harbors or wide rivers. In constricted areas, narrow channels, or
shallow nearshore areas, conventional land-based construction equipment may also be
considered.
Potential resuspension of in-situ contaminated material by impact of capping material should
be considered in selecting equipment and placement technique for the cap. There is no stan-
dardized method presently available to calculate the potential resuspension of sediment and
associated contaminant release due to such resuspension. Monitoring conducted at capping
sites has generally focused on cap thickness and coverage rather than sediment resuspension.
At an ISC demonstration in Hamilton Harbor, Environment Canada monitored the water column
47
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Chapter 4. Equipment and Placement Techniques
and tracked a small plume of suspended material. Analysis of the material in suspension
indicated that it was predominantly fines that had been washed off the sand capping material
during placement and not resuspended contaminated sediments (Zeman and Patterson 1996a
and 1996b).
SURFACE RELEASE FROM HOPPER DREDGE
SPREADING WITH PIPELINE AND SURFACE DISCHARGE WITH PIPELINE
BAFFLE PLATE OR BOX
SUBMERGED DIFFUSER WITH PIPELINE
DIRECT MECHANICAL PLACEMENT
BARGE EQUIPPED FOR GEOTEXTILE PLACEMENT
SPREADING BY CONTROLLED BARGE RELEASE
JETTING FROM BARGE
LAND - BASED DIRECT PLACEMENT
BARGE WfTH TRM1E
SAND SPREADER BAHOE
Figure 8. Conceptual illustrations of equipment which can be considered for capping.
48
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Chapter 4. Equipment and Placement Techniques
Equipment and Placement Techniques
for Granular Cap Materials
Granular cap material can be handled and placed in a number of ways. Materials that have
been mechanically dredged and soils excavated from an upland site or quarry have relatively
little free water, and can be handled mechanically in a "dry" state until released into the water
over the ISC site. These mechanical methods rely on the gravity settling of cap materials in
the water column, and may be depth-limited in their application. Granular cap materials can
also be entrained in a water slurry, and carried to the cap site where they are discharged into
the water column at the surface or at depth. These hydraulic methods offer the potential for
a more precise placement, although the energy required for slurry transport may require
dissipation to prevent resuspension of contaminated sediments.
Direct Mechanical Placement
If the area to be capped is nearshore and appropriate access is available, direct mechanical
placement of capping material with land-based equipment can be considered. The reach of
the equipment is the major limitation. The capping material would likely be trucked to the site
with this method, so access for the trucks and traffic should be considered. Land-based
methods might include backhoes, clamshells, end-dumping from trucks, spreading with dozers
(during low water periods) etc. A cap with layers of gravel and geotextile was placed using
land-based equipment (Figure 9) at a site on the Sheboygan River (Eleder 1992). At the GM
Figure 9. Land-based cap placement at Sheboygan River.
Superfund site in Massena, New York, sand and gravel cap materials were placed in the St.
Lawrence River with a backhoe bucket from a work barge (Kenna, pers com).
Surface Discharge Using Conventional
Dredging Equipment
Field experiences with dredged material capping operations in Long Island Sound and the New
York Bight have shown that contaminated sediment mounds have been successfully capped
with both mechanically-dredged material released from barges and with material released from
hopper dredges (O'Conner and O'Conner 1983, Morton 1987). The surface release of
mechanically-dredged material from barges results in a faster descent, tighter mound, and less
water column dispersion as compared to surface discharge of hydraulically-dredged material
49
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Chapter 4. Equipment and Placement Techniques
from a pipeline, while surface release of hydraulically-dredged material from a hopper dredge
has characteristics somewhat between barge and pipeline discharges.
Surface discharge of material from barges or hopper dredges would not normally be considered
for in-situ capping unless special provisions were made for gradual release of the material and
spreading the material over a larger area. Point discharges from hopper dredges or barges
would normally not be applicable for in-situ capping of soft fine-grained contaminated
sediments.
Spreading by Barge Movement
A layer of capping material can be spread or gradually built up using bottom-dump barges if
provisions are made for controlled opening or movement of the barges. This can be accom-
plished by slowly opening a conventional split-hull barge over a 30 to 60 minute interval,
depending on the size of the barge. Such techniques have been successfully used for con-
trolled placement of predominantly coarse-grained, sandy capping materials at the Denny Way
and other sites in Puget Sound (Sumeri 1989 and 1995). The gradual opening of the split-hull
or multi compartmented barges allows the material to be released slowly from the barge in a
sprinkling manner.
If two tugs are used to slowly move the barge sideways during the release, the material can be
spread in a thin layer over a large area (Figure 10). Multiple barge loads are necessary to cap
larger areas in an overlapping manner. The gradual release of fine-grained silts and clays
mechanically loaded into barges may not be possible due to potential "bridging" action; that
is, the cohesion of such materials may cause the entire bargeload to "bridge" the split-hull
opening until a critical point is reached at which time the entire bargeload is released. If the
water content of fine-grained material is high, as in the case of hydraulic filling of barges, the
material may exit the barge in a matter of seconds as a dense slurry, even though the barge
is only partially opened.
Spreading of thin layers of cap material over large areas can also be accomplished by gradually
opening a conventional split-hull barge while underway by tow. This technique has been
successfully used for capping operations at Eagle Harbor, WA (Nelson, Vanerberden and
Schuldt 1994, Sumeri 1995). Use of barges for spreading cap materials may not be suitable
in shallow water depths, because of the water depths needed for barge draft, door openings
and consideration of the propeller wash from tug boats.
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Chapter 4. Equipment and Placement Techniques
Figure 10. Spreading technique for capping by barge movement at Denny Way, Puget
Sound.
Figure 11. Hydraulic washing of coarse sand, Eagle Harbor, Puget Sound.
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Chapter 4. Equipment and Placement Techniques
Figure 12. Spreader plate for hydraulic pipeline discharge.
Hydraulic Washing of Coarse Sand
Granular capping materials such as sand can be transported to a site in flat-topped barges and
washed overboard with high-pressure hoses. Such an operation was used to cap a portion of
the Eagle Harbor Superfund site, forming a cap layer of uniform thickness (Figure 11). This
technique produces a gradual buildup of cap material, prevents any sudden discharge of a
large volume of sand, and may be suitable for water depths as shallow as 10 feet or less.
Pipeline with Baffle Plate or Sand Box
Where granular cap material is excavated by a hydraulic dredge or transported in a slurry form
through a pipeline, spreading placement capping operations can be easily accomplished with
surface discharge by an energy dissipating device such as a baffle plate or sand box attached
to the end of the pipeline. Hydraulic placement is well-suited to placement of thin layers over
large surface areas.
A baffle plate (Figure 12), sometimes called an impingement or momentum plate, serves two
functions. First, as the pipeline discharge strikes the plate, the discharge is sprayed in a radial
fashion and the dscharge is allowed to fall vertically into the water column. The decrease in
velocity reduces the potential of the discharge to erode material already in place. Second, the
angle of the plate can be adjusted so that the momentum of the discharge exerts a force which
can be used to swing the end of the floating pipeline in an arc. Such plates are commonly used
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Chapter 4. Equipment and Placement Techniques
in river dredging operations where material is deposited in thin layers in areas adjacent to the
dredged channel (Elliot 1932). Such equipment can be used in capping operations to spread
very thin layers of material over a large area, thereby gradually building up the required capping
thickness.
A device called a "sand box" (Figure 13) serves a similar function. This device acts as a
diffuser box with baffles and side boards to dissipate the energy of the discharge. The bottom
and sides of the box are constructed as an open grid or with a pattern of holes so that the
discharge is released through the entire box. The sand box was used to successfully apply a
sand cap at the Simpson Kraft Tacoma site in Puget Sound (Sumeri 1989).
Submerged Diffuser
A submerged diffuser (Figure 14) can be used to provide additional control for submerged
pipeline discharge (Neal, et al. 1978; and Palermo 1994). The diffuser consists of conical and
radial sections joined to form the diffuser assembly, which is mounted to the end of the
discharge pipeline. A small discharge barge is required to position the diffuser and pipeline
vertically in the water column. By positioning the diffuser several feet above the bottom, the
discharge is isolated from the upper water column. The diffuser design allows material to be
radially discharged parallel to the bottom and with a reduced velocity. Movement of the
discharge barge can serve to spread the discharge to cap larger areas. The diffuser can also
be used with any hydraulic pipeline operation including hydraulic pipeline dredges, pump-out
from hopper dredges, and reslurried pump-out from barges.
Sand Spreader Barge
Specialized equipment for hydraulic spreading of sand for capping has been used by the Japa-
nese (Kikegawa 1983, Sanderson and McKnight 1986). This equipment employs the basic
features of a hydraulic dredge with submerged discharge (Figure 15). Material is brought to
the spreader by barge, where water is added to slurry the sand. The spreader then pumps the
slurried sand through a submerged pipeline. A winch and anchoring system is used to swing
the spreader from side to side and forward, thereby capping a large area.
Figure 13. Spreader box or sand box for hydraulic pipeline
discharge, Simpson Kraft Tacoma, Puget Sound
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Chapter 4. Equipment and Placement Techniques
DERRICK
WATER SURFKE
Figure 14. Submerged diffuser system, including the diffuser and discharge barge.
"^_
BMGE UNIOMER MD SMD SPREADER *
Figure 15. Hydraulic barge unloader and sand spreader barge (from Kikegawa 1983).
54
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Chapter 4. Equipment and Placement Techniques
Gravity-fed Downpipe (Tremie)
Tremie equipment can be used for submerged discharge of either mechanically or hydraulically
handled granular cap material. The equipment consists of a large-diameter conduit extending
vertically from the surface through the water column to some point near or above the bottom.
The conduit provides the desired isolation of the discharge from the upper water column and
improved placement accuracy. However, because the conduit is a large-diameter straight
vertical section, there is little reduction in momentum or impact energy over conventional
surface discharge. The weight and rigid nature of the conduit requires a sound structural
design and consideration of the forces due to currents and waves.
The Japanese have used tremie technology in the design of specialized conveyor barges for
capping operations (Togashi 1983, Sanderson and McKnight 1986). This equipment consists
of a tremie conduit attached to a barge equipped with a conveyor (Figure 16). The material is
initially placed in the barge mechanically. The conveyor then mechanically feeds the material
to the tremie conduit. A telescoping feature of the tremie allows placement at depths of up to
approximately 40 feet. Anchor and winch systems are used to swing the barge from side to
side and forward so that larger areas can be capped, similar to the sand spreader barge.
W®!ii®!iMm
Figure 16. Conveyor unloading barge with tremie (from Togashi 1983).
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Chapter 4. Equipment and Placement Techniques
Figure 17. Tremie system employed at Hamilton Harbor.
A variation on the tremie system was used at the ISC demonstration in Hamilton Harbor
(Zeman and Patterson 1996a and 1996b). Sand, piled on a flat-deck barge, was placed into
a hopper using a small front-end loader. Inside the hopper, the sand was slurried and routed
into a number of 6-inch diameter, PVC plastic tubes (Figure 17). The tubes extended 30-feet
down, where the sand exited about 5-10 feet above the sediment. An anchor and winch system
was used to position the barge.
Hopper Dredge Pump-down
Some hopper dredges have pump-out capability by which material from the hoppers is dis-
charged like a conventional hydraulic pipeline dredge. In addition, some have further modi-
fications that allow pumps to be reversed so that material is pumped down through the dredge's
extended dragarms. Because of the expansion at the draghead, the result is similar to using
a diffuser section. Pump-out depth is limited, however, to the maximum dredging depth,
typically about 60-70 ft.
Equipment and Placement Techniques
for Armoring Layers
Placement of armor layers on in-situ caps can apply techniques commonly used for purposes
of streambank and shoreline erosion protection. The Sheboygan River ISC was constructed
using stone (1-2 inch cobbles) for erosion protection. Armor stone was also used at GM
Massena site. Although there is very little experience with armor stone at ISC applications,
guidance from streambank and shoreline erosion protection (USAGE 1990, 1994) may be
applicable to some ISC sites.
Methods that have been used for placing armor stone include placing by hand; machine
placing, such as from some form of bucket; and dumping from trucks and spreading by
bulldozer. Placement of cobbles at the Sheboygan River ISC was by bucket from a land-based
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Chapter 4. Equipment and Placement Techniques
crane with support from workers wading in the shallow river (see Figure 18). Gravel-sized
armor stone was placed onto the cap at Massena using a backhoe which was emptied a few
feet above the cap. Where gravel, cobbles or small stone must be placed in deeper water, it
may be possible to push them over the side of a flat deck barge or down a modified tremie.
Potential effects with such methods that should be considered include the disruption or
penetration of other cap components by the armor stone impact and the differential settling of
graded stone. In order to reduce the force of impact it may be necessary to handle the stone
by bucket and release it closer to the cap surface or pass the material down some type of slide
towed behind the barge.
As noted in the previous chapter, because of the uncertainties associated with underwater
placement of stone, the design thickness of the erosion component should be increased by 50
percent.
Figure 18. Stone placement at Sheboygan River.
Placement of Geosynthetic Fabrics
and Membranes
Experience with placement of geosynthetic fabrics in subaqueous conditions is limited. At the
Chicago Area Confined Disposal Facility (CDF), a plastic liner was pulled from a workbarge
in sections which were heat welded together on the barge surface (Savage 1986). Cranes have
been used to place geotubes prior to filling, directly lifting folded fabric tubes from working
barges. Longer lengths of tube have been deployed from large reels mounted on barges. A
membrane measuring 110 feet by 240 feet was placed as a temporary subaqueous cap at
Manistique River by crane from a workbarge and anchored using concrete blocks (Hahnenberg,
pers com). This operation required some manipulation of the cover by divers. A geotextile cap
was deployed using a reel at Eitrheim Bay in Norway (Instanes 1994). Geosynthetic fabric was
also used at Sheboygan, comprising two layers of the armoring.
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Chapter 4. Equipment and Placement Techniques
Geosynthetics have been fabricated with anchors around the perimeter and other locations to
simplify aquatic deployment. In most cases, the placement of geosynthetic fabrics at an ISC
will require the coordinated actions of several crews and vessels. The material will have to be
anchored quickly, especially where currents, waves or tidal conditions are subject to rapid
changes.
Positioning Requirements
The ability to keep barges and work vessels in position may require considerable effort at sites
subject to currents, waves and tidal movements. Where granular cap material is placed by
surface discharge, barge spreading, or hydraulic washing, vessels can be positioned by tug
boats or other support vessels. Spuds, long steel posts attached to some barges that are
lowered into sediments to maintain position, may not be appropriate for use during cap place-
ment, as the spuds might penetrate and damage the cap. Cables attached to large "deadman"
anchors deployed outside the cap footprint have been used to position work barges for ISC
construction at Hamilton Harbor (Zeman and Patterson 1996a and 1996b).
Once the equipment and placement techniques for the various cap components are selected,
the needs for land-based surveys or navigation and positioning equipment and controls can be
addressed. The survey or navigation controls must be adequate to insure that the cap can be
placed (whether by land-based equipment, bargeload, hopperload or by pipeline) at the desired
location in a consistently accurate manner. Global positioning equipment (GPS) using the
differential mode (DGPS) was used at the Hamilton Harbor capping demonstration (Zeman and
Patterson 1996b).
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5 Monitoring and Management
Monitoring Requirements
A monitoring program should be required as a part of any capping project design. The main
objectives of monitoring for ISC would normally be to insure that the cap is placed as intended
and that the cap is performing the basic functions (physical isolation, sediment stabilization and
chemical isolation) as required to meet the remedial objectives. Specific items or processes
that may be monitored include cap integrity, thickness, and consolidation, the need for cap
nourishment, benthic recolonization, and chemical migration potential.
Intensive monitoring is necessary at capping sites during and immediately after construction,
followed by long-term monitoring at less frequent intervals. In all cases, the objectives of the
monitoring effort and any management or additional remedial actions to be considered as a
result of the monitoring should be clearly defined as a part of the overall project design. The
cost and effort involved in long term monitoring and potential management actions should be
evaluated as part of the initial feasibility study.
Design of Monitoring Programs and Plans
The design of monitoring programs for any project should follow a logical sequence of steps.
Several excellent publications containing general guidance for monitoring in aquatic environ-
ments and specific guidance on physical and biological monitoring at aquatic sites for purposes
of site designation/specification and for permit compliance are available (Marine Board 1990;
Fredette et al. 1990a; Fredette et al. 1990b; and Pequegnat et al. 1990). These basic
references should be consulted in developing appropriate monitoring plans for capping projects
which suit the site and material specifics.
Fredette et al. (1990a) outlines five steps for developing a physical/biological monitoring
program for open water dredged material disposal:
a. Designating site-specific monitoring objectives,
b. Identifying elements of the monitoring plan,
c. Predicting responses and developing testable hypotheses,
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Chapter 5. Monitoring and Management
d. Designating sampling design and methods (to include selection of equipment and
techniques),
e. Designating management options.
These steps should be applicable in developing a monitoring program for ISC projects.
Guidance for dredged material disposal and dredged material capping recommends that the
monitoring program be multi-tiered (Palermo et al 1992; Fredette et al. 1986). Each tier has
its own unacceptable environmental thresholds, null hypotheses, sampling design, and
management options should the thresholds be exceeded. These are best determined by a
multidisciplinary advisory group whose technical advice is sought in organizing and conducting
the monitoring program. A sample tiered monitoring program developed for dredged material
capping projects is outlined in Table 2, showing how a tiered monitoring program could be
structured. This sample program is generally applicable to an in-situ capping project.
The monitoring program for in-situ capping does have some differences from those for dredged
material capping. At Great Lakes areas of concern, or other locations where in-situ capping
is conducted for purposes of sediment remediation, existing degraded conditions will have been
well defined as the justification for remedial action. The remedial objectives should outline the
desired impacts of the in-situ cap, which may include specific end points such as reductions in
fish contamination levels, improved water quality conditions or the restoration of beneficial
uses. The monitoring plans for ISC projects are therefore directed by the objectives of the
remedial action.
Each of the steps in developing an in-situ capping monitoring program is discussed in more
detail in the following paragraphs.
Monitoring Objectives
Monitoring can be generally considered in two phases; that occurring during and immediately
after construction, and long-term monitoring. The objectives of monitoring at these two
timeframes may be somewhat different.
The objectives of construction monitoring are to assure that the contractor follows the terms
of contract plans and specifications in the placement of the ISC, to identify any changes in site
conditions that may impact cap design or performance and modify the design or construction
techniques as necessary.
The objectives of long-term monitoring at an in-situ cap are rooted in the remedial objectives.
For instance, if the primary objective of sediment remediation was to reduce the contaminant
body burden in fish, the monitoring program might be devised to measure the performance of
the cap in physical and chemical isolation to determine if that objective had been met. If the
cap was designed primarily to stabilize the contaminated sediments, an entirely different mo-
nitoring program might be developed.
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Chapter 5. Monitoring and Management
Table 2. Sample Tiered Monitoring Program for dredged material capping
Monitoring Program
Define chemical
baseline conditions.
Tierl
*Bathymetery
*Subboottom profiles
*Side scan sonar
*Surface grab samples
"Cores
"Settling plates
"Water samples
Tier II
"Bathymetry
"Subbottom profiles
"Side scan sonar
"Sediment profile cam.
"Cores
"Water samples
"Settling Plates
"Consolidation
instrumentation
"Tissue samples
Tier III
"Bathymetery
"Subbottom profiles
"Sediment profile
camera
"Surface grab samples
"Cores
"Water samples
"Tissue samples
Monitoring
Frequency
Threshold
Post
Placement,
Annual
Quarterly to
Semi Annually
Monthly to
Semi Annual
"Mound approaches
being navigation hazard
"Cap thickness
decreases slightly
"Contaminant exceeds
limit in sediment or water
sample
"Cap thick decreases
significantly
"Contaminant exceeds
limit in sediment or water
sample
"Cap thick decreases
significantly
"Contaminant exceeds
limit in sediment or water
sample
"Contaminant exceeds
limit in tissue
Management
(Threshold Not
Exceeded)
Options
(Threshold
Exceeded)
"Continue to monitor at
same level
"Reduce monitoring
level
"Stop monitoring
"Continue to monitor at
same level
"Reduce monitoring
level
"Continue to monitor at
same level
"Reduce monitor level
*Go to next
Tier
"Increase cap
thickness
*Go to next
Tier
"Replace cap
material
"Increase cap
thickness
"Place armor
layer
"Replace cap
material
"Increase cap
thickness
"Change cap
sediment
"Place armor
layer
"Redredge
and remove
Note: This is only an example of a possible monitoring program. Each monitoring program is site specific.
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Chapter 5. Monitoring and Management
Aside from the evaluation of cap functional performance, another important objective of long-
term monitoring is to track the physical integrity of the cap under variable hydrodynamic
conditions and any man-made stresses. Cap designs are based on conditions and forces with
a significant degree of uncertainty, and long-term monitoring is needed to check the reason-
ableness of those assumptions and determine how the cap responds to unforeseen conditions.
Long-term monitoring is also used to guide cap maintenance plans and modify future
monitoring activities.
Construction Monitoring
Cap Materials
The elements of construction monitoring are typically defined in the quality assurance plan for
the remedial construction contract, and may be conducted by the construction contractor,
subcontractors and/or by independent agencies or contractors. The contract documents will
typically define criteria or standards for all capping materials. Samples of materials provided
by vendors and suppliers will be analyzed periodically to assure that they meet criteria speci-
fied in the contract, such as:
• acceptable grain size distribution of granular materials
• maximum/minimum levels of TOG in granular materials
• geologic characteristics of armor stone
• strength or puncture resistance of geotextiles
Granular materials and geosynthetics should be analyzed using accepted laboratory methods
(USAGE 1970; ASTM 1992).
Monitoring of granular cap materials will require inspections or the collection of samples at
various places and times, including:
• inspection/certification of quarry by geologist
• laboratory analysis of samples collected at quarry
• laboratory analysis of samples collected after placement
Quarry inspection/certification is important to ensure that armor stone is cut from rock with no
argillaceous inclusions or seams, which tend to swell when submerged (Johnson, pers com).
Samples collected at the quarry are typically analyzed for grain size distribution (and other
parameters as necessary) for compliance with contract specifications.
Analysis of granular materials following placement is especially important for in-situ caps.
Differential settling of granular materials during placement has the potential to cause
segregation of materials by grain size. Fine-grained or less dense materials may be
transported off-site during placement in waters with even small currents. Some cap placement
methods can reduce these effects. However, the collection and analysis of samples of granular
materials, post-placement, is the only way to determine if the cap, as constructed, meets the
contract requirements.
Granular cap materials (post placement) should be sampled as cores. Grab samplers are not
recommended because they don't maintain vertical integrity and may result in a loss of fines.
Gravity coring devices are generally suitable for deeper water than than typical of most ISC
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Chapter 5. Monitoring and Management
sites, and may not penetrate adequately except where the cap is more fine-grained and poorly
consolidated. Vibracore samplers, as used to monitor cap thickness at Hamilton Harbor
(Zeman and Patterson 1996b) can penetrate sand and finer materials. For coarse-grained cap
materials, divers may also be an effective means of collecting representative cores during
construction. A variety of sediment coring techniques are available (Mudroch and MacKnight
1991; USEPA/NCD1994).
Construction Methods
Depending on how the construction contract is advertised and awarded, the methods for
placement may be specifically defined or left to the contractor's selection so long as certain
performance goals and criteria are met. Construction performance criteria for ISC projects
might include:
• maximum/minimum tolerance for cap placement (laterally)
• maximum/minimum tolerance for cap component thickness
• maximum tolerance for "mixing" of sediment and cap material
• maximum levels of sediment resuspension
• maximum levels of sediment contaminants on cap surface following construction
Appropriate techniques for monitoring cap placement include bathymetric surveys, sediment
core sampling, and sediment profiling camera. For some sites, visual observation in relatively
shallow waters (i.e., up to 20ft. at GM Massena site) or diver observations may also be useful.
Precision bathymetric surveys are perhaps the most critical monitoring tool for capping projects.
Such surveys allow determination of the location, size, and thickness of the contaminated
material deposit and cap. For ISC, a series of surveys should be taken immediately prior to
placement of the cap, periodically during placement, and at the completion of placement. The
differences in bathymetry as measured by the consecutive surveys yields the location and
thickness of the deposits. Contractors will probably make bathymetric measurements on a
daily basis to keep track of their progress and plan work for the following days.
Lillycrop et al. (1991) discusses tidal elevations, bathymetry measurements, and equipment
capabilities. Acoustic instruments such as depth sounders (bottom elevations accurate to +/-
0.6 ft under favorable conditions), side scan sonar (mapping of areal extent of sediment and
bedforms), and subbottom profilers (measures internal mound and seafloor structure) are used
for these physical measurements. Survey track spacing can be 50 to 200 ft depending on the
areal coverage of the cap. Multi-beam depth sounding systems provide 100 percent coverage
of the bottom. Their additional expense may be justified for some projects.
The attainable accuracy of bathymetric surveys must be considered and limit the area and
thickness of the deposit which can be detected. Limits of accuracy are governed by a variety
of factors which include accuracy of positioning systems, water depth, wave climate, etc.
Engineer Manual EM 1110-2-1003 contains additional information on hydrographic survey
equipment and techniques.
The interpretation of bathymetric data needs to be coupled with an understanding of consoli-
dation processes. Consolidation that occurs in the cap, contaminated sediment, and the
original base material can result in substantial changes in bathymetry (Silva et al. 1991,
Poindexter-Rollings 1990) that could mistakenly be considered as an indication of inadequate
cap thickness. The ability to measure or predict consolidation can limit the utilization of
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Chapter 5. Monitoring and Management
bathymetric data for monitoring the total cap thickness. A schematic of a settling plate used
for monitoring cap consolidation is shown in Figure 19. This technique can provide a means of
measuring the consolidation of the contaminated sediments and underlying bed material
Figure 19. Schematic of a settling plate used for monitoring cap consolidation.
(together), which enables bathymetric data to be used to monitor total cap thickness and to
confirn predictions of sediment consolidation which are controlling short-term advective flux.
It should be noted that the installation of settling plates can be difficult and some cap placement
methods can easily disturb/destroy these plates.
The Sediment Profiling Camera (SPG) is a recently developed tool which can be used to detect
thin layering within sediment profiles. The SPC is an instrument which is lowered to the bottom
and is activated to obtain an image of sediment layering and benthic activity by penetrating to
a depth of 15-20 cm (Figure 20). SPC can be used to monitor the thickness of granular cap
components and examine the "mixing" of granular cap material and contaminated sediments.
As with bathymetric surveys, the SPC approach also has its limits. The depth of penetration
limits the thickness which can be viewed. The SPC was designed for penetration of relatively
soft cap materials, would not be appropriate for an armored cap (unless the armor layer was
removed by divers), and may be difficult to push more than a few inches into a cap of medium
or coarse sand.
The thickness of granular cap components and the presence of sediment contaminants in any
component can be determined from cores or borings of the ISC. In general, a core should
sample the full thickness of a cap and the underlying contaminated material. The selection of
boring techniques may be limited by site conditions and the cap design.
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Chapter 5. Monitoring and Management
Figure 20. Illustration of Sediment Profiling Camera.
Contract criteria for limiting sediment resuspension during ISC placement may require moni-
toring. At the Hamilton Harbor capping demonstration, water samples were collected around
the placement operation and analyzed for total suspended solids. The color of particulates on
the filter paper indicated that the suspended solids in the plume around the capping operation
were fines washed off the sand during placement, rather than resuspended bottom sediments
(Zeman and Patterson 1996a). At the Wyckoff/Eagle Harbor Superfund site, water samples
were collected during cap placement and analyzed for dissolved oxygen, total suspended
solids, ammonia and total sulfides (Nelson, Vanerberden, and Schuldt 1994). In addition,
sediment traps were deployed near the ISC site to collect and measure resuspended bottom
sediments.
Navigation and positioning equipment are needed to accurately locate sampling stations or
survey tracks in the disposal site area. State of the art positioning systems are recommended
for offshore sites. Land-based survey techniques may be acceptable for sites near shore. Taut
wired buoys are also excellent for marking disposal locations and as a reference for sampling
station locations.
Cap Performance Monitoring
Monitoring that is conducted to evaluate the performance of the ISC in regard to specific cap
functions can be conducted on a short- or long-term basis. Some elements of a monitoring
program, such as those evaluate the consolidation-induced advection) may only occur during
construction and weeks to months afterwards. Other elements of a monitoring program that
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Chapter 5. Monitoring and Management
might be conducted for a longer, but finite period (a few years) might include measurements
of changes in flow patterns and erosion at adjacent and downstream locations. Still other
elements of a monitoring program may be required to be conducted indefinitely, but at a
diminishing frequency. Methods for monitoring each of the basic cap functions (stabilizatyion,
physical isolation, and chemical isolation) are discussed in the following sections.
Sediment Stabilization
To evaluate the performance of the ISC in sediment stabilization, a monitoring program must
demonstrate that the stabilization component is intact and the cap completely covers the
contaminated sediment deposit. The elements of such a monitoring program might include
measurements of:
• bathymetry of the capped area
• cap/component thickness
• component integrity
Methods for measuring cap bathymetry and the thickness of cap components are the same as
discussed for construction monitoring. Component integrity refers to the physical integrity of
the stabilization component. Armor stone are subject to cracking and weathering. After many
years, even 7-inch armor stone can be reduced to gravel and monitoring is needed to measure
the character, as well as the thickness of the stabilization component.
The frequency of measurements in a long-term monitoring plan will vary with site conditions
and cap design. One approach is a time-based schedule, where monitoring occurs at a fixed
or expanding frequency. Another approach is an event-based schedule, where monitoring
occurs only after significant erosion events (i.e., storms, floods, etc.). The design of an ISC
erosion protection component is based on predictions of one or more hydrodynamic processes.
The design presumes that an event of some magnitude and recurrence interval will be able to
dislodge part of the cap, and that repair or replenishment of the cap will be needed following
such an event. Monitoring after erosion events is preferable, since it is after such events that
emergency maintenance or repair of the cap is more likely to be needed. In addition, the
development of monitoring data after events of known magnitude will enable the predictive
methods used in the design to be "fine-tuned" so that the magnitude of events capable of
causing major damage to the cap might be predicted more accurately. As the predictive
methods are "fine-tuned", monitoring can be scheduled to occur only after events capable of
causing damage to the cap.
An event-based monitoring program requires the ability to perform monitoring with little advance
notice. It also requires some means of measuring the event that triggers the monitoring, such
as the flood stage at a river gage, measured wave height at recording station, or meteorological
conditions at a recording station (e.g., the amount of precipitation or wind velocity from a certain
direction over a specified period of time). Flood stages are recorded at a number of river gages
operated by the U.S. Geological Survey, USAGE and some state and local agencies. The
National Oceanic and Atmospheric Administration maintains nine wave rider buoys on the
Great Lakes during the navigation season which transmit meteorologic and wave conditions
in real time. The installation of a recording gage should be considered at ISC sites in order to
get the most representative and dependable source of information.
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Chapter 5. Monitoring and Management
Physical Isolation
To evaluate the performance of the ISC in physical isolation, a monitoring program must
demonstrate that the cap is intact, covers the contaminated sediment deposit, prevents the
physical loss of contaminants, and that benthos are not able to penetrate the cap. The ele-
ments of such a monitoring program might include measurements of:
• bathymetry of the capped area
• cap/component thickness
• benthos colonizing the cap
• sediment traps
The methods for measuring bathymetry and cap/component thickness are described above.
Benthic organisms colonizing the cap may be surveyed to determine if organisms capable of
burrowing through the physical isolation component are present. Benthic sampling devices
include trawls, drags, box corers, and grab samplers. Trawls and drags are qualitative sam-
plers which collect samples at the bottom interface, and therefore are good for collecting
epifauna and shallow infauna (top few centimeters). Quantitative samples are usually obtained
with box corers and grab samplers. Generally these samplers collect material representing
0.02 to 0.5 m2 of surface area and sediment depths of 5 to 100 cm. Divers may be needed for
the collection of samples from caps with coarse material or an armor layer.
An armor layer rock coverage can be monitored by divers or remote sensing techniques. Sonar
and ground penetrating radar may assist in evaluating the presence of armor stone and
tracking changes in the armor layer elevation. Erosion pins, typically constructed of rebar, can
be used to measure changes in the relative elevation of the top of the armor stone. An erosion
pin mounted on a steel plate placed at a level under the cap may provide a more reliable
relative vertical reference point. Painted rock studies have been used to measure the transport
of cobbles in high energy streams and could be employed to monitor cap stone integrity.
Uniquely marked stones of the same size as the armor stone may assist in monitoring off-site
losses of stone. Recently Rosenfeld, et al (1996) has proposed to use passive radio tran-
sponders to track stone movement in open channel studies. Other researchers have used
magnetic tracers, radioactive tracers and active radio transponders to track the movement of
rock and gravels.
Chemical Isolation
In order to evaluate the chemical isolation function of an in-situ cap, the long-term migration of
contaminants must be measured. This is likely the most difficult element of a monitoring
program to perform because the predicted rates of long term chemical flux are so low, the
variability in physical and chemical properties of the capped sediments are so great, and the
logistical problems of collecting representative samples at an underwater cap are so numerous.
Methods for measuring the movement of contaminants through an ISC are in the early stages
of development. Predictive models (see Appendix B) suggest that available techniques should
not be able to discern a flux, other than the short-term flux during consolidation, unless there
is a significant advective movement (groundwater) through the cap.
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Chapter 5. Monitoring and Management
Methods for measuring contaminant migration that have been used or considered at in-situ
caps include chemical analysis of cap materials, collection chambers, solvent-filled bags, and
caged fish. Methods that rely on samples of water, fish or bioaccumulative materials collected
at the surface of the cap are less likely to be useful in tracking contaminant migration than
those which collect samples within the cap.
Chemical analysis of cap materials may used to detect any mixing of contaminated sediments
with these materials during placement, and has been used as an indicator of chemical
migration at dredged material caps (Sumeri et al 1994).
CLEAR
ACRYLIC
BODY
CELL
LEXANE
WEDGE
Figure 21. Semi-permeable bags or "peepers" filled with an organic solvent used for
monitoring the levels of hydrophobic contaminants in sediment pore water.
Small, semi-permeable bags filled with doubly distilled water have been used for monitoring the
levels of nutrients and metals in sediment pore water. These devices, known as "peepers",
have been adapted for use, as shown in Figure 21, at the Hamilton Harbor capping
demonstration (Rosa and Azcue 1993; Azcue, Rosa, and Lawson 1996; Zeman and Patterson
1996b).
A seepage meter considered for the Manistique Harbor ISC employed a 55-gallon drum that
had been cut in half, with the open end inserted into the cap surface (Figure 22). Water
seeping upward from the cap into the drum would be channeled into a collection vessel which
could be removed/replaced without disturbing the cap (Blasland, Bouck & Lee 1995). Such a
monitoring device has not yet been employed at an ISC site.
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Chapter 5. Monitoring and Management
Figure 22. Seepage meter used to measure groundwater flow.
Other Monitoring Methods
The elements of long-term monitoring that are directed by the remedial objectives are not
always measured immediately at the ISC. The impacts of sediment contamination may be over
a large area, and the effects of remediation may need to be evaluated at the same scale. For
example, if the remedial objective is to reduce the body burden of a contaminant in fish, this
might be best evaluated using the same monitoring approach used to define the problem in the
first place (e.g., periodic collection of fish at specific locations in a river/lake or collection of
selected fish tissues at fish cleaning stations).
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Chapter 5. Monitoring and Management
Management Actions
An in-situ cap is not an operating facility in the sense that a treatment facility or CDF is
operated. Nonetheless, an ISC does have some operational practices and controls that may
need to be implemented in order to assure that the in-situ cap functions as designed and
remains intact. These considerations may include planned maintenance of the cap, restrictions
on uses of the waterway at the capping site and other institutional controls. The management
plan for the ISC must also be adjusted as monitoring data indicates.
During Construction
The results of monitoring conducted during cap placement need to be evaluated rapidly so that
problems with materials or placement methods can be identified in time to effect the necessary
changes. For this reason, monitoring techniques that can generate results in real time, or with
a rapid turnaround are preferable. The construction contractor is typically responsible for
proposing actions to remedy any shortcomings in cap materials or placement methods.
Because of the difficulty in fixing deficient material or placement methods after the fact, it may
be appropriate to construct a small portion of ISC as a "test plot" before proceeding with the
larger capped area(s).
Routine Maintenance & Protection
Routine cap maintenance generally is limited to the repair or replenishment of erosion protec-
tion component material. The design of some dredged material caps includes a thickness of
granular material that is expected to be eroded during storm events of a known magnitude or
recurrence interval. For such a design, maintenance can be scheduled or planned for in
advance. This type of erosion control is not appropriate unless there is a dependable source
of capping material readily available. For an ISC, the ability to detect and quickly respond to
a loss of the erosion protection layer should also be taken into consideration. On the Great
Lakes, seasonal limitations, such as ice formation or closure of navigation structures (locks),
can limit the ability to monitor in-situ caps after a significant erosion event and respond with
maintenance if needed.
The long-term integrity of a cap requires that conditions which affect erosive forces are not
changed (for the worse). For instance, after a cap is constructed, the removal of an upstream
dam or modification to a breakwater could have significant impacts on the current- or wave-
induced erosion at the cap. The "owner" of the cap must be capable of protecting its integrity
from man-made activities.
Aside from erosion caused by natural phenomena, the greatest threat to the integrity of an ISC
is from navigational activity. As discussed in Chapter 3, and in Appendix A, the erosive forces
created by propellers of ships, tug boats, and even recreational watercraft can be quite pow-
erful, especially where water depths are reduced by the presence of an in-situ cap. Other
activities, such as bottom drag fishing, direct hull contact, and anchoring create bottom stresses
that can damage a cap (Truitt 1987a). An in-situ cap, particularly one with an armor layer, may
be attractive to some fish, and consequently may be attractive to fisherman.
In order to inform navigation users of the presence of the ISC, navigation maps, mariners
guides, and local land-use documents should be updated to show the presence of the cap and
any use restrictions. Information about the cap and restrictions might also be posted at boat
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Chapter 5. Monitoring and Management
launch areas, bait shops, and provided with fishing licenses. Signs should be posted at prom-
inent locations near the cap, and marker buoys deployed where appropriate. Active local public
education programs on the presence and purpose of the ISC may improve voluntary
compliance.
The ability to enforce restrictions on navigation activities in and around ISC sites should be
weighed in considering the overall feasibility of capping. Restrictions that are codified as local
or state statutes are more likely to be adhered to than voluntary ones. For instance,
development of waterfront facilities, marinas, and docks that might increase navigation in
proximity to the cap could be restricted in State Coastal Zone Management plans or local
zoning ordinances. Enforcement of any use restrictions in the waterway may require
considerable resources. The costs and ability to enforce use restrictions should be considered
in the evaluation of the feasibility of ISC.
Repair & Modification
If monitoring of cap performance indicates that one or more cap functions are not being met,
options for modifying the cap design may or may not be available. If monitoring shows that the
stabilization component is being eroded by events of lesser magnitude than planned, or the
erosive energy at the capping site was underestimated, eroded material may be replaced with
larger stone. If monitoring indicates that benthic organisms are penetrating the cap in
significant numbers, a layer of sand or gravel might be placed on top of the cap to inhibit
benthic colonization. These types of management options are feasible where additional cap
thickness, and the resulting decrease in water depths at the site do not conflict with other
waterway uses. Where an ISC has been closely designed to a thickness that will not limit
waterway use (i.e., recreational or commercial navigation), the options for modifying a cap
design after construction may be very limited.
When the cap design is performing as expected, monitoring results can be used to optimize
maintenance monitoring activities. If there is a failure of the ISC design to meet remedial
objectives (e.g., unanticipated advection of groundwater through the cap causes unacceptable
contaminant migration), removal may be the only management alternative available. Because
of the additional cost of removing, treating and/or disposing of cap materials in addition to
contaminated sediments, in-situ caps should only be proposed where the performance of cap
design functions required to meet remedial objectives can be assured.
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6 Summary
This document presents technical guidance for planning and design of in-situ subaqueous
capping projects. The guidance is summarized as follows:
a. In-Situ Capping (ISC) refers to placement of a covering or cap over an in-situ deposit
of contaminated sediment. The cap may be constructed of clean sediments, sand,
gravel, or may involve a more complex design with geotextiles, liners and multiple layers.
b. ISC is one of many options for the remediation contaminated sediments, which should
be considered using the full suite of guidance development under the ARCS Program.
c. An ISC operation must be treated as an engineered project with carefully considered
design, construction, and monitoring to ensure that the design is adequate.
d. There is a strong interdependence between all components of the design for a
capping project. By following an efficient sequence of activities for design, unnecessary
data collection and evaluations can be avoided and a fully integrated design is obtained.
e. The basic criteria for a successful capping operation is simply that the cap
components required to isolate the contaminated material from the environment be
successfully placed and maintained.
f. The contaminated sediment to be capped must be characterized as part of the project
design. The capping materials (granular sediments or other materials) must also be
characterized.
g. The evaluation of the site is a critical requirement for an ISC capping design.
Bathymetry, currents, water depths, waterway uses, bottom sediment characteristics,
potential groundwater flow, and operational requirements must be evaluated.
h. A number of different equipment types and placement techniques can be considered
for ISC operations. Conventional discharge of granular capping material from barges
and hydraulically dredged material from hopper dredges or pipelines can be considered
as well as use of diffusers, tremies, and other equipment needed for submerged
discharge. Controlled discharge and movement of barges and use of spreader plates or
boxes with hydraulic pipelines can be considered for spreading a capping layer over a
larger area. Specialized equipment may be needed for placement of geotextile or
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Chapter 6. Summary
membrane components. Armor stone may be placed using conventional placement
methods for riprap. Compatibility between equipment and placement technique for
contaminated and capping material is essential for any capping operation.
i. Accurate navigation and precise positioning during material placement are required for
capping operations. State-of-the-art equipment and techniques must be employed to
assure accurate placement to the extent deemed necessary. Diligent inspection of
operations to insure compliance with specifications is essential.
j. The composition and dimensions (thickness) of the components of a cap can be
referred to as the cap design. This design must perform one or more of the three
functions of a cap (physical isolation, stabilize sediment, and reduce flux of dissolved
contaminants). The design must also be compatible with available construction and
placement techniques.
k. Caps composed of multiple layers of granular materials as well as other materials
such as armor stone or geotextiles are often considered for ISC projects, and the in-situ
cap design cannot always be developed in terms of cap material thickness alone.
I. Monitoring of capped sites is required during and following placement of the
contaminated and capping material to insure that an effective cap has been constructed
and to insure that the cap as constructed is effective in isolating the contaminants and
that long term integrity of the cap is maintained. Design of monitoring programs must be
logically developed, prospective in nature, and tiered with each tier having its own
thresholds, null hypotheses, sampling design, and management responses based on
exceedance of predetermined thresholds.
m. Management of an ISC requires the routine maintenance of the cap, protecting cap
integrity through enforcement of waterway use restrictions, repair and modification of the
cap as needed to address changing conditions or design deficiencies indicated by
monitoring data.
Recommendations
As more designs are completed and additional field experience is gained, the technical guide-
lines in this report should be refined and expanded. Additional research is also recommended
to develop improved tools for capping evaluations. Specific recommendations for further
research are summarized as follows:
a. Refine and verify models that predict long-term erosion of caps.
b. Refine existing estimates of resuspension of contaminated material during cap place-
ment. This work will assist in determining the costs vs. benefits of "sprinkling" cap
material versus conventional bottom dumping of cap material.
c. Develop engineering guidance on acceptable rates and methods of application of
capping material over contaminated material of varying density and shear strength.
These techniques should consider the geotechnical behavior related to displacement and
mixing of contaminated and capping sediments and resistance of the sediments to
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Chapter 6. Summary
bearing failure. Extend the investigation to include penetration of dense (e.g., rock) cap
material into contaminated material or other cap layers.
d. Refine existing models for prediction of cap and sediment consolidation. This effort
will likely require developing or refining instrumentation for in situ geotechnical mea-
surements.
e. Develop predictive tools for evaluation of long term cap integrity, considering chemical
migration via advection, bioturbation, and diffusion. Both analytical and modeling
approaches should be considered.
f. Conduct laboratory and field verification studies of long-term cap integrity. Laboratory
approaches should include refinement of existing cap effectiveness tests. Field studies
should include periodic monitoring and sampling of capped sites to include analysis of
core samples.
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83
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-------
Guidance for In-Situ
Subaqueous Capping
of Contaminated Sediments:
Appendix A: Armor Layer Design
by
Steve Maynord
U.S. Army Engineer Waterways Experiment Station
Vicksburg, Mississippi
Prepared for U.S. Environmental Protection Agency
Great Lakes National Program Office
Assessment and Remediation of Contaminated Sediment Program
Chicago, Illinois 60604
Monitored by U.S. Army Engineer Division, North Central
Chicago, Illinois 60605-1592
-------
-------
Appendix A: Armor Layer Design
Appendix A: Armor Layer Design
If an evaluation of cap erosion indicates that the capping material will not be sufficiently resistant
to erosion, an armor layer can be considered. Such an armor layer would be incorporated into the
cap design and would replace any previously determined cap sediment thickness component for
erosion.
A design of capping armor layers has been developed as a part of the EPA ARCS program and
is presented in this Appendix. This section provides guidance for the design of armoring to ensure
the long term stability or integrity of the cap. Caps might be subjected to a variety of physical
stresses such as river or tidal currents, wind wave generated currents, ice and debris scour, or
propeller wash in navigation channels. Preliminary technical guidance is provided on the hydraulic
design of in-situ capping/armoring of contaminated sediments with riprap. Factors pertinent to
flood flows, navigation effects, and wind wave induced currents are presented and then formulas
and sample calculations are provided. Less predictable forces on ISC such as scouring from ice
and debris, flow from velocities generated by channel blockages such as ice dams, or massive
bank failure are not evaluated by this analysis. Designers of ISC should consider the significance
of these forces and potential effects in the evaluation of the feasibility of ISC.
Filter Design
Filters provide an interface between the riprap layer and the protected material and are an essential
element for protecting contaminated sediments, particularly poorly consolidated sediments. Filters
prevent turbulence and groundwater from moving sediments through the revetment. Filters serve
as foundations or load distributors for the riprap for poorly consolidated material which is typical of
many contaminated sediments. Filters can be either geotextile, granular, or a combination of the
two. Granular filters are generally more expensive but have been shown to provide long term
performance. Geotextile filters are less expensive but have not been around long enough to
completely evaluate the potential for clogging of the geotextile over long time periods. Problems
can occur with geotextiles if the permeability factor is too low. Gas and advective ground water may
displace a cap that has too low a permeability. Uncertainty in design should err on the side of
providing too large a permeability. A sand layer on top of fine-grained sediments may be required
prior to placement of either a granular or geotextile filter. A bedding layer of granular material
(sand or gravel) may be placed on top of the geotextile to prevent damage during placement of the
riprap. Guidance on design of geotextile filters can be found in Pilarczyk (1984) and PIANC (1987).
In determining the stability of intermediate granular layers subjected to velocity forces, the Worman
(1989) equation is
V! = c 4«L (1)
gS Du
A-1
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Appendix A: Armor Layer Design
Where V is the mean flow velocity above the granular layer, g is gravity, S is the granular layer
thickness, C is a coefficient that varies with the uniformity of the granular layer, d85 is the 85 percent
passing size of the base material, and D15 is the 15 percent passing size of the granular material.
Based on experimental work by Manamperi (1952), the coefficient for uniform riprap having D85/D15
= 1.3 is C = 24 and for Manamperi's graded riprap having D85/D15 = 6.7, C = 10. D85 is the 85
percent passing size of the riprap. For relatively uniform riprap having D85/D15 = 1.3, V = 7 ft/sec,
S = 1.0 ft, and D15 = 5 in., the required d85 of the intermediate granular layer is 0.32" or 8.1 mm.
Additional guidance on design of granular filters can be found in Pilarczyk (1984), EM 1110-2-1901
(USAGE 1986), and EM 1110-2-2300 (USAGE 1982).
Gradation and layer thickness considerations
Both riprap gradation and layer thickness play a significant role in defining the stability of the armor
layer. The gradation of rock produced by quarries across the country varies widely and
standardized gradations have not been widely adopted in the U.S. The gradations shown in Table
A1 are taken from EM 1110-2-1601 and give a maximum or upper limit and a minimum or lower
limit at the 100, 50, and 15 percent sizes. Any gradation falling between the maximum and
minimum limits is acceptable.
Minimum layer thickness requirements vary depending on the type of attack on the revetment. For
flood flows, the minimum layer thickness is 1D100(max) or 1.5D50(max), whichever is greater. D100
is the riprap size of which 100 percent is smaller, i.e. the largest riprap size. The (max) refers to
the upper or maximum limit curve. For propeller wash where turbulence is much greater than flood
flows, the minimum layer thickness is 1.5D100(max) or 2D50(max), whichever is greater.
Placement and Limits of Coverage
Placement of riprap and filters in dry conditions generally presents no problems and the minimum
layer thickness given above is applicable. Underwater placement presents uncertainties with even
coverage of stone and a 50 percent increase in granular filter and riprap volume is required.
Placement of geotextiles in shallow depths and low velocity can be accomplished as described in
the Appendix C case studies, by the method shown in the main body of this report or by attaching
the fabric to a framework and lowering the framework into position prior to stone placement.
Underwater placement in moderate to high velocity (> 2 ft/sec) would present significant problems
with geotextile placement. With a granular filter, a diver may be required to insure adequate
coverage in deep placement conditions.
A-2
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Appendix A: Armor Layer Design
Table A1 . Gradations For Specific Stone Weight of 165 LB/FT3,3
From USAGE (1994)
D100
(Max)
(in)
9
12
15
18
21
24
Limits of Stone Weight (Ib) for Percentage Lighter by
Weight"
100
Max
36
86
169
292
463
691
Min
15
35
67
117
185
276
50
Max
11
26
50
86
137
205
Min
7
17
34
58
93
135
15
Max
5
13
25
43
69
102
Min
2
5
11
18
29
43
D50
(Min)
(ft)
0.43
0.58
0.73
0.88
1.02
1.07
Ib/ft3 = 16.018kg/m3
b Stone weight limit data from USAGE (1994). Relationship between diameter
and weight is based on shape of a sphere.
The limits of protection and a typical cross-section are shown in Figure A1. Riprap protection
should extend 5 times the thickness of the riprap protection beyond the edge of the contaminated
material. The thickness of the edge extension should be 1.5 times the riprap thickness to allow for
scour along the edges of the protection. On the outer bank of channel bendways, significant scour
can be expected at the toe of the bank during flood flows. For contaminated sites on the outer
bank of bendways, refer to EM 1110-2-1601 (USAGE 1994) for design of toe scour protection. If
contaminated sediments on the bed are adjacent to the toe of the bank, protection should not only
cover the bed sediments, but should also extend partially up the side slope.
Stone Sizing for Flood Flows
Waterways that do not experience significant navigation may require protection for the maximum
flood flow or storm velocities near the capped sediments for the required life of the project. At sites
without navigation having flow velocities typically found in flood control channels, the riprap
protection requirements should follow the guidance provided in Chapter 3 of the EM 1110-2-1601
entitled "Hydraulic Design of Flood Control Channels" (USAGE 1994). The procedures for riprap
protection in EM 1110-2-1601 should be used for design guidance and revised as deemed
necessary to provide an adequate but practical protection for specific project conditions. Both the
guidance presented herein and EM 1110-2-1601 will be useful in evaluating design specifications
of riprap protection for capping projects.
A-3
-------
Appendix A: Armor Layer Design
CONTAMINATED SEDIMENT
GEDTEXTILE
Figure A-1. Cross section of riprap and edge protection.
A-4
-------
Appendix A: Armor Layer Design
Stone Size Equations
Velocity and flow depth are the two basic factors used in design of riprap protection. The method
of determining the stone size in EM 1110-2-1601 uses depth-averaged local velocity. Stone size
computations should be conducted for flow conditions that produce the maximum velocities at the
riprap boundary.
The following equation, modified from EM 1110-2-1601, relates velocity to stone size and is
applicable to any location in the channel. The changes from the EM include the use of the
gradation factor and basing stone size on D50 instead of D30. This was done to use the same
characteristic riprap size as in the navigation sizing presented subsequently.
SfCsCvCTCGd _
IB I
Y
w
SW,
1/2
I2.5
V
(2)
Where,
D50 = characteristic riprap size of which 50 percent is finer by weight.
Sf = safety factor,-minimum = 1.1
Cs = stability coefficient for incipient failure,
thickness = 1D100(max) or 1.5D50(max), whichever is greater,
D85/D15 = 1.7to5.2
= 0.30 for angular rock
= 0.375 for rounded rock
D85/D15 = gradation uniformity coefficient(typical range= 1.8 to 3.5)
CV = velocity distribution coefficient
= 1.0 for straight channels, inside of bends
= 1.283-.2log(R/W) for outside of bends (1 for R/W > 26)
= 1.25 downstream of concrete channels
= 1.25 at end of dikes
R = centerline radius of bend
W = water surface width at upstream end of bend
CT = blanket thickness coefficient(typically 1.0 for flood flows)
CG = gradation coefficient = (D85/D15)1/3
K! = side slope correction factor(see EM 1110-2-1601 for other slopes
d = local depth, use depth at 20 percent upslope from toe for side slopes
V = local depth averaged velocity, use velocity at 20 percent upslope from toe for side slope
riprap
Yw = unit weight of water
Ys = unit weight of stone (typical value of 165 Ib/ft3)
g = gravitational constant
A-5
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Appendix A: Armor Layer Design
A key element in any riprap design problem is the estimation of local depth-averaged velocity at
the protection location. The EM primarily addresses velocity estimation in areas where erosion is
expected which is normally the outer bank of channel bendways. Plate B-33 in the EM (Figure A2)
provides an estimate of the maximum velocity that will occur in a bend on the outer bank. For sites
where flow velocities are the predominate force, contaminated sediments needing protection may
be located on either the bed or bank at any position along the length of the channel. Bernard and
Schneider (1992) have developed a PC based depth-averaged numerical model that includes
secondary current effects that occur in channel bends. This model has been shown to give good
results in trapezoidal channels. This model will provide a velocity estimate at any position across
the channel and along the bend.
Normally the minimum safety factor for riprap design is 1.1; however, if the consequences of failure
are extremely hazardous, the designer should increase the safety factor accordingly. A computer
program incorporating the EM 1110-2-1601 procedures is available from the Hydraulics Laboratory
of the Waterways Experiment Station.
= 1.74-0.52 LOG (R/W)
0.8
NOTE:
4 6 8 10
R/W
V ss IS DEPTH-AVERAGED VELOCITY AT 20 PERCENT
OF SLOPE LENGTH UP FROM TOE
Figure A-2. Riprap design velocites.
A-6
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Appendix A: Armor Layer Design
Examples of Design for Flood Flows
Consider the Sheboygan River which has contaminated sites along the upper non-navigable reach.
The two-year average discharge is 3140 cfs, the five-year is 5000 cfs, and the ten-year is 6150 cfs.
For the purpose of this example design, assume design average channel velocity of 6 ft/sec, the
channel plan view in Figure 3, and design depths shown in the following table.
AREA DEPTH
1,5 9 Ft
8,10,11 6 Ft
The following analysis uses a unit stone weight of 165 #/ft3, minimum Sf = 1.1, angular rock (Cs =
0.30), blanket thickness = 1 D100 (C,. = 1.0), 1V:2H side slope (K, = 0.88) for all areas, and a
gradation having D85/D15 of 2.0.
Areas 1 and 5 in Figure 3 are on the outside of bendways where velocities are the highest. From
Figure 2, an assumed R/W = 3 gives a ratio between the outer bank velocity and the average
channel velocity of about 1.5, so the local velocity is 1.5(6) =9.0 ft/sec. Using equation (2) results
in DSQ = 0.57 ft. From Table A1, a gradation having D^niin) greater than or equal to the computed
value would have a D100(max) of 12 in. and if placed in the dry, a thickness of 12 in.
Areas 8, 10, and 11 in Figure 3 are in a relatively straight reach of channel not strongly affected
by upstream channel curvature. In these areas the right part of the natural channel curve (Figure
2) is applicable and bank velocity/average channel velocity = 1.0. This leads to a bank velocity of
1.0(6.0) = 6.0 ft/sec and equation (2) yields a D50 =0.19 ft.
In these examples, rock from a nearby source having D50 greater than the computed D50 would
have to be specified. In practice the largest rock size required is often specified for both areas due
to economics. It is assumed that the risk to human health and the environment is greater for a
failure of a contaminated sediment cap than for a failure of a bank erosion control riprap layer.
Therefore, additional margins of safety in stone sizing may be warranted for a ISC to protect the
cap from localized very high velocities resulting.
A-7
-------
Appendix A: Armor Layer Design
AREA*!5 (R)
0-1500 ppm
TECUMSEH
PRODUCTS
COMPANY
AREA'llA)
153 ppm
AREA*9 (R)
0-450O ppm
-AREA *IO (A)
13-58O ppm
AREA *8(A)
0-35 ppm
AREA *7 (A)
No Data
AREA*5(R)
5-830 ppm
AREA*5A(A)
59 ppm
AREA*3(R)
1.4-1290 ppm
AREA #4(R
15-2250 ppm
LEGEND:
[] = ACCESS AREA
AREA (R) = SEDIMENTS FOR
REMOVAL
AREA (A) = SEDIMENTS FOR
ARMORING
* CTF = CONFINED TREATMENT FACILITY
O
Figure A-3. Sheboygan River and Harbor site.
A-8
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Appendix A: Armor Layer Design
Stone sizing for navigation effects
Navigation can generally be divided into two categories, underway and maneuvering. For large
commercial vessels underway in relatively small channels, the vessel creates a variety of erosion
producing forces that are primarily water-level drawdown, return velocity acting opposite to the
direction of travel, transverse stern waves, and a limited attack of the propeller jet. For underway
vessels, these forces tend to increase with increasing speed and with decreasing channel size.
In harbor areas, typical underway speeds tend to be low and erosion producing forces will also be
low.
The second category of navigation, maneuvering vessels, produces erosion generating forces that
are primarily caused by the propeller jet and can be large. Rock sizing guidance that follows will
address the protection requirements for the propeller jet of maneuvering vessels.
Propeller Jet Stone Sizing Equations
The basic equations used in the analysis of riprap size are presented in Blaauw and van de Kaa
(1978). The equation for the maximum bottom velocities in the propeller wash of a maneuvering
vessel is
V^max) =
where
Vb(max) = maximum bottom velocity
C, = 0.22 for non-ducted propeller
= 0.30 for ducted propeller
U0 = jet velocity exiting propeller
Dp = propeller diameter
Hp = distance from propeller shaft to channel bottom
A-9
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Appendix A: Armor Layer Design
The ratio Dp/Hp is a measure of the clearance of the propeller above the channel bottom. High
values indicate the propeller is close to the channel bottom. Values of Dp/Hp > 1.2 are outside the
range of data used in developing Equation 3 and should be used with caution.
The jet velocity exiting a propeller is given by Blaauw and van de Kaa (1978) as
(4)
U0 = C2 P"
where
U0 = jet velocity exiting propeller in ft/sec
Pd = applied engine power/propeller in Hp
Dp = Propeller diameter in ft
C2 = 9.72 for non-ducted propellers
= 7.68 for ducted propellers
The applied engine power used in equation 4 is the most difficult question to answer and one of
the most important parameters in determining stone size. Blaauw et al (1984) gives the following
equation for rock size
Vb(max) = C3*(g*A*D50)1'2 (5)
where
C3 = coefficient
A = (Ys-Yw)/Yw
Blaauw et al. (1984) found C3=0.55 for no movement and C3=0.70 for small transport. Data from
Maynord (1984) using equations 3-5 show that C3 = 0.55 provides good agreement with
experimental results for no transport and should be used in harbor areas where repeated attack
can be expected and no movement can be allowed. For channel protection where infrequent attack
can be expected, C3 = 0.6-0.7 should be used in design.
Thrusters
Bow and stern thrusters are often used in deep draft vessels to permit maneuvering in navigation
channels. Thrusters are ducted propellers and, depending on the position of the vessel relative to
the bank, the maximum attack may be on either the channel bottom or channel bank. Due to the
uncertainty of the location of maximum attack, the general equation from which equation 3 was
derived must be used to determine velocity along the bed and bank. The general form of equation
3 from Blaauw and van de Kaa(1978) provides the distribution of jet velocity and is
A-10
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Appendix A: Armor Layer Design
D
-Ji=2.78-?exp[-15.43(±)2l
(6)
U
where Vx = velocity at coordinates x,z
D0 = 0.71 Dp for non-ducted propeller
= Dp for ducted propeller
x = horizontal distance from propeller
z = radial distance from axis of propeller
Thrusters generally operate at full power and a typical class 8 lake vessel has a bow thruster which
is 6.8 ft in diameter and 850 hp. Typical stern thrusters are the same diameter and 1000 hp.
Thruster centerlines are about 6.2 ft above the keel. Riprap sizing for thrusters would use equation
6 and solve for Vx at various point along the bottom and up the bank until the maximum Vx is found.
This maximum Vx will be the Vb(max) to use in equation 5.
Example designs for navigation
Two examples are presented in this subsection, one based on commercial vessel traffic and
another on recreational vessel traffic. On the Ashtabula River in Ohio, the possible areas for
capping are located in the Federal Navigation Channel where depths in this area vary from 2 to 16
ft. Small recreational craft normally use this reach with an infrequent commercial vessel. Contacts
with the U.S. Coast Guard led to the following findings regarding the largest commercial vessels
using this reach:
Table A2. Largest Commercial Vessels on the Ashtabula River
LENGTH
FT
42
72
59
WIDTH
FT
12.5
22.5
14.0
PROPELLER
DIA-INCHES
INCHES
40
60
60
DRAFT
FT
6
9.4
8
SHAFT
BELOW W.S.
FT
4.5
6.5
6.0
HP
300
1100
680
Using the 1100 hp vessel at 1/4 throttle which is typical of this vessel, the applied engine power is
Pd = 1100(0.25) = 275 hp, and with the propeller diameter Dp = 5 ft, equation (4) results in U0 =
21.6 ft/sec for a non-ducted propeller. With a 16 ft depth, Hp = 9.5 ft and from Equation (3), Vb(max)
= 0.22(21.6)5/9.5 = 2.50 ft/sec. From Equation (5) with C3 = 0.60, D50 = 0.33 ft. A blanket
thickness of 9 in. from Table A1 has a D50(min) greater than or equal to 0.33 ft.
A-11
-------
Appendix A: Armor Layer Design
12
14 16
CHANNEL DEPTH, FT
20
Figure A-4. Influence of channel depth on stone size,
Astabula River, 1100 hp vessel, 25 percent power.
Two of the significant variables in the propeller jet stone sizing equations are the channel depth and
the applied power. Figure 4 demonstrates the change in rock size D50 for changing channel depth
with all other parameters as above for the 1100 hp vessel on the Ashtabula River. Rock size
becomes large as the propeller approaches the bottom. Figure 5 demonstrates the change in rock
size for changing percent of total power applied for a depth of 13 ft and all other parameters as
above. Rock size becomes large for significant power increases.
In the second example, the largest vessels in a contaminated reach adjacent to a towing basin are
300 HP recreational craft with maximum draft of 3.5 ft. These vessels are twin propeller boats with
maximum propeller diameter of 1.44 ft with the centerline of the shaft 2 ft below the water level.
The maximum throttle is about 25 percent. Water depth varies from 4-11 ft. Based upon the basic
equations 3-5 and a water depth of 5 ft, the jet velocity for the maximum vessels would be based
on 150 hp per propeller. The applied power is Pd = 0.25(150) = 37.5 hp. From Equation (4),U0 =
25.5 ft/sec. For a 5 ft depth, Hp = 3 ft. From Equation (3), Vb(max) = 2.7 ft/sec. From Equation
(5), DM = 0.38 ft and a blanket thickness of 9" from Table A1 (EM1110-2-1601) provides D50(min)
greater than or equal to 0.38 ft. If depth were 10 ft., Hp = 10-2 = 8 ft. From equation (3), Vb(max)
= 1.0 ft/sec and Equation (5) gives D50 = 0.053 ft which would be equivalent to a large gravel
covering.
A-12
-------
Appendix A: Armor Layer Design
2.0-
1.5-
UJ 1.0
N
CO
Q.
2
Q_
tr
0.5-
0.0
0 10 20 30 40 50 60 70 80 90 100
APPLIED POWER, PERCENT OF TOTAL
Figure A-5. Influence of applied power on sstone size,
Ashtabula river, 1100 HP vessel, 13 ft depth.
A-13
-------
Appendix A: Armor Layer Design
Stone sizing for wave induced currents
Significant wind wave activity can create large bottom velocities that can erode an unprotected
sand cap. To define the required armor layer size to prevent scour, Equation 5 should be used with
with the maximum horizontal bottom velocity from the wave. For orbital velocities beneath waves,
a C3 = 1.7 is recommended.
Example Design for wave induced currents
Wave induced bottom velocities are calculated to be 7 fps for the design wave. Using equation 5
with C3 = 1.7 results in D50 = 3.8" for unit stone weight of 165 Ib/cf. A maximum/minimum stone
size of about 2 is recommended to reduce attack of underlying layers and the resulting stone
gradation is 2.5" to 5.0".
References
Bernard, R.S., and Schneider, M.L 1992. "Depth-Averaged Numerical Modelling For Curved
Channels," Technical Report HL-92-9, US Army Engineer Waterways Experiment Station,
Vicksburg, MS.
Blaauw, H. G., and van de Kaa, E. J. 1978. "Erosion of Bottom and Banks Caused by the Screw
Race of Maneuvering Ships," Publication No. 202, Delft Hydraulics Laboratory, Delft, The
Netherlands, presented at the Seventh International Harbor Congress, Antwerp, May 22-26.
Blaauw, H. G., van der Knaap, F. C. M., de Groot, M. T., and Pilarcyk, K. W. 1984 "Design of
Bank Protection of Inland Navigation Fairways," Publication No. 320, Delft Hydraulics Laboratory.
Headquarters, US Army Corps of Engineers. 1982. "Earth and Rockfill Dams," EM 1110-2-2300,
US Government Printing Office, Washington, DC.
Headquarters, US Army Corps of Engineers. 1986. "Seepage Analysis and Control for Dams," EM
1110-2-1901, US Government Printing Office, Washington, DC.
Headquarters, US Army Corps of Engineers. 1994. "Hydraulic Design of Flood Control Channels,"
EM 1110-2-1601, US Government Printing Office, Washington, DC.
Maynord, S. T. 1984. "Riprap Protection on Navigable Waterways, "U.S. Army Engineer
Waterways Experiment Station, Technical Report HL-84-3.
PIANC, 1987. "Guidelines for the Design and Construction of Flexible Revetments Incorporating
Geotextiles for Inland Waterways," supplement to Bulletin No 57, Permanent International
Association of Navigation Congresses.
Pilarczyk, K.W. 1984. "Filters," in Chapter 2, The Closure of Tidal Basins, edited by W. van Aalst,
Delft University Press, Delft, The Netherlands.
-------
Appendix A: Armor Layer Design
Worman, A. (1989). "Riprap protection without filter layers," J. of Hydraulic Engineering, ASCE,
115,no. 12, Dec.
Manamperi, H.D.S. (1952). "Tests of graded riprap for protection of erosible material," Thesis
submitted to Univ of Iowa.
A-15
-------
Guidance for In-Situ
Subaqueous Capping
of Contaminated Sediments:
Appendix B: Model for Chemical
Containment by a Cap
by
Danny D. Reible
Louisiana State University
Baton Rouge, Louisiana
Prepared for U.S. Environmental Protection Agency
Great Lakes National Program Office
Assessment and Remediation of Contaminated Sediment Program
Chicago, Illinois 60604
Monitored by U.S. Army Engineer Division, North Central
Chicago, Illinois 60605-1592
-------
Appendix B: Model for Chemical Containment by a Cap
Introduction
This Appendix describes a model for evaluation of chemical flux through a cap. Through use of
this model the effectiveness of chemical containment of a cap can be assessed. This model should
be applied once cap design objectives with respect to flux are determined, a specific capping
material has been selected and characterized, and a minimum cap thickness has been determined
based on components for isolation, bioturbation, erosion, consolidation, and operational
considerations. If an objective of the cap is attainment of a given contaminant flux, the model can
be used to estimate the required cap thickness.
The effective thickness, L^ , of a cap can be defined as the thickness available for long term
chemical containment. This thickness is reduced by consolidation of the cap, AL^, the thickness
affected by short term pore water migration due to consolidation in the underlying sediment, AL.ed,
and by bioturbation over a thickness, Lblo. Bioturbation, the normal life-cycle activities of benthic
organisms, leads to mixing and redistribution of contaminants and sediments in the upper layer.
The chemical migration rate within the bioturbated zone is typically much faster than in other
portions of a cap. In addition, consolidation typically occurs on a time scale that is rapid compared
to the design lifetime of a cap. Consolidation of the cap directly reduces the thickness of a cap and
the separation between contaminants and the overlying water and benthic organisms while
consolidation of the underlying sediment results in the expression of potentially contaminated
porewater. Note, however, that in addition to reducing the thickness of a cap, consolidation serves
to reduce both the porosity and permeability of a cap causing reductions in chemical migration
rates by both advection and diffusion.
Using AL^ to represent the thickness of a cap affected by a contaminant A during consolidation
of the underlying sediment, the effective cap thickness remaining for chemical containment is given
by
(1)
where LQ is the initial thickness of the cap immediately after placement.
The depth of bioturbation can be assessed through an evaluation of the capping material and
recognition of the type, size and density of organisms expected to populate this material. Because
of the uncertainty in this evaluation, the bioturbed zone is generally chosen conservatively, that is
considered to be as large as the deepest penetrating organism likely to be present in significant
numbers. Due to the action of bioturbating organisms, this layer is also generally assumed to pose
no resistance to mass transfer between the contaminated sediment layer and the overlying water.
The consolidation of the underlying contaminated sediment can be estimated through consolidation
models. The resulting movement of the chemical contaminants must be estimated, however, and
a model is described below. The effective cap thickness estimated by Equation (1) is still subject
to chemical migration by advection and diffusion processes. The long term chemical flux to the
water via these processes can also be modeled.
The complete model of chemical movement must be composed of two components:
B-1
-------
Appendix B: Model for Chemical Containment by a Cap
•An advective component considering the short term consolidation of the contaminated
sediment underlying the cap, and,
•A diffusive or advective-dispersive component considering contaminant movement as a
result of porewater movement after the cap has stabilized.
The first component is operative for all caps in which the underlying contaminated sediment layer
is compressible but only for a short period of time. The first component allows completion of the
determination of the effective cap thickness through Equation (1). The resulting effective cap
thickness can then be used to assess long term losses through the cap by advective and/or
diffusive processes. For simplicity and conservatism, the sediment underlying a cap could be
assumed to remain uniformly contaminated at the concentration levels prior to cap placement. In
reality, migration of contaminants into the cap reduce the sediment concentration and the long term
flux to the overlying water. The consideration of this situation, however, greatly complicates the
analysis and the models used to describe contaminant flux. Both of the model components will be
considered separately. Due to the different mechanisms operative in a system with porewater
motion present or absent, the second model component will be subdivided into submodels
appropriate for each.
Model for Short Term Cap Losses - Advection during Cap
Consolidation
After placement of capping materials, consolidation of both the cap and the underlying sediment
occurs. Consolidation of the cap results in no contaminant release since the cap is initially free of
contamination. Furthermore, the consolidation of the cap serves to reduce the permeability and,
to a lesser extent, the porosity of a cap. Both serve to reduce contaminant migration through the
cap by both diffusive and advective processes.
Consolidation of the underlying sediment due to the weight of the capping material, however, tends
to result in expression of porewater and the contaminants associated with that water. The ultimate
amount of consolidation may be estimated using standard methods or computer models. The
consolidation of the underlying sediment is likely to occur over a very short period (e.g. months)
compared to the lifetime of the cap. It is appropriate, therefore, to assume that the consolidation
occurs essentially instantaneously and estimate the resulting contaminant migration solely on the
basis of the total depth of consolidation and the porewater expressed. For a nonsorbing
contaminant, the penetration depth of the chemical is identical to that of the expressed porewater.
For a sorbing contaminant, the penetration depth is less as a result of the accumulation of
chemical on the sediment. Mathematically, if AL.ed represents the ultimate depth of consolidation
of the underlying contaminated sediment due to cap placement, the depth of cap affected by this
porewater (or nonsorbing contaminant), AL^edpw, is given by
where e is the porosity of the cap materials. The division by the cap porosity recognizes that the
expressed porewater moves only through the void volume formed by the spaces between the
grains of the capping material. Equation (2) assumes that the capping material is spatially uniform
and that porewater is not preferentially forced through an area a fraction of the total cap area.
B-2
-------
Appendix B: Model tor Chemical Containment by a Cap
Although the depth of cap affected by the expressed porewater is given by Equation (2), the
migration distance of a sorbing contaminant is less due to accumulation in the cap. The quantity
of contaminant that can be rapidly adsorbed by the cap material, coc (mg/kg), is generally assumed
to be proportional to the concentration in the porewater (Cpw, mg/L),
«c = <%„ (3)
where the constant of proportionality is the observed sediment-water partition coefficient. Note that
the observed partition coefficient is measured during sorption onto clean cap material. The value
of Kj0"8 may be predicted or measured as described in a subsequent section. Use of a measured
value, however, does not require linearity or reversibility of the sorption isotherm, nor does it require
specification of the form of the contaminant in the porewater (e.g. dissolved or bound to particles).
For a compound that sorbs to soil with an observed partition coefficient of K^"5 (U/kg), the ratio of
the total concentration in the soil to that in the porewater is given by the retardation factor, Rf,
R, = e + 9bK?s (4)
The distance that the contaminant migrates during underlying sediment consolidation of a distance
is then given by
This distance must be subtracted from the actual cap thickness to estimate effective cap thickness.
Note that this model suggests that the more sorbing a cap, the less important is consolidation in
the underlying sediment. Sorption for hydrophobic organics such as polyaromatic hydrocarbons
and polychlorinated biphenyls is strongly correlated with the organic carbon content of the
sediments. If a cap contains 0.5% organic carbon or more, the K^"8 is typically of the order of
hundreds or thousands for these compounds and the loss of effective cap thickness by
consolidation is a small fraction of the consolidation distance. Metals also tend to be strongly
associated with the solid fraction, again reducing the migration of contaminant out of the sediment
as a result of consolidation.
B-3
-------
Appendix B: Model for Chemical Containment by a Cap
Estimation of Long-Term Losses
Mechanisms and Driving Force
The effective cap thickness defined by Equation (1) is subject to advection or diffusion or a
combination of both throughout the lifetime of the cap. The long term contaminant release or loss
requires estimation of the contaminant flux by these processes. Diffusion is always present while
advection only occurs if there exists a significant hydraulic gradient in the underlying sediments.
The relative magnitude of diffusion to advection in the cap of effective thickness, L eff, can be
estimated by the Peclet number.
ULM
"•^
where U is the advective velocity (Darcy or superficial velocity) in the sediment and Deff is the
effective diffusion/dispersion coefficient. If the magnitude or absolute value of the Peclet number
is much greater than one, advection dominates over diffusion/dispersion while the opposite is true
for absolute values much less than one. Advection directed out of the cap will speed contaminant
release while advection directed into the sediment will effectively lengthen the cap.
The average groundwater flow velocity is estimated from the sediment conductivity (K, cm/sec) or
permeability (k, cm2) and the local hydraulic gradient.
Here, p is the density of water (~1 gm/cm3), g is the acceleration of gravity (980 cm-sec"2) and \i
is the viscosity of water (-0.01 govern"1 -sec"1). — is the local gradient in hydraulic head or
elevation with distance into the sediment. The average groundwater flow is also the volumetric
seepage rate divided by the sediment-water interfacial area. Thus lakes, with large sediment-water
interfacial areas tend to exhibit less potential for advective influences than small streams.
Estuarine systems subject to significant tidal fluctuations may also exhibit significant advective
transport. Losing streams, in which the advective transport is into the sediment may exhibit
advection but may not be important since the direction of transport is away from the sediment-water
interface and long travel distances may be required to impact groundwater of significance.
Similarly, advection may be less important in wetlands subject to frequent cycles of flooding
followed by infiltration due to the downward vector of advection. The presence of a cap will tend
to reduce any advective transport by preferentially channeling flow to uncapped sediment. The
permeability of the cap materials may also be selected to minimize advection.
The effect of advection includes both transport by the porewater flow and that by diffusion and
dispersion. Dispersion is the additional "diffusion-like" mixing relative to the average porewater
velocity that occurs as a result of heterogeneities in the sediments. Thus the description of
advection is more complicated than diffusion and the model for long term cap losses will be
subdivided into models appropriate when advection is important and a model appropriate only when
diffusion dominates.
_
-------
Appendix B: Model lor Chemical Containment by a Cap
Both processes, however, are operative only for that portion of the contaminant present in the
porewater. This might include contaminant dissolved in the porewater as well as contaminant
sorbed to fine particulate or colloidal matter suspended in the porewater. The pore-water
concentration in the underlying sediment, assuming linear partitioning between the sediment and
porewater, is given by
c
pw
. * SCO ~ *
if < C
y »1 (8)
"-,
where C? is the equilibrium solubility of the chemical in water and o>sed is the sediment loading (mg
chemical/ kg (dry) sediment). The Equation indicates that the porewater concentration increases
linearly with the sediment loading until the water is saturated, that is, until the solubility limit is
reached. Loading above that critical value cannot increase the sediment porewater concentration
or the driving force for diffusion. The porewater concentration can exceed this value, however,
if colloidal organic matter, typically measured by dissolved organic carbon, is present in large
quantities in the porewater. Sorption onto this colloidal matter can increase the total fraction of
contaminant present in the porewater. If the partitioning to the organic colloidal matter is assumed
to be given by K^ the organic carbon to water partition coefficient, and if poc represents the
colloidal organic carbon concentration, then the porewater concentration calculated above must
be corrected by the factor (1 + K^c). This approximately accounts for the enhanced chemical
solubility due to the presence of sorbing colloids. A similar correction for metal species could be
adopted, however, it is difficult to predict the partitioning of metals to soils and colloidal particles.
Degradation of contaminants over the long time of expected confinement is a significant benefit
of capping which should be incorporated into the design of a cap. If simple first order degradation
kinetics is employed the sediment loading changes with time according to
° ~kt y«w
(*>sed = wsecf (9)
where w°sec is the sediment loading at the time of cap placement and /rp the exponential time
constant is given by 0.693/t05, with tos the chemical half life in the sediment.
In the subsequent sections, the sediment porewater concentration estimated by Equation (5) is
used to evaluate diffusive and advective-dispersive transport.
Diffusion
Diffusion is a process that occurs at significant rates only within the pores of the sediment and is
driven by the difference in porewater concentration between the sediment and the cap. The initial
concentration of the contaminant in the cap porewater is generally 0 while the concentration in the
sediment is given by Equation(S), modified if appropriate by Equation (9). Even without
degradation, however, migration of contaminants into the cap will deplete the underlying sediments
as a result of the loss of mass by diffusion through the cap.
B-5
-------
Appendix B: Model lor Chemical Containment by a Cap
Thoma et al. (1993) developed a model of diffusion through a cap that explicitly accounts for
depletion in the underlying sediment. A simpler model of diffusion through the cap, however,
assumes that the contaminant concentration in the underlying sediment is essentially constant.
This would be most appropriate if the contaminant concentration in the sediment far exceeds the
critical concentration defined by Equation (8). Because the assumption of no depletion in the
underlying sediment overpredicts the driving force for diffusion, however, it also represents a
conservative assumption of the effectiveness of the cap. We will therefore employ it in the
description that follows.
Let us first estimate the steady long term flux of contaminants through the cap via diffusion. This
is the maximum flux that can occur through the cap by the diffusive mechanism.
Maximum Flux Estimation (Steady State) If diffusion is the only operative transport process
through the cap, the pseudo steady-state flux through the cap (assuming constant contaminated
sediment porewater concentration and no sorption effects in the cap layer) is given by
cpW*Kc,pCpW (10)
where F = chemical flux (ng-cm-2-sec~1)
Dw = the binary diffusivity of the chemical in water, (cm2/sec)
e = the sediment porosity (void volume/ total volume),
i^H = effective cap thickness
cpw ~ pore-water concentration (ng/cm3)
Kcap = effective mass transfer coefficient through cap (cm/sec)
Millington and Quirk (1961) suggest the factor e4'3 to correct for the reduced area and tortuous path
of diffusion in porous media. The overlying water concentration is assumed very much less than
the sediment porewater concentration.
In general, the chemical flux is influenced by bioturbation and a variety of water column processes.
Figure 1 shows the idealized concentration profile in a capped system at this pseudo steady state.
The flux of chemical through each layer is equal to the sum of the rate of evaporation and flushing.
Mathematically, in terms of mass transfer coefficients, we have:
M = KoMAs Cpw =
(11)
where
M = rate of chemical loss from the system (mg/day) = F*AS
Kov = overall mass transfer coefficient (cm/day)
As = contaminated sediment area (m2)
Ae = evaporative surface area (m2)
£,..„ = cap mass transfer coefficient = Dw e4/3 /L^ (cm/day)
cap
Cpw = porewater concentration within the contaminated sediment
Including dissolved and any sorbed to colloidal material
B-6
-------
Appendix B: Model for Chemical Containment by a Cap
cbio = porewater concentration at the top of the cap (ng/cm )
csw = porewater concentration at the sediment water interface (ng/cm3)
nDM Rf
Kbia = bioturbation mass transfer coefficient = —5^— (cm/day)
/7 = desorption efficiency of contaminant from sediment particles (0.1-0.2)
D = biodiffusion coefficient (cm2/day)
Rf = retardation factor = e
L^ = depth of bioturbation (cm)
KX = benthic boundary layer mass transfer coefficient (cm/day)
Kg = evaporation mass transfer coefficient (cm/day)
De = effective diffusivity = DW • e*3 (cm3/day)
Q = basin flushing rate (cm3/day).
cw = chemical concentration in the basin water (ng/cm3).
Kd = sediment water partition coefficient for the chemical = K^foe (cm3/g)
Koc = organic carbon-water coefficient for the chemical (cm3/g)
foc = sediment fractional organic carbon content.
ps = sediment bulk density.
The overall mass transfer coefficient, KOV, can be obtained from the following:
f • F- * j- * r * zr^ <12>
ov cop Wo bf CF» •—
An analysis of this relationship for reasonable values of L^ suggests that 1/KOV = 1/Kcap and
therefore the cap controls the flux to the overlying water and Equation ((10)) is valid.
This flux can be used to estimate water concentrations in the water (Cw) or at the sediment water
interface (Csw) or multiplied by the capped area to determine total release rate. For hydrophobic
organics, the concentration in the overlying water at steady-state is defined by a balance between
the flux through the cap, the rate of evaporation to the air and the rate of flushing of the water
column. For metals and elemental species not associated with volatile compounds, the flux
through the cap is balanced only with the flushing of the water column. The overlying water
concentration of the contaminant is given by:
(13)
The concentration at the sediment-water interface, which would be indicative of the level of
exposure of bottom surface dwelling organisms, is defined by the balance of the flux through the
cap with the flux through the benthic boundary layer. The contaminant concentration at the
sediment-water interface is:
B-7
-------
Appendix B: Model for Chemical Containment by a Cap
K
'Sw
+ c..
(14)
Either of these concentrations or the estimated fluxes may be compared to applicable criteria for
the chemical in question to determine if a specified cap thickness is adequate. A sample
calculation is presented below.
Transient Diffusion - Breakthrough time estimation The simple steady state analysis we have
presented above is not capable of predicting the time required for the contaminants) to migrate
through the cap layer. Until sorption and migration in the cap is complete, the flux to the water
column will be less than predicted by Equation (10). Time must be explicitly incorporated in the
differential mass balance to address this problem. The following partial differential equation
represents a differential mass balance on the contaminant in the pore-water of the cap as it
diffuses from the contaminated sediment below.
dc
pw
dt
-£r
*3 °^pw
(15)
We apply the conditions of a constant concentration at the sediment-cap interface as specified by
Equation (8) and effectively zero concentration at the height L^ in the cap. Carslaw and Jaeger
(1959) present a solution to the equivalent heat transfer problem which in terms of concentration
and mass diffusion is given by:
off
ant
(16)
where Deff represents Dwe4/3. Note that as /-»«> the expontential term in square brackets
approaches zero and the flux approaches the value obtained by the approximationKOV ~ Dgff/L^n
as indicated by Equation (10). From Equation (16) we can obtain relations for the breakthrough
time and the time required to approach the steady state flux.
We define breakthrough time, Tb, as the time at which the flux of contaminant from the
contaminated sediment layer has reached 5% of its steady state value, and we define the time to
reach steady state, T^, as the time when the flux is 95% of its steady state value. It is easily shown
that
0-541^1,
(17)
B-8
-------
Appendix B: Model for Chemical Containment by a Cap
and
Advective-Dispersive Models
When advection cannot be neglected during the operation of a cap, the basic equation governing
contaminant movement is
dcow dcm d2cow
- —-= —
where Cpw is the contaminant concentration in the porewater. U is again the Darcy velocity and
Deff is the effective diffusion/dispersion coefficient. The effective diffusion/dispersion coefficient is
often modeled by a relationship of the form
Deft=Dwen + aU (20)
The first term in this relation is associated with molecular diffusion and is identical to the effective
diffusivity used above.
The second term is mechanical dispersion associated with the additional mixing due to flow
variations and channeling, a is the dispersivity and is typically taken to be related to the sediment
grain size (uniform sandy sediments) or travel distance (heterogeneous sediments). Very little
guidance exists for the estimation of field dispersivities for vertical flow in sediments. In uniform
sandy sediments, the dispersivity is approximately one-half the grain diameter. Dispersion in
heterogeneous sediments would be expected to be larger.
If the effective dispersivity can be estimated, the contaminant concentration and flux through the
cap can be estimated by solutions to Equation (19). Let us first consider the long time behavior
of Equation (1 9) when the sediment originally exhibits a contaminant porewater concentration C0.
If the contaminant is not subject to depletion by either degradation or migration through the cap,
the flux through the cap, at infinitely long time periods, ultimately reaches that given by
u c
o "s ^°° (21)
That is, once the adsorbing capacity of the cap is exhausted, the contaminant flux due to advection
is identical to that which would be observed if no cap were placed over the sediment. Recognize
that any sorption in the cap must deplete the reservoir of contaminants in the contaminated layer.
The assumption of no depletion is therfore very conservative.
—
-------
Appendix B: Model for Chemical Containment by a Cap
In the advection dominated case, therefore, it is important to examine the transient release of the
contaminant. The conditions on Equation (10) that are appropriate for a cap include
cap-sed
cap
(22)
Available solutions, however, do not satisfy the cap-water interface condition. Instead there are
two solutions that are commonly applied.
dC
= o
at
as
= L
(finite cap)
(infinitt cap)
(23)
The first explicity recognizes the finite thickness of the cap while the second assumes that it is
infinitely thick. For Pe>1, however, the solution to Equation (10) subject to either condition is
essentially identical. Moreover, for Pe<1 when diffusion dominates, the finite cap condition is
inappropriate and causes the solution to underpredict the contaminant flux through the cap. The
solution for the infinite cap is also simpler to use. For these reasons, only the infinite cap thickness
model will be described here.
The solution to Equation(19) subject to the infinite cap condition is given by
erfc
2JRf Dt
exp
(f)
trft\
Ut
(24)
Here erfc represents the complementary error function which is given by 1- erf, the error function.
The error function is a tabulated function (e.g.,Thibodeaux, 1979) and is commonly available in
spreadsheets and computer languages. It ranges from 0 at a value of the argument equal to zero
to 1 at a value of the argument equal to infinity. The model is most useful in predicting the
penetration of the contaminant into the cap and the time until the sediment-water interface begins
to be significantly influenced by the cap, the breakthrough time. The breakthrough time can be
estimated by evaluating Equation (24) for z=l_eft and determining the time required until Cpw(Leff,t)
is equal to some fixed fraction of the concentration in the underlying sediment, for example until
Cpw(Leff,t)=0.05 C0. The flux at any time could also be evaluated by computing
= u
dc
eff
dt
(25)
The equation for the flux is lengthy, however, and, as indicated earlier, Equation(24) is most useful
to calculate the breakthrough time or the concentration profile within the cap at any given time.
B-10
-------
Appendix B: Model for Chemical Containment by a Cap
Parameter Estimation
Use of any of the equations presented above requires estimation of a variety of model parameters.
The most important of these parameters and an example calcuation are presented below.
These include the porosity(e), bulk density(pb) and organic carbon content (foc) of the cap material,
the partition coefficient^) for the chemical(s) between the pore-water and the cap material, the
diffusivity of the chemical(s) in water(Dw), the depth of bioturbation(b) and a biodiffusion
coefficient(Dfc/0), benthic boundary layer(/ffcw) and evaporation (Ke) mass transfer coefficients, and
for flowing systems the water depth(H) and current velocity^. Information should be obtained on
the degradadion half-life or reaction rate of chemicals of concern in the specific project if such
information is available.
obs
Contaminant properties These include water diffusivity and sediment-water or cap-water partition
coefficient. The water diffusivity of most compounds varies less than a factor of two from 1x10"5
cm2/sec. Higher molecular weight compounds such as PAH's tend to have a water diffusivity of
the order of 5x10^ cm2/sec. Estimation techniques can be found in Lyman et al. (1990). The
preferred means of determining the partition coefficient is through experimental measurement of
sediment and porewater concentration in the sediment or cap. In this manner, any sorption of
contaminant onto suspended particulate or colloidal matter is implicitly incorporated. If such
measurements are unavailable, it is possible to predict values of the partition coefficient, at least
for hydrophobic organic compounds. For other contaminants, including metals, very little predictive
guidance exists. For hydrophobic organics, the partition coefficient between the pore-water and
sediment for a given chemical can be estimated from the organic carbon-water partition coefficient
through the relation Kd = f^K^. K^. values are tabulated (e.g. Montgomery and Welkom, 1990) or
may be estimated from solubility or the octanol-water partition coefficient using the methods in
Lyman et al. (1990). If colloidal material in the porewater influences the partition coefficient, an
apparent or effective partition coefficient can be estimated from the dissolved organic carbon
concentration, p^, in the porewater and the relationThe porewater concentration to be used in this
case is then not the truly dissolved concentration but that corrected for the amount sorbed on the
colloidal matter. This is the same correction for the presence of colloidal matter referred to in the
discussion of Equation (8).
Physical characteristics The long term average current velocity and water depth should be
evaluated for the site to determine water side mass transfer resistances. Cap material properties
are dependent on the specific materials available and should be measured using standard
analytical methods. The water diffusivity can be estimated using the Wilke-Chang method (Bird
et al., 1960). Compilations of diffusivities are also available (Thibodeaux, 1979; Montgomery and
Welkom, 1990).
Mass transfer coefficients A turbulent mass transfer correlation (Thibodeaux, 1979) can be used
to estimate the value ofx^ in the water above the cap:
Sb = 0.036 Re aa Se 1/3 (27)
B-11
-------
Appendix B: Model for Chemical Containment by a Cap
where sb = Sherwood number = —-—
Reynolds number =
*>„
x • u
Sc = Schmidt number = —
*>w
v = kinematic viscosity of water, (0.01 cm2/sec at 20°C)
u = benthic boundary layer water velocity (cm/s)
x = length scale for the contaminated region - here we take
x = JAS (cm), where A,, is the surface area of the contaminated
region
As indicated previously, however, the benthic boundary layer mass transfer coefficient is rarely
significant in the estimation of contaminant flux through the cap.
Transport by bioturbation has often been quantified by an effective diffusion coefficient based on
particle reworking rates. A bioturbation mass transfer coefficient can then be estimated from the
following relation assuming linear partitioning between the sediment and water in the bioturbation
layer
(28)
where n is a desorption efficiency of the chemical once the particle carrying it has been reworked
to the sediment-water interface, n would tend to be small for more hydrophobic compounds
thattend to desorb slowly at the surface and large for compounds that are more soluble. In the
absence of experimental information to the contrary, n is assumed to be 1. The biodiffusion
coefficient and the depth of bioturbation are important factors in the determination of the required
cap thickness, and thus the best possible estimates should be used. The ranges for Dbio and Lbio
are quite large, and an extensive tabulation is presented by Matisoff (1982). An examination of this
data suggests that a depth of bioturbation of 2-10 cm is typical and that biodiffusion coefficients
are generally in the range of 0.3-30 cm2/yr. As indicated previously, however, the contaminant flux
is controlled by transport through the cap and is essentially insensitive to the bioturbation mass
transfer coefficient.
Evaporation mass transfer coefficient Evaporation from natural, unagitated surfaces is normally
water side controlled for sparingly soluble compounds such as those of interest in this discussion.
We will take the overall evaporation mass transfer coefficient as equal to the water-side mass
transfer coefficient. A water-side mass transfer coefficient for evaporative losses is given by Lunny
(1983) as
K= 19.6 u^D**
where Ux is the wind speed at 10m (miles/hr), DJias units of cm2/sec, and Ke has units of cm/hr.
B-12
-------
Appendix B: Model lor Chemical Containment by a Cap
Cap technical design Several design criteria are possible for specifying the physico-chemical
containment afforded by a cap. There are at least five quantities which may be of interest to the
cap designer and for which models were presented here. These are the breakthrough time, the
pollutant release rate (as an source term input to other fate and effects models), concentrations
at the sediment-water interface or in the overlying water column and the time to approach steady
state. The two physico-chemical properties of the cap material which have the largest effect on the
efficacy of the cap are the organic carbon content and the cap thickness. We will illustrate the
design procedure for choosing the proper cap thickness and estimating the breakthrough time in
the following example.
Example calculation of cap thickness Table 1 presents parameter values used for estimating
polychlorinated biphenyl release from New Bedford Harbor sediments (Thibodeaux and Bosworth,
1990).
Table 1 . Physico-Chemical Properties of Site Parameters
Cap Properties
Organic carbon content
Porosity
Bulk density
Colloid concentration
Effective cap thickness
(U
(e)
(Ph)
(O)
(U)
0.005
0.25
2.0 q/cm3
20 mg/L
35cm
Aroclor 1 242 Properties
Solubility (salt water)
Diffusivity in Water
Organic Carbon Partition Coeff.
Evaporative Mass Transfer Coeff.
(s)
(DJ
(KJ
(KJ
88 A/g/L
4.5x10~6cm2/sec
1 98000 L/kg
7 cm/hr (Thibodeaux and Bosworth,
1990)
Site Properties
Bioturbation Depth
Biodiffusion Coefficient
A1242 sediment loading
Extent of contamination
Evaporative mass transfer area
Benthic Boundary Layer Velocity
Basin flushing rate
Water Quality Criterion
(UJ
(Dhm)
((0.)
AJ
A.
(u)
(Q)
(Cwon)
10cm
1 0 cm2/yr
500 mg/kg
10000m2
10000m2
1 0 cm/sec
1.7x10'3cm3/day
30 nq/L
B-13
-------
Appendix B: Model for Chemical Containment by a Cap
Water
Benthic BL
Bioturbation
zone
\
V
Cap
Concentration profile
Contaminated
material
NA=(K eAe+Q)C w
:&
0
—
NA=K
NA=K
ft
NA=K cap(Cscd-Cbio)
ovCscd
Figure B-1. Idealized contaminant concentration in a cap and sediment profile
and flux relationships.
B-14
-------
Appendix B: Model for Chemical Containment by a Cap
References
Bird R.B., Stewart, W.E. and Lightfoot, E.N. (1960) Transport Phenomena John Wiley & Sons,
New York
Carslaw, H. S., and Jaeger, J. C. (1959) Conduction of Heat in Solids. Second Edition. Oxford
University Press: Oxford, England.
Lyman, W. J., Reehl, W. F., and Rosenblatt, D. H. (1990) Handbook of Chemical Property
Estimation Methods: Environmental Behavior of Organic Chemicals. American Chemical
Society: Washington DC.
Matisoff, G. (Mathematical Models of Bioturbation," Animal-Sediment Relations, P.L. McCall
and M.J. Tevesz, Ed. Plenum Press, New York, pp. 289-331.
Millington, R.J. and J.M. Quirk. 1961. Permeability of Porous Solids, Trans, of Faraday Soc.,
57, 1200-1207.
Montgomery, J. H., and Welkom, L M. (1990) Groundwater Chemicals Desk Reference. ,
Vol 1. Lewis Publishers, Inc., Chelsa, Ml.
Stark, T.D. 1991. Program Documentation and User's Guide: PCDDF89, Primary Consolidation
and Dessication of Dredged Fill, Instruction Report D-91-1, US Army Engineers Waterways
Experiment Station, Vicksburg, MS.
Thibodeaux, L. J. (1979) Chemodynamics: Environmental Movement of Chemicals in Air. Water
and Soil. Wiley & Sons: New York.
Thibodeaux, L. J., and Bosworth, W. S. (1990) A Theoretical Evaluation of the Effectiveness of
Capping PCB Contaminated New Bedford Harbor Bed Sediment, Final Report Baton Rouge,
LA: Hazardous Waste Research Center, Louisiana State University.
Thoma, G.J., D.D. Reible, K.T. Valsaraj and LJ. Thibodeaux. 1993 "Efficiency of Capping
Contaminated Sediments in Situ: 2. Mathematics of Diffusion-Adsorption in the Capping Layer,
Environmental Science and Technology, 27, 12, 2412-2419.
B-15
-------
Guidance for In-Situ
Subaqueous Capping
of Contaminated Sediments:
Appendix C: Case Studies on
Geotechnical Aspects of In-Situ Sand
Capping
by
Hoe Peter Ling
Dov Leshchinsky
Department of Civil Engineering
University of Delaware
Newark, DE 19716
Prepared for U.S. Environmental Protection Agency
Great Lakes National Program Office
Assessment and Remediation of Contaminated Sediment Program
Chicago, Illinois 60604
Monitored by U.S. Army Engineer Division, North Central
Chicago, Illinois 60605-1592
-------
Appendix C' Case Studies on GeotechnicalAspects ofln-Situ Sand Capping
Appendix C: Case Studies on
Geotechnical Aspects of In-Situ Sand
Capping
Introduction
Industrial activities have resulted in significant deposits of contaminated sediments in some US
harbors and waterways. Remediation of these contaminated sites can be costly and technically
difficult. In-situ sand capping has been identified as a feasible and cost-effective technique for on-
site remediation. The extremely low shear strength of these sediments presents unique engineering
problems. The geotechnical aspects of successful in-situ sand capping projects conducted in the
U.S., Japan, and Norway are reviewed and compiled in this report. Geotechnical assessment of
in-situ capping technique, based on bearing capacity and slope stability analyses, is made with
reference to these projects. Usage of geosynthetic of adequate strength and hydraulic conductivity
is recommended to improve the sand cap stability in case where extremely soft sediments are
encountered. Recommendations leading to improvement of sand capping design are included.
Significant deposits of contaminated submarine sediments are found within the U.S., in and around
the Great Lakes, typically as a consequence of industrial manufacturing activities. These materials
are physically characterized by a low shear strength and high compressibility. They are easily
transferred to the water column as a result of disturbance by natural currents and maritime
activities. For example, propeller wash from the traffic and movement of powerful vessels at
shallow depths are found to be a source of significant disturbance. The Army Corps of Engineers
has been involved in developing technical guidelines related to remediation by dredging and
capping (Palermo et al., 1993).
The level-bottom capping and contained aquatic disposal are two of the most common methods
of isolating dredged contaminants in the U.S. Clean materials, such as sand, have been used to
cap the contaminated sediments. The Army Corps of Engineers, New England Division, initiated
the first sand capping project on dredged sediments in Central Long Island Sound (CLIS) Disposal
Site at Connecticut in 1979, as part of Disposal Area Monitoring System (DAMOS). This is referred
to as Stamford/New Haven Project in which contaminated sediments were dredged from Stamford
and New Haven Harbor. The Stamford sediments were deposited as two mounds, one capped with
sand (2.1-3 m thick) and the other with silt (3.9 m thick). A successful sand capping project was
also reported for the Mud Dump Site in New York Bright in 1980 (O'Connor and O'Connor, 1983).
C-1
-------
Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
The contaminated dredge material was capped with fine sediments from the Bronx River and
Westchester Creek, then followed by sand from the Ambrose Channel. The cap was 1 m thick.
A comprehensive monitoring program was conducted when the Black Rock and New Haven
Habors were dredged in 1983. Black Rock Harbor sediments were reported to be composed of
organic silt and clay that were highly contaminated with oil, grease, heavy metals and RGB's. The
dredged sediments from this site were placed in two mounds in CLIS Disposal Site and capped
with silt from New Haven Harbor and sand from the nearby channel, respectively. A Field
Verification Program (FVP) was also conducted on the uncapped sediments at the northeastern
corner of CLIS site in order to evaluate the effectiveness of capping. A monitoring program was
established for these sites and documented by SAIC (1984). Additional cases of sand capping
projects may be found in Palermo et al. (1993).
Remediation of contaminated sediments by first dredging followed by disposal and capping at a
site different from the source may not be the most economical solution. As the volume of
contaminated material increases, an appropriate disposal site becomes limited. Risk of
resuspension of contaminants into the water column increases by disturbance during dredging and
disposal. Due to the extremely low shear strength of sediments immediately after dredging, cap
placement is technically very difficult. This has led to use of different technology in Japan in which
sand caps are placed directly over contaminated sediments without involving dredging (hereafter
known as in-situ capping). The purpose of this report is to document the geotechnical aspects of
several in-situ capping projects conducted in Japan, U.S. and other countries. The report also
highlights cases in which geotextiles were used to improve stability of the sand caps placed on
extremely soft sediments. Geotechnical evaluation of sand cap and foundation stability are made
with reference to these case histories.
In-Situ Capping: Case Histories
Successful Japanese sand capping projects were conducted primarily on fishery grounds near the
Seto Inland Sea (Figure C-1). This area has poor current circulation and is affected by heavy
industrial discharges carried by several major rivers. The red and blue tides have seriously
affected the fisheries. An experimental in-situ sand capping project was conducted in 1979 by the
Port Construction Bureau of the Ministry of Transport. Since then, several other projects were
conducted (see Table 1). Figure C-1 gives the individual location of these sites. Earlier studies
related to in-situ sand capping projects tend to focus on the chemical and biochemical aspects of
the sand-sediment-water column environment. It has, however, been recognized that success of
this technique depends also on geotechnical considerations. The following is a description of a few
of the well-documented cases with insight on geotechnical properties. Later, these cases are
utilized for geotechnical evaluation.
C-2
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
HIROSHIMA
(KURE)BAY
SUONADA
Kumamoto
^MIKAWABAY
XGOKASH08AY
"TSUDABAY
URANOUCHI BAY
UWAJIMABAY
Figure C-1. Sand capping sites in Japan.
C-3
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
Table 1. Detail of Sand Capping Sites
SITE
Hiroshima Bay
(Japan)
Uranouchi
Bay
(Japan)
Suonada Bay
(Japan)
Mikawa Bay
(Japan)
Minamata
Bay1 (Japan)
Tsuda bay
(Japan)
Lake Biwa2
(Japan)
Matsushima
Bay2 (Japan)
Gokasho Bay
(Japan)
Uwajima Bay
(Japan)
Soerfjorden1
(Norway)
Eagle Harbor
(US)
YEAR
1979
1980
1979
1986
1987
1988
1991
1992
1993
1992
1993
1991
1993
DEPTH
BELOW
SEA LEVEL
(m)
21
21
13-14
6-9
1
5
2-10
10-15
10-15
10-15
1.5
3
10
17
13
CAP
THICKNESS
(cm)
50
30
15
15
20
30
50
40-100
40-100
80
50
50
50
20
30
20
20
30-60
100
100
AREA
(lO3!!!2)
19.2
44.8
1.2
10.0
7.4
0.9
15.0
9.6
4.5
324.7
212.8
114.0
91.2
24.2
19.2
106.9
46.8
100
99.1
117.4
MAIN
REFERENC
E
Horie
(1991)
P.C.B.
(1994)
P.C.B.
(1994)
P.C.B.
(1994)
Namba
(1994)
P.C.B.
(1994)
Gomyoh et
al.
(1994)
P.C.B.
(1994)
P.C.B.
(1994)
Instanes
(1994)
Gilbert
(1994)
1 : Geotextile was installed
2: Sand capping
after dredging
C-4
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
Hiroshima/Kure Site
The chemical and biochemical aspects of this site are found in Kuroda and Fujita (1981), Fujita
(1980), Ichikawa et al.(1981), and Horie (1991). The sand capping project was conducted in two
phases. Phase 1 was conducted in 1979 covering an area of 160 mx120 m. Phase 2 was
conducted a year later and covered an area 2.3 times that of Phase 1. Sand dredged from the
nearby sea was used as capping material (mean diameter= 0.1-10 mm, Gs=2.62). The cap
thickness was 50 cm and 30 cm, respectively, for phase 1 and 2. As shown in Figure C-2, the two
sites overlap each other.
260m
200m
80 m
PHASE 2
PHASE 1
120m
200m
160m
Figure C-2 Configuration of Hiroshima Site
C-5
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
O
0
10
20
30
_0
UJ 40
Q
50
60
30 60 90 120
WATER CONTENT (%)
150
Figure C-3 Index Properties of Hiroshima Bay Sediments
(after Gomyoh et al., 1994)
o
0
10
20
30
40
50
60
t,=0.2+0.12h
HIROSHIMA BAY
I . I . I
0123456789 10
SHEAR STRENGTH, tf (kPa)
Figure C-4 Vane Shear Strength of Hiroshima Bay Sediments
(after Gomyoh et al., 1994)
C-6
-------
Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
The properties of the contaminated sediment were described by Gomyoh et al (1994). The natural
water content of the first 10-20 cm of the mud is close to 100% (Figure C-3). The value at greater
depths is about 80%, still higher than the liquid limit. Figure C-4 shows the typical value of vane
shear strength distribution with depth. The undrained shear strength in the top 20 cm is extremely
low, but increases linearly to 5 kPa as the depth increases to 50 cm. The sediments were slightly
overconsolidated at the surface.
Matsushima Bay and Lake Biwa Sites
Toa Corporation conducted experimental projects at these two sites (Toa Corporation, 1994;
Gomyoh et al., 1994). Sand capping at Lake Biwa covered an area of 110 m x 200 m. At
Matsushima Bay, the project was composed of three areas, each 15 mx15 m. At Lake Biwa site,
the upper 20 cm of sediment and then the area was covered with sand 20 cm thick. At Matsushima
site, a 1.9 m thick sediment deposit was first dredged followed by a sand cap of 30 cm. The index
properties of the bottom sediments at the sites are shown in Figures C-5 and C-6. Matsushima Bay
mud has a natural water content as high as 250% The vane shear strength of these sites are given
in Figures C-7 and C-8. The sediments at both sites show slight overconsolidated behavior.
Piezocone penetration tests were conducted before and after dredging at Lake Biwa. It was
reported that negligible strength reduction has resulted from dredging. Typical values of strength
variation with depth are shown in Figure C-8. The sand used at Lake Biwa has a mean diameter
of about 0.8 mm and a unit weight of 15.5 kN/m3. Two types of sand were used at Matsushima Bay,
one has a mean diameter of 0.25 mm and a unit weight of 11.7 kN/m3 (dredged sand), and the
other sand has a mean diameter of 0.45 mm and a unit weight of 15.6 kN/m3.
C-7
-------
Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
0
10
20
r: 30
_o
g 40
50
60
50 100 150 200 250 300
WATER CONTENT (%)
350 400
Figure C-5 Index Properties of Matsushima Bay Sediments
(after Gomyoh et al., 1994)
o
_n
111
10
20
30
40
50
60
30 60 90 120
WATER CONTENT (%)
150
Figure C-6 Index Properties of Lake Biwa Sediments
(after Gomyoh et al., 1994)
C-8
-------
Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
ui
Q
10
20
30
40
50
60
MATSUSHIMA BAY
tf=0.03+0.02h
A
t,=-0.67+0.05h
0.0 0.5 1.0 1.5 2.0 2.5
SHEAR STRENGTH, tf(kPa)
3.0
Figure C-7 Shear Strength of Matsushima Bay Sediments
(after Gomyoh, et al., 1994)
u
0
10
20
30
40
50
60
t ,=0.8+0.18h
LAKE BIWA
0123456789 10
SHEAR STRENGTH,tf (kPa)
Figure C-8 Shear Strength of Lake Biwa Sediments
(after Gomyoh et al., 1994)
C-9
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
Eagle Habor Site
The first in-situ sand capping project conducted in the US was that of the Eagle Harbor at the
Wyckoff/Eagle Harbor Superfund Site (Figure C-9). The site was highly contaminated with mercury
and polynuclear aromatic hydrocarbons. It was decided to cap two areas at the site with different
materials. Areas 1 and 2 are at a water depth of 17 and 13 m, respectively. A split hull barge was
used in Area 1 and the water jet washing of material off of a barge was used for Area 2. A sediment
sample obtained at a point between Areas 1 and 2 shows that the sediments are comprised of 80%
silt and 20% clay. Sediment properties were reported by Nelson, Vanerberden and Schuldt (1994)
as LL= 40-50%, PL= 30%, Gs=2.65. The average unit weight of the sand cap was 16.4 kN/m3. The
targeted cap thickness was 1 m, but post construction surveying indicated slight variation of the
final cap thickness over the site. Vane shear strengths obtained shortly after placement of the cap
is shown in Figure C-10. These values are considerably higher than most Japanese sites. The
measured in-situ shear strength indicated that the sediment is overconsolidated.
Figure C-9 Eagle Harbor Site (U.S A.)
C-10
-------
Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
30
5 10 15
SHEAR STRENGT^(kPa)
Figure C-10 Shear Strength of Eagle Harbor Site Sediments (after Gilbert, 1994)
C-11
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
In-sltu Sand Capping Utilizing
Geosynthetic: Case Histories
In-situ sand capping may not be feasible if the submarine sediment is extremely soft to the point
where the sediment is not capable of supporting a cap. The geosynthetic sheet, placed between
the cap and the soft sediment, allows the sand cap to be constructed over the soft foundation. With
the geosynthetic in place, sediments may consolidate under the sand cap load and gain strength.
The sand cap restrains the geosynthetic sheet and prevents migration of contaminated fines into
the water column. Two successful projects, from Japan and Norway, are summarized below.
Minamata Site
Geosynthetics have been used in nearshore reclamation works in Japan since the 1960's (e.g.,
Fukuzumi and Nishibayashi, 1967; Watari and Higuchi, 1985). This experience led to a successful
sand capping at Minamata site. The sediments at this site were highly contaminated with mercury.
Human consumption of contaminated fish from this area led to the well known Minamata disease.
It was decided that the sediments with mercury concentration greater than 25 ppm were to be
dredged and capped. Hirose and Yamaguchi (1990) reported on the general aspects of this project.
A schematic drawing of Minamata site is shown in Figure C-11. It has an extremely soft sediment
layer between 4.3 - 6.8 m deep. Some of the index properties are: Gs=2.71, LL=96%, PL=38.5%,
Pl=57.5% (Umehara and Zen, 1981). Figure C-12 shows the typical variation of strength with
depth. The shear strength for this site is considerably lower than other Japanese capping sites. It
exhibits normally consolidated behavior. Geotextile sheets, with a tensile strength of 78 kN/m and
a hydraulic conductivity of 4.4x10"2 cm/s, were used. These geotextile sheets, each 30 m x 51 m,
were laid over the dredged sediments with a 1 m overlay along the edges of the sheets to allow for
possible differential settlement. Sand ($=25°, v=10 kN/m3, D50=0.1 mm) was spread in two layers
under water. The water table was adjusted so that it was maintained at 50 cm during sand
spreading. Water was then removed and the contaminated sediments were capped permanently
with another type of sand (y=14.7 kN/m3, D50=0.7 mm), 2 m thick, on top.
Soerfjorden Site
Geosynthetic was used in a sand capping project in Soerfjord, Norway (Instanes, 1994). The site
was highly contaminated with heavy metals. The sediments have an undrained shear strength of
5-10 kPa and natural water content of 35%. The geosynthetic used was a composite material
manufactured from polyester, density is higher than the water. It is comprised of a nonwoven
geomembrane and a woven polyester geotextile which acted as separation/filter function and
tensile reinforcement (Colins, 1994). The strength of geosynthetic was 50 kN/m. Polyester is
denser than water, and thus, facilitated installation process. Fourteen geosynthetic sheets were
placed with a minimum overlay of 2.5 m to allow for settlement. Finally, a sand cap of 30-60 cm
was placed.
C-12
-------
Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
SAND (CAPPED AFTER
REMOVAL OF WATER)
SAND (CAPPED UNDER WATER)
2m
80 cm
GEOTEXTILE
SHEET
SOFT SEDIMENT LAYER
Figure C-11 Configuration of Sand Cap at Minamata Site (after Namba, 1994)
100
200
300
400
MINAMATA BAY
t,=0.02h
t(i=0.004h
0.0 1.0 2.0 3.0 4.0 5.0 6.0
SHEAR STRENGTIL.(kPa)
Figure C-12 Shear Strength of Minamata Site Sediments (after Namba, 1994)
C-13
-------
Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
Geotechnical Considerations
In a capping project, there are several objectives to be considered regarding the cap thickness. For
example, the sand cap should be sufficiently thick to offer chemical isolation, protection from
intrusion as the result of bioturbidity, and protection from breach as the result of erosion. From a
geotechnical view point, a larger cap thickness may lead to instability if the sediments have very
low shear strength.
The cap stability and settlement due to consolidation are two main geotechnical issues. However,
the most critical aspect of the cap would be its stability immediately after placement, before any
excess pore water pressure due to the weight of the sand layer has dissipated. The settlement is
related to long-term performance of the cap as the sediments consolidate simultaneously with the
dissipation of excess pore water pressure while gaining additional strength. In this report, (the (BF))
discussion will be focused on a short term stability analysis (i.e., the most critical state) of the sand
cap as viewed from bearing capacity and slope stability analysis.
Bearing Capacity Analysis
In bearing capacity analysis, the sand cap is considered as a footing acting over large area. The
footing contact pressure is replaced by an equivalent surcharge, q, due to the cap's effective unit
weight, y', and thickness, h. That is,
q=Y'h (1)
In undrained analysis, considering local shear failure (i.e., punching mode of failure) and a footing
embedded on a purely cohesive soil with zero depth into the foundation, the ultimate bearing
capacity, quft, is determined as (Terzaghi and Peck, 1967):
qult = 2/3 cu Nc (2a)
and
Nc = (2 + n) (2b)
where cu is the undrained shear strength, and Nc is the bearing capacity factor. The usage of local
failure is justified in sand capping projects because the bottom sediments are soft, and therefore,
do not allow the classical bearing capacity type of failure to occur.
In design, the allowable surcharge is obtained by reducing the ultimate bearing capacity by a safety
factor, typically of value 3. Thus, combining Eqs. (1) and (2), the allowable cap thickness, haiiow, is
determined as
h = 1.14 Cu/ Y' (3)
Assuming a typical value of Y'= 5 kN/m3 and cu= 1 to 2 kPa, the allowable cap thickness is between
20 and 50 cm. This range of value explained reasonably the success of most sand capping
projects.
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Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
It should be pointed out that traditional bearing capacity analysis (Eq. 3) assumes a constant value
of undrained shear strength. However, the review of case histories indicates that soft sediments
are having undrained shear strength that increases with depth. Therefore, it is recommended to
sing a small value of cu) and only when limited shear strength data of the foundation are available.
Cap Stability Analysis
Cap slope stability is analyzed using a computer program with the procedure proposed by
Leshchinsky (1987) and Leshchinsky and Smith (1989). It is based on a limit equilibrium approach
considering a log-spiral and a circular failure mechanisms in the sand cap and soft sediments,
respectively. If stability cannot be attained, the analysis will indicate whether geosynthetic
reinforcement is needed or whether additional consolidation must be allowed to occur prior to cap
placement. The analysis determines the geosynthetic strength required to restore stability if the
safety factor falls below a specified value. The notation used is shown in Figure C-13.
Since the in-situ cap is completely submerged, the buoyant unit weight (Y') and the design value
of the internal friction angle of the sand (c|)d) are specified. The water depth above the cap does not
affect its effective stresses (and thus the cap stability) if external forces, such as waves, are not
excessive. Different layers of sediments having depth (d,) and undrained shear strength (cm=cu/Fs,
Fs: safety factor) may be specified. The strength of each layer can be specified as a constant value
or varying linearly with depth (Figure C-14).
It should be noted that the water depth affects the falling velocity of sand particles placed in water
leading to different impact energy as they reach the sediments. This may result in different
penetration depths into the soft sediments and affect the unit weight of sand. The shear strength
of sand is also affected by this unit weight. However, since accurate identification of subaqueous
material properties is very limited, it seems justified to ignore these effects at this stage. That is,
the quantification of properties is not warranted considering the potential uncertainties in design.
CENTER OF ROTATION
Figure C-13 Cap Stability Analysis
C-15
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Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
B/2
B/2
SAND CAP
SEDIMENTS
SEDIMENTS
cu=cuo+Dcuh
Figure C-14 Variation of Undrained Strength with Depth
Stability analyses were conducted for several of the reported case histories (Hiroshima, Minamata,
Lake Biwa, Matsushima, and Eagle Harbor). Sediment properties required for stability analysis are
available only at these sites (Table C-2). The internal friction angle of the sand and the slope angle
of the cap are assumed as 35° and 30°, respectively. This is by assuming the largest possible angle
of repose under water since the actual value was not available. Since the submerged unit weight
of sand at the Hiroshima site is not available, it is assumed as 6.0 kN/m3 in the analysis.
Table C-2 shows the sand cap thickness analyzed using a safety factor of 1.0 applied to the soils.
Consequently, the calculated cap thickness signifies the maximum theoretical cap thickness. The
analysis shows that Hiroshima site, Lake Biwa and Eagle Habor sites are stable against potential
failure. In particular, the Eagle Harbor site has an extremely large safety margin. The analysis
indicates that the Minamata site requires the sand cap to be placed with the aid of geosynthetic.
The required geosynthetic strength is 7 N/m based on v'= 0.2 kN/m3. If v' is assumed as 6.0 kN/m3
(i.e., a reasonable design value), the required geosynthetic strength increases significantly to 3.2
kN/m. The analysis also indicates possible instability of the sand cap at the Matsushima site. The
required geosynthetic strengths are 4 N/m and 77 N/m for Y'= 1-9 kN/m3 and 5.8 kN/m3,
respectively. The successful sand cap placement at this site could have been due to dredging
away of the top extremely soft sediment layer so that the actual sediments strength was larger than
that used in the analysis. That is, dredging the top 20 cm of the sediment exposed the stronger
sediment layer as foundation for the sand cap without the use of geosynthetic reinforcement.
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Appendix C: Case Studies on Geotechmcat Aspects ofln-Situ Sand Capping
Table C-2. Computed and Constructed Sand Cap Thickness
Capping Site
Hiroshima
Minamata
Lake Biwa
Matsushima
Eagle
Harbor
Undrained Strength of
Contaminated Sediments
Cu = Cuo + Acu x depth
GUO (kPa)
0.2
0
0.8
0.03
5
Acu(kPa/m)
12
0.4
18
2
22
Effective
Unit Weight
of Cap
Material
Y;{kN/m3)
6.0*
0.2
5.7
1.9
5.8
6.6
Constructed
Cap
Thickness
(cm)
50
80
20
30
30
100
Computed
Cap
Thickness
(max stable
thickness)
(cm)
58
**
295
22***
5***
>17m
*assumed value
**construction is infeasible without reinforcement. In actual construction, geosynthetic
reinforcement was used.
***actual sediment strength was likely greater than that before dredging.
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Appendix C: Case Studies on Geotechnical Aspects ofln-Situ Sand Capping
Conclusions and Recommendations
Successful case histories related to in-situ sand capping projects are reviewed and presented. This
technique has been evaluated and proved feasible from a geotechnical view point. At the sites
where the sediments are of extremely low strength, geosynthetics of adequate strength and
permeability can improve the stability of the sand cap. Dredging away the top layer (10-20 cm)
may also be a feasible solution. There are several topics that need to be further studied so that in-
situ capping technique and its design procedure may be verified and refined:
1. It appears that construction technique is an important factor in the success of a sand capping
projects. Sand dumped in lumps may penetrate the soft sediments and may cause resuspension
of contaminants into the water column. Conversely, "raining" the sand in layers will allow gentle
spreading and result in a stable sand cap. It is recommended that laboratory model tests be
conducted and the performance monitored and quantified. This should lead to an optimized
construction procedure which takes the geotechnical properties of the sediments into account.
2. It is suggested to develop an analytical technique which may be used to predict the density of
sands pluviated in water. Experimental work should also be conducted to verify the theory. The
effect of soil grain size, water depth and foundation compressibility should be considered as the
parameters in the analytical and experimental studies.
3. It is recommended that the roles of a geosynthetic (reinforcement, separation, and filtration) in
maintaining the cap integrity be considered in future research. This should also be studied and
quantified using a well-controlled experimental work, including "control tests" which do not have a
geosynthetic layer.
4. A reliable procedure to estimate the in-situ distribution of sediments strength is needed.
5. Potential external forces, in particular waves, need to be included in future studies.
6. Finally, a versatile procedure which considers the deformations, generation and dissipation of
excess pore water pressure from the sediments should be developed.
Acknowledgment
The study as described herein was funded by the U.S. Army Engineer Waterways Experiment
Station, Vicksburg, Miss. Prof. T. Yamagami of the University of Tokushima, Prof. H. Ochiai of
Kyushu University, Prof. T. Pradhan of Yokohama National University, Drs. T. Horie and T.
Tsuchida of the Port and Harbour Research Institute, Mr. T. Namba of Kumamaoto Port
Construction Office, and Dr. H. Hanzawa of Toa Habor Works Co. Ltd., have assisted the authors
in one way or another with both published and unpublished information related to the Japanese
case histories. Dr. T. Collins of the Huesker, Inc., offered details related to the Soerfjorden Site.
However, the authors are responsible for the accuracy of results presented.
C-18
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Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
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Appendix C: Case Studies on Geotechmcal Aspects ofln-Situ Sand Capping
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