&EPA
United Stet0$
6nviportm$ntal
Office of Emergency and
Remedial Ro$pons$
Washington, DO 20460
Research and Development
Cincinnati, OH 452G&
Supertund
Engineering Bulletin
Landfill Covers
Purpose
Section 121 (b) of the Comprehensive Environmental Re-
sponse, Compensation, and Liability Act (CERCLA) mandates
the Environmental Protection Agency (EPA) to select remedies
that "utilize permanent solutions and alternative treatment
technologies or resource recovery technologies to the maxi-
mum extent practicable" and to prefer remedial actions in
which treatment "permanently and significantly reduces the
volume, toxicity, or mobility of hazardous substances, pollut-
ants, and contaminants as a principal element." The Engineer-
ing Bulletins are a series of documents that summarize the
latest information available on selected treatment and site
remediation technologies and related issues. They provide
summaries of and references for the latest information to help
remedial project managers, on- scene coordinators, contrac-
tors, and other site cleanup managers understand the type of
data and site characteristics needed to evaluate a technology
for potential applicability to their Superfund or other hazardous
waste site. Those documents that describe individual treatment
technologies focus on remedial investigation scoping needs.
Addenda will be issued periodically to update the original
bulletins.
Abstract
Landfill covers are used at Superfund sites to minimize
surface water infiltration and to prevent exposure to the
waste. In many cases, covers are used in conjunction with other
waste treatment technologies, such as slurry walls, ground-
water pump- and-treat systems, and in situ treatment.
This bulletin discusses various aspects of landfill covers,
their applicability, and limitations on their use and describes
innovative techniques, site requirements, performance data,
current status, and sources of further information regarding
the technology.
Technology Applicability
Covers may be applied at Superfund sites where contami-
nant source control is required. They can sense one or more of
the following functions:
• Isolate untreated wastes and treated hazardous wastes to
prevent human or animal exposure
* [reference number, page number]
• Prevent vertical infiltration of water into wastes that
would create contaminated leachate
• Contain waste while treatment is being applied
• Control gas emissions from underlying waste
• Create a land surface that can support vegetation and/or
be used for other purposes
Covers may be interim (temporary) or final. Interim
covers can be installed before final closure to minimize genera-
tion of leachate until a better remedy is selected. They are
usually used to minimize infiltration when the underlying waste
mass is undergoing most of its settlement. A more stable base
will thus be provided for the final cover, reducing the cost of
post-closure maintenance.
Covers also may be applied to waste masses that are so
large that other treatment is impractical. At mining sites for
example, covers can be used to minimize the entrance of water
to contaminated tailings plies and to provide a suitable base for
the establishment of vegetation. In conjunction with water
diversion and detention structures, covers may be designed to
route surface water away from the waste area while minimiz-
ing erosion.
The effectiveness of covers on underlying soils and ground-
water containing contaminants is shown in Table 1. Effective-
ness is defined as the ability of the cover to perform its
function over the long term without being damaged by the
chemical characteristics of the underlying waste. Examples of
constituents within contaminant groups are provided in the
"Technology Screening Guide for Treatment of CERCLA Soils
and Sludges" [1, p. 10].
The degree of effectiveness shown in Table 1 is based
on currently available information or on professional
judgment when no information was available. The effec-
tiveness of the technology for a particular site or waste
does not ensure that it will be effective at all sites. Demon-
strated effectiveness means that, at some scale, chemical
resistance tests showed that landfill covers were resistant
to that particular contaminant in a soil or groundwater
matrix. The ratings of potential effectiveness and no
expected effectiveness are based on expert judgment.
Where potential effectiveness is indicated, the technology is
Printed on Recycled Paper
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believed capable of successfully containing the contaminant
groups so indicated in a soil or groundwater matrix. If the
technology were not applicable or probably would not work for
a particular combination of contaminant group and matrix, a
no expected effectiveness rating is given. Note that this rating
does not occur in Table 1 for any of the contaminant groups.
Limitations
Landfill covers are part of landfilling technology, which is
generally considered a technology of last resort in remediating
hazardous waste sites. Landfilling of hazardous waste is not
permitted without first applying the best available treatment.
Landfilling technology does not lessen toxicity, mobility, or
volume of hazardous wastes. However, when properly de-
signed and maintained, landfills can isolate the wastes from
human and environmental exposure for very long periods of
time.
Covers are most effective where most of the underlying
waste is above the water table. A cover, by itself, cannot
prevent the horizontal flow of groundwater through the waste,
only the vertical entry of water into the waste. Other proce-
Tabtol
Effectiveness of Covers on General Contaminant
Groups for Soil and Groundwater
Contaminant Croups
!
Inorgani
1
Halogenated volatile*
Halogenated semivolatiles
Nonhalogenated volatiles
Nonhalogenated semivolatiles
PCBs
Pesticides (halogenated)
Dioxins/Furans
Organic cyanides
Organic corrosives
Volatile metals
Nonvolatile metals
Asbestos
Radioactive materials
Inorganic corrosives
Inorganic cyanides
Oxidizers
Reducers
Effectiveness
of
Covers
T
•
T
• Demonstrated Effectiveness: Short-term effectiveness demonstrated at
field-scale.
V Potential Effectiveness: Expert opinion that technology will work.
Q No Expected Effectiveness: Expert opinion that technology will not
work.
dures (e.g., landfill liners, slurry walls, extraction wells) may
be needed to exclude, contain, or treat contaminated
groundwater.
It is generally conceded that landfill components (liners
and covers) will fail eventually, even though failure may occur
after many tens or hundreds of years. Their effective life can be
extended by long-term (30 years or more) inspection and
maintenance [20]. Vegetation control and repairs associated
with construction errors, cover erosion, settlement and subsid-
ence are likely to be required. The need for cover repairs can
be lessened considerably by adherence to a rigorous quality
assurance program during construction.
Technology Description
The U.S. Environmental Protection Agency (EPA) has pub-
lished several documents that provide guidance on the technol-
ogy of cover construction at land disposal facilities [2] [3] [4] [5]
[6] [7]. Other documents specifically address remediation of
radiologically-contaminated Superfund sites, including the use
of covers [8] [9]. Design and construction of clay liners (not
covers specifically), properties of clay, testing methods, soil
permeabilities, liner performance, and failure mechanisms are
discussed at length in Reference 10.
The design of covers is site-specific and depends on the
intended functions of the system. Many natural, synthetic, and
composite materials and construction techniques are available.
The effectiveness of covers (and other structural components of
engineered landfills) has been shown to be primarily a function
of the attention given to quality in choosing, installing, and
inspecting those materials and techniques [24].
Covers can range from a one-layer system of vegetated soil
to a complex multi-layer system of soils and geosynthetics. In
general, less complex systems are required in dry climates and
more complex systems are required in wet climates. The most
complex systems are usually found on engineered landfills in
the humid eastern United States, where the cover must meet
the erosion and moisture requirements of the associated liner
designed to contain the waste. Figure 1 depicts a vertical
section of such a cover. Table 2 summarizes the function,
materials of construction, and purpose of each of the compo-
nents. Covers on Superfund sites usually contain some, but not
necessarily all, of these components.
The materials used in the construction of covers include
low-permeability and high-permeability soils and geosynthetic
products. The low-permeability materials (geomembrane/soil
layer) divert water and prevent its passage into the waste. The
high-permeability materials (drainage layer) carry water away
that percolates into the cover. Other materials may be used to
increase slope stability.
The most critical components of a cover in respect to
selection of materials are the barrier layer and the drainage
layer. The barrier layer can be a geomembrane or low- perme-
ability soil (clay), or both (composite).
Geomembranes are supplied in large rolls and are available
in several thicknesses (20 to 140 mil), widths (15 to 100 ft), and
Engineering Bulletin: Landfill Covers
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Figure 1
Cross-section of Mum-layer Landfill Cover
vegetation —^-
protection layer —^-
drainage layer —^-
geomembrane/soil
barrier layer
I I ri_l-LLLlJJJUUJ±LLL
^ ^
waste Q9
^ O 0
h— topsoil
— granular or geotextile filter
geomembrane
w/overlying protective geotextile
geotextile gas
collection layer
Table 2
Configuration of Cover Systems
Layer
1 . Surface Layer
2. Protection Layer
3. Drainage Layer
4. Barrier Layer
5. Gas Collection
Layer
Primary Function
Promotes vegetative growth
(Most covers); Decrease erosion;
Promote evapotranspiration.
Protect underlying layers from
intrusion and barrier layer
from desiccation and freeze/thaw
damage; Maintain stability;
storage of water
Drain away infiltrating water
to dissipate seepage forces
Reduce further leaching of waste
by minimizing infiltration of water
into waste; Aid in directing gas to
the emissions control system by
reducing the amount leaving
through the top of the cover
Transmit gas to collection
points for removal and/or
cogeneration
Usual Materials
Topsoil (humid site); Cobbles
(arid Site); Ceosynthetic erosion
control systems
Mixed soils; Cobbles
Sands; gravels; geotextiles;
geonets; geocomposites
Compacted clay liners;
Geomembranes; Geosynthetic
clay liners; Composites
Sand; geotextiles; geonets
General Considerations
Usually required for control of
water and/or wind erosion
Usually required; May be
combined with the protective
layer into a single "cover soil"
layer
Optional; Necessary where
excessive water passes through
protection layer or seepage
forces are excessive
Usually required;
May not be needed at
extremely arid sites
Usually required if waste produces
excessive quantities of gas
lengths (180 to 840 ft). The polymers currently used include
polyvinyl chloride (PVC) and polyethyienes of various densities.
Geomembranes are much less permeable than clays; measur-
able leakage generally occurs because of imperfections created
during their installation; however, the imperfections can be
minimized [15].
Soils used as barrier materials generally are clays that are
compacted to a hydraulic conductivity (usually referred to as
permeability) no greater than 1 x 10"6 cm/sec or a combination
of bentonite and other soil that will achieve a comparable or
even lower permeability. Compacted soil barriers are generally
installed in 6-inch minimum lifts to achieve a thickness of 2 feet
or more.
A composite barrier uses both soil and a geomembrane,
taking advantage of the properties of each. The geomembrane
is essentially impermeable, but, if it develops a leak, the soil
component prevents significant leakage into the underlying
waste. A composite liner has proven to be the most effective in
decreasing hydraulic conductivity [2, p. A-2].
Engineering Bulletin: Landfill Covers
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Geosynthetic clay barriers are beginning to be used in
place of both the geomembrane and clay components. The
geosynthetic clay barriers are constructed of a thin layer of
bentonite sandwiched between two geosynthetic materials. In
use, the bentonite expands to create a low-permeability,
resealable ("self-healing") barrier. It is supplied in rolls, but
does not require seaming as geomembranes do [21].
Other identified alternative barrier materials are flyash-
bentonite-soil mixtures; super absorbent geotextiles; sprayed-
on geomembranes and soil-particle binders; and custom-made
bentonite composites with geomembranes or geotextiles [11,
p. 63] [12, p. 6]. Potential advantages of alternative barriers
include quick and easy installation, better quality control, cost
savings potentially greater than use of compacted soil or stan-
dard soil/geomembrane composite, reduction in volume of
material, lighter construction equipment required, and some
self-healing capabilities [11, p. 65] [12, p. 6] [13, p. 225].
The following discussion briefly describes the construction
of a multi-layer cover. It does not attempt to describe all of the
possible configurations and materials.
Covers are usually constructed in a crowned or domed
shape with side slopes as low as is consistent with good runoff
characteristics. The bottom layer, which may be a granular gas
collection layer, forms the base on top the waste mass for the
remainder of the cover. The clay component of the bamer
layer is constructed on this base layer. The clay is spread and
compacted in "lifts" a few inches thick until the desired barrier
thickness is reached (usually 24 inches or more).
Each lift is scarified (roughed up) after compaction so there
will be no discernible surface between it and the next higher lift
when the latter is compacted. The top lift is compacted and
rolled smooth so the geomembrane may be laid on it in direct
and uniform contact. During the entire process the clay must
be maintained at a near-optimum moisture content in order to
attain the necessary low permeability upon compaction.
Low hydraulic conductivity is the most important property
of the clay/soil barrier. Hydraulic conductivity is significantly
influenced by the method of compaction, moisture content
during compaction, compactive energy, clod size, and the
degree of bonding between lifts [11, p. 6].
Geomembranes require a great deal of skill in their installa-
tion. They must be laid down without wrinkles or tension.
Their seams must be fully and continuously welded or ce-
mented and they must be installed before the underlying clay
surface can desiccate and crack. If vent pipes protrude through
the cover, boots must be carefully attached to the membrane
to prevent tearing if the cover subsides later. Care must be
taken that the membrane is not accidentally punctured by
workers or tools.
Extremes of temperature can adversely affect geomembrane
installation, e.g., stiffness and brittleness are associated with
low temperatures and expansion is associated with high tem-
peratures. Thus, air temperature and seasonal variation are
important design considerations [15].
A geotextile may be laid on the surface of the geomembrane
for the geomembrane's protection, particularly if relatively coarse
and sharp granular materials are applied as the drainage layer.
Another geotextile can then be put on top of the drainage layer
to prevent clogging of the drainage layer by soil from above.
Fill soil and topsoil are then applied (compaction is not so
critical) and the topsoil seeded with grass or other vegetation
adapted to local conditions.
The drainage layer in a cover is designed to carry away
water that percolates down to the barrier layer. It may be either
a granular soil with high permeability or a geosynthetic drain-
age grid or geonet sandwiched between two porous geotextile
layers. A geotextile may be used as a filter at the top of a
granular soil drainage material to separate it from an overlying
soil of different characteristics to prevent the drainage layer
from becoming plugged with fine soil. A geotextile may also
be used at the bottom of a granular drainage layer to protect
the underlying geomembrane barrier from abrasion or punc-
ture by sharp particles.
Other component layers may be used in landfill covers.
Wider tolerances are generally acceptable in the material and
construction requirements for these layers. Topsoil and subsoil
from the vicinity are likely to be suitable for the surface and
protection layers, respectively. The gas collection layer may be
similar to the drainage layer in its characteristics, but it does not
need to be. For example, gravel or coarse sand may be
appropriate. Geosynthetic drainage materials may be used
here too, but the chemical resistance to volatile wastes may be
of greater concern due to the proximity of the waste and
possibility for contact with it. However, EPA has no data that
suggest damage to covers by volatiles.
Many laboratory tests are needed to ensure that the mate-
rials being considered for each of the cover components are
suitable. Tests to determine the suitability of soil include grain
size analysis (ASTM D422), Atterberg limits (ASTM D4318), and
compaction characteristics (ASTM D698 or D1557). These
tests generally are performed on the source material (called
"borrow" material) before and during construction at predeter-
mined intervals. EPA is expected to publish a new manual on
construction quality assurance in the spring of 1993 [23].
The major engineering soil properties that must be defined
are shear strength and hydraulic conductivity. Shear strength
may be determined with the unconfined compression test
(ASTM D2166), direct shear test (ASTM D3080), or triaxial
compression test (ASTM D2850). Hydraulic conductivity of
soils may be measured in the laboratory with either ASTM
D2434 or D5083. Field hydraulic conductivity tests are gener-
ally recommended and may be performed, prior to actual cover
construction on test pads to ensure that the low-permeability
requirements can actually be met under construction condi-
tions. EPA strongly encourages the use of test pads [3] [4].
Laboratory tests are also needed to ensure that geosynthetic
materials will meet the cover requirements. For example,
geosynthetics in covers may be subjected to tensile stresses
caused by subsidence and by the gravitational tendency of a
geomembrane or material adjacent to it to slide or be pulled
Engineering Bulletin: Landfill Covers
-------
down slopes. Hydraulic conductivity of geomembranes is not
defined but leakage should not be significant in undamaged
materials. Geosynthetic drainage materials (reinforcement type
products such as geonets and geotextiles) can become clogged
or compressed under pressure and lose some or all of their
drainage capacity.
The geosynthetics in a cover generally are not in direct
contact with the underlying waste, so chemical resistance to the
waste is not often a limitation [14, p. 79] [3, p. 109]. On the
other hand, vapors from volatile contaminants have the poten-
tial to degrade cover materials. Note in Table 1 that although
the organic volatiles are the only chemical groups with less than
demonstrated effectiveness, the opinion of experts is that the
use of geosynthetics in cover systems will work. EPA has no
evidence to suggest damage to covers by volatile organic com-
pounds.
High-quality seams are essential to geomembrane integ-
rity. Test-strip seaming, in which the actual seaming process is
imitated on narrow pieces of excess membrane, can help to
ensure high seam quality. The test strips should be prepared
and subjected to strength (shear and peel) testing whenever
equipment, personnel, or climatic changes are significant [15,
p. 14]. Failure to meet specifications with the test strips indi-
cates the necessity for destructive testing of actual field seams
and correction of deficiencies in the seaming process.
Although construction quality assurance, including testing,
will increase the installation cost about 10 to 15 percent and the
time required to complete the project, it has been shown to
improve the performance of the installation [22].
Steeply mounded landfills can have a negative effect on
the construction and stability of the cover. A steep slope can
make it difficult to compact soil properly due to the limited
mobility and reduction of compacting effort of some compac-
tion equipment. The rate of erosion is also a function of slope.
Difficulty may arise in anchoring a geomembrane to prevent it
from sliding along the interfaces of the geomembrane and soils.
In some instances, geosynthetic reinforcement grids may be
used to increase slope stability. Engineering design guidance
addressing geomembrane stability can be found in Reference
16.
When constructing a new landfill or when covering an
existing landfill where the surface of the waste mass can be
graded, EPA suggests that side slopes of a landfill cover not be
less than 3 per cent or exceed 5 per cent [4, p. 24].
High air temperatures and dry conditions during construc-
tion may result in the loss of moisture from a clay barrier layer,
causing desiccation cracking that can increase hydraulic con-
ductivity. Desiccation cracking can be prevented by adding
moisture to the clay surface and by installing the geomembrane
in a composite barrier quickly after completion of the clay layer.
The hydraulic conductivity of compacted soil is also signifi-
cantly influenced by the method of compaction, soil moisture
content during compaction, compactive energy, clod size, and
the degree of bonding between the individual lifts of soil in the
barrier layer [11, p. 6].
Geomembranes are negatively influenced by different fac-
tors than soils during the construction process. Generally more
care must be taken to prevent accidental punctures. Sunlight
can heat the material, causing it to expand. If installed while
hot, the geomembrane can then shrink to the point of seam
rupture if compensating actions are not taken. Seams must be
carefully constructed to ensure continuity and strength. They
should run up and down slopes rather than horizontally in
order to reduce seam stress. Details of geomembrane installa-
tion can be found in Reference 15.
Site Requirements
The construction of covers requires a variety of construc-
tion equipment for excavating, moving, mixing, and compact-
ing soils. The equipment includes bulldozers, graders, various
rollers, and vibratory compactors. Additional equipment is
required in moving, placing, and seaming geosynthetic materi-
als, e.g., forklifts and various types of seaming devices.
Storage areas are necessary for the materials to be used in
the cover. If site soils are adequate for use in the cover, a
borrow area needs to be identified and the soil tested and
characterized. If site soils are not suitable, other low-permeability
soils may have to be trucked in. An adequate supply of water
may also be needed for application to the soil to achieve
optimum soil density.
Performance Data
Once a cover is installed, it may be difficult to monitor or
evaluate the performance of the system. Monitoring well
systems or infiltration monitoring systems can provide some
information, but it is often not possible to determine whether
the water or leachate originated as surface water or groundwa-
ter. Few reliable data are available on cover performance other
than records of cover condition and repairs.
The difficulty in monitoring the performance of covers
accentuates the need for strict quality assurance and control for
these projects during construction. It is important to note that
no landfill cover is completely impervious. It is also important
to note that small perforations or poorly seamed or jointed
materials can increase leakage potential significantly.
Technology Status
The construction of landfill covers is a well-established
technology. Several firms have experience in constructing
covers. Similarly, there are several vendors of geosynthetic
materials, bentonitic materials, and proprietary additives for
use in constructing these barriers.
In EPA's FY 1989 ROD Annual Report [17], 154 RODs
specified covers as part of the remedial action. Table 3 shows a
selected number of Superfund sites employing landfill cover
technology. While site-specific geophysical and engineering
studies are needed to determine the appropriate materials and
construction specifications, covers can effectively isolate wastes
from rainfall and thus reduce leachate and control gas emis-
sions. They can also be implemented rather quickly in conjunc-
Engineering Bulletin: Landfill Covers
-------
Tabto3
S*toct«d Sup«rfund Sites Employing Landfill Covers
SITE
Chemtronics
Mid-State Disposal Landfill
Bailey Waste Disposal
Cleve Reber
Northern Engraving
Ninth Avenue Dump
Charles George Reclamation
E.H. Shilling Landfill
Henderson Road
Ordinance Works Disposal
Industri-Plex
Combe Fill North
Combe Fill South
Location (Region)
Swannada, NC (4)
Cleveland Township, Wl (5)
Bridge City, TX (6)
Sorrento, LA (6)
Sparta, Wl (5)
Gary, IN (5)
Tyngsborough, MA (1)
Ironton, OH (5)
PA (3)
WV(3)
Woburn, MA(1)
Mount Olive Township, N) (2)
Chester and Washington Township, NJ (2)
Status
In design phase
In pre-design phase
In design phase
In design phase
In operation since 1988
In design phase
In operation
In design phase
In design phase
In design phase
In design phase
Completed in 1991
In design phase
tion with other anticipated remedial actions. Long-term moni-
toring is needed to ensure that the technology continues to
function within its design criteria.
EPA Contact
Technology-specific questions regarding landfill covers may
be directed to:
Robert E. Landreth or David A. Carson
U.S. Environmental Protection Agency
Risk Reduction Engineering Laboratory
26 W. Martin Luther King Drive
Cincinnati, Ohio 45268
(513)569-7871
Acknowledgments
This bulletin was prepared for the U.S. Environmental Pro-
tection Agency, Office of Research and Development (ORD),
Risk Reduction Engineering Laboratory (RREL), Cincinnati, Ohio,
by Science Applications International Corporation (SAIC), un-
der contract no. 68-C8-0062. Mr. Eugene Harris served as the
EPA Technical Project Monitor. Mr. Gary Baker was SAIC's
Work Assignment Manager. This bulletin was written by Mr.
Cecil Cross of SAIC. The author is especially grateful to Mr. Eric
Saylor of SAIC who contributed significantly to the develop-
ment of this bulletin.
The following contractor personnel have contributed their
time and comments by participating in the expert review meet-
ings or in peer reviewing the document:
Dr. David Daniel
Mr. Robert Hartley
Ms. Mary Boyer
University of Texas
Private Consultant
SAIC
Engineering Bulletin: Landfill Covers
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REFERENCES
1. U.S. Environmental Protection Agency. Technology
Screening Guide for Treatment of CERCLA Soils and
Sludges. EPA/540/288/004. 1988.
2. U.S. Environmental Protection Agency. Seminars: Design
and Construction of RCRA/CERCLA Final Covers. CERI
90-50. July-August 1990.
3. U.S. Environmental Protection Agency. Seminar Publica-
tion: Requirements for Hazardous Waste Design, Con-
struction, and Closure. EPA/625/4-89/022. August
1989.
4. U.S. Environmental Protection Agency. Technical
Guidance Document: Final Covers on Hazardous Waste
Landfills and Surface Impoundments. EPA/5 30-SW-89-
047. July 1989.
5. U.S. Environmental Protection Agency. Guide to Techni-
cal Resources for the Design of Land Disposal Facilities.
EPA/625/6-88/018. December 1988.
6. Lutton, R.J. Design, Construction, and Maintenance of
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7. McAneny, C.C., P.G. Tucker, J.M. Morgan, C.R. Lee, M.F.
Kelley, and R.C. Horz. Covers for Uncontrolled Hazardous
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8. U.S. Environmental Protection Agency. Technological
Approaches to the Cleanup of Radiologically Contami-
nated Superfund Sites. EPA/540/2-88/002. August
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9. U.S. Environmental Protection Agency. Assessment of
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Contaminated Superfund Sites. EPA/540/2-90/001.
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the AWWA 84th Annual Meeting and Exhibition,
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15. U.S. Environmental Protection Agency. Technical
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16. U.S. Environmental Protection Agency. Geosynthetic
Design Guidance for Hazardous Waste Landfill Cells and
Surface Impoundments. EPA/600/2-87/097. Cincinnati,
OH. 1987.
17. U.S. Environmental Protection Agency. ROD Annual
Report: FY 89. EPA/540/8-90/006. April 1990.
18. U.S. Environmental Protection Agency. Prediction and
Mitigation of Subsidence Damage to Hazardous Waste
Landfill Covers. EPA/600/2-87/025. Cincinnati, OH.
1987.
19. Daniel, D.E. and R.M. Koerner. Final Cover Systems.
Chapter 18 in Geotechnical Aspects of Waste Disposal.
D.E. Daniel, Ed. To be published by Chapman & Hall,
London, England. 1992.
20. Bennett, R.D. and R.C. Horz. Recommendations to the
NRC for Soil Cover Systems Over Uranium Mill Tailings
and Low Level Radioactive Wastes. NUREG/CR-5432.
U.S. Army Engineer Waterways Experiment Station. 1991.
21. U.S. Environmental Protection Agency. Report of
Workshop on Geosynthetic Clay Liners. To be published
fall 1992.
22. Bonaparte, R. and B.A. Gross. Field Behavior of Double-
Liner Systems. Am. Soc. Civil Eng. Geotech. Publ. No. 26.
November 1990.
23. U.S. Environmental Protection Agency. Guidance Manual
on Construction Quality Assurance of Geosynthetics and
Clay Soils. To be published spring 1993.
24. U.S. Environmental Protection Agency. Construction
Quality Assurance for Hazardous Waste Land Disposal
Facilities. EPA/530/SW-86-031. Washington, D.C. 1986.
Engineering Bulletin: Landfill Covers
•U.S. Government Printing Office: 1903 — 780-071/60102
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United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/540/S-93/500
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