United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
Washington DC 20460
EPA 530-SW-89-047
July 1989
PB89-233»80
EPA
Technical Guidance
Document:
Final Covers on
Hazardous Waste
Landfills and Surface
Impoundments
REPRODUCED BY
U.S. DEPARTMENT OF COMMERCE
NATIONAL TECHNICAL INFORMATION SERVICE
SPRINGFIELD, VA. 22161
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TECHNICAL REPORT DATA
(rleait rreti Innruelionj «w» the rrrtne
NO.
EPA/530-SW-39-047
3. RECIPIENT'S ACCESSION NO.
TITLE AND SUBTITLE
ECHNICAL GUIDANCE DOCUMENT: FINAL COVERS ON HAZARDOUS
ilASTE LANDFILLS AND SURFACE IMPOUNDMENTS
. REPORT DATE
July 1989
6. PERFORMING ORGANIZATION CODE
AUTMOR(S)
8.PERFORMING ORGANIZATION REPORT NO.
J.S. Environmental Protection Agency
ffice of Solid Waste & Risk Reduction Engineering Lab
PERFORMING ORGANIZATION NAME AND ADDRESS
401 M Street, SW
Washington, DC 20460
26 W. Martin Luther King Drive
Cincinnati, OH 45268
10. PROGRAM ELEMENT NO.
»». CONTRACT/GRANT NO.
1. SPONSORING AGENCY NAME AND ADDRESS
EPA Office of Solid Waste
Washington, DC 20460 and
Risk Reduction Engineering Laboratory
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
6. SUPPLEMENTARY NOTES
Project Officer: Robert E. Landreth
FTS: 684-7836 COMM: 513/569-7836
6. ABSTRACT
This document recommends and describes a design for landfill covers that will meet the
requirements of RCRA regulations. It is a multilayered system consisting, from the top
down, of:
a top layer of at least 60 cm of soil, either vegetated or armored at the surface;
a granular or geosynthetic drainage layer with a hydraulic transmissivity no less
than 3 x 10"5 cm /sec; and
a two-component low permeability layer comprised of (1) a flexible membrane liner
installed directly on (2) a compacted soil component with an hydraulic conductivity
no greater than 1 x 10~7 cm/sec.
Optional layers may be added, e.g., a biotic barrier layer or a gas vent layer, depending
on the need.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTIFIERS/OREN ENDED TERMS C. COSATI Field/Group
IB. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
SPA r~n» 5230-1 (R.». 4-77) PMCVIOUI COITION is oaiouCTt .
19. SECURITY CLASS (THll Rtfortl
UNCLASSIFIED
21. NO. OF
GES
JO. SECURITY CLASS (Thitf*t<)
UNCLASSIFIED
33. PRICE
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EPA/530-SW-89-047
July 1989
TECHNICAL GUIDANCE DOCUMENT
FINAL COVERS ON HAZARDOUS WASTE LANDFILLS
AND SURFACE IMPOUNDMENTS
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
In cooperation with
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
The preparation of this document has been funded wholly by
the United States Environmental Protection Agency. It has been
subjected to the Agency's peer and admininistrative review, and
it has been approved for publication as an EPA document. Mention
of trade names or commercial products does not constitute
endorsement or recommendation for use.
11
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FOREWORD
Today's rapidly developing and changing technologies and
industrial products and practices frequently carry with them the
increased generation of solid and hazardous wastes. These
materials, if improperly dealt with, can threaten both public
health and the environment. Abandoned waste sites and accidental
releases of toxic and hazardous substances to the environment
also have important environmental and public health implications.
The Risk Reduction Engineering Laboratory assists in providing an
authoritative and defensible engineering basis for assessing and
solving these problems. Its products support the policies,
programs, and regulations of the U.S. Environmental Protection
Agency; the permitting and other responsibilities of State and
local governments; and the needs of both large and small
businesses in handling their wastes responsibility and
economically.
This document provides design guidance on final cover
systems for hazardous waste landfills and surface impoundments.
We believe that the final cover, if properly designed and
constructed, can provide long-term protection of the unit from
moisture infiltration due to precipitation. The cover system
presented herein is a multilayer design consisting of a vegetated
top layer, drainage layer, and low-permeability layer. Optional
layers which may be required for site-specific conditions are
also discussed. Rationale is provided for the design parameters
to give designers and permit writers background information and
an understanding of cover systems.
This document is intended for use by organizations involved
in permitting, designing, and constructing hazardous waste land
disposal facilities.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
111
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PREFACE
Subtitle C of the Resource Conservation and Recovery Act
(RCRA) requires the U.S. Environmental Protection Agency (EPA) to
establish a Federal hazardous waste management program. This
program must ensure that hazardous wastes are handled safely from
generation until final disposition. EPA issued a series of
hazardous waste regulations under Subtitle C of RCRA that are
published in Title 40 Code of Federal Regulations (40 CFR). The
principal 40 CFR Part 264 and 265 regulations were issued on
July 26, 1982 for treatment, storage, and disposal (TSD)
facilities and establish performance standards for hazardous
waste landfills, surface impoundments, land treatment units, and
waste piles. The regulations have been amended several times
since then.
In support of the regulations, EPA has been developing three
types of documents to assist preparers and reviewers of RCRA
permit applications for hazardous waste TSD facilities. These
include RCRA Technical Guidance Documents, Permit Guidance
Manuals, and Technical Resource Documents (TRDs).
RCRA Technical Guidance Documents, such as this one, present
design and operating parameters or design evaluation techniques
that generally comply with, or demonstrate compliance with, the
Design and Operating Requirements and the Closure and Post-
Closure Requirements of 40 CFR Part 264.
The Permit Guidance Manuals are being developed to describe
the permit application information the Agency seeks, and to
provide guidance to applicants and permit writers in addressing
information requirements. These manuals will include a
discussion of each set of specifications that must be considered
for inclusion in the permit.
The Technical Resource Documents present summaries of state-
of-the-art technologies and evaluation techniques determined by
the Agency to constitute good engineering designs, practices, and
procedures. They support the RCRA Technical Guidance Documents
and Permit Guidance Manuals in certain areas (i.e., liners,
leachate management, final covers, and water balance) by
describing current technologies and methods for designing
hazardous waste facilities, or for evaluating the performance of
a facility design. Although emphasis is given to hazardous waste
facilities, the information presented in these TRDs may be used
iv
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for designing and operating nonhazardous waste TSD facilities as
well. Whereas the RCRA Technical Guidance Documents and Permit
Guidance Manuals are directly related to the regulations, the
information in these TRDs covers a broader perspective and should
not be used to interpret the requirements of the regulations.
This document is a Technical Guidance Document prepared by
the Risk Reduction Engineering Laboratory of EPA's Office of
Research and Development in cooperation with the Office of Solid
Waste and Emergency Response. The document has undergone
extensive technical review and has been revised accordingly.
With the issuance of this document, all previous drafts are
obsolete and should be discarded.
Comments are welcome at any time on the accuracy and
usefulness of the information in this document. Comments will be
evaluated, and suggestions will be incorporated, wherever
feasible, before publication of any future revisions. Written
comments should be addressed to EPA RCRA Docket (OS-305), 401 M
Street S.W., Washington, DC 20460. The document for which
comments are being provided should be identified by title and
number.
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ABSTRACT
The owner or operator of a landfill, or a surface
impoundment closed as a landfill, must meet the closure
requirements specified under 40 CFR 264.310 (permitted units) or
40 CFR 265.310 (interim status units).
This guidance document addresses landfill covers and
recommends a multilayer final cover design that includes the
following elements, from top to bottom:
o a top layer consisting of two components: (1) a vegetated
or armored surface component, either of which is selected
to minimize erosion and, to the extent possible, promote
drainage off the cover, and (2) a soil component with a
minimum thickness of 60 cm [24 in.] comprised of topsoil
and/or fill soil as appropriate, the surface of which
slopes uniformly at least 3 percent but not more than 5
percent;
o a soil drainage layer with a minimum thickness of 30 cm
(12 in.) and a minimum hydraulic conductivity of l x 10"2
cm/sec that will effectively minimize water infiltration
into the low-permeability layer, and a final bottom slope
of at least 3 percent after settlement and subsidence; or
the drainage layer may consist of geosynthetic materials
with equivalent performance characteristics; the drainage
layer also serves as a protective cover for the flexible
membrane liner (FML) component of the underlying low-
permeability layer;
o a two-component low-permeability layer, that limits water
infiltration into the underlying wastes to a rate less
than or equal to the rate of leachate migration out of the
bottom liner system and consists of (1) a 20-mil minimum
thickness [or greater depending on the material and
design] FML component and (2) a 60-cm [24-inch] minimum
thickness compacted soil component with an in-place
saturated hydraulic conductivity no less than 1 x 10"7
cm/sec. (NOTE: The requirement for FMLs in the cover are
for all permitted units and interim status units with an
FML in the bottom. For interim status units with only a
clay bottom liner, an FML may not be required.)
VI
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Optional layers that may be used on a site-specific basis
include (1) a gas vent layer to remove gases produced within the
wastes, and/or (2) a biotic barrier layer to protect the cover
from animal or plant intrusion.
The Agency recommends a detailed construction quality
assurance (CQA) program for each layer of the final cover system.
CQA records should document quality and demonstrate compliance
with plans and specifications. The cover design process must
consider many site-specific factors, such as precipitation,
construction materials, freeze-thaw phenomena, waste
characteristics, potential subsidence, and other environmental
factors.
via
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CONTENTS
Disclaimer ii
Foreword iii
Preface iv
Abstract vi
Figures x
Tables xi
Acknowledgment xii
1. Introduction 1
1.1 Purpose 1
1.2 Closure and Post-Closure Regulations 1
1.3 Liquids Management Strategy 4
1.4 General Cover System Recommendations 5
1.4.1 Design Recommendations 5
1.4.2 Construction Quality Assurance 8
1.4.3 Settlement and Subsidence 8
2. Top Layer 11
2.1 Design 11
2.2 Discussion 12
2.2.1 Upper Component of Top Layer 12
2.2.1.1 Vegetation . .- 13
2.2.1.2 Other Erosion-Impeding Materials . 13
2.2.2 Lower Component of Top Layer 14
3. Drainage Layer 16
3.1 Design 16
3.2 Discussion 18
4. Low-Permeability Layer 22
4.1 Design 23
4.2 Discussion 24
4.2.1 FML Component 25
4.2.2 Low-Permeability Compacted Soil Component . 27
Vlll
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CONTENTS (continued)
5. Optional Layers 31
5.1 Gas Vent Layer . . 31
5.1.1 Design 31
5.1.2 Discussion 32
5.2 Biotic Barrier Layer 33
5.2.1 Design 33
5.2.2 Discussion 34
References * 36
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FIGURES
Number Page
1. EPA-recommended cover design 6
2. EPA-recommended cover design with optional layers ... 7
3a. Cover with sand drainage layer 17
3b. Cover with geogrid drainage layer 17
4. Cover and liner edge configuration with example ....
toe drain 19
5. Detail of FML/soil composite low-permeability layer . . 22
6. Regional depth of frost penetration 30
7. Cover with gas vent and vent layer 31
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TABLES
Number Page
1. Closure and Post-Closure Regulatory Requirements .... 3
2. Synopsis of Minimum Technology Guidance for Covers ... 9
XI
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ACKNOWLEDGMENT
The EPA Project Manager who directed the draft preparation
of this document was Les Otte with the assistance of Ana Aviles,
both of the Environmental Protection Agency's Office of Solid
Waste, Land Disposal Branch. Early drafts of the document were
prepared by David C. Anderson of K. W. Brown & Associates, Inc.
Later drafts were prepared by Jeffrey D. Magaw, Charles W. Young,
and Clay Spears of Alliance Technologies Corporation. This draft
has been prepared by Robert P. Hartley of EPA's Risk Reduction
Engineering Laboratory, after peer reviews by Dr. Gordon Boutwell
of Soil Testing Engineers, Inc.; Dr. Richard C. Warner,
University of Kentucky; Leo Overman of Colder Associates, Inc.;
Dirk Brunner of E. C. Jordan, Inc.; the Solid and Hazardous Waste
Management Committee of the American Society of Civil Engineers,
Environmental Engineering Division; and by various members of the
EPA's Risk Reduction Engineering Laboratory.
XII
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1. INTRODUCTION
1.1 PURPOSE
This document provides design guidance on final covers for
hazardous waste units. The recommended design satisfies the
requirements of 40 CFR 264 and 265 Subparts G (closure and
post-closure), K (surface impoundments), and N (landfills). The
Environmental Protection Agency (hereafter referred to as "the
Agency") emphasizes that recommendations are guidance only and
not regulations. The Agency acknowledges that other final cover
designs may be acceptable, depending upon site-specific
conditions and upon a determination by the Agency that an
alternative design adequately fulfills the regulatory
requirements. It is, however, the responsibility of the facility
owner or operator to prove that the alternate design will provide
a level of performance that is at least equivalent to that of the
final cover system described in this document.
The Agency's liquids management strategy for landfills, and
the role that final covers serve in that strategy, are outlined
in general terms for background. Regulatory requirements for
landfill and surface impoundment covers are also outlined, as
well as differences in requirements between interim status and
permitted units. The Agency-recommended final cover system
design is presented in detail, as well as considerations for
construction quality assurance. Attention is given to erosion,
settlement, and subsidence, and their potential cover-damaging
effects.
A separate section of this document is devoted to the design
details of each layer of the recommended cover. A discussion of
the rationale for the recommended specifications is included.
1.2 CLOSURE AND POST-CLOSURE REGULATIONS
All of the regulations dealing with hazardous waste landfill
and surface impoundment cover requirements are found in/Title 40,
Parts 264 and 265, of the Code of Federal Regulations-"t40 CFR 264
and 40 CFR 265) . Part 264 deals with permitted fa»cilities and
Part 265 with interim status facilities. Interim status
facilities are, in general, those facilities that were in
existence on November 19, 1980. Three Subparts of each of Parts
264 and 265 deal with general closure requirements: Subpart G -
Closure and Post-Closure; Subpart K - Surface Impoundments; and
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Subpart N - Landfills. Each Subpart contains several sections
important to cover planning, design, and construction, as
outlined in Table 1.
There are few difference between permitted and interim
status unit closure and post-closure regulations under Subpart G
of Parts 264 and 265. The major difference is that, for interim
status units, public notice for changes to the approved closure
and post-closure plans is not required. Changes to plans for
permitted units reguire permit modifications which, in turn,
require public notice and comment.
There are three significant differences between permitted
and interim status unit final cover regulations under Subparts K
and N of Parts 264 and 265. Part 264.303 requires monitoring and
inspection to ensure that synthetic and soil materials used in
the cover are watertight and structurally uniform. Such a
requirement was not included in Part 265 for interim status
units. The Agency recommends that a Construction Quality
Assurance (CQA) program, establishing inspection activities, be
utilized for covers being built at both permitted and interim
status units. The Agency believes that a site-specific CQA
inspection program is necessary to ensure that cover design
specifications are met.
A second difference in requirements is that, while leachate
collection and removal activities are required after closure
under 40 CFR 264.310, for permitted units, they are not required
under Part 265 for interim status units. The absence of a stated
post-closure leachate collection and removal requirement makes
cover performance for interim status units even more important.
It should be noted that, under the broader performance standards
of 40 CFR 265.111, the Agency may still require leachate
collection during post-closure at an interim site.
The third, and perhaps most significant, difference is in
the requirements of 40 CFR 264.310(b)(1)(v) and 40 CFR 265.310
(b)(1)(v). These subsections require that the cover have a
permeability less than or equal to any bottom liner or natural
subsoil present. For interim status units, without an engineered
liner, the cover could presumably be of relatively permeable
materials. But here again, the Agency may impose the standards
of 40 CFR 265.111, and require a more impermeable cover.
For permitted landfills, to meet the requirements of 40 CFR
264.310, the cover must have a permeability no greater than that
of the double liner required under 40 CFR 264.301(c). The Agency
does not consider this to mean that the final cover for a
permitted unit must actually contain a double liner. Rather, the
Agency recommends that the final cover include a layer whose
liquid-rejection performance is equal to or better than the
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Table 1. Closure and Post-Closure Regulatory Requirements
Section
Part 264
Part 265
Subpart G - Closure and Post-Closure
111
112
113
115
116
117
118
120
Closure performance
standard.
Closure plan; amendment
of plan.
Time allowed for closure.
Certification of Closure.
Survey plat.
Post-closure care.
Post-closure plan;
amendment of plan.
Certificate of completion
of postclosure care.
Closure performance
standard.
Closure plan; amendment
of plan.
Time allowed for closure.
Certification of closure.
Survey plat.
Post-closure care.
Post-closure plan;
amendment of plan.
Certificate of completion
of post-closure care.
226
228
Subpart K - Surface Impoundments
Monitoring and inspection. Inspections.
Closure and post-closure
care.
Closure and post-
closure care.
301
302
303
310
Design and operating
requirements.
N/A
Subpart N - Landfills
Design requirements,
Closure and post-closure
care.
General operating
requirements.
Monitoring and inspection. N/A
Closure and post-
closure care.
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bottom composite liner (flexible membrane liner [FML] underlain,
and in full contact with, compacted soil) of the double-liner
system detailed in the "Minimum Technology Guidance on Double
Liner Systems for Landfills and Surface Impoundments - Design,
Construction and Operation" (EPA, 1987i). The Agency-recommended
design for the cover does, in fact, include a composite barrier
layer as outlined in Section 4. In all cases where a FML is used
in the bottom liner, one should also be used in the cover. This
does not mean, however, that the Agency necessarily recommends
the use of exactly the same barrier materials in both the liner
and cover. For example, different FML materials of equivalent
performance may be used, such as high density polyethylene for
the bottom liner and polyvinyl chloride in the cover.
The Agency also recommends using the composite FML/clay
barrier in interim status unit covers. However, for interim
status units, compacted clay with a permeability equal to or less
than 1 x 10"7 cm/sec may be used without a FML if the clay is
less permeable than the landfill bottom liner or natural subsoil
beneath the site. While 40 CFR 265.310(a)(5) might allow a less
effective design, we believe the long-term protection from
infiltration provided by the recommended cover design justifies
its use for all units. With the Agency-recommended composite
design, it is more certain that the cover will be no more
permeable than the bottom of the unit. In addition, the
installation of the composite design on interim status units
takes advantage of the practical opportunity to more effectively
minimize water infiltration, leachate generation, and leachate
migration.
1.3 LIQUIDS MANAGEMENT STRATEGY
The general closure performance standards are specified in
40 CFR 264.111 and 265.111 (Subpart G) for permitted and interim
hazardous waste disposal facilities, respectively. The standards
state that:
"The owner or operator must close the facility in a manner
that: :
a. Minimizes the need for further maintenance; and
b. Controls, minimizes, or eliminates, to the extent
necessary to protect human health and the
environment, post-closure escape of hazardous
waste, hazardous constituents, leachate,
contaminated runoff, or hazardous waste
decomposition products to the ground or surface
waters or to the atmosphere . . "
The requirements apply to hazardous waste landfills and to
hazardous waste surface impoundments closed as landfills.
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Landfill closure requirements are based on a two-part
liquids management strategy of (1) minimizing the leachate
generation by keeping liquids out of the unit, and (2) detecting,
collecting, and removing leachate within the unit. Closure
requirements are specified in 40 CFR 264.310 and 40 CFR 265.310
and include a final cover and post-closure care.
The Agency considers keeping water out of the unit to be the
prime element of the strategy. Thus, the Agency believes that a
properly designed and constructed cover becomes, after closure,
the most important feature of the landfill structure. The Agency
requires that the cover be designed and constructed to provide
long-term minimization of the movement of water from the surface
into the closed unit. Where the waste mass lies entirely above
the zone of ground-water saturation, a properly designed and
maintained cover can prevent, for all practical purposes, the
entry of water into the closed unit, and thus minimize the
formation and migration of leachate. In the absence of damage,
the cover design recommended here, including the FML/soil low-
permeability layer, should" restrict infiltration, to the extent
of the design, for the long term.
1.4 GENERAL COVER SYSTEM RECOMMENDATIONS
The cover system should be a major consideration during site
selection, planning, and initial design of the landfill
containment structure. Factors for consideration include
location and availability of low-permeability soil, stockpiling
of topsoil, restricting height to provide stable slopes, and site
use beyond the post-closure care period.
1.4.1 Design Recommendations
The final cover recommended in this guidance document is a
multilayer design (Figure 1) comprised as follows, from top to
bottom:
o a top layer consisting of two components: (1) either
a vegetated or armored surface component, selected to
minimize erosion and, to the extent possible, promote
drainage off the cover, and (2) a soil component with
a minimum thickness of 60 cm [24 in.], comprised of
topsoil and/or fill soil as appropriate, the surface
of which slopes uniformly at least 3 percent but not
more than 5 percent; a soil component of greater
thickness may be required to assure that the
underlying low-permeability layer is below the frost
zone;
o either a soil drainage (and FML-protective bedding)
layer with minimum thickness of 30 cm (122 in.) and a
minimum hydraulic conductivity of 1 x 10' cm/sec that
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will effectively minimize water infiltration into the
low-permeability layer, and will have a final slope of
at least 3 percent after settlement and subsidence; or
a drainage layer consisting of geosynthetic materials
with equivalent performance characteristics; and
a two-component low-permeability layer, lying wholly
below the frost zone, that provides long-term
minimization of water infiltration into the underlying
wastes, consisting of (1) a 20-mil [0.5 mm] minimum
thickness flexible membrane liner [FML] component and
(2) a compacted soil component with a minimum
thickness of at least 60 cm [24 in.] and a maximum in-
place saturated hydraulic conductivity of 1 x 10"
cm/sec.
vegetation/soil
top layer
drainage layer
low-permeability
FML/soil layer
waste
60cm
-^— filter layer
30cm
" E-E^E^E^E^E-EZ^ 60 cm
20-mil FML
O
0?
0
0
O
O
e> ^
Q3
o o
Figure l. EPA-recommended cover design.
Optional layers may be used on a site-specific basis.
Figure 2 depicts a cover design that includes optional layers.
Two such layers include (1) a gas vent layer to remove gases that
are produced within the wastes, and/or (2) a biotic barrier layer
to protect the cover from animal or plant intrusion.
Geosynthetic filter materials may also be used to prevent
migration of fine materials from one layer into another or to
prevent clogging of the drainage layer.
The Agency recognizes, for specific cases, that alternative
designs (e.g., fewer layers or optional layers) may be
applicable. For instance, in extremely arid regions, a gravel-
armored top surface component might serve to compensate for a
naturally reduced vegetation coverage and the erosion control
that it provides. Also, in arid regions the drainage layer might
not be required. In areas where burrowing animals may damage the
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low-permeability layer, the damage may be prevented by use of an
overlying "biotic barrier" layer of large-size material, such as
cobbles. A gas vent layer between the waste and the low-
permeability layer may be installed, as shown in Figure 2, at
units that produce gases.
Alternative designs must provide long-term performance at
least equivalent to the recommended design outlined in this
guidance. All alternative designs must be approved by the
appropriate Regional Administrator of the Agency.
cobbles/soil -
top layer
biotic barrier
(cobbles)
drainage layer
low-permeability.
FML soil layer
gas vent layer
waste
o <0 o o (7 0 o ° o
e>-J: c
o o
o C, o
60cm
30cm
30 cm
geosynthetic filter
geosynthetic filter
20-mil FML
60cm
30cm
geosynthetic filter
Figure 2. EPA-recommended cover design
with optional layers.
In some cases, where the waste is of such character that
vertical migration of gases is impeded, full-depth vent
structures to the bottom of the waste mass may be needed. These
structures would be designed to prevent the horizontal migration
of gases out of the landfill into the surrounding soil. Active
rather than passive systems may be required in some cases to
adequately remove accumulated gases.
Filter layers are likely to be needed above the drainage
layer and between layers that are comprised of soils of greatly
different particle sizes, to prevent one from migrating into the
other. The filters may 'be constructed of soils of intermediate
grain size, or they may be geosynthetic materials. Three
between-layer locations where geosynthetic filters may be
appropriate are shown in Figure 2.
Table 2 presents a synopsis of the Agency-recommended
components of a landfill and their principal design parameters.
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1.4.2 Construction Quality Assurance (CQA)
The Agency believes that the landfill owner or operator
should implement a detailed construction quality assurance (CQA)
program for the final cover system based on written plans for
inspecting the quality of construction materials and the
construction practices employed in their placement. The Agency
believes that use of a CQA program is essential for determining,
with a reasonable degree of certainty, whether a completed final
cover system meets or exceeds all design criteria, plans, and
specifications. The Agency has issued technical guidance that
includes final cover CQA (EPA, 1987i).
The Agency has proposed CQA rules for both permitted and
interim status units (EPA, 1987b). These proposed rules would
require a CQA program for installing the following components of
landfills, surface impoundments and waste piles: foundations;
low-permeability soils; FMLs; dikes; leachate detection,
collection, and removal systems; and final covers. The CQA plan
would be site-specific. It should address activities such as
inspecting, monitoring, and sampling of the individual
components. For the cover, the CQA plan should provide assurance
that: 1) all layers of the final cover are uniform and damage-
free; 2) the materials for each layer are as specified in the
design specifications; and 3) each layer is constructed as
specified in the design.
1.4.3 Settlement and Subsidence
Settlement within a closed hazardous waste landfill can
disrupt the integrity and function of the final cover system.
Settlement of the waste may be uniformly distributed and may
occur primarily before placement of the final cover. Subsidence,
however, is considered to be an unevenly distributed settlement
(i.e., differential settlement) after closure that can disrupt
the integrity of the final cover by creating depressions and
cracks. In addition, subsidence due to the collapse of drums
(this will occur mainly in older units), the leaching of soluble
waste constituents, or biodegradation of organic matter in the
waste, may not begin until several years after closure or it may
occur gradually over decades.
To reduce the potential for damage from settlement and
subsidence, the final cover should be designed and constructed to
allow for the total estimated settlement. The final grade after
subsidence of the'cover should be at the actual desired design
elevation. The cover design process used to establish the final
grade elevation should include consideration of the following:
o consolidation of all waste layers (the primary
consideration) and daily and intermediate soil covers;
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Table 2. Synopsis of Minimum Technology Guidance for Covers
Layer
Thickness
Slope
Requirements
Top Layer
Vegetation
OR
Surface Armor 5-10 in.
(13-25 cm)
ON
Soil
Drainage Layer
Soil
OR
> 24 .in.
(> 60 cm)
j> 12 in.
(> 30 cm)
Geosynthetic variable
Low-Permeability Layer
FML
ON
> 20 mils
(j> 0.5 mm)
Low-Perme-
ability Soil
> 24 in.
(> 60 cm)
3-5%
> 3%
> 3%
> 3%
> 3%
Optional Layers (site-specific design)
Gas Vent Layer > 12 in. > 2%
(> 30 cm)
Biotic Barrier animal or
root-dependent
Persistent,
drought-resistant,
adapted to local
conditions.
Cobbles, gravel.
Erosion rate
<2 ton/acre/yr
(5.5 MT/ha/yr).
SP (USCS) soil
K > 1 x 10"2 cm/s;
gravel toe drain.
Performance equi-
valent to soil,
hydraulic transmis-
sivity > 3 x 10"5
m/sec.
In EPA Report No,
EPA 600/2-88-052.
In-place
K < 1 x 10"' cm/s
and test fill.
,-7
Similar to
drainage layer.
Large materials,
e.g., cobbles.
-------
o consolidation of soils and foundation materials
underlying the site;
o consolidation of liner and leachate collection
systems; and
o consolidation of all final cover components.
The Agency has published two technical research reports on
cover settlement and subsidence (EPA, 1985c and 1987d) that
address both the theoretical and practical aspects.
Interim covers have been proposed when a significant amount
of settlement and subsidence is expected in a fairly short time
(say 2-5 years) that could result in the premature failure of a
final cover. An interim cover could be maintained until settling
is judged to be virtually complete. After settlement occurs, the
interim cover could be removed and replaced or overlain by a new
final cover. If components of the interim cover can meet the CQA
requirements for the final cover, the interim cover could be made
an integral part of the final design.
In no case can an interim cover be used that does not
satisfy the performance standards of 40 CFR 264.111 to protect
human health and the environment. Use of an interim cover on a
permitted unit will generally result in a longer closure period
during which the stipulations of 40 CFR 264.113 must be met,
i.e., the applicant must take all necessary steps "to prevent
threats to human health and the environment from the unclosed but
not operating hazardous waste management unit or facility,
including compliance with all applicable permit requirements."
10
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2. TOP LAYER
The Agency recommends a two-component top layer for a
landfill cover system (Figure 1). The upper component should be
vegetation or other surface treatment, designed to impede erosion
but allowing surface runoff from major storm events. The Agency
believes that, in most cases, vegetation underlain by soil, at
least part of which is topsoil, will best accomplish these
objectives. However, in some areas the prevailing climate may
inhibit the establishment and maintenance of vegetation, or a
planned alternative use of the site may preclude vegetation. In
those cases, an armored surface without vegetation (Figure 2),
and underlain by fill soil, might be used if it will minimize
erosion and abrasion of the cover and allow, to the maximum
practicable extent, surface drainage off the cover.
2.1 DESIGN
The Agency recommends that the vegetation component of the
top layer meet the following specifications:
o Locally adapted perennial plants.
o Resistant to drought and temperature extremes.
o Roots that will not disrupt the low-permeability
layer.
o Capable of thriving in low-nutrient soil with minimum
nutrient addition.
o Sufficient plant density to minimize cover soil
erosion to no more than 2 tons/acre/year (5.5
MT/ha/yr), calculated using the USDA Universal Soil
Loss Equation.
o Capable of surviving and functioning with little or no
maintenance.
In landfill situations where the environment or other
considerations make it inappropriate for maintaining sufficiently
dense vegetation, armoring material may be substituted as the
upper component of the top layer or in rare cases the whole
layer. It is recommended that the material possess the following
characteristics:
11
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o capable of remaining in place and minimizing erosion
of itself and the underlying soil component during
extreme weather events of rainfall and/or wind;
o capable of accommodating settlement of the underlying
material without compromising the purpose of the
component;
o surface slope approximately the same as the underlying
soil (at least 3 percent slope); and
o capable of controlling the rate of soil erosion from
the cover to no more than 2 tons/acre/year (5.5
MT/ha/yr), calculated by using the USDA Universal Soil
Loss Eguation.
Agency-recommended specifications for the lower soil
component of the top layer include the following:
o for vegetation support, a minimum thickness of 60 cm
(24 in.) including at least 15 cm (6 in.) of topsoil
(soil of lower quality may be used beneath an armored
surface); greater total thickness where required,
e.g., where maximum frost penetration exceeds this
depth, or where greater plant-available water storage
is necessary or desirable;
o medium texture to facilitate seed germination and
plant root development;
o final top slope, after allowance for settling and
subsidence, of at least 3 percent, but no greater than
5 percent, to facilitate runoff while minimizing
erosion; and
o minimum compaction to facilitate root development and
sufficient infiltration to maintain growth through
drier periods.
The owner or operator of the landfill should prepare a
separate section specific to monitoring construction of the top
layer to be included in the construction quality assurance (CQA)
plan.
2.2 DISCUSSION
2.2.1 Upper Component of Top Layer
As noted in the design recommendations above, the upper
component of the top layer may be vegetation (Agency-preferred
where possible) or other erosion-impeding materials. These are
discussed separately below.
12
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2.2.1.1 Vegetation
Plant species is an important consideration in the
establishment of vegetation when it is selected as the upper
component of the top layer. The use of shrubs and trees is
usually inappropriate because the root systems extend to a depth
that would normally invade the drainage layer or the low-
permeability layer. A large number of suitable plant species
such as grasses and low-growing plants are available for various
climates (EPA, 1983c and 1987c). The timing of seeding is also
very important to successful vegetation establishment.
The Agency advises landfill owners or operators to contact a
consulting agronomist, Cooperative Extension Service agent, or
local university for recommendations of adapted plant varieties
and other guidance on local crop cultivation. Several references
provide lists of available vegetation and discussions on site-
selection criteria (EPA, 1976, 1979, 1983a, 1983c, 1985a, and
1987c; Lee, et al., 1984; Thornburg, 1979; and Wright, 1976).
These references provide essential information about plant
species, seeding rate, time of seeding, and areas of adaptation.
2.2.1.2 Other Erosion-Impeding Materials
In areas where vegetation is inappropriate or difficult to
establish and maintain, other materials may be selected as the
upper component of the top layer. The materials should be
selected to prevent erosion of the cover and yet allow, as much
as practicable, for surface drainage. Several materials have
been suggested for use in lieu of vegetation, including broken
rock or cobbles that may prevent deterioration of the cap due to
wind, heavy rain, or temperature extremes (EPA, 1982b and 1985a;
Nyhan, et al., 1985; Pertusa, 1980). An example of such an upper
component is a layer several (perhaps eight or more) inches thick
comprised of 5- to 10-cm (2- to 4-in.) cobbles of hard durable
rock. The cobbles allow infiltration of rain water but retard
erosion due to water and wind action (see Figure 2). Asphalt or
concrete might be used if promoting runoff is a prime objective,
but they are likely to deteriorate, for example, by cracking due
to thermal effects and subsidence deformation (EPA, 1979 and
1987a) thus causing concern for their long-term performance.
Substantial maintenance could be expected for these materials.
Asphalt can be very permeable unless special attention is given
to eliminating the air voids during mixing and application (Repa,
et al., 1987) .
A surface armor component of very coarse materials promotes
infiltration rather than runoff. Thus, it may be more applicable
in arid areas. In those areas, leachate generation due to water
infiltration may not be a major concern, but it can happen during
infrequent short-duration storms of great intensity (EPA, 1987c).
13
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2.2.2 Lower Component of Top Layer
When the upper component of the top layer is vegetation, the
EPA recommends that the associated lower component be composed of
at least 60 cm (24 in.) of soil. The soil should be capable of
indefinitely sustaining plant species that will minimize erosion.
The minimum thickness of the soil component is based upon the
Agency's judgment that:
o it accommodates the root systems of most non-woody
plant species (EPA, 1983c);
o for most locales, it provides adequate water-holding
capacity to attenuate rainfall infiltration to the
drainage layer and to sustain vegetation through dry
periods; and
o it provides sufficient soil thickness to allow for
expected long-term erosional losses.
A layer thicker than 60 cm (24 in.) may be required to
prevent freezing and thawing from damaging the low-permeability
layer, or to increase plant-available water storage capacity in
drier climates.
Medium-textured soils such as loam soils, have the best
overall characteristics for seed germination and plant root
system development. Fine-textured soils, such as clays, are
often fertile but may be beset by management problems such as
puddling of water on the surface or difficulty in initial
establishment of plant cover during wet periods. Sandy soils are
often a problem due to low water retention and loss of nutrients
by leaching. It may be cost-effective to stockpile the topsoil
initially removed from a site for later use during cover
construction. Where only a minimum amount of native topsoil can
be saved by stockpiling, the remainder needed to provide at least
the minimum thickness of 60 cm (24 in.) may be made up by
selecting local borrow material having appropriate qualities.
The Agency recommends that the lower component of the top
layer (and thus the entire top layer) be slightly convex, or be
low in height above the surrounding terrain and uniformly sloped.
In non-level terrain, diversion structures should be installed to
prevent the run-on of surface water onto the cover. To prevent
ponding of rainwater due to irregularities of the surface of the
lower component, the final slope should be uniform and at least
3 percent, after allowance for settlement and subsidence (EPA,
1982a, p. 42). Slopes greater than 5 percent, however, are
likely to promote erosion unless controls are included in the
design. The design of surface water controls is well-documented
(EPA, 1979 and 1982b). The Agency believes that slopes greater
than 5 percent will increase erosion, decrease slope stability,
14
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and, in general, increase the long-term maintenance of the cover
system. Owners and operators using final slopes based on
site-specific conditions should determine that the slopes will
not result in the formation of erosion rills and gullies and will
limit total erosion to less than 2.0 tons/acre/year (5.5
MT/ha/yr). The U.S. Department of Agriculture's Universal Soil
Loss Equation (USLE) is recommended as the tool for use in
evaluating erosion potential (EPA, 1982a). The Agency believes
that a maximum erosion rate of 2.0 tons/acre/year (5.5 MT/ha/yr)
is realistically achievable for a wide range of soils, climates,
and vegetation. The Agency also believes that reliance on this
criterion will minimize gully development and cover maintenance.
15
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3. DRAINAGE LAYER
The recommended final cover design includes a drainage layer
for the removal of water which infiltrates through the top layer
(see Figures 3a and 3b). The drainage layer should be designed
to minimize the amount and residence time of water coming into
contact with the low-permeability layer, thereby decreasing the
potential for leachate generation. In other words, the drainage
layer construction materials and configuration should facilitate
the rapid and efficient removal of water to an exit drain.
The drainage layer should be designed, constructed, and
operated to function without clogging. Physical clogging may be
prevented by incorporating a filter layer of soil or geosynthetic
material between the top layer and the drainage layer. The
prevention of biological clogging may range from limiting
vegetation .to shallow-rooted species to the installation of a
biotic barrier (see Figure 2). Any or all of these features may
be included in a single cover design.
In arid locations, the need for, and design of, a drainage
layer should be based on consideration of precipitation event
frequency and intensity, and sorptive capacity of other soil
layers in the cover system. It may be possible to construct a
top layer that will absorb most, if not all, of the precipitation
that infiltrates into that layer, eliminating the need for a
drainage layer.
3.1 DESIGN
If composed of granular material such as sand (Figure 3a),
the Agency recommends that the cover drainage layer meet the
following specifications:
o Minimum thickness of 30 cm (12 in.) and minimum slope
of 3 percent at the bottom of the layer; greater
thickness and/or slope if necessary to provide
sufficient drainage flow as determined by site-
specific hydrologic (e.g., HELP) modeling.
o Hydraulic conductivity of drainage material should be
no less than 1 x 10"2 cm/sec (hydraulic transmissivity
no less than 3 x 10"5 m2/sec) at the time of
installation.
16
-------
Granular material should be no coarser than 3/8 inch
(0.95 cm), and classified as SP; it should be smooth
and rounded and should contain no debris that could
damage the underlying flexible membrane liner (FML),
nor should it contain fines that might lessen
permeability.
A filter layer (granular or geosynthetic) should be
included between the drainage layer and top layer if
necessary to prevent clogging of the drainage layer by
fine particles.
vegetation/soil
top layer
20-mil FML
>w-pernv
FML/soll layer
wast*
low-permeability -\ t£^El-El-EZ-EZ-E
filter layer
drainage layer
000
Q G
vegetation/soil J
top layer (
filter layer —«.
20-mll FML—»•
low-permeablllty
FML/soll layer
waste
0 •
L/vwvwv*
MB=O:O:O=Ei:t
o
0
Q
O
o 0
°0*
0 0
0
0
O 0
o 0
C5
O
0
-i geosynthetic
- drainage layer
Figure 3a,
Cover with sand
drainage layer.
Figure 3b. Cover with geosyn-
thetic drain layer.
If composed of geosynthetic materials (Figure 3b), the
Agency recommends that the drainage layer meet the following
specifications:
Same minimum flow capabilility as a granular drainage
layer in the same situation; hydraulic transmissivity
no less than 3 x
overburden for the design life.
10'5 m2/sec under anticipated
Inclusion of a geosynthetic filter layer above the
drainage material to prevent intrusion and clogging by
the overlying top layer soil material.
Inclusion of geosynthetic bedding beneath the drainage
layer, if necessary, to increase friction and minimize
slippage between the drainage layer and the underlying
FML, and to prevent intrusion, by deformation, of the
FML into the net or grid of the drainage layer.
17
-------
The owner or operator should prepare a written construction
quality assurance (CQA) plan to be used during construction and
installation of the drainage layer (see EPA, 1987b).
3.2 DISCUSSION
The primary functions of the drainage layer are to intercept
water that percolates through the top layer and to transport the
water out of the cover (for example, by gravity flow to an outlet
at the toe of the cover). The Agency believes that the criteria
presented above are the minimums required to provide cover
drainage and FML protection. The criteria for permeability and
FML bedding are equivalent to those cited in "Minimum Technology
Guidance on Double Liner Systems for Landfills and Surface
Impoundments — Design, Construction and Operation" (EPA, I987i)
for the leachate detection, collection, and removal system.
The recommended 30-cm (12-in.) minimum thickness of the
drainage layer allows sufficient cross-sectional area for
transport of drainage in most situations and for protection of
the FML during construction. In some cases, particularly where
unusually long drainage slopes may be part of the design,
drainage layers thicker than 30 cm (12 in.) and/or slopes greater
than 3 percent may be necessary. The minimum value of 1 x 10"2
cm/sec for permeability was chosen because granular materials
widely used as drainage media (i.e., SP soils) can provide this
minimum hydraulic conductivity. In situations where the minimum
criteria are insufficient or questionable, the design should
utilize flow modelling in arriving at the flow-controlling design
parameters. The HELP model (EPA, 1984a) can be of assistance for
this purpose.
Rounded grains with a maximum size of 3/8 inch (0.95 cm)
have been recommended, because they have been shown to be non-
damaging to most FMLs when in direct contact with them (EPA,
1984b). Crushed stone would not normally be appropriate due to
sharpness of the particles.
The drainage layer must slope to an exit drain which allows
percolated water to be efficiently removed. An example of an
exit drain is shown in Figure 4. Further information is provided
in EPA (1985a) and Bureau of Reclamation (1977) publications.
Care should be taken in the design to control the velocity of the
exiting water, within and beyond the exit drains, to prevent soil
loss and destabilization. Large safety factors may be needed to
accommodate unexpected events.
Materials used to construct the drainage layer should be
washed or screened prior to construction to remove fines that may
promote clogging. To further prevent clogging of the drainage
18
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drainage layers
FML
FML anchors
.(separate anchor trench for each geosynthetlc)
low-permeability soil
waste
FML
Figure 4. Cover and liner edge configuration with example
toe drain.
layer, the Agency recommends that a granular or geosynthetic
filter be placed directly over the drainage layer to minimize the
migration of fines from the overlying topsoil into the drainage
layer. If a graded granular filter is used, care should be taken
to design the relationship of grain sizes according to the
criteria presented below (Cedergren, 1967).
To prevent
piping:
J15
Filter
D85 Top soil layer
D15 Drainage layer
<4-5, and
<4-5
D85 Filter
To maintain
permeability:
D
>15 Filter
D.
D15 Top soil layer
>15 Drainage layer
D15 Filter
19
>4-5, and
>4-5
-------
D50 Filter
To achieve uniformity <25, and
of grain size distribution D50 Top soil layer
curves among top soil
layer, filter, and
drainage layer:
D50 Drainage layer
<25
D50 Filter
These criteria are cited by the Army Corps of Engineers for
selection of a filter layer in relation to a soil to prevent the
soil from piping through the filter. D85 refers to the size of
particle in the gradation, below which 85 percent by weight of
the particles have a smaller particle size. D15 and D50 have
similar definitions. The criteria must be satisfied for all
layers or media in the drainage system, including protected soil,
filter media, and drainage media. Criteria for granular and
geotextile filter design are found in numerous references (Horz,
1984; Bureau of Reclamation, 1984 and 1977; EPA, 1987e; and
Koerner, 1989) .
Innovative drainage systems, such as those using
geosynthetic materials (see Figure 3b), may be used if it can be
shown that they are at least equivalent to the recommended
granular system in hydraulic transmissivity, in performance
longevity (transmissivity must be maintained for cover's design
life), and in their ancillary function as FML bedding. Criteria
which should be addressed in determining equivalence of
geosynthetic and soil drainage materials include, but are not
limited to, the following:
o hydraulic transmissivity (the rate at which liquid can
be removed) no less than 3 x 10"5 m2/sec;
o compressibility (the ability to maintain open pore
space and thus transmissivity, under expected
overburden);
o deformation characteristics (the ability to conform to
changes in the shape of the surrounding materials);
o mechanical compatibility with the FML (the tendency
for the drainage material and the FML to deform each
other);
o useful life of the system; and
o ability to resist physical, chemical and biological
clogging.
20
-------
Geosynthetic drainage materials are manufactured in a
variety of configurations, which continue to evolve with
experience in manufacturing and use. "Geonets" and "geogrids"
are drainage components designed for rapid flowthrough. They are
manufactured as single components that usually must be separated
from overlying and underlying soils that could clog them. The
separating materials are also geosynthetics in the form of filter
fabric. The geogrid, and top and bottom filters (which may also
serve as protective bedding and slide-resistant materials), may
all be factory-bonded together in one unit. These bonded-
together materials, one form of "geocomposites," may be applied
in one operation as the entire drainage layer. The various forms
of geocomposites are .well-described by Koerner (1989). In
geosynthetic materials are continually being improved by the
manufacturers for durability and design.
21
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4. LOW-PERMEABILITY LAYER
The final cover system is required by 40 CFR 264.228,
264.310, and 265.310 to provide long-term minimization of
migration of liquids through the closed land disposal unit and to
have a permeability less than or equal to the permeability of the
bottom liner system or natural subsoils present. The Agency has
interpreted this to mean that the cover should contain a FML/soil
composite layer (Figure 5) similar in concept (but not
necessarily identical construction materials) to the composite
bottom liner detailed in "Minimum Technology Guidance on Double
Liner Systems for Landfills and Surface Impoundments — Design,
Construction and Operation" (EPA, 1987i) and in proposed
regulations (EPA, 1987a). The two components (FML and soil) of
the low-permeability layer recommended in this document are
considered to function as one system. They should be designed,
constructed, and operated to maximize removal of water by the
20-milFML —»
60-cm soil
waste
) O o
O O
*-n 0
O O,
O o
?o °
smooth even soil surface
15-cm lifts after compaction
0
0
0
0
s\
°
0
drainage layer
FML/soll
low-permeability
layer
Figure 5. Detail of FML/soil composite
,low-permeability layer.
overlying drainage layer and to minimize infiltration of water
into the waste. The low-permeability layer should require little
or no maintenance during and after the post-closure period. The
Agency recommends the same design for both permitted and interim
status units, although it may not be required for some interim
status units.
22
-------
4.1 DESIGN
The Agency recommends that the low-permeability layer be
located below the maximum depth of frost penetration and, at a
minimum, consist of the following two components:
1. An upper FML component with the following
characteristics:
a. The FML should be at least 20 mils (0.5 mm) in
thickness, but some units and/or some FML
materials may require a greater thickness to
prevent failure under potential stress of the
post-closure care period, or during construction.
The Agency recognizes that some types of FMLs must
be thicker to accommodate unique seamability
requirements, or to increase long-term durability
(e.g., increase resistance to puncture).
b. The surface of the FML should have a minimum
3 percent slope after allowance for settlement.
c. There should be no surface unevenness, local
depressions, or small mounding that create
depressions capable of containing or otherwise
impeding the rapid flow and drainage of
infiltrating water.
d. The Agency recommends the use of material and seam
specifications such as those in "Lining of Waste
Containment and other Impoundment Facilities"
(EPA, 1987h).
e. The FML should be protected by an overlying
drainage layer of at least 30 cm (12 in.) of soil
material no coarser than 3/8-in. (0.95-mm)
particle size, Unified Soil Classification System
(USCS) SP sand, free of rock, fractured stone,
debris, cobbles, rubbish, roots, and sudden
changes in grade (slope) that may impair the FML.
The overlying drainage layer should suffice as
bedding in most cases, but care should be taken
that any included drainage pipes are not placed in
a way that will damage the FML.
f. The FML should be in direct contact with the
underlying compacted soil component and should be
installed on a smoothed soil surface.
23
-------
i*4* .,...„„.,**.. <
g. The number of penetrations of the FML by designed
structures (e.g., gas vents) should be minimized.
Where penetrations are necessary, the FML should
be sealed securely around the structure.
h. Bridging or similar stressed conditions in the FML
should be avoided by providing slack allowances
for temperature-induced shrinkage of the FML
during installation and during the period prior to
placement of the protective layer or drainage
layer.
i. Slack should not be excessive to the extent that
folds are created that later may crack.
2. A bottom low-permeability soil component with the
following characteristics:
a. The soil should be at least 60 cm (24 in.) of
compacted, low-permeability soil with an in-place
saturated hydraulic conductivity of 1 x 10"7
cm/sec or less.
b. The compacted soil must be free of clods, rock,
.fractured stone, debris, cobbles, rubbish, and
roots, etc., that would increase the hydraulic
conductivity or serve to promote preferential
water flow paths.
c. The upper surface of the compacted soil (which is
in contact with the FML) should have a minimum
slope of 3 percent after allowance for settlement.
d. The soil layer should be constructed so that it
will be entirely below the maximum depth of frost
penetration upon completion of the cover system.
The written CQA plan prepared by the owner/operator should
include a separate section specific to monitoring the
installation of both the FML and compacted soil liner (see EPA,
1987i).
4.2 DISCUSSION
The Agency believes that the recommended two-component low-
permeability layer design (Figure 5) is the best practicable, in
most cases, to minimize infiltration of surface water into the
underlying waste. Both the FML and the compacted soil components
have excellent characteristics to prevent infiltration into
underlying waste over the long term when properly designed,
installed, and operated in accordance with site-specific
conditions. Their characteristics tend to complement each other,
24
-------
so that the long-term effectiveness of the two components
together is greater than each alone. A summary discussion of the
comparative effectiveness of the composite liner in the bottom
liner application appears in a Federal Register notice (EPA,
1987f). A more complete discussion appears in "Background
Document on Bottom Liner Performance in Double-Lined Landfills
and Surface Impoundments" (EPA, 1987g). In short, the FML will
tend to roof over the inconsistencies in the underlying compacted
soil, while the compacted soil will tend to significantly impede
the flow of any leakage through a hole in the overlying FML. In
addition, each component tends to back up the other in the event
of a failure of either.
In the past, due to lack of data on durability, the Agency
has considered the FML to be short-lived compared to compacted
soil. Thus, the Agency has thought of the FML as fulfilling a
function of "short-term prevention" of infiltration, while the
soil provides for "long-term minimization." With increasing
knowledge of FML characteristics and performance, and the
increasing technical ability to custom-tailor FML materials to
the containment need, it is now the consensus that they, too, can
be made to last for very long periods of time (EPA, 1988a). Of
course, this implies that care be taken in the construction, and
later operation of the facility, that all design requirements are
met, that certain waste consolidation conditions are met to
minimize settlement problems, and that physical damage does not
occur. The same implication applies to the soil component even
though the design requirements and potential physical damage are
significantly different.
The following subsections provide more detail on the design
rationale for each of the two components of the low-permeability
layer.
4.2.1 FML Component
The Agency recommends that, in no case, should the thickness
of the FML be less than 20 mils (0.5 mm). The Agency believes
that this is the minimum acceptable thickness to meet cover
objectives and still be sufficiently rugged to withstand expected
stresses during construction and operation. In many, if not
most, cases the thickness should be greater. The adequacy of the
selected thickness should be demonstrated by an evaluation
considering the type, strength, and durability of the proposed
FML material, its seamability, and site-specific factors such as:
steepness of slopes, physical compatibility with the material
used in the underlying and overlying layers, stresses of
installation, expected overburden, climatic conditions,
settlement, and subsidence.
25
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FML failure mechanisms are discussed in several reports (EPA
1985a, 1983b, 1987h). Most failures result from inadequacies in
the design and construction processes. It follows then that most
failures can be prevented if a strict quality assurance program
is adhered to during the construction process. The Agency has
placed great emphasis on construction quality assurance,
particularly in the construction of barrier layers, and has
published guidance in that area for landfill waste containment
liners (EPA, 1986).
One of the causes of FML failure in landfill and surface
impoundment lining systems is chemical incompatibility. However,
the FML in a final cover should not come in direct contact with
any wastes and chemical incompatibility should not be of concern.
This makes it possible to accept a wider range of FML materials
in cover systems. It should be remembered here that it was not
the Agency's intent in the regulations that the bottom liner and
cover barrier necessarily be constructed of the same material.
Another of the primary causes of FML failure is damage
during installation or operation. To aid in preventing damage,
such as punctures, rips and tears, at least 30 cm (12 in.) of
bedding material above and below the membrane is recommended.
Since the FML is in direct contact with the low-permeability soil
layer, that layer will serve as the underlying FML bedding. In
most cases, the drainage layer above the membrane will suffice as
the overlying FML bedding. A minimum underlying bedding
thickness of 30 cm (12 in.) is recommended, the same as for the
drainage layer.' The actual bedding thickness should, however, be
based upon consideration of failure mechanisms and construction
methods potentially harmful to the FML (e.g., if construction
equipment or methods are capable of penetrating the 30-cm [12-
in.] drainage layer and tearing, ripping or puncturing the FML,
then the thickness should be increased). If the design
thicknesses for drainage and bedding differ, then the greater
thickness should be used. Geosynthetic drainage materials may
also serve as protective bedding if they can provide equivalent
protection for the design life of the cover system.
Penetration of the FML by gas vents or drainage pipes should
be minimized. Where a vent is necessary, it is essential to
obtain a secure, liquid-tight seal between the structure and the
FML to prevent leakage of water around the vent (see Section 5).
Settlement of the material around the structure may create
destructive stresses in the FML, which should be taken into
account in the design of both the structure and the FML collar.
Differential settlement across the cover may also cause
disruptive stresses that should be accounted for in the FML
design. Care should be taken to make allowance for these and
other stresses. For example, wrinkles and folds might be created
26
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intentionally to reduce stress, but they may, in turn, result in
stresses in the folds that can lead to long-term failure of the
FML (EPA, 1988b).
The subgrade for the FML must be carefully prepared and
smoothed so that no small-scale stress points are created due to
protrusions of rocks or other materials. In most cases, this
should cause no difficulty, since the subgrade will be the low-
permeability soil component, comprised of fine material.
Field-seaming of the FML must be done carefully by
technicians qualified and experienced in seaming the particular
FML being installed. Holes can result from discontinuous seams
or those not sufficiently sturdy to withstand unavoidable
stresses. Some FMLs require destructive surface preparation
(e.g., grinding) prior to seaming; all will expand and shrink
with temperature changes. These characteristics may promote
later leakage if not carefully considered in the construction
process. All of the potential failure causes can be minimized or
prevented by using expert installers and adhering to a strict
construction quality assurance program (EPA, 1986).
4.2.2. Low-Permeability Compacted Soil Component
The Agency believes that a compacted soil component beneath,
and in direct contact with, the FML will:
o minimize, over the long term, liquid migration into
the waste in the event of FML failure or through
imperfections (holes, tears, etc.) inadvertently left
during the construction process;
o provide a firm foundation for the overlying layers of
the cover system;
o serve as bedding material for protection of the
overlying FML; and
o in conjunction with the FML, satisfy the regulatory
requirement for the cover to be no more permeable than
the bottom liner of the facility.
The design of the soil layer will depend on site-specific
factors including the properties and engineering characteristics
of the soil being compacted, the degree of compaction attainable,
the total expected load, and the expected precipitation.
The Agency recommends a minimum thickness of 60 cm (24 in.)
for the low-permeability soil component. The minimum thickness
is based upon constructability considerations and the ability to
provide uniformity in overall permeability. Sixty centimeters
allows for the installation of four lifts (see Figure 5),
27
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considered sufficient to overcome any inconsistencies in the
underlying surface. The four lifts also allow the localized
inconsistencies in permeability in one lift to be "sealed" by the
overlying lift.
As in the case of landfill liners, the Agency recommends the
use of a test fill (EPA, 1986, 1987i, and 1988c) prior to actual
construction of the soil component. The purpose is to
demonstrate, where appropriate soil is available, that the
compacted soil component actually can be constructed to an
hydraulic conductivity no greater than 1 x 10"7 cm/sec. (Most of
the ensuing discussion assumes that soil will be available that
can meet the 1 x 10"7 criterion.) The Agency believes that
construction of a test fill utilizing the soil, equipment, and
procedures to be used in construction of the low-permeability
layer will ensure that design specifications are attainable with
the available materials and equipment.
The test fill need not be constructed on the waste. The
Agency believes that, if the waste consolidation or compressive
strength of the waste is insufficient to allow adequate
compaction of the low-permeability soil component, that problem
should be corrected before installation of the compacted soil.
One of the possible solutions is to install an interim cover, as
noted earlier for the mitigation of subsidence. If components of
the interim cover can meet the CQA requirements for the final
cover, the interim cover could be made an integral part of the
final design.
Potential failure mechanisms that must be considered in
evaluating the design of the compacted soil component include
subsidence, dessication cracking, and freeze-thaw cycling.
Subsidence has been discussed in the Introduction. The main
factor of concern in design to counter subsidence is the
consolidation potential remaining at the time of cover
installation. That potential is difficult to estimate, but, in
the estimation, information regarding the presence of voids and
compressible materials in the underlying waste is all-important.
Ordinarily, most of the consolidation that will take place in
hazardous waste landfills has occurred by the time of cessation
of waste placement (EPA, 1985c and 1987d). An important benefit
is the ability of the compacted soil component to deform somewhat
without rupturing, a desirable characteristic related to the
soil's compressive and tensile strengths under expected field
conditions of moisture, density, etc.
The potential for desiccation of the compacted clay
component will depend on the physical properties of the clay,
design moisture content, local climatology, and moisture content
of the underlying waste. The actual clay-size particle content
of the soil, the type of clay, and properties such as liquid
28
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limit, plasticity, and shrinkage should be used to select a soil
with low cracking potential or in determining placement
procedures to reduce cracking potential (RTI, 1983; EPA, 1979).
Compaction of the soil component wet of optimum is
recommended by the Agency to assure that the lowest permeability
may be attained with standard Proctor densities (RTI, 1983). To
guard against drying in this case, the applicant may propose
immediate installation of the FML above the soil. If this is
done, it must first be assured that the installation of the soil
is complete, including a smooth surface on which to directly
apply the FML.
Freeze-thaw conditions are an important potential source of
damage to the soil component of the low-permeability layer.
Cycles of freezing and thawing may cause material cracking,
lessening of density, and loss of strength. This is brought
about by volume expansion of liquids in pore spaces during
freezing which, after thawing, increases the accessibility of
liquids to the pore spaces (EPA, 1983b). Cracking may be created
due to the expansion associated with freezing. For these
reasons, the Agency recommends that, upon completion of cover
system construction, the low-permeability layer be entirely below
the maximum depth of frost penetration estimated for the area in
which the facility is constructed. In other words, the top layer
and drainage layer of the cover together should be thicker than
the maximum depth of frost penetration. In northern areas of the
United States this recommendation would necessitate a top layer
thicker than the recommended 60-cm (24-in.) minimum.
Figure 6 is provided to show the variability of mean frost
penetration across the United States (Stewart, et. al., 1975).
The figure is provided only for perspective. It should not be
used to find the maximum depth of frost penetration at any
particular site. In determining the site-specific maximum depth
of frost penetration, advice may be sought from the Soil
Conservation Service, utility companies, construction
contractors, and universities in the area of concern.
Penetration of the low-permeability soil component by gas
vents or drainage pipes should be minimized. Adequate attention
must be given to the design of seals for such penetrations and
possible complications induced by differential settlement of
natural and man-made materials at penetration points. The Agency
has no information,specific to the adequacy of seals in the soil
component of the low-permeability layer.
29
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Figure 6. Regional average depth of frost penetration
in inches (Stewart, et al., 1975).
30
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5. OPTIONAL LAYERS
The optional layers discussed in this section are the gas
vent and biotic barrier layers. Other layers may be needed on a
site-specific basis. The Agency does not have information on the
performance of these layers in full-scale multilayer cover
systems.
5.1 GAS VENT LAYER ;
The function of a gas vent layer (Figure 7) is to control
combustible or toxic gases released from wastes buried in a
disposal facility. Hazardous waste disposal facilities that are
most likely to require a gas vent layer are co-disposal
facilities that receive organic waste material such as that found
in municipal waste. However, certain chemicals may also emit
gases or vapors in sufficient quantity to require venting.
gas vent
drain layer 1
FML
vent layer {f£l§J|
C?
top layer
low-permeability
FML/soil layer
waste
Figure 7. Cover with gas vent outlet and vent layer.
5.1.1 Design
The Agency offers the following design recommendations,
based upon engineering judgment, for a gas vent layer:
o The layer should be a minimum of 30 cm (12 in.) thick
and should be located between the low-permeability
soil liner and the waste layer (see Figure 2).
31
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o Materials used in construction of the gas vent layer
should be coarse-grained, porous materials such as
those used in the drainage layer.
o Geosynthetic materials may be substituted for granular
materials in the vent layer if equivalent performance
can be shown.
o Venting to an exterior collection point for disposal
or treatment should be provided by means such as
horizontal perforated pipes, patterned laterally
throughout the gas vent layer, which channel gases to
vertical risers.
o The number of vertical risers through the cover should
be minimized and located at.high points in the cross-
section, and designed to prevent water infiltration
through and around them.
An alternative design, particularly useful for layered
landfills where vertical migration is impeded, may include
perforated vertical collector pipes penetrating to the bottom of
the landfill. In this case, several cover penetrations may be
required, one for each standpipe. Here again, the pipes should
be securely sealed to the low-permeability layer. The standpipes
may be 30 cm (12 in.) or more in diameter and may be dual
purpose, serving also to provide access for measurement of
leachate levels.
The written CQA plan prepared by the owner/operator should
contain a specific section which covers monitoring the
construction and installation of the gas vent system.
5.1.2 Discussion
Materials used in construction of the gas vent layer should
have specifications similar to the granular material used for the
drainage layer. The materials should, be chosen and placed in a
way that facilitates the emplacement and compaction of the
overlying low-permeability soil component. Once placed, the
granular material should allow free movement of gases to
collection pipes and/or outlet points.
The outlets may consist of pipes or vents allowing the gas
to be collected, vented, or treated. The vent layer and outlet
should be designed to minimize cover penetrations which could
allow possible liquid infiltration through the cover. Outlet
vents should be constructed through the barrier layer at the
highest elevation of the gas vent layer to allow maximum
evacuation of gas (see Figure 7).
32
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A minimum thickness of 30 cm (12 in.) is recommended to
assure that a continuous layer of reasonable thickness (to allow
free movement of gases) is provided after placement on a non-
uniform waste surface.
In addition to providing gas removal, the gas vent layer may
provide a protective foundation upon which to construct the
compacted soil liner. The vent layer must be placed over the
waste up to design elevation, allowing for estimated settlement,
prior to placement and compaction of the soil liner. A filter,
either granular or geotextile, may be required between the gas
vent layer and the low-permeability soil to prevent clogging.
Alternative gas layer designs (e.g., using geosynthetic
materials) may be considered if it can be shown that they provide
a level of performance equivalent to a 30-cm (12-in.) granular
layer. Equivalence is based upon the ability of the design to
efficiently remove any gases produced, resist clogging, prevent
infiltration, withstand expected overburden pressures, and
function under the stresses of construction and operation.
Designs for gas vent layers can be found in several EPA
publications (EPA 1979, EPA 1985a).
Alternative, vertical standpipe gas collectors are
constructed of perforated sections, being built up as the unit is
filled with waste. They may be constructed of concrete and
wrapped with geosynthetic filter material to prevent clogging of
the perforations.
5.2 BIOTIC BARRIER LAYER
Plant roots or burrowing animals (collectively described as
biointruders) may disrupt the integrity of the drainage and low-
permeability layers. The drainage layer may be especially
susceptible to the intrusion of plant roots, which could
interfere with the drainage capability of the material. The
danger of FML penetration by plant intrusion has not been proven.
Burrowing animals may be a greater threat to FMLs, if a threat
indeed exists. In the absence of an FML, the low-permeability
soil layer could be exposed to both root and animal penetration.
Physical barriers, such as layers of cobbles or coarse
gravel beneath the top layer, and chemical barriers, have been
proposed to discourage or reduce the threat of biointrusion.
5.2.1 Design
The Agency knows of no full-scale application that would
prove the effectiveness of a biotic barrier in a landfill
situation. Therefore, the design of such a barrier must rely on
the results of small-scale field experiments. Experiments with
33
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barrier layers of cobbles have been carried out in arid or semi-
arid situations, with plants unique to such habitats (Cline, 1979
and Cline, et al, 1982). Some research suggests that three feet
(90 cm) of cobbles, or six inches (15 cm) of gravel over 30
inches (75 cm) of cobbles, may be effective in stopping root
penetration of some deep-rooted plants (DePoorter, 1982). It may
also be effective in stopping the invasion of burrowing animals.
The biotic barrier layer would directly underlie the soil
component of the top layer, perhaps separated by a geosynthetic
filter layer.
A polymeric herbicide carrier/delivery (PCD) system, used to
release herbicide, as discussed by Cline, et al. (1981), might be
installed within a cover, also just above the drainage layer to
stop the intrusion of roots below the system. The PCD system
would contain an herbicide designed to be released slowly over
many years. Note here the probable reluctance of the Agency in
approving this alternative, because it may introduce a hazardous
waste to the cover system, and/or it may not last through the
30-year post-closure period.
5.2.2 Discussion
Research by Cline (1979) and Hokanson (1986) found that if
objects, such as cobbles, placed in a burrowing animal's path are
too large or tightly packed, the animal's progress is effectively
stopped. Hokanson also found that large void spaces, which lack
water and nutrients, within the layer of stone, reduced the
intrusion of plant roots. On the other hand, the layer of very
coarse materials, at least in arid areas, may favor the growth of
grasses by impeding the downward percolation of moisture, thus
helping to retain it in the top soil layer.
Cline et al. (1982) also looked at the effectiveness of
several phytotoxins impregnated into polymeric sheets and buried
in soil. Some of them met the goal of being effective in
stopping the downward progress of root growth, with no other
effects. Some of the phytotoxins killed the plants when the
roots encountered the sheet, while others had no effect.
Obviously, a chemical biotic barrier must be chosen carefully, if
at all, to avoid potentially adverse environmental effects.
Most of the research on the effectiveness of biotic barriers
has been done in arid areas. Thus, the results must be used with
caution in areas of greater precipitation. The design and
resulting effectiveness of a biotic barrier are site-specific and
dependent upon the overlying topsoil layer, biotic barrier
material, natural precipitation, and anticipated biointruders.
34
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IH >
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