&EPA
United States
Environmental Protection
Agency
Technology Transfer
EPA/625/4-89/022
Seminar Publication
Requirements for
Hazardous Waste Landfill
Design, Construction, and
Closure
-------
-------
Technology Transfer
EPA/625/4-89/022
Seminar Publication
Requirements for Hazardous
Waste Landfill Design,
Construction, and Closure
August 1989
Center for Environmental Research Information
Office of Research and Development
U.S. Environmental Protectin Agency
Cincinnati, OH 45268
-------
NOTICE
The information in this document has been funded wholly or in part by the United States
Environmental Protection Agency under Contract 68-C8-0011 to Eastern Research
Group, Inc. It has been subject to the Agency's peer and administrative 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.
-------
CONTENTS
Page
Preface vi
1. Overview of Minimum Technology Guidance and
Regulations for Hazardous Waste Landfills 1
Background 1
Double Liners and Leachate Collection and
Removal Systems 2
Leak Detection Systems 6
Closure and Final Cover 9
Construction Quality Assurance 9
Summary of Minimum Technology Requirements 10
References 10
2. Liner Design: Clay Liners 11
Introduction 11
Materials 11
Clay Liners versus Composite Liners 12
Darcy's Law, Dispersion, and Diffusion 13
Laboratory Tests for Hydraulic Conductivity 17
Field Hydraulic Conductivity Testing 20
Field Tests versus Laboratory Tests 23
Attack by Waste Leachate 24
References 26
3. Flexible Membrane Liners 27
Introduction 27
Composite Liners: Clay versus Synthetic Components 27
Material Considerations 27
Design Elements 30
References 39
4. Elements of Liquid Management at Waste Containment Sites 53
Introduction 53
Overview 53
Primary Leachate Collection and Removal (PLCR) Systems 57
Leak Detection, Collection, and Removal (LDCR) Systems 62
Surface Water Collection and Removal (SWCR) Systems 65
Gas Collector and Removal Systems 66
References 70
5. Securing a Completed Landfill 75
Introduction 75
Flexible Membrane Caps 75
Surface Water Collection and Removal Systems 75
Gas Control Layer , 75
Biotic Barriers 78
Vegetative Layer 73
Other Considerations 81
iii
-------
6. Construction, Quality Assurance, and Control:
Construction of Clay Liners • • • 39
Introduction • • °9
Compaction Variables • • °9
The Construction Process ;• • 94
Construction Quality Assurance (CQA) Testing i. 95
Test Fills ;• • 96
7. Construction of Flexible Membrane Liners • • 99
Introduction • • 99
Responsibility and Authority '• • "
CQA Personnel Qualifications .• 10"
Inspection Activities 100
Sampling Strategies • 101
Documentation • • 103
8. Liner Compatibility with Wastes •• 109
Introduction • 1(^9
Exposure Chamber • 1°9
Representative Leachate •
Compatibility Testing of Components •
Blanket Approvals
Interpreting Data
9. Long-Term Considerations: Problem Areas and Unknowns ,. 113
Introduction
Flexible Membrane Liners •
Clay Liners •, •
Leachate Collection and Removal Systems :•
Cap/Closure Systems .•••'•
10. Leak Response Action Plans : •
Background • •
Action Leakage Rate (ALR) • • • •
Rapid and Large Leakage (RLL) 122
Response Action Plans (RAPs) ......;. 124
Preparing and Submitting the RAP • • • 124
Summary • f • 125
List of Abbreviations • 127
IV
-------
ACKNOWLEDGEMENTS
This seminar publication is based wholly on presentations made at the U.S.
Environmental Protection Agency (EPA) Technology Transfer seminars on
Requirements for Hazardous Waste Landfill Design, Construction, and Closure. These
seminars were held from June 20 to September 16, 1988 in San Francisco, California;
Seattle, Washington; Dallas, Texas; Chicago, Illinois; Denver, Colorado; Kansas Cityi
Missouri; Philadelphia, Pennsylvania; Atlanta, Georgia; New York, New York; and
Boston, Massachusetts. The presenters were:
Sarah A. Hokanson, the Earth Technology Corporation, Alexandria, Virginia
(Chapters 1 and 10)
Dr. David Daniel, University of Texas, Austin, Texas (Chapters 2 and 6)
Dr. Gregory N. Richardson, Soil & Materials Engineers, Inc., Raleigh, North
Carolina (Chapters 3, 5, and 7).
Dr. Robert M. Koerner, Drexel University, Geosynthetic Research Institute,
Philadelphia, Pennsylvania (Chapters 4 and 9).
Robert Landreth, U.S. Environmental Protection Agency, Risk Reduction
Engineering Laboratory, Cincinnati, Ohio (Chapter 8)
Susan Edwards, Linda Saunders, and Heidi Schultz of Eastern Research Group, Inc.,
Arlington, Massachusetts, prepared the text of this document based on the speakers'
transcripts and slides. Orville Macomber (EPA Center for Environmental Research
Information, Cincinnati, Ohio) provided substantive guidance and review.
-------
PREFACE
The U.S. Environmental Protection Agency's (EPA's) minimum technological require-
ments for hazardous waste landfill design were set forth by Congress in the 1984 Hazard-
ous and Solid Waste Amendments (HSWA). HSWA covered requirements for landfill lin-
ers and leachate collection and removal systems, as well as leak detection systems for
landfills, surface impoundments, and waste piles. In response to HSWA and other Con-
gressional mandates, EPA has issued proposed regulations and guidance on the design of
these systems, and on construction quality assurance, final cover, and response action
plans for responding to landfill leaks.
This seminar publication outlines in detail the provisions of the minimum technology
guidance and proposed regulations, and offers practical and detailed information on the
construction of hazardous waste facilities that comply with these requirements. Chapter
One presents a broad overview of the minimum technology guidance and regulations.
Chapter Two describes the use of clay liners in hazardous waste landfills, including the
selection and testing of materials for the clay component of double liner systems. Chap-
ter Three discusses material and design considerations for flexible membrane liners, and
the impact of the proposed regulations on these considerations. Chapter Four presents an
overview of the three parts of a liquid management system, including the leachate collec-
tion and removal system; the secondary leak detection, collection, and removal system;
and the surface water collection system. Chapter Five describes the elements of a closure
system for a completed landfill, including flexible membrane caps, surface water collec-
tion and removal systems, gas control layers, biotic barriers, and vegetative top covers.
Chapters Six and Seven discuss the construction, quality assurance, and control criteria
for clay liners and flexible membrane liners, respectively. Chapter Eight discusses the
chemical compatibility of geosynthetic and natural liner materials with waste ileachates.
Chapter Nine presents an overview of long-term considerations regarding hazardous
waste landfills, surface impoundments, and waste piles, including flexible membrane
and clay liner durability, potential problems in liquid management systems, and aesthet-
ic concerns. Chapter Ten reviews proposed requirements for response action plans for
leaks in hazardous waste landfills.
This publication is not a design manual nor does it include all of the latest knowledge
concerning hazardous waste landfill design and construction; additional sources should
be consulted for more detailed information. Some of these useful sources can bei located in
the reference sections at the end of several chapters. In addition, State and local authori-
ties should be contacted for regulations and good management practices applicable to lo-
cal areas.
VI
-------
1. OVERVIEW OF MINIMUM TECHNOLOGY GUIDANCE AND
REGULATIONS FOR HAZARDOUS WASTE LANDFILLS
This chapter presents a summary of existing and
proposed regulations and guidance on the design of
double liners and leachate collection and removal
systems, leak detection systems, final cover, and
construction quality assurance. An overview of
proposed regulations concerning leak response
action plans is given in Chapter Ten. More technical
discussion of these and other components of landfill
design and construction are given in Chapters Two
through Nine.
Background
EPA's minimum technological requirements for
hazardous waste landfill design and construction
were introduced by Congress in the 1984 Hazardous
and Solid Waste Amendments (HSWA). In HSWA
Section 3004(o)(l)(A), Congress required all new
landfills and surface impoundments to have double
liners and leachate collection and removal systems
(LCRS). In Section 3004(o)(4), Congress also required
leak detection systems at all new land disposal units,
including landfills, surface impoundments, and
waste piles. In response to other Congressional
mandates, EPA has issued proposed regulations or
guidance on the design of these systems. In addition,
EPA has issued guidance on construction quality
assurance programs and final cover. While not
specified in HSWA, the guidance and regulations in
the additional areas were issued by the U.S.
Environmental Protection Agency (EPA) to ensure
protection of human health and the environment.
For these new hazardous waste landfills and surface
impoundments, EPA and Congress have set forth
performance objectives of preventing hazardous
constituent migration out of a unit through the end
of post-closure care (or approximately 30 to 50 years).
The approach EPA has developed to meet those
performance objectives is called the Liquids
Management Strategy. The goal of the strategy is to
minimize leachate generation through both
operational practices and the final cover design, and
to maximize leachate collection and removal through
use of the lining system and LCRS.
To date, EPA has issued regulations and guidance
primarily focusing on double liners and leachate
collection and removal systems. Four Federal
Register notices and guidance documents have been
published by EPA in the last 4 years in this area (see
Table 1-1). EPA has issued proposed regulations
and/or guidance in the additional areas listed in
Table 1-2. The draft guidance on the final cover
issued in July 1982, which was never widely
distributed, is being revised for reissuance by the end
of 1989. EPA also plans to issue final regulations for.
double liners and for leak detection systems,
including construction quality assurance and
response action plans.
Table 1-1. Guidance and Regulations Issued to Date
(Double Liners and LCRS)
• Codification Rule (July 15, 1985)
• Draft Minimum Technology Guidance (May 24, 1985)
• Proposed Rule (March 28, 1986)
• Notice of Availability of Information and Request for Comments
(April 17, 1987)
Table 1-2. Guidance and Regulations Issued to Date
(Additional Areas)
Leak Detection Systems
• Proposed Rule (May 29, 1987)
Construction Quality Assurance
• Proposed Rule (May 29, 1987)
• Technical Guidance Document (October 1986)
Response Action Plan
• Proposed Rule (May 29, 1987)
Cover Design
• Draft Guidance (July 1982)
-------
Double Liners and Leachate Collection
and Removal Systems
Figure 1-1 is a simplified schematic diagram of a
hazardous waste landfill, showing the geometry and
placement of double liners and LCRSs in a landfill.
In a double-lined landfill, there are two liners and
two LCRSs. The primary LCRS is located above the
top liner, and the secondary LCRS is located between
the two liners. In this diagram, the top liner is a
flexible membrane liner (FML) and the bottom liner
is a composite liner system consisting of a FML
overlying compacted low permeability soil (or
compacted clay).
Existing (Draft) Guidance for Double
Liners
The EPA draft guidance issued in July 1985
discusses three types of liners: flexible membrane
liners (FMLs); compacted clay liners; and composite
liner systems (a FML overlying a compacted low
permeability soil layer). Material specifications in
the guidance for FMLs and compacted clay liners are
briefly reviewed below, along with existing and
proposed regulations regarding all three liner
systems.
The minimum thickness specification for a FML top
liner covered with a layer of soil is 30 mils; for a FML
without a soil cover layer, the specification is 45
mils. A FML in a composite bottom liner system
must be at least 30 mils thick. Even though these
FML thicknesses meet EPA specifications, 30 mils is
not a suitable thickness for all FML materials. In
fact, most FML materials installed at landfills are in
the range of 60 to 100 mils in thickness. Other key
factors affecting selection of FML materials include
chemical compatibility with waste leachate, aging
and durability characteristics, stress and strain
characteristics, ease of installation, and water
vapor/chemical permeation. These factors are
discussed in greater detail in Chapter Three.
For compacted, low permeability soil liners, the EPA
draft guidance recommends natural soil materials,
such as clays and silts. However, soils amended or
blended with different additives (e.g., lime, cement,
bentonite clays, borrow clays) also may meet the
current selection criteria of low hydraulic
conductivity, or permeability, and sufficient
thickness to prevent hazardous constituent
migration out of the landfill unit. Therefore, EPA
does not currently exclude compacted soil liners that
contain these amendments. Additional factors
affecting the design and construction of compacted
clay liners include plasticity index, Atterburg limits,
grain sizes, clay mineralogy, and attenuation
properties. These factors are discussed further in
Chapter Two.
Existing and Proposed Federal
Regulations for Double Liners
Figure 1-2 shows cross sections of three double liner
designs that have been used to; meet existing or
proposed regulations. The double liner design on the
left side of the figure meets the existing minimum
technological requirements (MTR) as codified in July
1985. The center and right-hand designs meet the
MTR as proposed by EPA in March 1986. The
existing regulations for MTR call for a double liner
system consisting of a FML top liner and a
compacted clay bottom liner that is 3 feet thick and
has a maximum saturated hydraulic conductivity of
no more than 1 x 10-7 centimeters per second
(cm/sec). The 1986 proposed rule on double liners
gives two design options for MTR landfills: one
similar to the existing MTR design (differing only in
that the compacted clay liner must be sufficiently
thick to prevent hazardous constituent migration);
and one calling for a FML top liner and composite
bottom liner. ',
EPA is currently leaning toward requiring a
composite bottom liner in the' final rule to be
published in the summer of 1989. The Agency also is
considering allowing use of a composite liner as an
optional top liner system, instead of a FML. The final
rule, however, probably will not have minimum
thickness or maximum hydraulic conductivity
standards associated with the compacted clay
component of such a composite top liner.
EPA's rationale for favoring the; composite bottom
liner option in the final double liner rule would be
based on the relative permeability of the two liner
systems. Figures 1-3 through 1-5 show the results of
numerical simulations performed by EPA (April
1987) that compare the performance of a composite
bottom liner to that of a compacted soil bottom liner
under various top liner leakage scenarios. In these
scenarios, liquids pass through defects in the top
FML and enter the secondary' LCRS above the
bottom liners. As illustrated in these numerical
results, the hydraulic conductivities of these bottom
liner systems greatly affect the amounts of liquids
detected, collected, and removed by the secondary
LCRS.
Figure 1-3 compares the compacted soil and
composite bottom liner systems in terms of
theoretical leak detection sensitivity, or the minimal
leak rate that can be detected, collected, and
effectively removed in the secondary LCRS. The
theoretical leak detection sensitivity is less than 1
gallon per acre per day (gal/acre/day) for a composite
liner having an intact FML component. This leak
detection sensitivity value reflects water vapor
transmission rates for FMLs with no defects. In
contrast, with well-constructed clay bottom liners
-------
Double Liners and Leachate Collection System
Components
- Protective Soil or
Cover (Optional)
r- Top Liner (FML)
— Drain Pipe (Typ)
Solid Waste
. . ,•';'. Drainage '•;.'. '•!•.•'•' : • .•'. ' • •.•'./'•':•.'•..'
'•'•' '• '• •: 'o'.' Material 'Q . '. •. •.'. :.•'.;''. .' Q ' '•' ', '-"•
Leachate Collection
and Removal System
Leachate Detection, .
Collection, and
Removal System (LDCRS)
.' • • j •••:••.•.••".. Drainage • • •'. •••.••';.'.•'.•••••• •. •
V'. • .'* '.!>'• ^/v. Material :.n • •>'.•»'• n'-'.'. '4'. .'•.•'
>>. • • o. . w • . • • u . i . •
Compacted Low-Permeability Soil
Native Soil Foundation
,"- 3'
Source: EMCON, 1988
Figure 1-1. Schematic of a double liner and leachate collection system for a landfill.
Bottom
Composite Liner
Upper Component
(FML)
Lower Component
(compacted soil)
Leachate Collection
System Sump
(Monitoring Compliance
Point)
cm/sec permeability), liquids entering the
secondary LCRS may go undetected and migrate into
the bottom liner until the leak rates approach 100
gal/acre/day. With a slightly more permeable
compacted clay bottom liner with 10-6 cm/sec
permeability, the secondary LCRS'may not detect,
collect, or remove the liquid flowing from a leak in
the top liner until leak rates are very serious (on the
order of 1,000 gal/acre/day).
Figure 1-4 compares theoretical leachate collection
efficiencies for landfills having compacted soil
bottom liners with those having composite bottom
liners. Leachate collection efficiency is the amount of
liquid collected and removed in the secondary
leachate collection system divided by the total
amount entering into the secondary LCRS through a
breach in the top liner. For low leakage rates, the
leachate collection efficiency of a landfill with a
composite bottom liner system, even a composite
system with tears or small defects in the FML, is
very high (above 95 percent for leak rates in the
range of 1 to 10 gal/acre/day). In comparison,
landfills with compacted clay bottom liners have 0
percent leachate collection efficiency for low leak
rates, and only 50 percent efficiency for leak rates of
approximately 100 gal/acre/day. These results
demonstrate that leachate collection efficiency of the
secondary LCRS improves significantly simply by
installing a FML over the compacted clay bottom
liner.
Figure 1-5 shows the total quantity of liquids
entering the two bottom liner systems over a 10-year
time span with a constant top liner leak rate of 50
gal/acre/day. A composite bottom liner with an intact
FML accumulates around 70 gal/acre, primarily
through water vapor transmission. Even with a 10-
foot tear, which would constitute a worst-case
leakage scenario, a composite liner system will allow
47,000 to 50,000 gal/acre to enter that bottom liner
over a 10-year time span. Compacted soil liners
meeting the 10-7 cm/sec permeability standard will
allow significant quantities of liquids into the bottom
liner, and potentially out of the unit over time, on the
order of hundreds of thousands of gallons per acre.
The numerical results indicate superior performance
of composite liner systems over compacted clay liners
in preventing hazardous constituent migration out of
the unit and maximizing leachate collection and
removal. Consequently, many owners of new units
subject to the double liner requirement of HSWA are
proposing and installing composite bottom liners or
double composite liner systems, even though they
are not required currently. A survey conducted in
February of 1987 and revised in November of that
year has indicated that over 97 percent of these MTR
-------
landfills and surface impoundments have one of
these two double liner designs.
Existing (Draft) Guidance for Leachate
Collection and Removal Systems
Double-lined landfills have both primary and
secondary LCRS. The design of the secondary LCRS
in the landfill receives particular attention in EPA's
proposed leak detection requirements. Described
below are the existing guidance and proposed
regulations applicable to both LCRSs in double-lined
landfills.
The components of a LCRS include the drainage
layer, filters, cushions, sumps, and pipes and
appurtenances. Of these components, the drainage
layer receives the most attention in the guidance and
regulations. The drainage layer can consist of either
granular or synthetic material. If granular, it must
be either clean sand or gravel with very little fines
content in order to facilitate the rapid collection and
removal of the liquids that accumulate above the top
liner and between the two liners. This minimizes
hydraulic head on both liner systems.
According to the draft guidance, the main selection
criteria for granular drainage materials are high
hydraulic conductivity and low capillary tension, or
suction forces. Figure 1-6 shows a range of hydraulic
conductivities for natural granular materials. For
typical drainage layer materials, permeabilities
range between 10-3 cm/sec and 1 cm/sec. A silty sand
drainage layer with significant fines content will
have a lower permeability (i.e., liO-3 cm/sec) and
significant capillary tension. At the-upper end of the
scale, drainage layers consisting of plean gravel can
achieve a permeability on the order of 1 cm/sec to 100
cm/sec. In this upper range of permeability, capillary
tension is negligible. Therefore clean sands and
gravels are preferred over silty sands.
Table 1-3 shows the correlation between
permeability and capillary rise (the elevation height
of liquids retained by granular particles within the
drainage layer by surface tension under unsaturated
conditions). At 10'3 cm/sec, there is significant
capillary rise (approximately 1 meter) while at the
upper end of the permeability scale (1 cm/sec), the
capillary rise is only on the order of an inch.
Reduction in fines content, therefore, significantly
reduces capillary rise while increasing hydraulic
Interim
Statutory Design
Proposed Designs
Waste
Leachate Collection
System Between
Liners
Design 1
Waste
O
o
3 Feet
Leachate Collection
System Between
Liners
Design 2
Waste
O
O
Native Soil
Native Soil
Top Liner
Designed, Constructed, and
Operated to Prevent
Migration—AL & PCCP
Bottom Liner
Composite
Upper Component Prevent (FML)
Lower Component Minimize (clay)
Compacted Soil
Sufficient Thickness
* to Prevent Migration
During AL & PCCP
Leachate Collection '
: System Between
Liners
o : o
Native Soil
Figure 1-2. Interim statutory and proposed double liner designs.
-------
1000
800
1
o
0
c
Q.
O
I-
o> 600
o
CO
oc
§* 400
^
co
CD
O
CD
Q
E
3
E
'E
i
200
860
Compacted Soil
k = 1 x 1 o-6 cm/sec
86
0.001
Compacted Soil
k = 1 x 1 0-7 cm/sec
Composite
(intact)
100
Type of Bottom Liner
Source: 52 FR 12570, April 17, 1987
Figure 1-3. Comparison of leak detection sensitivities for 3-
foot compacted soil and composite liners (one-
dimensional flow calculations).
conductivity. Increasing hydraulic conductivity, in
turn, results in rapid collection and removal of
liquids.
Synthetic drainage, materials-have-only recently
been introduced to^the wasfe management industry.
Unlike granular materials, synthetic drainage
materials come in various forms and thicknesses:
• Nets (160-280 mils)
• Needle-punched nonwoven geotextiles (80-200
mils)
• Mats (400-800 mils)
• Corrugated, waffled, or alveolate plates (400-800
mils)
50
o
O
I
I
CD
• Composite
(intact)
• Composite with
Small FML Hole
Compacted Soil -.
k = 1 x 10'7 cm/sec
11 Compacted Soil
= 1 x 10-6 cm/sec
1 10 100 1,000 10,000
Top Liner Leakage Rate (gal/acre/day)
Source: 52 FR 12572, April 17, 1987
Figure 1-4. Comparison of leachate collection efficiencies
for compacted soil and composite bottom liners.
Construction materials also vary. The most common
synthetic materials are polypropylene, polyester, or
polyethylene. More detailed discussion of these
drainage materials is presented in Chapter Four.
Because synthetic drainage layers are much thinner
(less than 1 inch) than granular drainage layers (1
foot) and have similar design liquids capacity, their
use in a landfill results in increased space for waste
storage and disposal. This advantage translates into
increased revenues fo"f"the owner/operator of a
-landfill.
The main selection criteria for synthetic drainage
materials are high hydraulic transmissivities, or
inplane flow rates, and chemical compatibility with
the waste leachate. Discussion of chemical
compatibility of synthetic liners and drainage layers
is given in Chapter Eight.
Hydraulic transmissivity refers to the value of the
thickness times the hydraulic conductivity for that
drainage layer. Over the lifetime of af facility, the
actual hydraulic transmissivities of synthetic
drainage layers are affected by two key factors: (1)
overburden stress and (2) boundary conditions. The
first factor pertains to the increasing loads (i.e.,
-------
200.000
la
150,000
£ 100,000
50,000
160,000
Compacted Soil
k = 1 x 10'7 cm/sec
47,000
Type of Bottom Liner
Source: 52 FR 12574 , April 17, 1987
Figure 1-5. Cumulative 10-year leakage into the bottom
liner for a leak of 50 gal/acre/day through the
side wall of the top liner.
Table 1-3. Capillary Rise as a Function of the Hydraulic
Conductivity of Granular Materials
Hydraulic Conductivity of
Drainage Medium (k)
cm/sec
Capillary Rise (h)
in
1 x 10-3
1 x 10-2
1
38.6
12.2
1.2
Source: EPA, May 1987
wastes, operating equipment, and final cover)
applied to the liner that an LCRS experiences over
the lifetime of the facility. The second factor pertains
to the stress-strain characteristics of adjacent layers
(i.e., PMLs, filters, cushions, compacted clay). Over
time and with increasing stress, adjacent layers will
intrude, or extrude, into the drainage layer and
result in clogging, or reduced transmissivity, of the
LCRS. '
Proposed Regulations for Leachaie
Collection and Removal Systems
Proposed regulations applicable tp LCRSs in double-
lined landfills (March 1986) differ in two principal
ways from existing standards for LCRS in single-
lined landfills and waste piles. First, LCRSs must be
designed to operate through the end of the post-
closure care period (30 to 50 years), and not simply
through the active life of the unit. Secondly, in a
double-lined landfill with primary and secondary
LCRSs, the primary LCRS need only cover the
bottom of the unit (i.e., sidewall coverage is
optional). The secondary LCRS, hbwever, must cover
both the bottom and the side walls.
As in the existing standards for single-lined landfills
and waste piles, the proposed regulations also
require that LCRSs be chemically resistant to waste
and leachate, have sufficient strength and thickness
to meet design criteria, and be able to function
without clogging. ;
Applicability of Double Liner and LCRS
Requirements
According to HSWA Section 3004,(o)(l), all new units
(landfills and surface impoundments) and lateral
expansions or replacements of;existing units for
which permit applications were submitted after
November 8,1984 (the date HSWA was enacted) will
be required to comply with these double-liner and
LCRS requirements once they are finalized. If permit
applications for these units were submitted before
this date, new units need not have double liners and
LCRSs unless the applications were modified after
the date HSWA was enacted. However, EPA can use
the omnibus provision of HSWA to require double
liners and LCRSs that meet the liner guidance on a
case-by-case basis at new facilities, regardless of
when the permit applications were submitted. Table
1-4 identifies facilities that will be required to
comply with the new regulations.
Leak Detection Systems
Described in this section are proposed leak detection
system requirements that apply to the secondary
LCRS between the two liners in a landfill. These
requirements focus on the drainage layer component
of the LCRS. Figure 1-7 illustrates the location of a
leak detection system in a double-lined landfill that
meets these requirements. ;
Proposed Design Criteria <
The proposed minimum design standards for
granular drainage layer materials require a
minimum thickness of 1 foot and a minimum
-------
Table 1-4. Landfills and Surface Impoundments Subject to
Proposed Regulations
APPLICABLE UNITS
Permit Applications Submitted
Before 11/8/84
New Facilities
If permit
modified after
11.8.84:
Interim Status
Facilities
If permit modified
after 11/8/84:
Permitted Facilities
New units at
previously Interim
Status facilities
No
Yes
No
Yes
No*
After 11/8/84
New Facilities
Interim Status
Facilities
Existing units:
New units
(operational)
after 5/8/85):
Permitted Facilities
New units at
previously Interim
Status facilities:
yes
No
Yes
Yes
'Proposing to require MTR for these units on site-by-site basis.
hydraulic conductivity of 1 cm/sec. In order to meet
this proposed minimum hydraulic conductivity
standard for granular drainage materials, the
secondary LCRS, or leak detection system, must be
constructed of clean gravels.
For synthetic drainage materials, EPA has proposed
a minimum hydraulic transmissivity of 5 x 1O4
square meters per second (m2/sec). The hydraulic
transmissivity of a drainage material refers to the
thickness of the drainage layer multiplied by the
hydraulic conductivity. The transmissivity of
granular drainage layers (1 foot x 1 cm/sec) is within
an order of magnitude of the 5 x 1O4 m2/sec standard
for synthetic drainage layers. The proposed
hydraulic transmissivity value for synthetic
drainage materials was developed to ensure that the
design performance for a geonet, geocomposite, or
other synthetic drainage layer is comparable to that
for a 1-foot thick granular drainage layer.
The proposed standards for leak detection systems
also specify a minimum bottom slope of 2 percent, as
is recommended in the existing draft guidance for
LCRS, and require the installation of a leak
detection sump of appropriate size to allow daily
monitoring of liquid levels or inflow rates in the leak
detection system. Specifically, the sump should be
designed to detect a top liner leakage rate in the
range of the action leakage rate (ALR) specified in
the proposed leak detection rule. Chapter Ten
discusses the proposed ALR in more detail.
Proposed Design Performance
Requirements
The proposed leak detection rule also establishes
design performance standards for the leak detection
system. Design performance standards mean that
the facility design must include materials and
systems that can meet the above-mentioned design
criteria. If the liners and LCRS materials meet the
design criteria, then the design performance
standards will be met. Compliance with design
performance standards can be demonstrated through
Coefficient of Permeability (Hydraulic Conductivity) in CM/SEC
(Log Scale)
102
101
1.0
10-2
10-3
10-4
ID'5
10-6
10-7
Drainage
Potential
Good
Poor
Almost
Impervious
Soil
Types
Clean gravel
Clean sands, and
clean sand and
gravel mixtures
Very fine sands, organic
and inorganic silts, mixtures
of sand, silt and clay, glacial
till, stratified clay deposits,
etc.
Adapted from Terzaghi and Peck (1967)
Figure 1-6. Hydraulic conductivities of granular material!;.
-------
1 -ft Granular Drainage Layer
Compacted Soil
2% min '.'•••'•i''•.:••:•;: •:•"•
MfeMtfMYfHNnwWWVv*"**9** " _ • . * « • *
;. •.';';; •' :•'.••'. •\'\'•:•'.'•'• /•' 2% min •;'•';
Protective Soil Cover
Leachate Collection and
Removal System (above top liner)
—- Top Liner (Composite)
Leachate Detection, ;
Collection, and Removal System
— Bottom Liner (Composite)
t granular drainage layer
(k > 1 cm/sec)
- 3-ft min compacted soil
(k < 1 x 10-7 cm/sec)
Legend
Geotextile
(synthetic fibers—woven, nonwoven or knit)
Geonet
(plastics formed into an open, netlike
configuration (used here in a redundant manner))
Flexible Membrane Liner (FML)
Figure 1-7. Location of a leak detection system in a double-lined landfill that meets proposed requirements.
numerical calculations rather than through field
demonstrations.
The proposed leak detection rule outlines two design
performance standards: (1) a leak detection
sensitivity of 1 gal/acre/day and (2) a leak detection
time of 24 hours. The leak detection sensitivity refers
to the minimum top liner leak rate that can
theoretically be detected, collected, and removed by
the leak detection system. The leak detection time is
the minimum time needed for liquids passing
through the top liner to be detected, collected, and
removed in the nearest downgradient collection pipe.
In the case of a composite top liner, the leak detection
time refers to the period starting at the point when
liquids have passed through the compacted soil
component and ending when they are collected in the
collection pipe.
EPA bases its 1 gal/acre/day leak detection
sensitivity on the results of calculations that show
that, theoretically, a leak detection system overlying
a composite bottom liner with an intact FML
component can detect, collect, and remove liquids
from a top liner leak rate less .than 1 gal/acre/day
(see Figure 1-3). This performance standard,
therefore, can be met with designs that include a
composite bottom liner. Based on numerical studies,
one cannot meet the leak detection sensitivity with a
compacted soil bottom liner, even one with a
hydraulic conductivity of 10-7 cm/sec. Therefore, the
emphasis of this standard is on selecting an
appropriate bottom liner system.
Meeting the 24-hour leak detection time, however, is
dependent on the design of the leak detection system.
A drainage layer meeting the design criteria,
together with adequate drain spacing, can
theoretically meet the 24-hour detection time
standard. The emphasis of the proposed standards,
therefore, is on designing and selecting appropriate
materials for the secondary LCRS.
As stated previously, compliance with EPA's
proposed design performance standards can be
demonstrated through one-dimensional, steady-state
flow calculations, instead of field tests. For detection
sensitivity, the calculation of flow rates should
-------
assume uniform top liner leakage. For detection
time, factors such as drain spacing, drainage media,
bottom slope, and top and bottom liners should all be
considered, and the worst-case leakage scenario
calculated.
Applicability of Leak Detection System
Requirements
Owners and operators of landfills, surface
impoundments, and waste piles on which
construction begins 6 months after the date the rule
is finalized will be required to install double liners
and leak detection systems.
Closure and Final Cover
The following section reviews existing guidance and
regulations concerning the design .of the final cover
on top of closed landfills. EPA is' currently revising
the guidance for final covers. The recommended
design differs little from that contained in the July
1982 draft version, with the exception that some of
the design values for components of the final cover
have been upgraded. EPA plans to issue the revised
guidance for final covers in 1989.
Draft Guidance and Existing Regulatory
Requirements
EPA issued regulations and draft guidance
concerning closure and final cover for hazardous
waste facilities in July 1982. Basically, the
regulations require that the final cover be no more
permeable than the liner system. In addition, the
cover must be designed to function with minimum
maintenance, and to accommodate settlement and
subsidence of the underlying waste. The regulations
do not specify any design criteria for liner materials
to meet the performance standard for permeability.
The draft guidance issued in July 1982 recommends
a three-layer cap design consisting of a vegetative
top cover, a middle drainage layer, and a composite
liner system composed of a FML over compacted low
permeability soil. The final cover is to be placed over
each cell as it is completed.
Since the regulations do not specify designs of
materials for the final cover, or cap, design engineers
can usually use their own judgment in designing the
final cover and selecting materials. For example, if
the lining system contains a high density
polyethylene (HOPE) membrane, the final cover does
not necessarily need to have a HOPE membrane. The
amount of flexibility in selecting FML materials for
the final cover varies from region to region, based on
how strictly'the statutory phrase "no more
permeable than" is interpreted. Nevertheless, from a
design perspective, the selection of FML materials in
the final cover should emphasize the physical rather
than the chemical properties of the liner material,
since the main objective is to minimize precipitation
infiltration. Precipitation infiltration is affected
mainly by the number of holes or tears in the liner,
hot by the water vapor transmission rates.
For the vegetative cover, EPA's guidance
recommends a minimum thickness of 2 feet and final
upper slopes of between 3 and 5 percent, after taking
into account total settlement and subsidence of the
waste. The middle drainage layer should have a
minimum thickness of 1 foot and minimum
hydraulic conductivity of 10-3 cm/sec. EPA's revised
draft guidance upgrades that standard by an order of
magnitude to 10-2 cm/sec to reduce capillary rise and
hydraulic head above the composite liner system. For
the composite liner system at the bottom of the cap, it
is critical that both the FML and the compacted soil
components be below the average depth of frost
penetration. The FML should also have a minimum
thickness of 20 mils, but 20 mils will not be a
sufficient thickness for all FML materials. The soil
component under the FML must have a minimum
thickness of 2 feet and a maximum saturated
hydraulic conductivity of 10-7 cm/sec. The final upper
slope of the composite liner system must be no less
than 2 percent after settlement. Table 1-5
summarizes specifications for each part of the final
cover.
Construction Quality Assurance
The final component of the regulatory/guidance
summary discusses construction of a hazardous
waste landfill. The following section summarizes
EPA's construction quality assurance (CQA)
program, as it is presented in existing guidance
(October 1986) and proposed regulations (May 1987).
Chapter Seven contains a more detailed discussion of
CQA implementation.
Guidance and Proposed Regulations
The proposed regulations and existing CQA
guidance require the owner/operator to develop a
CQA plan that will be implemented by contracted,
third-party engineers. The owner/operator also must
submit a CQA report containing the following:
• Summary of all observations, daily inspec-
tion/photo/video logs.
• Problem identification/corrective measures
report.
• Design engineer's acceptance reports (for errors,
inconsistencies).
» Deviations from design and material
specifications (with justifying documentation).
-------
Table 1-5. Cover Design
Vegetative Cover
• Thickness 2: 2 ft
• Minimal erosion and maintenance (e.g., fertilization, irrigation)
• Vegetative root growth not to extend below 2 ft
• Final top slope between 3 and 5% after settlement or
subsidence. Slopes greater than 5% not to exceed 2.0
tons/acre erosion (USDA Universal Soil Loss Equation)
• Surface drainage system capable of conducting run-off across
cap without rills and gullies
Drainage Layer Design
• Thickness > 1 ft
• Saturated hydraulic conductivity > 10-3 cm/sec
• Bottom slope > 2% (after settlement/subsidence)
• Overlain by graded granular or synthetic filter to prevent
clogging
• Allow lateral flow and discharge of liquids
Low Permeability Liner Design
FML Component.
• Thickness > 20 mil
• Final upper slope > 2% (after settlement)
• Located, wholly below the average depth of frost
penetration in the area
Soil Component.
• Thickness a 2 ft
• Saturated hydraulic conductivity < 1 x 10-7 cm/sec
• installed in 6-in lifts
• Summary of CQA activities for each landfill
component.
This report must be signed by a registered
professional engineer or the equivalent, the CQA
officer, the design engineer, and the owner/operator
to ensure that all parties are satisfied with the
design and construction of the landfill. EPA will
review selected CQA reports.
The CQA plan covers all components of landfill
construction, including foundations, liners, dikes,
leachate collection and removal systems, and final
cover. According to the proposed rule (May 1987),
EPA also may require field permeability testing of
soils on a test fill constructed prior to construction of
the landfill to verify that the final soil liner will meet
the permeability standards of 10-"7 cm/sec. This
requirement, however, will not preclude the use of
laboratory permeability tests and other tests
(correlated to the field permeability tests) to verify
that the soil liner will, as installed, have a
permeability of 10-7 cm/sec.
Summary of Minimum Technology
Requirements
EPA's minimum technology ; guidance and
regulations for new hazardous waste land disposal
facilities emphasize the importance of proper design
and construction in the performance of the facility.
The current trend in the regulatory programs is to
develop standards and recommend designs based on
the current state-of-the-art technology. Innovations
in technology are, therefore, welcomed by EPA and
are taken into account when developing these
regulations and guidance.
References
1. EMCON Associates. 1988. Draft background
document on the final double liner and leachate
collection system rule. Prepared for Office of
Solid Waste, U.S. EPA. NUS Contract No. 68-01-
7310, Work Assignment No. 66.
2. U.S. EPA. 1987a. Liners and leak detection for
hazardous waste land disposal units: notice of
proposed rulemaking. Fed. Regi Vol 52, No. 103,
20218-20311. May 29. :
3. U.S. EPA. 1987b. Hazardous waste management
systems: minimum technology requirements:
notice of availability of information and request
for comments. Fed. Reg. Vol. 52, No. 74, 12566-
12575. April 17.
4. U.S. EPA. 1987c. Background document on
proposed liner and leak detection rule. EPA/530-
SW-87-015. :
5. U.S. EPA. 1986a. Technical guidance document:
construction quality assurance for hazardous
waste land disposal facilities. EPA/530-SW-86-
031.
6. U.S. EPA. 1986b. Hazardous waste management
systems: proposed codification rule. Fed. Reg.
Vol. 51, No. 60,10706-10723. March 28.
7. U.S. EPA. 1985a. Hazardous waste management
systems: proposed codification rule. Fed. Reg.
Vol. 50, No. 135, 28702-28755. July 15.
8. U.S. EPA. 1985b. Draft minimum technology
guidance on double liner systems for landfills
and surface impoundments - design,
construction, and operation. EPA/530-SW-84-
014. May 24.
9. U.S. EPA. 1982. Handbook for remedial action at
waste disposal sites. EPA-625/6-82-006.
Cincinnati, OH: U.S. EPA.
10
-------
2. LINER DESIGN: CLAY LINERS
Introduction
This chapter discusses soil liners and their use in
hazardous waste landfills. The chapter focuses
primarily on hydraulic conductivity testing, both in
the laboratory and in the field. It also covers
materials used to construct soil liners, mechanisms
of contaminant transport through soil liners, and the
effects of chemicals and waste leachates on
compacted soil liners.
Materials
Clay
Clay is the most important component of soil liners
because the clay fraction of the soil ensures low
hydraulic conductivity. In the United
States.however, there is some ambiguity in defining
the term "clay" because two soil classification
systems are widely used. One.system, published by
the American Society of Testing and Materials
(ASTM), is used predominantly by civil engineers.
The other, the U.S. Department of Agriculture's
(USDA's) soil classification system, is used primarily
by soil scientists, agronomists, and soil physicists.
The distinction between various particle sizes differs
between ASTM and USDA soil classification systems
(see Table 2-1). In the ASTM system, for example
sand-sized particles are defined as those able to pass
a No. 4 sieve but not able to pass a No. 200 sieve,
fixing a grain size of between 0.075 millimeters (mm)
and 4.74 mm. The USDA soil classification system
specifies a grain size for sand between 0.050 mm and
2mm.
The USDA classification system is based entirely
upon grain size and uses a three-part diagram to
classify all soils (see Figure 2-1). The ASTM system,
however, does not have a grain size criterion for
classifications of clay; clay is distinguished from silt
entirely upon plasticity criteria. The ASTM
classification system uses a plasticity diagram and a
sloping line, called the "A" line (see Figure 2-2) to
distinguish between silt and clay. Soils whose data
Table 2-1. ASTM and USDA Soil Classification by Grain Size
ASTM USDA
Gravel
Sand
Silt
Clay
4.74
(No. 4 Sieve)
0.075
(No. 200 Sieve)
None
(Plasticity Criterion)
0.050
0.002
points plot above the A line on this classification
chart are, by definition, clay soils with prefixes C in
Unified Soil Classification System symbol. Soils
whose data points plot below the A line are classified
as silts.
EPA requires that soil liners be built so that the
hydraulic conductivity is equal to or less than 1 x
10-7 cm/sec. To meet this requirement, certain
characteristics of soil materials should be met. First,
the soil should have at least 20 percent fines (fine silt
and clay sized particles). Some soils with less than 20
percent fines will have hydraulic conductivities
below 10-7 cm/sec, but at such low fines content, the
required hydraulic conductivity value is much
harder to meet.
Second, plasticity index (PI) should be greater than
10 percent. Soils with very high PI, greater than 30
to 40 percent, are sticky and, therefore, difficult to
work with in the field. When high PI soils are dry,
they form hard clumps that are difficult to break
down during compaction. On the Gulf Coast of Texas,
for example, clay soils are predominantly highly
plastic clays and require additional .processing
during construction. Figure 2-3 represents a
collection of data from the University of Texas
laboratory in Austin showing hydraulic conductivity
as a function of plasticity index. Each data point
represents a separate soil compacted in the
11
-------
Clay Loam \Silty Clay\^
Sandy y\x—*
ClayLoairf ^^ ^
Percent Sand
Figure 2-1. USDA soil classification.
laboratory with standard Proctor compaction
procedures and at a water content about 0 to 2
percent wet of optimum. Hydraulic conductivities
are consistently below 10-7 cm/sec for soils with Pis
greater than 10 percent.
Third, coarse fragments should be screened to no
more than about 10 percent gravel-size particles.
Soils with a greater percentage of coarse fragments
can contain zones of gravel that have high hydraulic
conductivities.
Finally, the material should contain no soil particles
or chunks of rock larger than 1 to 2 inches in
diameter. If rock diameter becomes a significant
percentage of the thickness of a layer of soil, rocks
may form a permeable "window" through a layer. As
long as rock size is small compared to the thickness
of the soil layer, the rock will be surrounded by the
other materials in the soil.
Blended Soils
Due to a lack of naturally occurring soils at a site, it
is sometimes necessary to blend imported clay
minerals with onsite soils to achieve a suitable
blended material. The most common blend is a
combination of onsite sandy materials and imported
sodium bentonite.
Figure 2-4 shows the influence of sodium bentonite
on the hydraulic conductivity of the silt/sand soil.
The addition of only 4 or 5 percent sodium bentonite
to this particular soil drops the hydraulic
conductivity from 10-4 to 10-"7 cm/sec, a rather
dramatic reduction.
i
Calcium bentonite, though more permeable than
sodium bentonite, has also been used for soil blends.
Approximately twice as much calcium bentonite
typically is needed, however, to achieve a hydraulic
conductivity comparable to that of sodium bentonite.
One problem with using sodium bentonite, however,
is its vulnerability to attack by chemicals and waste
leachates, a problem that will be discussed later in
the chapter.
Onsite sandy soils also can be blended with other
clay soils available in the area, biit natural clay soil
is likely to form chunks that are difficult to break
down into small pieces. Bentonites, obtained in dry,
powdered forms, are much easier tjo blend with onsite
sandy soils than are wet, sticky clods of clay.
Materials other than bentonite can be used, such as
atapulgite, a clay mineral that is insensitive to
attack by waste. Soils also can !be amended with
lime, cement, or other additives. ;
Clay Liners versus Composite Liners
Composite liner systems should 'outperform either
flexible membrane liners (FMLs) or clay liners alone.
Leachate lying on top of a clay liner will percolate
down through the liner at a rate controlled by the
hydraulic conductivity of the liner, the head of the
leachate on top of the liner, and the liner's total area.
With the addition of a FML placed directly on top of
the clay and sealed up against its upper surface,
leachate moving down through a hole or defect in the
FML does not spread out between the FML and the
clay liner (see Figure 2-5). The composite liner
system allows much less leakage .than a clay liner
acting alone, because the area of flow through the
clay liner is much smaller. '
The FML must be placed on. top of the clay such that
liquid does not spread along the interface between
the FML and the clay and move downward through
the entire area of the clay liner. A FML placed on a
bed of sand, geotextiles, or other highly permeable
materials, would allow liquid to move through the
defect in the FML, spread over the whole area of the
clay liner, and percolate down as if the FML was not
there (see Figure 2-6). With clay liner soils that
contain some rock, it is sometimes proposed that a
woven geotextile be placed, on top of the soil liner
under the FML to prevent the ipuncture of rocks
through the FML. A woven geotextile between the
FML and the clay, however, ^creates a highly
transmissive zone between the FML and the clay.
The surface of the soil liner instead should be
f compacted and the stones removed so that the FML
can be placed directly on top of the clay.
12
-------
JO
CL
50
40
30
20
10
7
4
0
C
For classification of fine-grained soils
and fine-grained fraction of coarse-grained
soils.
Equation of "A" - Line
Horizontal at PI =4 to LL = 25.5,
then PI = 0.73 (LL - 20)
Equation of "U" - line
Vertical at LL = 1 6 to PI = 7,
then PI = 0.9 (LL - 8)
/.
yf s
S///, CL - ML
X I i
/
X
X
X
;-^^
/
X
/
. *
ov
S^
ML o
/
/
X
X
nv ^/
X
rOL
/•
*
/
/
X
^ S
MHc
X
-£j^
rOH
/
X
r
10 16 20 30 40 50 60 70 80 90 100 111
Liquid Limit (LL)
Figure 2-2. ASTM plasticity determination for fine-grained soils.
o
O
g
10-6
10-7
TO'8
10-9
10-10
Upper Bound
• Lower Bound
i
10 20 30 40
Plasticity Index (%)
50
60
Figure 2-3. Hydraulic conductivity as a function of plasticity
index for soils in Austin Laboratory Tests.
Darcy's Law, Dispersion, and Diffusion
Figure 2-7 illustrates Darcy's law, the basic equation
used to describe the flow of fluids .through porous
materials. In Darcy's law, coefficient k, hydraulic
conductivity, is often called the coefficient of
permeability by civil engineers.
Darcy's law applied to a soil liner shows the rate of
flow of liquid q directly proportional to the hydraulic
conductivity of the soil and the hydraulic gradient, a
measure of the driving power of the fluid forcing
itself through the soil and the cross-sectional area
"A" of the liner (see Figure 2-7).
If hydraulic conductivity is 10-7 cm/sec, the amount
of leakage for a year, per acre, is 50,000 gallons. If
the conductivity is 10 times that value (1 x. 10-6
cm/sec), the leakage is 10 times greater, or 500,000
gallons. Table 2-2 summarizes quantities of leakage
per annum for a 1-acre liner with an amount of liquid
ponded on top of it, assuming a hydraulic gradient of
1.5. Cutting the hydraulic conductivity to 10-8 cm/sec
reduces the quantity of leakage 10-fold to 5,000
gallons per acre per year. These data demonstrate
how essential low hydraulic conductivity is to
minimizing the quantity of liquid passing through
the soil liner.
Contam/nants
The transport of contaminants through the soil liner
occurs by either of two mechanisms: advective
13
-------
o
Leachate
Leachate
10-9 -
10-10
04 8 12 16 20 24
Percent Bentonite
Figure 2-4. Influence of sodium bentonite on hydraulic
conductivity.
Clav Liner
Composite Liner
FML
A = Area of Entire Area < Area of Entire
Liner Liner
Figure 2-5. Leachate infiltration in clay and composite liner
systems.
transport, in which dissolved contaminants are
carried by flowing water, and molecular diffusion of
the waste through the soil liner. Darcy's law can be
used to estimate rates of flow via advective transport
by calculating the seepage velocity of the flowing
water. Seepage velocity is the hydraulic conductivity
times the hydraulic gradient, divided by the effective
porosity of the soil. The effective porosity is defined
as the volume of the pore space that is effectively
Do , Don't
Figure 2-6. Proper placement of FMLs on clay liners.
Influent
Liquid
ent J
id -/
Soil
Effluent
Liquid
q = rate of flow
k = hydraulic
conductivity
Hi = headless
L = length of flow
A; = total area
V L
Leachate
Subsoil
q = kiA
i = Hydraulic Gradient
_ H + D |
D |
(Assumes No Suction Below Soil Liner)
Figure 2-7. Application of Darcy's Law.
14
-------
Table 2-2. Effects of Leakage Quantity on Hydraulic
Conductivity for a 1-Acre Liner
Hydraulic Conductivity (cm/sec) Annual Leakage (gallons)
1 X 10-6 .
1 x 10-7
1 X ID'8
500,000
50,000
5,000
Note: Hydraulic Gradient Assumed to be 1.5
conducting the flow, divided by the total volume of
the soil sample (Figure 2-8).
divided by the hydraulic conductivity times the
hydraulic gradient (Figure 2-9).
Leachate
Subsoil
. Ln
TOT = — = -=-2
V ki
Flux
I
Leachate
Subsoil
H
i = Hydraulic
Gradient
_ H +T
T
(No Suction)
i
Leachate
Subsoil
seepage velocity
ne = effective porosity
Figure 2-8. Advective transport.
If the liquid uniformly passes through all the pores
in the soil, then the effective and total porosities are
equal. However, if the flow takes place in only a
small percentage of the total pore space, for example,
through fractures or macropores, the effective
porosity will be much lower than the total porosity.
Judging the effective porosity is one of the problems
of estimating seepage velocities.
If effective porosity and other parameters are known,
the time of travel (TOT) for a molecule of waste
transported by flowing water through the soil liner
can be calculated. TOT equals the length of the
particular flow path times the effective porosity,
Figure 2-9. Time of Travel (TOT).
It is possible to confirm these calculations and
measure some of the parameters needed to make
them by performing laboratory permeability
experiments. In these experiments, clean soil is
placed into columns in the laboratory, and the
leachate or some other waste liquid is loaded on top
of each soil column, forcing the liquid through the
column over a period of time, while keeping the
concentration of influent liquid constant. The
concentration of one or more chemicals in the
effluent liquid is measured over time.
A plot called a breakthrough curve shows the
effluent liquid concentration c divided by the
influent liquid concentration c0 as a function of pore
volumes of flow (see Figure 2-10). One pore volume of
flow is equal to the volume of the void space in the
soil. The effective porosity of the soil is determined
by measuring a breakthrough curve.
It can be expected that as the leachate invades the
soil, none of the waste chemical will appear in the
effluent liquid at first, only remnant soil and water.
Then at some point, the invading leachate will make
its way downstream through the soil column, and
come out in more or less full strength. An
instantaneous breakthrough of the waste liquid
never occurs, however. The breakthrough is always
gradual because the invading leachate mixes with
the remnant soil water through a process called
mechanical dispersion.
Many of the waste constituents in the leachate are
attenuated or retarded by the soil. For example, lead
migrates very slowly through soil, while chloride and
bromide ions migrate very quickly. With no
retardation or attenuation, breakthroughs would
occur at c/c0 of 0.5 to 1 pore volume of flow and below
(see Figure 2-11). With effective and total porosities
equal, a much delayed breakthrough of chemicals
15
-------
Leachate
Concentration
of Solute = C0
Effluent
Concentration
of Solute = c
No Retardation
I
Retardation
(n = ne)
0 1 2
Pore Volumes of Flow
Figure 2-11. Effective porosity of soils with retardation and
without retardation of waste ions.
(for n = ne)
Dispersion
0 1
Pore Volumes of Flow
Figure 2-10. Effective porosities.
the soil. At the start of the experiment, the
concentration c is equal to c0 in the waste liquid. The
soil is clean. Even though no water flows into the soil
by advection, chemicals move into the soil by the
process of molecular diffusion. Eventually, the
concentration of the waste liquid and the soil will be
one and the same (see Figure 2-12).
Leachate
(constant c0)
t = 0
Concentration (c)
Figure 2-12. Molecular diffusion.
that have been absorbed or attenuated by the soil
could be expected.
The best way to determine effective porosity is to
perform a test using a "tracer" ion that will not be
absorbed significantly by the soil, such as chloride or
bromide. If the breakthrough occurs in one pore
volume of flow, the effective and total porosity is
equal. If, instead, the breakthrough occurs at half a
pore volume of flow, then the effective porosity is half
the total porosity.
Molecular Diffusion
Chemicals can pass through soil liners by molecular
diffusion, as well as by advective transport. One can
study the molecular diffusion of chemicals in the soil
by compacting soil in the bottom of an impermeable
beaker and ponding waste liquid or leachate on top of
Calculations show that after 10 to 30 years,
molecular diffusion begins to transport the first
molecules of waste 3 feet downwards through a
compacted soil liner. Accordingly, even with a
perfectly impermeable liner with 0 hydraulic
conductivity, in 1 to 3 decades contaminants will
begin to pass through the soil liner, due to molecular
diffusion. ;
The rate of diffusion is sensitive; to a number of
parameters. For conservative ions that are not
attenuated, the transfer time is 1 to 3 decades. For
ions that are attenuated, transfer time is much
longer. The mass rate of transport by molecular
diffusion, however, is so slow that even though
chemicals begin to show up in 1 to 3 decades, the
total amount released per unit of area is small.
Flexible membrane liners permit the release of
organics and vapors via molecular diffusion by
16
-------
almost exactly the same process. Transport times for
organic chemicals through FMLs typically range
from a few hours to a few days.
Laboratory Tests for Hydraulic
Conductivity
The hydraulic conductivity of a soil liner is the key
design parameter. The important variables in
hydraulic conductivity testing in the laboratory are:
• Representativeness of the sample.
• Degree of water saturation.
• Stress conditions.
• Confinement conditions.
• Testing details.
flepresentaf/Veness of Sample: Case
Histories
Representativeness of the soil sample being tested is
the most crucial factor. Two case histories illustrate
the importance and the difficulty of obtaining
representative samples.
Klingerstown, PA
A test pad constructed under EPA sponsorship in
Klingerstown, Pennsylvania, consisted of a pad of
clay soil 30 feet wide, 75 feet long, and 1 foot thick.
The clay liner was built in three lifts, or layers, each
lift being 4 inches thick. The liner was built up on a
concrete pad so that researchers could crawl under
and collect and measure the liquid coming out of the
bottom. A shelter was built over the test pad and
about 1 foot of water ponded over the surface.
The principal investigator, Dr. Andrew Rogowski,
divided the collection units into a number of
subunits, each subunit measuring 3 feet by 3 feet. A
total of 250 different collection units underneath the
soil liner were monitored independently to
determine rate of flow. Dr. Rogowski's objective was
to correlate the variability of the hydraulic
conductivity of the liner with the molding water
content of the soil and with the dry density of the
compacted soil.
Dr. Rogowski also installed 60 1-foot diameter rings
in the surface of the liner, so that he could measure
independently 60 different infiltration rates on the
surface of the liner. Each of the 3-square-foot (ft2)
blocks was assigned an average hydraulic
conductivity based on many months of testing and
observation. Figure 2-13 shows the results. The zone
at the top of the diagram with a high hydraulic
conductivity of 10-5 cm/sec probably resulted from
the way the liner was built. The sheepsfoot roller
used to compact the liner probably bounced on the
ramp causing lower compaction, which resulted in a
relatively high conductivity at the end. The
conductivity for the rest of the liner varies between
10-6 and lO-8 cm/sec, a 100-fold variation of hydraulic
conductivity.
10-5 cm/s
10-6 cm/s
10'7 cm/s
10'8 cm/s
75ft
30ft
Figure 2-13. Hydraulic conductivity zones from Klingerstown,
PA Tests.
For a laboratory test on this soil, the test specimen
would need to measure about 3 inches in diameter
and 3 inches in height. Finding a 3-inch diameter
1 sample representative of this large mass of soil
presents a challenge, since small samples from
larger quantities of material inevitably vary in
hydraulic conductivity.
Dr. Rogowski's experiments resulted in two
interesting sidelights. First, the average of all the
hydraulic conductivities was 2 to 3 x 10-7 cm/sec. Dye
was poured into the water inside some of the 1-foot
diameter rings installed in the surface of the liner to
determine if the dye came out directly beneath the
ring or off to the side. In some cases it came out
directly beneath the ring and in some it wandered off
to the side. It took only a few hours, however, for the
dye to pass through the soil liner, even with an
average conductivity only slightly greater than 1 x
ID-7 cm/sec. A few preferential flow paths connected
17
-------
to some of the rings allowed very rapid transit of the
dye through the soil liner.
The second interesting sidelight was Dr. Rogowski's
conclusion that no,relationship existed between in
situ hydraulic conductivity and either molding water
content of the soil or the dry density of the compacted
soil.
The soil used in the experiment was a low plasticity
sandy material with a PI of about 11 percent. The
variations in hydraulic conductivity probably
reflected zones of material that contained more sand
in some places and more clay in others. Tests have
been performed on a couple of liners in the field
where liquid flowing into the soil liners has been
dyed and traced by cutting a cross section or trench
through the liner. Typically, a pattern such as that
shown in Figure 2-14 emerges, with the horizontal
dots indicating lift interfaces. The results seem to
indicate that dyed liquid finds a defect in the top lift,
moves down and spreads along a more permeable
zone between lifts; finds another defect, moves
downward, spreads; finds another defect and so forth.
2.
Dyed water
3R
6 In.
I I I
Figure 2-14. Liquid flow between lift interfaces in a soil liner.
The problem arises in determining from where a
representative sample should be taken. Even if 25
samples were picked randomly in a grid pattern from
that zone for 25 independent measures of hydraulic
conductivity, it would be unclear how to arrive at a
single representative measure. The flow through a 3-
inch diameter specimen is much too small to mimic
the patterns of fluid flow that occur in the field under
similar conditions.
Houston, TX
A second case history that demonstrates the
difficulty of obtaining representative samples
involves a trial pad constructed in Houston in 1986.
A 1-foot thick clay liner was compacted over a gravel
underdrain with an area roughly 50 feet by 50 feet.
The entire area of the liner was drained and the flow
from an area roughly 16 feet by 16 feet was carefully
collected and measured.
The liner was first built on top of the underdrain, the
soil compacted with a padfoot roller, and water
ponded on top of the liner. Infiltro'meters measured
the rate of inflow, and a lysimeter measured the rate
of outflow. The soil used in the experiment was
highly plastic with a PI of 41 percent.
The liner was compacted with two lifts, each 6 inches
thick. A 1-ft3 block of soil was carved from the liner,
and cylindrical test specimens we|re trimmed from
upper and lower lifts and measured for hydraulic
conductivity. A 3-inch diameter specimen also was
cut, and hydraulic conductivity parallel to the lift
interface was measured. i
Table 2-3 summarizes the results! of these various
tests. The actual in situ hydraulic conductivity, a
high 1 x 10-4 cm/sec, was verified both by the
infiltration measurements and^ the underdrain
measurements.
Table 2-3. Hydraulic Conductivities from, Houston Liner Tests
Actual k: 1 x 10/4 cm/s '.
Lab K's:
Location
Sampler
K (cm/s)
Lower Lift
Upper Lift
Lift Interface
Lower Lift
Upper Lift
3-in Tube
3-in Tube
3-in Tube
Block :
Block
4x10-9
1 X 10-9
1 x 10-7
8 x 10-5
1 x 1 0-8
The tests were replicated under controlled conditions
using soil collected from the liner in thin-walled 3-
inch diameter sample tubes. The laboratory
measures of hydraulic conductivity were consistently
1 x 10-9 cm/sec, five orders of magnitude lower than
the field value of 1 x 10-4 cm/sec. The laboratory tests
yielded a hydraulic conductivity 100,000 times
different than that from the field test. Apparently
the flow through the 3-inch specimens did not mimic
flow on a larger scale through the entire soil liner.
The sample trimmed horizontally at the lift interface
was actually obtained by taking a 3-inch diameter
sample from a sample collected, with a 5-inch
diameter tube. The hydraulic conductivity with flow
parallel to the lift interface was two orders of
magnitude higher. :
Of all the values recorded from the lab tests, only the
one obtained from the upper lift of the block sample
was close to the field value of ,1 x 10-4 cm/sec.
18
-------
Apparently that one block sample happened to hit
one of the more permeable zones and, more or,less by
accident, yielded a lab measurement that agreed
with the field measurement.
Degree of Water Saturation
The hydraulic conductivity obtained in a laboratory
test also can be affected by the amount of gas present
in the soil. Dry soils are less permeable than wet
soils. A dry soil primarily is filled with air. Because
invading water does not flow through air-filled voids,
but only through water-filled voids, the dryness of a
soil tends to lower permeability.
Some engineers believe that hydraulic conductivity
tests on compacted clay soil should be performed on
fully saturated soils in an attempt to measure the
highest possible hydraulic conductivity. Most if not
all of the gas can be eliminated from laboratory
hydraulic conductivity tests by backpressure
saturation of the soil. This technique pressurizes the
water inside the soil, compressing the.gas and
dissolving it in the water. Increasing the
backpressure will increase the degree of water
saturation and reduce the amount of air, thereby
increasing hydraulic conductivity.
Stress Conditions
Another factor substantially influencing the
hydraulic conductivity of compacted clay soil is the
overburden, or confining pressure, acting on the soil.
The weight of 1 foot of soil overburden is roughly
equivalent to 1 pound per square inch (psi). If two
identical samples of soil are buried, one near the
ground surface and one at depth, the soil near the
ground surface is likely to be more permeable than
the soil buried at depth, simply because the soil
buried at depth is squeezed into a more compact
configuration by the overburden pressure. Thus, soil
has a lower porosity with increasing depth.
In a series of experiments performed a few years ago,
slabs of clay were compacted in the lab and then
trimmed to produce cylindrical test specimens. One
sample of the clay was compacted and then trimmed
for a test specimen immediately, while the other was
allowed to desiccate for a period of time before being
trimmed. The one that desiccated had tiny cracks as
a result of the desiccation process, and was much
more permeable than the soil that had not been
desiccated. As confining stress increased, the
hydraulic conductivity decreased because the soil
was compacted into a less porous condition.
Although the sample that was cracked from
desiccation was obviously more permeable, at a very
high stress the hydraulic conductivities were
essentially identical (see Figure 2-15). With enough
confining pressure acting on the soil, the cracks that
had existed earlier closed up completely so that the
soil was no longer highly permeable.
(kPa)
50
100
10-7
TO'8
E
o
O
o
1
I-
10-9
. Sample Containing
Desiccation
Cracks
Sample Containing
No Desiccation
Cracks
0 4 8 12
Effective Confining Pressure (psi)
16
Figure 2-15. Hydraulic conductivity as a function of confining
pressure.
One implication of these experiments for laboratory
hydraulic conductivity testing is that conductivity
values can vary remarkably depending on the
confining stress. It is essential that the confining
stress used in a laboratory test be of the same
magnitude as the stress in the field.
Another important implication is that highly
permeable soil liners generally have defects, such as
cracks, macropores, voids, and zones that have not
been compacted properly. One opportunity to
eliminate those defects is at the time of construction.
Another opportunity arises after the landfill is in
operation and the weight of overlying solid waste or
of a cover over the whole system further compresses
the soil. This compression, however, occurs only on
the bottom liners, as there is not much overburden
stress on a final cover placed over a solid waste
disposal unit. This is one reason it is more difficult to
design and implement a final cover with low
hydraulic conductivity than it is a bottom liner. Not
only is there lower stress acting on a cover than on a
liner, but the cover is also subjected to many
environmental forces which the liner is not.
19
-------
Double-ring and Flexible Wall Permeameters
A double-ring permeameter separates flow that
occurs through the central part of the soil sample
from flow that occurs near the side wall. The
permeameter is designed such that a ring sticks into
the bottom of the soil sample, thereby detecting
sidewall leakage that might invalidate the results of
laboratory conductivity tests. Almost all of the rigid
wall permeameters now being installed in the
University of Texas laboratories have double rings.
Another kind of permeameter cell is a flexible-wall
permeameter in which the soil specimen is confined
by a thin, flexible membrane, usually made of latex.
The latex membrane conforms to any irregularities
in the sample, an advantage when collecting
irregularly shaped specimens from the field.
Termination Criteria
When conducting laboratory hydraulic conductivity
tests, two criteria should be met before testing is
terminated. First, the rate of inflow should be within
10 percent of the rate of outflow. Measuring both the
rate of inflow and the rate of outflow is necessary to
detect problems such as a leak in the system or
evaporation from one of the reservoirs. Second, a plot
of hydraulic conductivity versus time or pore volume
of flow should essentially level off, indicating that
hydraulic conductivity is steady.
ASTM has no standards at the present time for
testing low-hydraulic- conductivity soil, but is in the
final stages of developing a standard for tests with
flexible wall permeameters that should be available
within the next 2 years.
Field Hydraulic Conductivity Testing
In situ, or field, hydraulic conductivity testing
operates on the assumption that by testing larger
masses of soil in the field one can obtain more
realistic results. There are actually four kinds of in
situ hydraulic conductivity tests: borehole tests,
porous probes, infiltrometer tests, and underdrain
tests. To conduct a borehole test one simply drills a
hole in the soil, fills the hole with water, and
measures the rate at which water percolates into the
borehole.
The second type of test involves driving or pushing a
porous probe into the soil and pouring water through
the probe into the soil. With this method, however,
the advantage of testing directly in the field is
somewhat offset by the limitations of testing such a
small volume of soil.
A third method of testing involves a device called an
infiltrometer. This device is embedded into the
surface of the soil liner such that the rate of flow of a
liquid into the liner can be measured. Infiltrometers
have the advantage of being able to permeate large
volumes of soil, which the first two devices cannot.
A fourth type of test utilizes an underdrain, such as
the one at the Houston test site discussed earlier.
Underdrains are the most accurate in situ
permeability testing device because they measure
exactly what comes out from the bottom of the liner.
They are, however, slow to generate good data for
low permeability liners because they take a while to
accumulate measurable flow. Also, underdrains
must be put in during construction, so there are
fewer in operation than there are other kinds of
testing devices. They are highly recommended for
new sites, however.
The two forms of infiltrometers popularly used are
open and sealed. Four variations are illustrated in
Figure 2-16. Open rings are less desirable because
with a conductivity of 1O7 cm/sec, it is difficult to
separate a 0.002 inches per day drop in water level of
the pond from evaporation and other losses.
\
Open, Single Ring
\
Open, Double Ring
Sealed, Single Ring
\
Sealed, Double Ring
Figure 2-16. Open and sealed single- and double-ring
infiltrometers.
With sealed rings, however, very low rates of flow
can be measured. Single-ring infiltrometers allow
lateral flow beneath the ring, complicating the
interpretation of test results. Single rings are also
susceptible to the effects of temperature variation; as
the water heats up, the whole system expands and as
it cools down, the whole system contracts. This
situation could lead to erroneous^ measurements
when the rate of flow is small.
The sealed double-ring infiltrometer has proven the
most successful and is the one used currently. The
outer ring forces the infiltration from the inner ring
to be more or less one dimensional. Covering the
20
-------
inner ring with water insulates it substantially from
temperature variation.
Figure 2-17 shows the double ring device currently
being used. It consists of a 12-foot by 12-foot outer
ring and a 5-foot diameter inner ring. Tensiometers
are embedded at various depths to establish the
depth of water penetration into the soil so that
hydraulic conductivity can be calculated.
Sealed Inner Ring
Flexible Bag N
Tensionmeters
Outer Ring
Figure 2-17. Details of a sealed double-ring infiltrometer.
Rate of infiltration is measured by a small flexible
bag. As water infiltrates from the inner ring into the
soil, the flexible bag is gradually compressed as
water leaves it to enter the ring. To determine how
much flow has taken place, the flexible bag is
disconnected, dried off, and weighed. Then it can
either be refilled or replaced with a fresh bag.
The flexible bag also serves to stabilize pressure
between the inner and outer rings. If the water level
in the outer ring changes, the hydrostatic pressure
on the flexible bag changes by precisely the same
amount. Thus, even though the water level in the
outer ring fluctuates, the differential pressure
between the inner and outer rings is always zero.
Overall, this simple device compensates for water
level changes and allows a range of measurements,
Installation of the Sealed Double-ring
Infiltrometer
The sealed double ring infiltrometer is best used on a
test pad. The width of the test pad is usually about 40
feet by 50 feet; the thickness of the test pad usually 2
or 3 feet. The test pad is always covered to prevent
desiccation after construction has been completed.
The 12-foot by 12-foot outer ring is made of four
aluminum panels that are assembled at the site. A
prefabricated design allows the panels to be bolted
together to prevent breaching. The outer box can
then be lifted up and put into position embedded in
the liner. If the site is sloping, the elevation of the
four corners is measured at the site with a handheld
level or a transit, so that the top of the infiltrometer
is more or less horizontal and the water level is even
with the top of the infiltrometer.
A rented ditching machine is used to excavate a
trench about 18 inches deep and 4 inches wide for the
outer ring. The ring is embedded into the trench and
the elevations are checked again.
The sealed inner ring typically is made of fiberglass
and measures 5 feet by 5 feet. It slopes from left to
right and from side to side in a dome-shaped slope
such that it has a high point. As the ring fills with
water from the bottom up, gas is displaced out the
top. When the inner ring is completely full of water,
the gas is purged out of the system.
The trench for the inner ring is not dug with the
ditching device because the vibration and churning
action might open up fractures in the soil and change
the measurements. Instead, the trench is dug in one
of two ways: by a handheld mason's hammer or by a
chain saw. A chain saw with a specially equipped
blade is the state-of-the-art in excavation of trenches
for the inner ring.
While the excavation is being done, the working area
is covered with plastic. Before the system is ready to
be filled with water, a pick or rake is used to scrape
the surface thoroughly to ensure that smeared soil
has not sealed off a high hydraulic conductivity zone.
After the trench has been excavated, it is filled with
a grout containing bentonite that has been mixed
with water to the consistency of paste. A grout mixer
rather than a concrete mixer is used to provide more
complete mixing. The inner ring is then forced into
the grouted trench. The grout is packed up against
the ring to obtain the best possible seal. To pretest
the seal, the inner ring is filled with about 3 inches of
water. If there is a gross defect at the seal, water will
spurt out of it. Next, the outer ring is placed in its
grout-filled trench.
The next step is to tie steel cords in the middle of the
four sections to prevent the outer ring from bowing
out from the pressure of the water. Tensiometers are
installed in nests of three at three different depths to
monitor the depth of the wetting process. To cushion
the tensiometers, soil is excavated and mixed with
water to form a paste. The tensiometer is then
inserted into the hole which is then sealed with
bentonite. The depths of the porous tips are typically
6 inches, 12 inches, and 18 inches in each nest of
three. Finally, the system is ready to fill with water.
The small flexible bag used can be an intravenous
bag from the medical profession, available in a range
21
-------
of sizes from a few hundred milliliters to larger sizes.
A ruler taped to the inside of the outer ring can be
used to monitor the water level, which should be kept
to within +/-1 inch of its original level.
When the construction process is complete, the entire
unit is covered with a tarp. The tarp minimizes water
evaporation and keeps the temperature from
fluctuating.
After the infiltrometer has been installed,
measurements are taken over a period lasting at
least 2 weeks and often as much as 1 to 2 months.
Readings involve removing the bag, weighing the
bag, and refilling it with water. Readings might be
taken as infrequently as once a week or as frequently
as once a day, depending on the situation.
An experienced group of people can put in a sealed
double ring infiltrometer in 1 day. Two days is more
typical for less experienced people.
The cost of the equipment to build a sealed double
ring infiltrometer is about $3,000. The tensiometers,
grout, and equipment rental typically add another
$1,500. The total cost for equipment and installation,
plus the periodic monitoring of the flow rate and
analysis of test data is approximately $10,000, not
including the cost of a trial pad. The sealed double
ring infiltrometer itself is reusable, therefore the
$3,000 cost of the rings is recoverable. In comparison
to the cost of infiltrometer installation and
operation, a single laboratory hydraulic conductivity
test costs only $200 to $400.
Issues Associated with Field Hydraulic
Conductivity Testing
A number of issues are associated with all field
hydraulic conductivity tests. Most importantly, the
tests do not directly measure the hydraulic
conductivity (k) of the soil. Instead they measure the
infiltration rate (I) for the soil. Since hydraulic
conductivity is the infiltration rate divided by the
hydraulic gradient (i) (see equations in Figure 2-18),
it is necessary to determine the hydraulic gradient
before k can be calculated. The following equation
(with terms defined in Figure 2-18) can be used to
estimate the hydraulic gradient:
i = (D+Lf)/Lf
This equation assumes the pressure head at the
wetting front equal to zero. The value of the pressure
head is, however, a source of disagreement and one of
the sources of uncertainty in this test. The
assumption that the pressure head is zero is a
conservative assumption, tending to give a high
hydraulic conductivity.
I = Infiltration Rate
= (Quantity of Flow/Area)/Time
= (Q/A)/t
k = Hydraulic Conductivity
= Q/(iAt) = l/i
Figure 2-18. Hydraulic gradient.
A second issue is that of effective stress or
overburden stress. The overburden stress on the soil
is essentially zero at the time the test is performed,
while under operating conditions, it may be
substantial. The influence of overburden stress on
hydraulic conductivity cannot be estimated, easily in
the field, but can be measured in the laboratory.
Using conservative estimates, the shape of the field
curve should be the same as that; obtained in the
laboratory (see Figure 2-19). If thiere is significant
overburden stress under actual field performance,
the infiltrometer test measurements would need to
be adjusted according to the laboratory results.
A third issue that must be considered is the effect of
soil swelling on hydraulic conductivity (see Figure 2-
20). Tests on highly expansive soils almost always
take longer than tests with other soils, typically
lasting 2 to 4 months. This is a particular problem
with soils along the Texas Gulf Coast.
A hydraulic conductivity test is terminated when the
hydraulic conductivity drops below 10-7 cm/sec (see
Figure 2-21). It usually takes 2 to;8 weeks to reach
that point, and is usually clear after about 2 months
whether or not that objective will be achieved.
The ASTM standard for double-ring infiltrometers is
currently being revised. The existing double-ring
test (D3385) was never intended for low hydraulic
conductivity soil and should not ibe used on clay
liners. A new standard for double-ring infiltrometers
22
-------
Termination of Test
t
1
•o
S
g
o
o
O
to
•§,
10-7
Effective Stress
Time
Figure 2-19. Hydraulic conductivity as a function of effective
stress.
Figure 2-21. Termination of testing.
Figure 2-20. Soil swelling.
intended for low hydraulic conductivity soils will
probably be available in 1990.
Field Tests versus Laboratory Tests
A comprehensive program of testing soil liner
materials will involve both laboratory and field tests.
Field tests provide an opportunity to permeate a
larger, more representative volume of soil than do
laboratory tests. A field test is also more
comprehensive and more reliable.
A primary advantage of laboratory tests is that they
are less expensive so more of them can be performed.
Also, certain conditions can be simulated in a lab
that cannot be duplicated in the field. One can
saturate the soil fully in the laboratory, getting rid of
all the gas. One can also vary the overburden stress
in the lab, which cannot be done conveniently in the
field. Finally, in the lab, actual waste liquids can be
passed through a column of material for testing, a
condition that could not be duplicated in the field.
There is a radical variation in the reliability of field
tests versus laboratory tests. In the Houston test pad
discussed earlier the real value for hydraulic
conductivity in the field was 1 x 10-4 cm/sec while the
lab values were 1 x 10-9 cm/sec, a 100,000-fold
difference in the values.
At the Keele Valley landfill, just outside Toronto,
however, some excellent field data have been
obtained. At this particular site, a 3-foot clay liner
spanning 10 acres is monitored by a series of
underdrains. Each underdrain measures 15 m2 and
is made of high density polyethylene. The
underdrains track the liquid as it moves down
through the soil liner. The underdrains have been
monitored for more than 2 years and have
consistently measured hydraulic conductivities of
. about 1 x 10-8 cm/sec. Those field values essentially
are identical to the laboratory values.
:The clay liner at Keele Valley was built very
carefully with strict adherence to construction
quality assurance. The laboratory and field values
are the same because the liner is essentially free of
defects. Lab and field values differ when the soil
liner in the field contains defects that cannot be
simulated accurately on small specimens. If the soil
23
-------
is homogeneous, lab and field tests should compare
very well.
Attack by Waste Leachate
Acids and Bases
Acids can attack soil by dissolving the soil minerals
into other constituents. Typically, when acids are
passed through soil, hydraulic conductivity drops
because the acids dissolve the materials that help to
neutralize them. After large amounts of acid wash
into the soil, hydraulic conductivity decreases.
There is real concern over waste impoundments used
to store acidic liquid. Small amounts of acid such as
that contained in a barrel in a solid waste landfill
underlain by a 3-foot thick liner will not inflict major
damage on the soil liner. A large volume of liquid in
the impoundment, however, can damage the soil
seriously.
Neutral Inorganic Compounds
Nonacidic liquids can change hydraulic conductivity
in other ways. Soil is made up of colloidal particles
that have negative charges along the surface. Water
is a polar molecule, with atoms arranged or aligned
asymetrically. This alignment allows the water
molecule to be attracted electrochemically to the
surfaces of the negatively charged soil particles (see
Figure 2-22).
of water and ions surrounding the clay particles,
known as the diffuse double layer.
The water and ions in the double layer are attracted
so strongly electrochemically to the clay particles
that they do not conduct fluids. Fluids going through
the soil go around the soil particles and, also, around
the double layer. The hydraulic conductivity of the
soil,' then, is controlled very 'strongly by the
thickness of these double layers. When the double
layers shrink, they open up flow paths resulting in
high hydraulic conductivity. When the layers swell,
they constrict flow paths, resulting in low hydraulic
conductivity. '
The Gouy-Chapman Theory relates electrolyte
concentration, cation valence, and dielectric constant,
to the thickness of this double layer (see Figure 2-
23). This theory was originally developed for dilute
suspensions of solids in a liquid. However,
experience confirms that the principles can be
applied qualitatively to soil, even compacted soil that
is not in suspension.
Oouv-Chapman Theory: .
Thickness «
D
D = Dielectric Constant
n0 = Electroylte Concentration
V = Cation Valence
Figure 2-23. Gouy-Chapman Theory.
+ + •»• +•+ + + +
y'V ^-^ V^ f^ *"^ ¥^* V^* «**^
Figure 2-22. Water and clay particle molecules.
It is also possible for ions in the water, especially
positively charged ions, or cations, to be attracted to
the negatively charged surfaces. This leads to a zone
The following application of the Gouy-Chapman
Theory uses sodium bentonite. The ion in the soil is
sodium, which has a charge of +1. The electrolyte
valence in the Gouy-Chapman Theory is v = 1. The
permeating liquid is rich in calcium, and calcium has
a charge of +2. As calcium replaces sodium, the
valence (v) in the Gouy-Chapman equation goes from
1 to 2. A rise in v increases the (denominator, thus
decreasing the thickness (T). As Tj decreases and the
double layer shrinks, flow paths open up making the
soil more permeable, as shown in Figure 2-24.
Since calcium bentonite, typically, is 100 to 1,000
times more permeable than sodium bentonite, the
24'
-------
Flow
Clay
Particle
Figure 2-24. The diffuse double layer.
Double
Layer
introduction of this permeating liquid could change
hydraulic conductivity substantially.
Table 2-4 shows that soils containing polyvalent
cations having high valence and high electrolyte
concentration have a high conductivity, while the
soils containing monovalent cations, like sodium,
have a low k. Distilled water at the extreme end of
the spectrum is Tree of electrolytes. In the Gouy-
Chapman equation, then, no the electrolyte
concentration, would be 0. The denominator,
therefore, would go to 0 and the T value to infinity.
Table 2-4. Electroylte Concentration
High k Water with Polyvalent Cations
Tap Water (Note Variation)
Water with Monovalent Cations
Low k Distilled Water
Consequently, if the free ions in the soil water are
leached out, the double layers swell tremendously,
pinching off flow paths and resulting in very low
hydraulic conductivity. Data have shown hydraulic
conductivity to be as much as two to three orders of
magnitude lower when measured with distilled
water than with other kinds of water. For this
reason, distilled water should not be used in testing
the hydraulic conductivity of a clay liner.
An ASTM standard under development recommends
using 0.005 normal calcium sulfate as1 the standard
permeating water, because of its medium range
electrolyte concentration. Calcium sulfate, with
divalent calcium, will usually not reduce hydraulic
conductivity.
Neutral Organic Compounds
Organic chemicals can cause major changes in
hydraulic conductivity. The dielectric constant (D) of
many of the organic solvents used in industry is very
low. For example, the dielectric constant of water is
about 80, while the dielectric constant of
trichloroethylene is about 3. Using the Gouy-
Chapman equation, if D decreases, which means the
numerator decreases, the value for T will also
decrease, causing the double layer to shrink. The
effect of replacing water with an organic solvent then
is to shrink the double layer and open up flow paths.
In addition to opening up flow paths, as the double
layers shrink, the solvent flocculates the soil
particles, pulling them together and leading to
cracking in the soil. Permeation of the soil with an
organic chemical, such as gasoline, may produce
cracking similar to that associated with desiccation.
The organic solvent, however, produces a chemical
desiccation rather than a desiccation of the soil by
drying out.
Laboratory test data indicate that if the organic
chemical is present in a dilute aqueous solution, the
dielectric constant will not be dangerously low. A
dielectric constant above 30 generally will not lower
the conductivity substantially enough to damage the
soil. Two criteria need to be met for a liquid not to
attack'clay liners: (1) the solution must contain at
least 50 percent water, and (2) no separate phase or
organic chemicals can be present.
Termination Criteria
Chemical compatibility studies with hydraulic
conductivity tests must be performed over a long
enough period of time to determine the full effects of
the waste liquid. Termination criteria include equal
inflow and outflow of liquid, steady hydraulic
conductivity, and influent/effluent equilibrium. At
least two pore volumes of liquid must be passed
through the soil to flush out the soil water and bring
the waste leachate into the soil in significant
quantities (see Figure 2-25). Reasonable
equilibrations of the influent and effluent liquids
occur when the pH of the waste influent and effluent
liquids are similar and the key organic and inorganic
ions are at full concentrations in the effluent liquid.
Resistance to Leachate Attack
It is possible to make soils more resistant to chemical
attack. Many of the same methods used to lower
hydraulic conductivity can stabilize materials
against leachate attack, including greater
25
-------
Water
Organic Chemical
0 1
Pore Volumes of Flow
Figure 2-25. Hydraulic conductivity as a function of pore
volumes of flow.
compaction, an increase in overburden stress, and
the mixing of additives such as lime cement or
sodium silicate with the natural soil materials.
Figure 2-26 shows the results of an experiment
conducted using a soil called SI, an illitic clay
containing chlorite from Michigan. Two sets of data
show the results of permeation of the regular soil, .
first with water and then with pure reagent grade
heptane. The heptane caused the hydraulic
conductivity of the regular compacted soil to
skyrocket. About 8 percent cement was then added to
the soil.
After treatment of the soil with Portland cement,
however, the heptane did not affect the soil even
after a pore volume of flow. The Portland cement
glued the soil particles together so that the soil
became invulnerable to attack, rather than causing
it to undergo chemical desiccation.
References
1. Daniel, D.E. 1987. Earthen liners for land
disposal facilities. Proceedings, Geotechnical
Practice for Waste Disposal 1987, Univ. of
Michigan, Ann Arbor, Michigan, June 1987: 21-
39. New York, NY: American Society of Civil
Engineers.
2. Trautwein Soil Testing Equipment Company.
1989. Installation and operating instructions for
the sealed double-ring infiltrometer. Houston,
Texas: Trautwein Soil Testing Equipment
Company.
10-5
ID'6
O 10-7
o
|
£ 10-8
10-9
10-10
Clay Stabilization Research;
Soil: S1
Stabilization: Cement
Organic Chomical: Heptane
Cement Stabilized
Key
Unstabilized
Stabilied
•2.00 0.00 2.00 4.00 6.00
Pore Volumes of Flow
8.00
Figure 2-26. Illltic-chlorated clay treated with heptane and
with Portland cemant.
3. U.S. EPA. 1986a. U.S. Environmental Protection
Agency. Design, construction,; and evaluation of
clay liners for waste management facilities.
Technical Resource Document. Report No.
EPA/530/SW-86/007, NTIS Order No. PB86-
184496/AS. Cincinnati, Ohio: EPA.
i
4. U.S. EPA. 1986b. U.S. Environmental Protection
Agency. Saturated hydraulic conductivity,
saturated leachate conductivity, and intrinsic
permeability. EPA Method 9ltiO. EPA.
26
-------
3. FLEXIBLE MEMBRANE LINERS
Introduction
This chapter discusses several material and design
considerations for flexible membrane liners (FMLs).
It highlights some of the problems encountered in
designing "bathtub" systems for hazardous waste
landfills and describes the impact of proposed
regulations on material and design considerations.
Composite Liners: Clay versus Synthetic
Components
After a landfill site has been chosen and a basin has
been excavated, the basin is lined with one or more
layers of water-retaining material (liners) that form
a "leachate bathtub." The contained leachate is
pumped out through a network of pipes and collector
layers. Liners may be constructed of synthetic
polymer sheets or of clay. EPA's minimum
technology guidance (discussed in Chapter One)
relies on a composite liner that utilizes advantages
obtained from combining both liner systems.
Understanding the basic hydraulic mechanisms for
synthetic liners and clay liners is very important in
appreciating the advantages of a composite liner.
Clay liners are controlled by Darcy's law (Q = kiA)
(Darcy's law is discussed in more detail in Chapter
Two). In clay liners, the factors that most influence
liner performance are hydraulic head and soil perme-
ability. Clay liners have a higher hydraulic
conductivity and thickness than do synthetic liners.
Additionally, leachate leaking through a clay liner
will undergo chemical reactions that reduce the
concentration of contaminants in the leachate.
Leakage through a synthetic liner is controlled by
Pick's first law, which applies to the process of liquid
diffusion through the liner membrane. The diffusion
process is similar to flow governed by Darcy's law
except it is driven by concentration gradients and not
by hydraulic head. Diffusion rates in membranes are
very low in comparison to hydraulic flow rates even
in clays. In synthetic liners, therefore, the factor that
most influences liner performance is penetrations.
Synthetic liners may have imperfect seams or
pinholes, which can greatly increase the amount of
leachate that leaks out of the landfill.
Clay liners, synthetic liners, or combinations of both
are required in landfills. Figure 3-1 depicts the
synthetic/composite double liner system that appears
in EPA's minimum technology guidance. The system
has two synthetic flexible membrane liners (FMLs):
the primary FML, which lies between two leachate
collection and removal systems (LCRS), and the
secondary FML, which overlies a compacted clay
liner to form a composite secondary liner. The ad-
vantage of the composite liner design is that by
putting a fine grain material beneath the membrane,
the impact of given penetrations can be reduced by
many orders of magnitude (Figure 3-2). In the figure,
Qg is the inflow rate with gravel and Qc is the inflow
rate with clay.
Figure 3-3 is a profile of a liner that appeared in an
EPA design manual less than a year ago. This
system is already dated. Since this system was
designed, EPA has changed the minimum hydraulic
conductivity in the secondary leachate collection
system from 1 x 10-2 cm/sec to 1 cm/sec to improve
detection time. To meet this requirement, either
gravel or a net made of synthetic material must be
used to build the secondary leachate collection
system; in the past, sand was used for this purpose.
Material Considerations
Synthetics are made up of polymers—natural or
synthetic compounds of high molecular weight.
Different polymeric materials may be used in the
construction of FMLs:
• Thermoplastics—polyvinyl chloride (PVC)
• Crystalline thermoplastics—high density poly-
ethylene (HOPE), linear low density polyethylene
(LLDPE)
27
-------
Primary FML
Secodary FML
ompacted Clay Liner
Native Soil Foundation
(Not to Scale)
Figure 3-1. Synthetic/composite double liner system.
Qn « 105 CL
Figure 3-2. Advantage of composite liner.
• Thermoplastic elastomers—chlorinated poly-
ethylene (CPE), chlorylsulfonated polyethylene
(CSPE)
• Elastomers—neoprene, ethylene propylene diene
monomer (EPDM)
Typical compositions of polymeric geomembranes
are depicted in Table 3-1. As the table shows, the
membranes contain various admixtures such as oils
and fillers that are added to aid manufacturing of the
FML but may affect future performance. In addition,
many polymer FMLs will cure once installed, and the
strength and elongation characteristics of certain
FMLs will change with time. It is important
Minimum
Thickness
—r~—
15 cm
30 cm
j.076 cm
30 cm
.076 cm
90 cm
,•.; ;••-•.-!••.••- «>.-•?. -•.r..<.-.-:,^,:;v
:o;.-;:r;''- Solid Waste .'•"••^i
.*-r.r*::.i •• , r.r.-•'.--^..^'-^i
Filter Media
Hydraulic Conductivity
10-2 cm/secr"~y
/^\a 1
Hydraulic Conductivity
> 1 x 10-2
s~\ > 1
'/-\
Hydraulic Conductivity
< 1 x 10'7 cm/sec
Primary LCR
• Primary FML
Secondary LCR
• Secondary FML
Clay Liner
Native Soils
Unsaturated Zone
; Saturated Zone
r//////j0m!(f0(m
"Minimum hydraulic conductivity is now 1 cm/sec.
Figure 3-3. Profile of MTG double liner system.
therefore to select polymers for FML construction
with care. Chemical compatibility, manufacturing
considerations, stress-strain characteristics, sur-
28
-------
vivability, and permeability are some of the key
issues that must be considered.
Chemical Compatibility
The chemical compatibility of a FML with waste
leachate is an important material consideration.
Chemical compatibility and EPA Method 9090 tests
must be performed on the synthetics that will be
used to construct FMLs. (EPA Method 9090 tests are
discussed in more detail in Chapter Nine.) Unfor-
tunately, there usually is a lag period between the
time these tests are performed and the actual
construction of a facility. It is very rare that at the
time of the 9090 test, enough material is purchased
to construct the liner. This means that the material
used for testing is not typically from the same
production lot as the synthetics installed in the field.
The molecular structure of different polymers can be
analyzed through differential scanning calorimeter
or thermogravimetric testing. This testing or
"fingerprinting" can ensure that the same material
used for the 9090 test was used in the field. Figure 3-
4 was provided by a HOPE manufacturer, and the
fingerprints depicted are all from high density
polyethylenes. Chemical compatibility of extrusion
welding rods with polyethylene sheets is also a
concern.
Manufacturing Considerations
Polyethylene sheets are produced in various ways:
• Extrusion—HOPE
• Calendaring—PVC
• Spraying—Urethane
In general, manufacturers are producing high
quality polyethylene sheets. However, the
Table 3-1. Basic Composition of Polymeric Geomembrane
180°C, SOOpsig
28-
24-
20-
LL
to 12-1
0)
8-
0 10 20 30 40 50 60 70 80 90 100110120
Time (min)
Figure 3-4. Comparison of "fingerprints" of exothermic peak
shapes.
compatibility of extrusion welding rods and high
density polyethylene sheets can be a problem. Some
manufacturing processes can cause high density
polyethylene to crease. When this material creases,
stress fractures will result. If the material is taken
into the field to be placed, abrasion damage will
occur on the creases. Manufacturers have been
working to resolve this problem and, for the most
part, sheets of acceptable quality are now being
produced.
Stress-Strain Characteristics
Table 3-2 depicts typical mechanical properties of
HDPE, CPE, and PVC. Tensile strength is a
Composition of Compound Type
(parts by weight)
Component
Polymer or alloy
Oil or plasticizer
Fillers:
Carbon Black
Inorganics
Antidegradants
Crosslinking system:
Inorganic system
Sulfur system
Crosslinked
100
5-40
5-40
5-40
1-2
5-9
5-9
Thermoplastic
100
5-55
5-40
5-40
1-2
-
' Semicrystalline
100
0-10
2-5
1
'-- .,
Source: Haxo, H. E. 1986. Quality Assurance of Geomembranes Used as Linings for Hazardous Waste Containment. In: Geotextiles and
Geomembranes, Vol. 3, No. 4. London, England.
29
-------
fundamental design consideration. Figure 3-5 shows
the uniaxial stress-strain performance of HDPE,
CPE, and PVC. As 600, 800,1,100, and 1,300 percent
strain is developed, the samples fail. When biaxial
tension is applied to HDPE, the material fails at
strains less than 20 percent. In fact, HDPE can fail at
strains much less than other flexible membranes
when subjected to biaxial tensions common in the
field.
Another stress-strain consideration is that high
density polyethylene, a material used frequently at
hazardous waste facilities, has a high degree of
thermal coefficient of expansion - three to four times
that of other flexible membranes. This means that
during the course of a day (particularly in the
summer), 100-degrees-Fahrenheit (°F) variations in
the temperature of the sheeting are routinely
measured. A 600-foot long panel, for example, may
grow 6 feet during a day.
Survivability
Various tests may be used to determine the
survivability of unexposed polymeric geomembranes
(Table 3-3). Puncture tests frequently are used to
estimate 'the survivability of FMLs in the field.
During a puncture test, a 5/16 steel rod with rounded
edges is pushed down through the membrane. A very
flexible membrane that has a high strain capacity
under biaxial tension may allow that rod to
penetrate almost to the bottom of the chamber
rupture. Such a membrane has a very low
penetration force but a very high penetration elonga-
tion, and may have great survivability in the field.
High density polyethylenes will give a very high
penetration force, but have very high brittle failure.
Thus, puncture data may not properly predict field
survivability.
Permeability
Permeability of a FML is evaluated using the Water
Vapor Transmission test (ASTM E96). A sample of
the membrane is placed on top of a small aluminum
cup containing a small amount of water. The cup is
then placed in a controlled humidity and
temperature chamber. The humidity in the chamber
is typically 20 percent relative humidity, while the
humidity in the cup is 100 percent. Thus, a
concentration gradient is set up across the
membrane. Moisture diffuses through the membrane
and with time the liquid level in the cup is reduced.
The rate at which moisture is moving through the
membrane is measured. From that rate, the
permeability of the membrane is calculated with the
simple diffusion equation (Pick's first law). It is
important to remember that even if a liner is
installed correctly with no holes, penetrations,
punctures, or defects, liquid will still diffuse through
the membrane. '•
Design Elements
A number of design elements must be considered in
the construction of flexible membrane liners: (1)
minimum technology guidance, (2) stress
considerations, (3) structural details, and (4) panel
fabrication.
Minimum Technology Guidance
EPA has set minimum technology guidance for the
design of landfill and surface impoundment liners to
achieve de minimis leakage. De minimis leakage is 1
gallon per acre per day. Flexible membrane liners
must be a minimum of 30 mils thick, or 45 mils thick
if exposed for more than 30 days. There may,
however, be local variations in the requirement of
minimum thickness, and these variations can have
an impact on costs. For example, membranes cost
approximately $.01 per mil per square foot, so that
increasing the required thickness of the FML from
30 mils to 60 mils, will increase the price $.30 cents
per square foot or $12,000 per acre.
I
Stress I
Stress considerations must be considered for side
slopes and the bottom of a landfill. For side slopes,
self-weight (the weight of the membrane itself) and
waste settlement must be considered; for the bottom
of the facility, localized settlement and normal
compression must be considered. !
The primary FML must be able to support its own
weight on the side slopes. In order to calculate self-
weight, the FML specific gravity, friction angle,
FML thickness, and FML yield stress must be known
(Figure 3-6). ;
Waste settlement is another consideration. As waste
settles in the landfill, a downward force will act on
the primary FML. A low friction component between
the FML and underlying material prevents that
force from being transferred to the underlying
material, putting tension on the primary FML. A 12-
inch direct shear test is used to measure the friction
angle between the FML and underlying material.
An example of the effects of waste settlement can be
illustrated by a recent incident at a hazardous waste
landfill facility in California. At this facility, waste
settlement led to sliding of the waste, causing the
standpipes (used to monitor secondary leachate
collection sumps) to move 60 to 90 feet downslope in
1 day. Because there was a very low coefficient of
friction between the primary liner and the geonet,
the waste (which was deposited in a canyon) slid
down the canyon. There was also a failure zone
between the secondary liner and the clay. A two-
30
-------
Table 3-2. Typical Mechanical Properties
Density, gm/cm3
Thermal coefficient of expansion
Tensile strength, psi
Puncture, Ib/mil
HOPE
>.935
12.5 x 10-5
4800
2.8
CPE
1.3 - 1.37
4 x 10-5
1800
1.2
PVC
1 .24 - 1 .3
3x10-5
2200
4000
To 3860 PSI at 1180%
100
200
300
Strain, %
400
500
Figure 3-5. FML stress-strain performance
(uniaxial-Koerner, Richardson; biaxial-Steffen).
dimensional slope stability analysis at the site
indicated a factor of safety greater than one. A three-
dimensional slope stability analysis, however,
showed the safety factor had dropped below one.
Three-dimensional slope stability analyses should
therefore be considered with canyon and trench
landfills.
Since more trenches are being used in double FML
landfills, the impact of waste settlement along such
trenches should be considered. Figure 3-7 is a simple
evaluation of the impact of waste settlement along
trenches on the FML. Settlements along trenches
will cause strain in the membrane, even if the trench
is a very minor ditch. Recalling that when biaxial
tension is applied to high density polyethylene, the
material fails at a 16 to 17 percent strain, it is
possible that the membrane will fail at a moderate
settlement ratio.
Another consideration is the normal load placed on
the membranes as waste is piled higher. Many of the
new materials on the market, particularly some of
the linear low density polyethylene (LLDPE) liners,
will take a tremendous amount of normal load
without failure. The high density polyethylenes, on
the other hand, have a tendency to high brittle
failure.
Structural Details
Double liner systems are more prone to defects in the
structural details (anchorage, access ramps,
collection standpipes, and penetrations) than single
liner systems.
31
-------
Table 3-3. Test Methods for Unexposed Polymeric Geomembranes
Membrane Liner Without Fabric Reinforcement
Property
Analytical Properties
Volatiles
Extractables
Ash
Specific gravity
Thermal analysis:
Differential scanning
calorimetry (DSC)
Thermogravimetry
(TGA)
Physical Properties
Thickness - total
Coating over fabric
Tensile properties
Tear resistance
Modulus of elasticity
Hardness
Puncture resistance
Hydrostatic resistance
Seam strength:
In shear
In peel
Ply adhesion
Environmental and Aging
Effects
Ozone cracking
Environmental stress
cracking
Low temperature testing
Thermoplastic
MTM-1a
MTM-2a
ASTM D297, Section 34
ASTM D792, Method A
NA
Yes
ASTM D638
NA
ASTM D882,
ASTM D638
ASTM D1004
(modified)
NA
ASTM D2240
Dura A or D
FTMS101B,
Method 2065
NA
ASTM D882, Method A
(modified)
ASTM D413, Mach
Method Type 1
(modified)
NA
ASTM D1149
NA
ASTM D1790
Crosslinked
MTM-1a
MTM-2a
ASTM D297, Section 34
ASTM D297, Section 15
NA
Yes
ASTMD412
NA
ASTM D412
ASTM D624
NA
ASTM D2240
Duro A or D
FTMS 101B,
Method 2065
NA
ASTM D882, Method A
(modified)
ASTM D413, Mach
Method Type 1
(modified)
NA
ASTM D1 149
NA
ASTM D746
Semicrystalline
MTM-ia
MTM-2a
ASTM D297, Section 34
ASTM D792, Method A
\/r\ff
Yes
Yes
ASTM D638
NA
ASTM D638
(modified)
ASTM D1004
DieC
ASTM D882, Method A
ASTM D2240
Duro A or D
FTMS 1 01 B,
Method 2065
ASTM D751, Method A
ASTM D882, Method A
(modified)
ASTM D41 3, Mach
Method Type 1
(modified)
NA
NA
ASTM D1 693
ASTM D1790
ASTM D746
Fabric Reinforced
,
MTM-ia
(on selvage and
reinforced sheeting)
MTM-2a
(on selvage and
reinforced sheeting)
ASTM D297, Section 34
(on selvage)
ASTM D792, Method A
(on selvage)
>
MA '
IMr\ ,
Yes
ASTM D751, Section 6
Optical method
AStM D751, Method A
and. B (ASTM D638 on
selvage)
ASTM D751, Tongue
method (modified)
NA
ASTM D2240 Duro A
or 0 (selvage only)
FTMS 1.01 B,
Methods 2031 and 2065
ASTM D751, Method A
ASTM D751, Method A
(modified)
ASTM D413, Mach
Method Type 1
(modified)
ASTM D413, Mach
Method Type 1
ASTM D751 , Sections
39:42
ASTM D1 149
NA
ASTM D2136
Tensile properties at
elevated temperature
Dimensional stability
ASTM D638 (modified) ASTM D412 (modified) ASTM D638 (modified) ASTM D751 Method B
(modified)
ASTM D1204
ASTM D1204
ASTM D1204
ASTM D1204
32
-------
Table 3-3. Test Methods for Unexposed Polymeric Geomembranes (continued)
Membrane Liner Without Fabric Reinforcement
Property Thermgplastc Crosslinked ; Semicrystalline Fabric Reinforced
Air-oven aging ASTM D573 (mod fied) ASTM D573 (modified) ASTM D573 (modified) ASTM D573 (modified)
Water vapor transmission ASTM E96, Method BW ASTM E96, Method BW ASTM E96, Method BW ASTM E96, Method BW
Water absorption ASTM D570 ASTM D471 ASTM D570 ASTM D570
Immersionin standard ASTM D471, D543 ASTM D471 ASTM D543 ASTM D471 D543
liquids '
Immersion in waste
^uids EPA 9090 EPA 9090 EPA 9090 EPA 9090
Soil burial ASTM D3083 ASTM D3083 ASTM D3083 ASTM D3083
Outdoor exposure ASTM D4364 ASTM D4364 ASTM D4364 ASTM D4364
Tub test b b b b
aS
"S
Nf
So
ee reference (8).
ee reference (12).
^ = not applicable.
urce: Haxo, 1987
Cell Component: FLEXIBLE ME.MBRAWE Lmep
Consideration !~Ttw*'LE ^TRE« - LIUER UEIGHT • t™ u»,e
A5ILITY OF FML TO zureaHT
Required Material Properties Range
F«i<=-n<>u £>"<;Lf-
• FML-TO-LCE ^ %L \°~-,. *<>'
FML TiWuijti-i ,-t to TO no
Oilli1 MIC limi," "£*':> > ^Y I««ero5»o
Analysis Procedure:
"" lifer1
(4U^-u«. D«,«M aoi. SimA~
PR. GI /s
Design Ratio: Referenc
T* OKJ U ^JCi^HT ow THE
Test Standard
Oi«££T -5HCAA PftoPoato ASTM
0 "Te^jsue A-STM Dtss
>-' l'x t
-)ffY
es:
FML ^rtc.Fii <^«»uily , Cj. O.S4I
• p>- 5D*
(O tfr.ucj^j^jt FML TtU4iLt foeZ(TE T
•°'° 1
= 2.2.2. Ib/fr
T' ~\o.e, ^.vj -J0°_ 22.2
(z1 ^^LcuL^Tt FMI_ lei_,-5at STRESS s"
..,VO,^;¥,,,
'te~
20 ' 450 /"./ •.
»« -'*-/'*- '« o^
Example No. 5.H
Figure 3-6. Calculation of self-weight.
33
-------
0.1 0.2
Settlement Ratio, S/2L
Figure 3-7. Settlement trough models (Knipschield, 1985).
Anchorage
Anchor trenches can cause FMLs to fail in one of two
ways: by ripping or by pulling out. The pullout mode
is easier to correct. It is possible to calculate pullout
capacity for FMLs placed in various anchorage
configurations (Figures 3-8 and 3-9). In the "V;"
anchor configuration, resistance can be increased by
increasing the "V" angle. A drawback to using the
"V" design for getting an accurate estimate of
pullout capacity is that it uses more space. The
concrete trench is not presently used.
Ramps
Most facilities have access ramps (Figure 3-10),
which are used by trucks during construction and by
trucks bringing waste into the facility. Figure 3-11
depicts a cross section of a typical access ramp. The
double FML integrity must be maintained over the
entire surface of the ramp. Because ramps can fail
due to traffic-induced sliding, roadway
considerations, and drainage, these three factors
must be considered during the design and
construction of access ramps.
The weight of the roadway, the weight of a vehicle on
the roadway, and the vehicle braking force all must
be considered when evaluating the potential for
slippage due to traffic (Figure 3-12). The vehicle
braking force should be much larger than the dead
weight of the vehicles that will use it. Wheelloads
also have an impact on the double FML system and
the two leachate collection systems below the
roadway. Trucks with maximum axle loads (some
much higher than the legal highway loads) and 90
psi tires should be able to use the ramps. Figure 3-13
illustrates how to verify that wheel contact loading
will not damage the FML. Swells or small drains
may be constructed along the inboard side of a
roadway to ensure that the ramp will adequately
drain water from the roadwjay. Figure 3-14
illustrates how to verify that a ramp will drain water
adequately. The liner system, which must be
protected from tires, should be armored in the area of
the drainage swells. A sand subgrade contained by a
geotextile beneath the roadway can prevent local
sloughing and local slope failures along the side of
the roadway where the drains are located. The sand
subgrade tied together with geotextile layers forms,
basically, 800-foot long sandbags stacked on top of
one another.
Vertical Standpipes
Landfills have two leachate collection and removal
systems (LCRSs): a primary LCRS and a secondary
LCRS. Any leachate that penetrates the primary
34
-------
Cell Component: FLEXIBLE ME.KI&RAJ-JE
Consideration* ^ ^"" '^""^•'iS -CALCULATE
Required Material Properties Range
•^•IL/FML Fw^n«u Au^ie. 12-20*
• S.iL F«i<=Tiou AuqL6 2S-J8"
Dull "1C Mnrun
Analysis Procedure:
* iC^toMETKV * MA.T*IR IAL,
- SLOPE At-i^lE fe -SoiL fd
-ENBePMtMr Lcu?-s.^r
Standard
A»TS, P«0P0«0
<•
I V
LvJfj -i
Design Ratio: References:
MOT APPLICABLE.
Example:
^iv/E M *• ^EoMtTR V 5^, L
• DR« 1.5. M.^.M-JM
(0 DtFiuE^piA6LE.^ /^ iw" U'
-s^\+ t«*
^^ ^--X,«JM'«"«*.M-P-
H - ' •) T 21"«6XTAMll' -Jglt/
b!I*.U\ ^ 1 I.SixlB.V-S.ulM'TA,UIS* "1° ^*"T
- ., x" — ' 'W""Av*' /-i/.|'
1 ^ ^^^ ^J.^
"d*"'-*l'M'5 7,3-q^r 4»-i- '"•''•'"]
T |.331 3Hlb/Fr
0,U«E1f A.^B.« _x^ «^"*W*
'"t''*'rt''^Ll»-J..^)"1\.
l-aai ^^ /'T
--^^•-••^^^^^^^^^
KP =="
[T]<° '•-•*•'
Example No. 2>.17
Figure 3-8. Calculation of anchor capacity.
system and enters the secondary system must be
removed. Vertical standpipes (Figure 3-15) are
used to access the primary leachate collection sumps.
As waste settles over time, downdrag forces can have
an impact on standpipes. Those downdrag forces can
lead to puncture of the primary FML beneath the
standpipe. Figure 3-16 illustrates how to verify that
downdrag induced settlement of standpipes will not
cause the underlying leachate collection system to
fail.
•To reduce the amount of downdrag force on the waste
pile, standpipes can be coated with viscous or low
friction coating. Standpipes can be encapsulated
with multiple layers of HDPE. This material has a
very low coefficient of friction that helps reduce the
amount of downdrag force on the waste piles. Figure
3-17 illustrates how to evaluate the potential
downdrag forces acting on standpipes and how to
compare coatings for reducing these forces.
Downdrag forces also affect the foundation or
subgrade beneath the standpipe. If the foundation is
rigid, poured concrete, there is a potential for sig-
nificant strain gradients. A flexible foundation will
provide a more gradual transition and spread the
distribution of contact pressures over a larger
portion of the FML than will a rigid foundation
(Figure 3-18). To soften rigid foundations,
encapsulated steel plates may be installed beneath
the foundation, as shown in Figure 3-15.
Standpipe Penetrations
The secondary leachate collection system is accessed
by collection standpipes that must penetrate the
primary liner. There are two methods of making
these penetrations: rigid or flexible (Figure 3-19). In
the rigid penetrations, concrete anchor blocks are set
behind the pipe with the membranes anchored to the
concrete. Flexible penetrations are preferred since
these allow the pipe to move without damaging the
35
-------
Tcosg
Cover Soil
mrnTiF iTrnrnT
tsinp
^ < *L
<},= Ycs^cs
Horizontal Anchor
"V" Anchor
Tcosp
tsing
Concrete Anchor
Figure 3-9. Forces and variables—anchor analysis.
liner. In either case, standpipes should not be welded
to the liners. If a vehicle hits a pipe, there is a high
potential for creating major tears in the liner at
depth.
36
-------
Ramp structure
Figure 3-10. Geometry of typical ramp.
FML—•
FML—•
18' Typical
Figure 3-11. Cross section of typical access ramp.
Wind Damage
During the installation of FMLs, care must be taken
to avoid damage from wind. Figure 3-20 shows
maximum wind speeds in the United States.
Designers should determine if wind will affect an
37
-------
Cell Component: RA.MP
Consideration: -^UD'Sj<:\ ' -"g-a-Ff THAT RAMP -sj6-&A-sE. •»
5TA8L6 JUPE.R LOAD,
Required Material Properties
S.IL • PML fa.cr— j Au^u, $fnl.
Dull Kit ««nn
Analysis Procedure:
Range
to - IS'
14 • «*
\o-ia'
^.-ivj" V
Test
^3^=4
Standard
«TM RoRrrfO
^
fa * rrt ie.T.»»ML FtRe£ (Q pA*£ o r K OAOVI A v
* C W^^ U*4 ) V ^tf < ft 1* TAtJ 2
UMEAE ,£M.U i»7fie MIMIMJH fc'^r.^j norec?FA^e. AM^LE
(^ • KoHt^iou FoRiE
i«r*,~t Dt*^(?AT,»,DR
^rt (?E*i5Tluq R>R«:e-b
WK" (U^W^'^^e,
Design Ratio:
B«»M- «/
rt
References:
Example:
**iL-L«iR JJ" Zoa,p»f?
TML-I^O; 11° o ! ^ fiuOat e« irtaiA
UIM-" . i»' Itu^rw • 1*°' THKKUEW £4" ft- 8°
U.-iJ^r.rBp.^ -;^"*'^^- •*fa-"" ft*1^
F8' &^A.«,U, R.^6 - .9*n»K,P- i|.K,p
, r
F^» (7o2 4 no) *CQ<» £>° + TXkj 12°
r ITo,^ Kip-^
O1P*FM-IE pE,-»^KJ R^T.o*^ pf^
^^ l"70.«l 17**.^
^E
-------
Cell Component: I? A.MP-
Consideration: WHt£L
g - VER.FI
Required Material Properties Range Test Standard
2.S - 40'
Analysis Procedure:
(0
PR --
Design Ratio:
PRMIPJ 5>° <9|
References:
Example:
lUfc F'ELP 6.KJTA
.? 6..?
-------
Cell Component:
Consideration t
Required Material Properties
lu-piAjji now -r i-tre , £
Pl«MlA6iur»,K)flF'S
1*3
CO E-S.TIM1AT6- F^o1^ f^APAiiTY «r LCC
1.2 fi^ -sit.
|S
Example No. 4.2.
Figure 3-14. Calculation of ramp drainage capability.
40
-------
Placing low friction HOPE around a standpipe.
41
-------
Steel Plate &. .
• ^ f!f) *—
36"-48" RCP
•* +•
Concrete Base
Gravel
FML
,LCR
Sand
Figure 3-15. Details of standpipe/drain.
42
-------
Figure 3-16., Verification that downdrag induced settlement will not cause LCR failure.
43
-------
Cell Component: STXUDP.PE.
fof*. REOJ^-nwo of
Required Material Properties
Range
Test
ASTM PROPOSED
Standard
Analysis Procedure:
P»UUpgA<^ WiTHouT.
(MWFAtf DM7.2*)
'tf> STALJOPJ>e PffUlMOftAC^ U*TH &lT»lHtM
Design Ratio:
References:
V6«cC".T7,
(J*vr-/v« PM T.l ( 1^82)
Example:
- DIAMETER , D - ^ O"
- PfcPTH 2 » |£S'
C MfeotUM
CO STA.MPPIPE Poukj
- 655 kifi
Figure 3-17. Evaluation of potential downdrag forces on standpipes with and without coating.
a .73
5
—i_ALL PILCS<
1 I t
.VW SOTT.
rrr*>":±
RATIO OF
CA/C
oCLC
Example No.
44
-------
•s
J
111
2.0
edge
1.0
A) Elastic Settlement Constant
r/a
Clay Subgrade
Sand Subgrade
(after Terzaghi and Peck)
Elastic Subgrade
B) Distribution of Contact Pressures
Figure 3-18. Standpipe induced strain in FML.
45
-------
Geotextile
Weld
FML
Steel Clamp [
Pipe Sleeve
Weld
Rigid Penetrations
Flexible Penetrations
Weld
Pipe Sleeve
ygBBisagBEaM*. /..... • -.
\ -Pipe ' 1 Geotextile
FML
•Pipe
Primary FML
Weld i
Secondary FML
Figure 3-19. Details of rigid and flexible penetrations.
46
-------
Figure 3-20. Design maximum wind speeds.
47.
-------
Cell Component: FU^.euL Me.Me.RAue.
p -1J . UllUO L,CT: • 7.3/)o.£>^ 073 we,
o FT =*• 5-"7/'o.o = 0.57 fjq
Example No. &>.!
Figure 3-21. Calculation of required sandbag spacing for FML/FMC panels.
Table 3-4. Wind-Uplift Forces, PSF (Factory Mutual System)
Height
Above "
Ground _
(ft)
0-15
30
50
75
Wind Isotach, mph :
City, Suburban Areas, Towns,
70
10a
10
12
14
SO
11
13
15
18
90
14
17
19
22
and Wooded Areas
100
17
21
24
27
110
20
25
29
33
Flat, Open Country,
70
14
16
18
20
80
18
21
24
26
or Open Coastal Belt > 1500 ft from Coast
90
23
27
30
33
•I 00
29
33
37
40
; 110
35
40
; 44
49
120
14
48
35
85
'Uplift pressures in PSF
48
-------
Cell Component: Fi.eyi6Le MEMBBAME LIUI
L Art<> v
IM F~M L .
Required Material Properties Range Test Standard
S.,L^e>»-.FML
FML-LCE Faic
Yieio STRESS oF
Drill UIC Unrxm
Analysis Procedure:
EXAMPLE:
Design Ratio:
\o-Zo'
a -is°
iooo-tZ6t>\
A.XTM 04J0
= ISoo p«t
References:
Example':
•SOLVE Fog Snoiug STABILITY
pj,- loo»«2.4« stJS* ' SfcS l
." SOLVE Foa MfcMftRfcME ~feKJ*'°M
- ISooox
Bur
.'. T» O
lkjCE 1 -O
(Example No. 3.18
Figure 3-22. Calculation of soil cover stability.
49
-------
Creation of "whales."
50
-------
-Liner
. Geotextile or
Drainage Composite
Place Vent Higher than Maximum Liquid Level
at Over-Flow Conditions ',
Two-Inch Minimum
Wind Cowl Detail
Gas Flow
Openings in Vent to be Higher than
"Top of Berm or Overflow Liquid Level
Geotextile or
Drainage Composite
Air/Gas Vent Assembly
— Approx. Six-Inches
Bond Skirt of Vent to Liner
Gas Flow
Geomembrane
Concrete
Figure 3-23. Gas vent details.
51
-------
A A A
7C
N /o^\
\ *£y
\
(2§)
(26)
©
/§\S—
i i '
I I i
i i
^» Toe of Slope /^
i
0
r~ T "~i T T i
i . ' . i © ' ®
i i i
%1
f
©
©
©
©\
A A A
^^ Panel Number
/\ Seam Number
Figure 3-24. Panel-seam identificaiton scheme.
52
-------
4. ELEMENTS OF LIQUID MANAGEMENT AT WASTE CONTAINMENT
SITES
Introduction
The drainage system for removing leachate or other
aggressive liquids from landfills, surface impound-
ments, and waste piles is critically important. Even
if a liner has no leaks, the phenomenon of molecular
diffusion will allow some of the organics from the
liquids ponded on top of the liner system to leach
through the flexible membrane liner and the clay.
The timely collection and removal of that leachate is
at the heart of this chapter.
This chapter presents an overview of collector design
and materials, followed by a discussion of the three
parts of a liquid management system: the leachate
collection and removal system above the primary
liner, the secondary leak detection collection and
removal system between the primary and secondary
liners, and the surface water collection system above
the closure of the completed facility. The chapter
concludes with a discussion of gas collector and
removal systems. The following topics will be
examined:
• Overview
Drainage Materials
Filtration Materials
Geosynthetics
Design-by-function Concepts
• Primary Leachate Collection and Removal
(PLCR) Systems
Granular Soil (Gravel) Drainage Design
Perforated Collector Pipe Design
Geonet Drainage Design
Granular Soil (Sand) Filter Design
Geotextile Filter Design
Leachate Removal Systems
• Leak Detection, Collection, and Removal (LDCR)
Systems
Granular Soil (Gravel) Drainage Design
Geonet Drainage Design
Response Time
Leak Detection Removal Systems
• Surface Water Collection and Removal (SWCR)
Systems
• Gas Collector and Removal Systems
Overview
Leachate refers to rainfall and snowmelt that
combines with liquid in the waste and
gravitationally moves to the bottom of a landfill
facility. During the course of its migration, the liquid
takes on the pollutant characteristics of the waste
itself. As such, leachate is both site specific and
waste specific with regard to both its quantity and
quality. The first part of the collector system to
intercept the leachate is the primary leachate
collection and removal (PLCR) system located
directly below the waste and above the primary
liner. This system must be designed and constructed
on a site-specific basis to remove the leachate for
proper treatment and disposal..
The second part of a leachate collection system is
between the primary and secondary liners. Varying
with State or region, it is called by a number of
names including the secondary leachate collection
and removal (SLCR) system, the leak detection
network, or the leak questioning system. It will be
referred to here as the leak detection, collection, and
removal (LDCR) system. The main purpose of this
system is to determine the degree of leakage, if any,
of leachate through the primary liner. Ideally, this
system would collect only negligible quantities of
leachate; however, it must be designed on the basis of
a worst-case scenario.
The third part, called the surface water collection
and removal (SWCR) system, lies above the waste
system in a cap or closure above the closed facility.
Its purpose is to redirect surface water coming
through the cover soil from off of the flexible
membrane in the cap to the outside perimeter of the
53
-------
system. The location of all three parts of the liquid
management system is illustrated in Figure 4-1.
Drainage Materials
The drainage materials for the liquid management
system must allow for unimpeded flow of liquids for
the intended lifetime of the facility. In a leachate
collection system, the drains may consist of pipes,
soil (gravel), geonets, or geocomposites. These
materials will be described in the following sections.
Perforated drainage pipes have the advantage of
common usage and design, and they transmit fluids
rapidly. They do, however, require considerable
vertical space, and are susceptible to particulate
clogging, biological clogging, and creep (deflection).
Creep is of concern for both poly vinyl chloride (PVC)
and high density polyethylene (HDPE) pipe
materials.
According to proposed EPA regulations, the
hydraulic conductivity value for soil used as the
drainage component of leachate collection systems
will increase over previous regulations by two orders
of magnitude, from 0.01 cm/sec to I cm/sec, in the
very near future. This regulation essentially
eliminates the use of sand, and necessitates the use
of gravel. Gravel that meets this regulation has
particle sizes of 1/4 to 1/2 inches and must be quite
clean with no fines content. While gravels of this
type are durable and have! high hydraulic
conductivities, they require a filter soil to protect
them. They also tend to move when waste is loaded
onto the landfill or personnel walk on them. For the
latter reason, they are practically ^mpossible to place
on side slopes. ',
The synthetic materials that best1 meet inplane flow
rate regulations are called geonets. Geonets require
less space than perforated pipe or gravel, promote
rapid transmission of liquids, and, because of their
relatively open apertures, are less likely to clog.
They do, however, require geotextile filters above
them and can experience problems with creep and
intrusion. Geonets have the disadvantage of being
relatively new and, therefore, less familiar to owners
and designers than are sand and gravel drainage
materials.
-tf"—&
Cover Soil
T°\~*g cwc—- —... ,
^^
^
Figure 4-1. The three elements of a liquid management drainage system in a double-lined solid waste facility.
54
-------
Another new synthetic material is called a drainage
geocomposite, many types of which are available.
Geocomposites have most of the same advantages
and disadvantages of geonets. They generally are not
used for primary or secondary leachate collection
systems, however, because of their relatively low
crush strength. The crush strength, or normal
strength perpendicular to the plane, of currently
available products is not sufficient to carry the
weight of a large landfill. Geocomposites are useful,
however, for surface water collector systems, where
the applied normal stresses are quite low.
Filtration Materials
The openings in drainage materials, whether holes
in pipes, voids in gravel, or apertures in geonets,
must be protected against invading fine particle-
sized materials. An intermediate material, having
smaller openings than those of the drainage
material, must be used as a filter. Commonly in a
pipe or gravel drain, a medium-coarse to fine sandy
soil is used as a filter. Sand, however, has the
disadvantages of taking up vertical space and
moving under various loading conditions.
Geotextiles used as filters avoid these problems. The
open spaces in the fabric allow liquid flow while
simultaneously preventing upstream fine particles
from fouling the drain. Geotextiles save vertical
space, are easy to install, and have the added
advantage of remaining stationary under load. As
with sand filters, clogging can occur, and because
geotextiles are a new technology much about them is
not known. Geotextiles are being used more and
more not only for filters, but also as cushioning
materials above and/or below FMLs.
Geosynthetics
Geosynthetic materials play a key role in liquid
management systems. The five major categories of
geosynthetics are:
• Geotextiles
• Geogrids
• Geonets
• Geomembranes
• Geocomposites
A brief discussion of each type follows.
Geotextiles are either woven or nonwoven fabrics
made from polymeric fibers. Woven geotextiles are
fabrics made up of webbed fibers that run in
perpendicular directions. For filtration, the spaces
between the fibers are the most important element.
These spaces or voids must be large enough to allow
unimpeded liquid flow but be small enough to keep
out invading particulates. The geotextiles also must
be sufficiently strong to cover and reinforce the
apertures, or openings, of the drainage materials
they are meant to protect.
In nonwoven geotextiles the fibers are much thinner
but far more numerous. The various types are
needle-punched, resin-bond, and melt-bond. All
contain a labyrinth of randomly oriented fibers that
cross one another so that there is no direct line of
flow. The fabric must have enough open space to
allow liquid to pass through, while simultaneously
retaining any upstream movement of particles. The
needle-punched nonwoven type is very commonly
used as a filter material.
Geogrids are very strong in transverse and
longitudinal directions, making them useful as
reinforcing materials for either soil or solid waste.
Generally, they are used to steepen the side slopes of
interior cells or exterior containment slopes of a
facility. Recently they also have been used in the
construction of "piggyback" landfills, i.e., landfills
built on top of existing landfills, to reinforce the
upper landfill against differential settlements within
the lower landfill.
Geonets are formed with intersecting ribs made from
a counter-rotating extruder. A typical geonet is
about 1/4-inch thick from the top of the upper 'rib to
the bottom of the lower rib, yet the flow capability is
approximately equivalent to that of 12 inches of sand
having a 0.01 cm/sec permeability. (The proposed
regulation will increase this value to 1 cm/sec, as
mentioned earlier.) The rapid transmission rate is
due to clear flow paths in the geonets, as opposed to
particle obstructions in a granular soil material.
There are two main concerns with geonets. First, the
crush strength at the rib's intersection must be
capable of maintaining its structural stability
without excessive deformation or creep. Second,
adjacent materials must be prevented from intruding
into the rib apertures, cutting off or reducing flow
rates.
Foamed geonets are relatively new products made
with a foaming agent that produces a thick geonet
structure (up to 1/2-inch) with very high flow rates.
These improved flow rates result from the thicker
product, but eventually the nitrogen gas in the rib
voids diffuses through the polymer structure, leaving
behind a structure with reduced thickness. The
result over the long term is a solid rib geonet
thickness equivalent to other nonfoamed geonets.
The fourth type of geosynthetic is a geomembrane, or
FML. It is the primary defense against escaping
leachate and of crucial importance. FMLs are the
focus of Chapter Three.
55
-------
The final category of geosynthetics is drainage,
geocomposites. These are polymeric materials with
built-up columns, nubs, cuspations, or other
deformations that allow planar flow within their
structure. A drainage geocomposite having 1-inch
high columns can carry the flow -of a 4- to 5-inch
diameter pipe. Many products, however, have low
crush strengths that are inadequate for deep
landfills or surface impoundments. They are useful,
however, for surface water collector systems above
the closed facility where they only need to support
approximately 4 feet of soil and construction
placement equipment.
Design-by-function Concepts
Whatever parameter of a specific material one is
evaluating, a required value for the material must be
found using a design model and an allowable value
for the material must be determined by a test
method. The allowable value divided by the required
value yields the design ratio (DR), or the resulting
factor of safety (FS). This design-by-function concept
is necessary to design and evaluate new materials
that are both feasible and safe for a variety of
situations.
In evaluating drainage and filtration materials, an
allowable flow rate is divided by a required flow rate
to obtain the design ratio or factor of safety according
to the equations below:
(a) For Drainage:
DR = qallow/qreqd (D
or
DR = Va
where DR = design ratio
q = flow rate per unit width
V = transmissivity
(b) For Filtration:
DR = qallow/qreqd
or
DR =
(4)
where DR = design ratio
q = flow rate per unit area
W = permittivity
Transmissivity is simply the coefficient of
permeability, or the hydraulic conductivity (k),
within the plane of the material multiplied by the
thickness (t) of the material. Because the
compressibility of some polymeric materials is very
high, the thickness of the material needs to be taken
into account. Darcy's law, expressed by the equation
q = kiA, is used to calculate rate of flow, with
transmissivity equal to kt and i equal to the
hydraulic gradient (see Figure 4-2):
Figure 4-2. Variables for calculating inplane flow rates
(transmissivity).
q = kiA . ; ' (5)
= k(Ah/L) (w x t)|
q/w = (kt) (Ah/L) '
. if 9 = kt
q/w = 0(i) I
where q/w = flow rate per unit width
9 = transmissivity
Note that when i = 1.0, (q/w) = 9; otherwise it does
not. :
With a liquid flowing across ;the plane of the
material, as in a geotextile filter, the permeability
perpendicular to the plane can be divided by the
thickness, t, to obtain a new valu£, permittivity (see
Figure 4-3). In crossplane flow, t is in the
denominator; for planar flow it is in the numerator.
Crossplane flow is expressed as:
q = kiA (6)
= k(Ah/t)A
q = (k/t)AhA
u; = (k/t) = (q/AhA)
where W = permittivity
q/A = flow rate per unit area ("flux")
56
-------
Thus, both transmissivity and permittivity values
allow for the thickness to be avoided in subsequent
analyses.
Figure 4-3. Variables for calculating crossplane flow rates
(permittivity).
Table 4-1 shows some of the ASTM test methods and
standards for drainage and filter materials used in
primary leachate collection and leachate detection
and collection systems. Test methods are determined
by D18, the Soil and Rock Committee of ASTM, and
by D35, the Committee on Geosynthetics.
Primary Leachate Collection and
Removal (PLCR) Systems
The various design options for primary leachate
collection systems are granular soil drains,
perforated pipe collectors, geonet drains, sand filters,
and geotextile filters. Figure 4-4 shows a cross
section of a primary leachate collection system with
a geonet drain on the side slope leading into a gravel
drain on the bottom. This gravel drain then leads
into a perforated pipe collector. A geotextile acts as a
filter protecting the geonet and sand acts as a filter
for the drainage gravel. Quite often the sideslope
geotextile extends over the bottom sand filter as
shown in Figure 4-4.
Granular Soil (Gravel) Drainage Design
Current minimum technology guidance (MTG)
regulations require that granular soil drainage
materials must:
• Be 30 centimeters (12 inches) thick.
• Have 0.01 cm/sec (= 0.02 ft/min) permeability
(hydraulic conductivity).
• Have a slope greater than 2 percent.
• Include perforated pipe.
• Include a layer of filter soil.
• Cover the bottom and side walls of the landfill.
There are two ways to calculate the required flow
rate, q, in granular soil drainage designs. One is
based on the above MTG values; the other is based on
the Mound Model (see Figure 4-5). Based on MTG
values:
q = kiA
.= (0.02) (0.02) (1 x 1)
= 4x10-4 ft3/min
(7)
Note that if MTG increases the required hydraulic
conductivity of the drainage soil to 1 cm/sec, the
above flow rate will be increased to 0.04 ft3/min.
In the Mound Model, the maximum height between
two perforated pipe underdrain systems is equal to:
h =
max
L/C
9
tan a
tana , o
+ 1 - V tan a + c
(8)
where c = q/k
k = permeability
q = inflow rate
The two unknowns in the equation are L, the
distance between pipes, and c, the amount of
leachate coming through the system. Using a
maximum allowable head, hmax, of 1 foot, the
equations are usually solved for L.
One method of determining the value of c is using the
Water Balance Method:
PERC = P - R/0 - ST - AET
(9)
where PERC =
percolation, i.e. the liquid
that permeates the solid
waste (gal/acre/day).
P = precipitation for which the
mean monthly .values are
typically used.
R/O = surface runoff.
ST = soil moisture storage, i.e.,
moisture retained in the soil
after a given amount of
accumulated potential water
loss or gain has occurred.
AET = actual evapotranspiration,
i.e., actual amount of water
loss during a given month.
57
-------
Table 4-1, Test Methods and Standards
ASTM Test Designation
(or other)
D2434
D2416
F405, F667
D4716
D4491
D4751
CW-022153
GRI-GTlb
Used to Determine
Permeability
Strength
General specification
Transmissivity
Permittivity
Apparent opening size
Gradient ratio
Long-term flow
Material
Soil
Underdrain pipe
HOPE pipe
Geonet, geocomposite
Geotextile
Geotextile
Geotextile
Geotextile
i
jValue Used for
1 PLCR, LDCR
PLCR, LDCR
PLCR, LDCR
PLCR, LDCR
1 PLCR filter
PLCR filter
' PLCR filter
PLCR filter
a U.S. Army Corps of Engineers Test Method.
bQeosynthetic Research Institute Test Method.
Perforated
Pipe
Figure 4-4. Cross section of primary leachate collection systems.
Inflow
I 1 i i 1 1 • 1 1 i 1
Drainage Layer 7^— ^ hn
Figure 4-5. Flow rate calculations: Mound Model.
The range of percolation rates in the United States is
15 to 36 inches/year (1,100 to 2,700 gal/acre/day)
(U.S. EPA, 1988). !
The computer program Hydro;logic Evaluation
Landfill Performance Model (HELP) can also be used
to calculate c. HELP was developed to assist in
estimating the magnitude o|f water-balance
components and the height of water-saturated soil
above the barrier layers. HELP [can be used with
three types of layers: vertical percolation, lateral
drainage, and barrier soil liner. By providing
climatological data for 184 cities throughout the
United States, HELP allows the user to incorporate
extended evaluation periods without having to
assemble large quantities of data (Schroeder et al,
1984). :
58
-------
Perforated Collector Pipe Design
The original perforated collector pipes in landfills
were made of concrete like those used in highway
underdrain systems. As landfills became higher, the
strength of such pipes became inadequate. Today,
perforated PVC pipes are commonly used, as are
HDPE pipes. New regulations require that all
materials be tested for chemical resistance as part of
the permit-approval process.
The three steps in designing perforated collector
pipes are:
1. Obtain the required flow value using known
percolation and pipe spacing.
2. Obtain the required pipe size using the required
flow and the maximum slope.
3. Check the pipe strength and obtain its ring
deflection to determine tolerance against
crushing.
Knowing the percolation and pipe spacing from the
previous calculations, the required flow can be
obtained using the curve in Figure 4-6. The amount
of leachate percolation at the particular site is
located on the x-axis. The required flow rate is the
point at which this value intersects with the pipe
spacing value determined from the Mound Model.
Using this value of flow rate and the bottom slope of
the site, one can find the required diameter for the
pipe (see Figure 4-7). Finally, the graphs in Figures
4-8 and 4-9 show two ways to determine whether or
not the strength of the pipe is adequate for the
landfill design. In Figure 4-8, the vertical soil
pressure is located on the y-axis. The density of the
backfill material around the pipe is used to
determine ring deflection. Plastic pipe is not
governed by strength, so it will deform under
pressure rather than break. Twenty percent is often
used as the limiting deflection value for plastic pipe.
Using Figure 4-9 the applied pressure on the pipe is
located and traced to the trench geometry, and then
the pipe deflection value is checked for its adequacy.
Geonet Drainage Design
Table 4-2 presents a compilation of currently
available geonets. The structure and properties of
each are also identified. Geonets used in drainage
design must be chemically resistant to the leachate,
support the entire weight of the landfill, and be
evaluated by the ASTM test D4716 as to allowable
flow rate or transmissivity. This allowable value
must then be compared to the required value in the
design-by-function equation presented earlier.
In the D4716 flow test, the proposed collector cross
section should be modeled as closely as possible. The
candidate geonet usually will be sandwiched
C
'e
120
2
_o
13
pi
c
100
80
P 60
o 40-
fc
Q.
I
DC
20-
1 2 3 4.5
Percolation, in inches per month
"Where b = width of area contributing to leachate collection pipe
Figure 4-6. Required capacity of leachate collection pipe
(after U.S. EPA, 1983).
between a FML beneath and a geotextile above. Soil,
perhaps simulating the waste, is placed above the
geotextile and the load platen from the test device is
placed above the soil. Applied normal stress is
transmitted through the entire system. Then planar
flow, at a constant hydraulic head, is initiated and
the flow rate through the geonet is measured.
Figure 4-10 shows the flow rate "signatures" of a
geonet between two FMLs (upper curves) and the
same geonet with the cross section described
abovedower curves). The differences between the two
sets of curves represent intrusion of the
geotextile/clay into the apertures of the geonet.
Irrespective of the comparison in behavior, the
curves are necessary in obtaining an allowable flow
rate for the particular geonet being designed.
The required flow rate can be calculated by three
different methods: (1) directly from minimum
technology guidance, (2) using an equation
developed in the design manual, or (3) on the basis of
surface water inflow rate. To be conservative, all
three calculations should be performed and the
worst-case situation (e.g., that with the highest flow
rate) used for the required flow rate. The various
equations to determine the required flow rate or
transmissivity appear below:
1. Geonet must be equivalent to MTG regulations
for natural materials:
59
-------
G.P.M. C.F.S.
Pipe Flowing Full
Based on Manning's Equation n = 0.010
'0.1 ' 0.2 0.3 0.4 0.50.6 0.8 1.0 2.0 3.0 4.0 5.06.0 8.0 10
Slope of Pipe in Feet per Thousand Feet
Figure 4-7. Sizing of leachate collection pipe (U.S. EPA, 1983).
9 S: 0.02 ft3/min-ft
(10)
2. Based on estimated leachate inflow (Richardson
and Wyant, 1987):
Q _
reqd
2L sina
(11)
3. Based on surface water inflow (U.S. EPA, 1986):
Q = CIA (12)
where Q= surface water inflow
C= runoff coefficient
I = average runoff intensity
A= surface area
Generally geonets result in high factors of safety or
design ratios, unless creep becomes a problem or if
adjacent materials intrude into the apertures.
Granular Soil (Sand) Filter Design
There are three parts to an analysis of a sand filter to
be placed above drainage gravel. The first
determines whether or not the filter allows adequate
flow of liquids through it. The second evaluates
whether the void spaces are small enough to prevent
solids being lost from the upstream materials. The
third part estimates the long-term clogging behavior
of the filter. !
Required in the design of granular soil (sand) filter
materials is the particle-size distribution of the
drainage system and the particle-size distribution of
the invading (or upstream) soils. The filter material
should have its large and small size particles
intermediate between the two extremes (see Figure
4-11). Adequate flow and adequate retention are the
two focused design factors, but perhaps the most
important is clogging. The equations for adequate
flow and adequate retention are:
« Adequate Flow: d85f>(3 to 5) di5ds (13)
• Adequate Retention: disf < (3 to 5) dg5wf (14)
There is no quantitative method to assess soil filter
clogging, although empirical guidelines are found in
geotechnical engineering references.
Geotextile Filter Design :
Geotextile filter design parallels sand filter design
with some modifications. The three elements of
60
-------
I
Q.
'5
15
o
II
Q_
12,000
10,000
8,000
6,000
4,000
2,000
95%
Soil
Density
Initial Effect of
Ring Stiffness
85%
Soil
Density
75% Soil Density
Plot of Vertical
Soil Strain 6
0 5 10 15 20 25
Ring Deflection, AY/D (%) = € Except as Noted
Figure 4-8. Vertical ring deflection versus vertical soil pressure for 18-inch corrugated polyethylene pipe in high pressure soil
cell.
adequate flow, soil retention, and clogging
prevention remain the same.
Adequate flow is assessed by comparing the
allowable permittivity with the required
permittivity. Allowable permittivity uses the ASTM
D4491 test method, which is well established. The
required permittivity utilizes an adapted form of
Darcy's law. The resulting comparison yields a
design ratio, or factor of safety, that is the focus of
the design.
where
(15)
= permittivity from ASTM Test
D4491
' reqd
— = inflow rate per unit area
A.
hmax =12 inches
The second part of the geotextile filter design is
determining the opening size necessary for retaining
the upstream soil or particulates in the leachate. It is
well established that the 95% opening size is related
to the particles to be retained in the following type of
relationship
095 < fct. (d50, CU, DR)
(16)
where Ogs = 95% opening size of geotextile (U.S.
Army Corps of Engineers CW 02215
test method)
dso = 50% size of upstream particles
CU = uniformity of the upstream particle
sizes
61
-------
10
O
LJrgiU
>****
-aSS^!
^"
^
^^
/
^^
XI
^^
3/D - 1.
3
B/D = 1.5
^- B/D = 1 .8
- -^"Vertical So\\ £
for Native So
757<
..-B/D
5 10 15 20 2
> Density
= o 1
5 3
train e
l@
0
Ring Deflection, A Y/D (%)
Figure 4-9. The effect of trench geometry and pipe sizing
on ring deflection (after Advanced Draining
Systems, Inc., 1988)
DR = relative density of the upstream
particles
Geotextile literature documents the relationship
further.
The Ogs size of a geotextile in the equation is the
opening size at which 5 percent of a given size glass
bead passes through the fabric. This value must be
less than the particle-size characteristics of the
invading materials. In the test for the Ogg size of the
geotextile, a sieve with a very coarse mesh in the
bottom is used as a support. The geotextile is placed
on top of the mesh and is bonded to the inside so that
the glass beads used in the test cannot escape around
the edges of the geotextile. This particular test
determines the Ogs value. To verify the factor of
safety for particle retention in the geotextile filter,
the particle-size distribution of retained soil is
compared to the allowable value using any of a
number of existing formulae.
The third consideration in geotextile design is long-
term clogging. The test method that probably will be
adopted by ASTM is called the Gradient Ratio Test.
It was originally formulated by the U.S. Army Corps
of Engineers and is listed in CW 02215. In the test,
the hydraulic gradient of 1 inch of soil plus the
underlying geotextile is compared with the hydraulic
gradient of 2 inches of soil. If the gradient ratio is
less than 3, the geotextile probably will not clog. If
the gradient ratio is greater than 3, the geotextile
probably will clog. An alternate to this procedure is a
long-term column flow test that also is performed in
a laboratory. The test models a given soil-to-fabric
system at the anticipated hydraulic gradient. The
flow rate through the system is monitored. A long-
term flow rate at a constant value indicates an
equilibrium between the soil and the geotextile
system. If clogging occurs, the flow rate will
gradually decrease until it stops altogether.
Leachate Removal Systems
Figure 4-12 shows a low volume sump in which the
distance from the upper portion of the concrete
footing to the lower portion is approximately 1 foot.
One foot is an important design number because
EPA regulations specify a maximum leachate head
of 1 foot. Low volume submersible sumps present
operational problems, however. Since they run dry
most of the time, there is a likelihood of their
burning out. For this reason, landfall operators prefer
to have sumps with depths between 3 and 5 feet
instead of 1 (Figure 4-13), even though the leachate
level in a high volume sump will be greater than the
1-foot maximum.
The leachate removal standpipe must be extended
through the entire landfill from liner to cover and
then through the cover itself. It also must be
maintained for the entire post-closure care period of
30 years or longer.
Leak Detection, Collection, and Removal
(LDCR) Systems
The leak detection, collection, and removal system
(LDCR) is located between the primary and
secondary liners in landfills, surface impoundments,
and waste piles. It can consist of either granular soils
(i.e., gravels) or geonets.
Granular Soil (Gravel) Drainage Design
As with the primary leachate collection system
above the liner, leak detection systems between
liners are designed by comparing allowable flow
rates with required flow rates. The allowable flow is
evaluated as discussed in the section on granular soil
(gravel) drainage design for PLCR systems. The
required flow is more difficult to estimate. This value
might be as low as 1 gal/acre/day or many times that
amount. It is site specific and usually is a rough
estimate. Past designs have used 100 gal/acre/day for
the required flow rate. Data from; field monitoring of
response action plans (RAPs) will! eventually furnish
more realistic values. A pipe network for leachate
removal is required when using granular soils.
Geonet Drainage Design
For a geonet LDCR system, the flow rate for the
geonet is determined in the laboratory from ASTM
D4716 test method, and the value is modified to meet
site-specific situations. The geonet flow rate design
62
-------
Table 4-2. Types and Physical Properties of Geonets (all are polyethylene)
Roll Size, width/length
Thickness
Approx. Apperture Size
Manufacturer/Agent Product Name
Structure
-- ft.
mils
mm
in.
mm
Carthage Mills
Conwed Plastics
Fluid Systems Inc.
• Tex-Net (TN)
• Poly-Net (PN)
Geo-synthetics
Gundle
Low Brothers
Tenax
Tensar
FX-2000 Geo-Net
FX-2500 Geo-Net
FX-3000 Geo-Net
XB8110
XB8210
XB8310
XB8410
XB8315CN
TN-1001
TN-3001
TN-4001
TN-3001 CN
PN-1000
PN-2000
PN-3000
PN-4000
GSI Net 100
GSI Net 200
GSI Net 300
'Gundnet XL-1
Gundnet XL-3
Lotrak 8
Lotrak 30
Lotrak 70
CE1
CE2
CE3
CE600
DN1-NS1100
DN3-NS1300
-NS1400
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
foamed, and
extruded ribs
extruded ribs
extruded ribs
foamed, and
extruded ribs
foamed, and
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded mesh
extruded mesh
extruded mesh
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
extruded ribs
7.5/300
7.5/300
7.5/220
6.9/300
6.9/300
6.9/300
6.9/220
6.9/300
7.5/300
7.5/300
7.5/300
7.5/300
6.75/300
6.75/300
6.75/300
6.75/300
..
—
- •
6.2/100
6.2/100
6.6/164
6.6/164
6.6/164
4.8/66
7.4/82
7.4/82
5.5/100
5.2/98
6.2/98
6.2/98
2.3/91
2.3/91
2.3/67
2.1/91
2.1/91
2.1/91
2.1/67
2.1/91
2.3/91
2.3/91
2.3/91
2.3/91
2.0/91
2.0/91
2.0/91
2.0/91
„
—
-
1.9/30
1.9/30
2.0/50
2.0/50
2.0/50
1.5/20
3.8/25
2.2/25
1.67/30.5
1.6/30
1 .9/30
1 .9/30
200
250
300
250
160
200
300
200
250
200
300
200
250
160-
200
300
250
160
200
250
200
120
200
290.
250
200
160
160
220
150
200
5.1
6.3
7.6
6.3
4.1
5.1
7.6
5.1
6.3
5.1
7.6
5.1
6.3
4.1
5.1
7.6
6.3
4.1
5.1
6.3
5.1
3.0
5.2
7.3
6.3
5.1
4.1
4.1
5.6
3.8
5.1
0.3 x 0.3
0.35 x 0.35
0.3 x 0.4
0.25 x 0.25
0.3 x 0.3
0.3 x 0.3
0.3 x 0.4
0.35 x 0.35
0.25 x 0.25
0.3 x 0.3
0.3 x 0.3
0.3 x 0.3
1.2x1.2
2.8 x 2.8
0.3 x 0.25
0.3 x 0.35
0.3 x 0.25
0.3 X 0.25
0.3 x 0.3
0.3 x 0.3
0.3 x 0.3
8x8
9x9
8x10
6x6
8x8
8x8
9x9
8x10
6x6
8x8
8x8
8x9
30x27
70 x70
8x6
9x9
8x6
8x6
8x8
8x8
8x8
ratio is then determined in the same way as for the
granular system. No pipe network is needed.
A concern when using geonets with a composite
primary liner design is the effect of geotextile
intrusion and creep on the allowable flow rate (see
Figure 4-14). In composite primary liner systems, the
geonet is placed immediately below a clay liner with
a geotextile as an intermediate barrier. The design of
this geotextile is important because clay particles
can go through large voids in an open woven
geotextile, necessitating the use of a needle-punched
nonwoven geotextile of at least 8 to 10 ounces per
square yard (oz/yd2) mass per unit area. Even with
this precaution, the laboratory test to evaluate the
allowable flow rate should simulate the anticipated
cross section in every detail.
Response Time
EPA specifies that the minimum detection time for
leachate entering the leak detection system of a
LDCR system is less than 24 hours. Response time
calculations are based on velocity in the geonet
and/or granular soil drainage layer. Darcy's law is
used to calculate flow velocity in the geonet, and a
"true" velocity must be used for granular soil.
Figure 4-15 shows the response time calculation for a
leachate leak through a primary liner traveling 40
feet through the geonet on the side wall and 20 feet
through the sand at the bottom. The resulting
response times are 1.5 hours in the geonet and 6.2
hours in the soil; giving a total response time of 7.7
hours.
The travel time in a geonet is very short; so a 24-hour
response time can easily be achieved. With granular
soils, the travel time will be much longer.
Leak Detection Removal Systems
Leak detection removal systems require monitoring,
sampling, and leachate removal. Any leachate that
63
-------
it:
c
I
42
C
CC
5,000 10,000 15,000
Normal Stress (Ibs/sq. ft)
(a) FML - Geonet - FML Composite
20,000
5,000 10,000 15,000
Normal Stress (Ibs/sq. ft)
(b) FML - Geonet - Geotextile - Clay Soil Composite
Figure 4-10. Flow rate curves for geonets in different composite situations.
20,000
64
-------
100
Gravel Sand Silt
Particle Size (log)
Clay
Figure 4-11. Design based on particle-sized curves.
penetrates the primary liner system and enters the
secondary system must be removed. During
construction the LDCR system may accept runoff
water, but once the landfill is in operation it only
removes any leakage coming through the primary
liner. The most common removal system consists of a
relatively large diameter-pipe running down the side
wall between the primary and secondary liners to the
low point (sump) in the LDCR. The pipe must
penetrate the primary liner at the top. A submersible
pump is lowered through the pipe periodically for
"questioning" of the quantity of fluid coming into the
system (see Figure 4-16). The choice of monitoring
and retrieval pump depends on the quantity of
leachate being removed.
An alternate system, one based on gravity, requires
penetration of both the FML and clay components of
the secondary composite liner system as shown in
Figure 4-17. It also requires a monitoring and
collection manhole on the opposite side of the landfill
cell (see Figure 4-18). The manhole and connecting
pipe, however, become an underground storage tank
that needs its own secondary containment and leak
detection systems.
Surface Water Collection and Removal
(SWCR) Systems
The third part of liquids management is the surface
water collection and removal system (SWCR); It is
placed on top of the completed facility and above the
cover FML. The rainwater and snowmelt that
percolate through the top soil and vegetative cover
must be removed to a proper upper drainage system.
Figure 4-19 illustrates the major components of a
surface water collector system. The design quantity
for the amount of fluid draining into the surface
water collector system can be determined by either a
water balance method or the computer program
HELP discussed previously (see Figure 4-20).
Steel Plate
36" -48" RCP
•*—#-
Concrete Base
Gravel
>»LCR
***•••••••£•«•••••••••••••••••••••••••••••••••••
FML-T
MW^^
Figure 4-12. Leachate removal system with a low volume sump.
65
-------
-Standpipe
- Air Space
Figure 4-13. Leachate removal system with a high volume
sump.
J I I i *.*.>. *.^P-™L
•FML ''vuii'Possible Intrusion
^
'." *;' Clay Beneath Secondary FML- -'/'_ -.
-S-FML
Figure 4-14. Geotextile used as barrier material to prevent
extrusion of upper clay into geonet drain.
Surface water drainage systems can be composed of
granular soils, geonets, or geocomposites, but the
majority of drainage systems use granular soil. This
is particularly true in frost regions where it is
necessary to have 3 to 6 feet of soil above the FML to
satisfy the requirements for frost penetration. In
such cases, 1 foot of granular soil thickness can serve
as the surface water collector. If good drainage
materials are not available, if the site is too
extensive, or if natural materials would add
undesired thickness, a geonet or geocomposite can be
used. The advantage of drainage geocomposites is
their higher flow rate capabilities over geonets or
granular soils. Table 4-3 lists a number of
geocomposites that can be used for drainage systems.
All of these systems have polymer cores protected by
a geotextile filter. Although many of the polymers
cannot withstand aggressive leachates, this is not an
issue in a surface drainage collector where the only
contact is with water. The crush strengths of the
geocomposites are generally lower than for geonets,
but that too is not a problem in a surface water
collector. The heaviest load the geocomposite would
be required to support probably would be
construction equipment used to place the cover soil
and vegetation on the closed facility.
The design for the surface water collector system is
determined by an allowable flow rate divided by a
required flow rate. Allowable rates for geocomposites
are determined experimentally by iexactly the same
method as for geonets. Figure 4-21 shows the flow
rate behavior for selected drainage geocomposites.
The specific cross section used in the test procedure
should replicate the intended design as closely as
possible. For the required flow rate, Darcy's law or
HELP can be used. Then the design-by-function
concept is used to determine the design ratio (DR), or
factor of safety (FS).
DR = FS =
allowable flow rate
required flow rate
Gas Collector and Removal Systems
Degradation of solid waste materials in a landfill
proceeds from aerobic to anaerobic decomposition
very quickly, thereby generating gases that collect
beneath the closure FML. Almost 98 percent of 'the
gas produced is either carbon dioxide (CC>2) or
methane (CH4). Because COa is heavier than air, it
will move downward and be removed with the
leachate. However, CH4, representing about 50
percent of the generated gas, is lighter than air
and.therefore, will move upward and collect at the
bottom of the facility's "impermeable" FML. If the
gas is not removed, it will produce a buildup of
pressure on the FML from beneath.
In gas collector systems, either a granular soil layer
or a needle-punched nonwoven geotextile is placed
directly beneath the FML or clay of a composite cap
system. Gas compatibility and air transmissivity are
the design factors that must be considered. Methane,
the most predominant gas, should be compatible
with most types of geotextiles including polyester,
polypropylene, and polyethylene.
The thickness design should be based on gas
transmissivity tests. Since water has a viscosity of
1,000 to 10,000 times that of gas, qaiiow for gas flow
should compare very favorably with the results of a
water transmissivity test. As an example, Figure 4-
22 shows air transmissivity versus normal stress for
a 12 oz/yd2 needle-punched nonwoven geotextile.
Alternatively, one could look directly at
permeability coefficients where geotextile air flow is
several orders of magnititude greater than the MTG-
required values as shown in Figure 4-23. In the test
method, the geotextile specimen fits underneath a
load bonnet. Then the load, equivalent to the cover
soil, is added and gas is brought to the inside of the
geotextile. The gas flows through the geotextile and
into a shroud that goes on the outside of the flanges
and registers on an air meter. The resulting applied
66
-------
Example:
vTe 'Taut.'
- .CO I M/
ftCc.
f ao
• "
0.3
Figure 4-15. Example problem for calculation of primary liner leak response time.
stresses, gas pressures, and gas permeabilities are
then recorded, and, if necessary, converted into gas
transmissivity. The allowable gas transmissivity is
then divided by the required gas transmissivity to
yield the design ratio, or factor of safety.
Gas generation occurs over a period of 70 to 90 years,
so gas collector and removal systems must work for
at least that long to avoid gas pressure on the
underside of the cover.
Gas generation might also cause problems in
"piggyback" landfills, landfills that have been built
on top of one another. It is still unknown what
happens to gas generated in an old landfill after a
hew liner is placed on top of it. To minimize
problems, the old landfill should have a uniform
67
-------
Leakage
Removal
Requires
Penetration
of P-FML
Submersible Pump
within Pipe
Figure 4-16. Secondary leak detection removal system via pumping between liners and penetration of pimary liner.
A . _ _ „ Penetration
Anti-Seep Collars Of S-F
; and Clay
Figure 4-17. Secondary leak detection removal system via gravity monitoring via penetration of secondary liner.
68
-------
Finish Grade to Slope .
Away from Manhole xj:-. :• •
Discharge Line from Leak
Collection/Detection System
(Solid Wall-6" Dia or Larger)
Manhole Frame and Cover
with Vented Lid
Granular Backfill
_;>• 4' LD Manhole
Point of Continuous or
Intermittent Monitoring
Note: Manhole will be equipped with discharge line to
leachate removal system or with discharge pump
Figure 4-18. Monitoring and collection manhole (E. C. Jordan, 1984).
Vegetative
Layer
Drainage
Layer
Low Permeability
Layer
FMI
(> 20 mils in
thickness)
Compacted
Soil Layer
a 12'
, J.
> 24
1
Functions
Vegetation'or Other
Erosion Control Material
at and Above Surface
Top Soil for Root
Growth
Remove Infiltrating
Water
Increases Efficiency
of Drainage Layer and
Minimizes Infiltration
into Unit
Figure 4-19. Surface Water Collection and Removal (SWCR) system.
69
-------
Computer Code "Help" will give design (required) flow rate
Leachate Collection Pipe
To Leachate Collection Sump
Figure 4-20. Design methodology to estimate cover soil infiltration to SWCR system.
slope and possibly an accordian-pleated bottom cross
section. Then the gas could escape from the
underside and be collected from the high gradient
side of the site.
As seen in Figure 4-24, the details of a gas collection
system are quite intricate and yet very important to
the proper functioning of the system.
References
1. Advanced Drainage Systems, Inc. 1988. Report.
2.
3.
Columbus,
Inc.
OH: Advanced Drainage Systems,
Richardson, G.N. and D.W. Wyant. 1987.
Construction criteria for geotextiles. Geotextile
Testing and the Design Engineer. ASTM STP
952.
Schroeder, P.R., A.C. Gibson and M.D. Smolen.
1984. The Hydrologic Evaluation of Landfill
Performance (HELP) Model: Vol. II,
Documentation for Version I. EPA/530/SW-
84/010. Cincinnati, OH: EPA Municipal
Environmental Research Laboratory.
U.S. EPA. 1983. U.S. Environmental Protection
Agency. Lining of waste impoundment and
disposal facilities. SW-869. Washington, DC:
Office of Solid Waste and Emergency Response.
70
-------
(A
CO
1
2
Q
2
'35
o
U>
E
L vailable Geocc
«i
1
i
o
o
able 4-3.
i-
'co
J-^
"S ^~
'§ ^
"cl! ®
S
ffi
CO 'co
CC Q.
g in
LL '-
©
1
CO .C"
CO
CO
CO
l.f
o £
H
~—s
.§1
cos
=5S
cc i
.32
1
O
CL
§
O
§
1
CO
03
O
O
CD
c*
Product Nan
+*
03
I
£
2
•*=
i
CO
00
CO
co
in
r-»
o
CD
C
03
Polypropyll
g
1
S
o
o
•a
03
a.
Q.
co
S
Amerdrain 480
S-
8
c
'co
Q
*£.
O
>
American V
itii
: ; : i
! ! ! !
oooo
°.§°§
ii^«
Polyester
Polyester
co co co co
c c cz c
o o o o
>< Ss >> ;*
-z.-z.-z-z
03 03 03 O
8888
c c c c
03 CD 03 03
Q. Q. Q. Q.
oooo
Enkamat 7010
Enkamat 7020
Enkamat 9010
Enkamat 9 120
(-,
o
O
LU
CO
ffl
r- CO
CM C33 !
CO 9 CM
Q) IO '
CO CO Is-
in in in
in co t-~
CM r-~ in
3.0/450
3.6/125
3.5/80
ggg
Polypropyll
Polypropyl
Polypropyl
iyethylene
iyethylene
iyethylene
ooo
CL CL CL
03 fl) 03
111
CO CO CO
333
OOO
O 0
ooo
CO CM •*
S S S
XXX
CO
.g
3
•o
C
CO
3
m
00 c\J
05 oj
in
CO ^
§0
o
CO O
o 9
g
Polyprople
03
C
03
1
f
CD
CJ
03 S
•£2 CO
CO Q-
& CO
3 CL
O LU
Tiger Drain
Geotech Drain
Board
CO
03
CO
£5 CD
LU O
co co
i .x • iii co
i in r- III CO
j ?• co in m m {5
•* in o in
! O CM CO £31
r- t—
SO O OOO O
. in in co co in o
CM CM CM m m f» o
o o O
oo in in o 2
§ 88"§ §
T-
03 CD 03 03 03 03
tc c e c e
03 03 03 CD CD
•> "5, "5. "5. "5,
Ct CL CL CL Q.
2 2 2 2 S 2
a. a. CL Q. Q. a.
CL Q. Q. D. CL CL CL
JQ3 03 03
C C 03 03 CD C
•^ CD 03 C C C CD
S ••>.•>. £ £ E •>.
2 £ S S'S'S' S
Q. CD 03 W CO 03 03
CX. • ^ ^ Q. Q. Q. Q_
5 .g .Q .g .g
11 II 1 ;§3
LU LU LU LU LU CC
03
S
1 8.8 S 2
I 88 Iff °
i c c c5 co co 5
5 11 in I
X -^ 2S5 x
1' S
-£ CO
C ' "C
>i CL
"' 03 O
|c 1 |
X -5 2 2.
1 *
in r-
ro •*
CM CM
co ^
"S
Sfe
tO h*
1.6/49,98
3.6/98
m S
Polypropyl
Polypropyl
Iyethylene
Iyethylene
o o
CL CL
.
i
Q
2
CL
^*
S •*
1 '
o o
CO <*
CM CM
5.3/100
5.3/100
03 03
"o 'o
CL CL
g g
0 03
>, >>
CL £
Extruded rib
Extruded rib
88
r- CM
00
Q Q
1
|
'S
i
e
E
2
>s
g •
^
c
03
£
_
1
I
CO
CD
3
§
03
.C
H
.-S"
11
2 ra
"CO" O
oT"^
is numerica
nd concepts
«± CO
i!
c •—
9|
Bw
03
^_ «
aulic gradiem
1, manner of f
•n
>,^
CO g
03 03
^3 "^
11
P (0
I'l
C§ 03
s«
2-S
^ *CO
*- c
> O
S §
= CD
0 |
-C
The values
literature w
*
-------
15
100
10
o>
I
CE
1.0
Q
0.1
Normal Pressure Ib/in2/lb/ft2
.-0/0
5/720
10/1440
15/2160
20/2880 •
\\_
10
0.01
0.10 1.0
Hydraulic Gradient
(a)
10.0
700
600
500
400
•i 300
200
100
OOOL
0.0 0.1
U-L.
0.2 0.3 0.4 0^5 0.6
Thickness of Ma|t, d(in)
i i
0.7
0 ' 25 50 75
Residual Thickness, n(%)
(b)
100
Figure 4-21. Flow rate behavior of selected geocomposite drainage systems.
(a) Mirardrain 6000 at hydraulic gradients of 0.01 to 1.0
(b) Enkadrain at hydraulic gradient of 1.0
72
-------
4.0
£• 3.0
1.0
Ua = 3.5 psi
Ua = 1.0 psi
Ua = 0.1 psi
j I
0 500 1000 1500 2000 2500
Stress (psf)
Figure 4-22. Air transmissitivity versus applied normal stress
for one layer of Fibretex 600R.
250
200
I 150
CD
CD 100
50
Ua = 3.5 psi
Ua = 0.1 psi
J I
0 500 1000 1500 2000 2500
Stress (psf)
Figure 4-23. Inplane coefficient of air permeability versus
applied normal stress for one layer of Fibretex
600R.
73
-------
Steel Clamp .
WELDS
. Mastic
*»«»»«»«^« JJ» M
.«»*•... •*/£&•
Boot Seal at FMC
Gasket
^
Flange Seal at FMC
Vent to Atmosphere /^_?
•4 ¥*•
Riser
Cover Soil
• Filter
: SWCR '•••'•'•.- .'
FMC
j> Compacted Soil
\ ;
i
:. Perforated Pipe '.- . '.;. •_ o'as Vent .
Operational Cover
- Liner
Air/Gas Vent
Place Vent Higher than Maximum Liquid Level
at Over-Flow Conditions
Two-Inch Minimum
•^^^fesP^—
. Geotextile or "^SsS^J?—Gas Flow
Drainage Composite
Openings in Vent to be Higher than
Top of Berm or Overflow Liquid Level
Air/Gas Vent Assembly
—Approx. Six Inches
~Geotextile or^srsf
Drainage Composite
Wind Cowl Detail I/' —
r —
Geomembrane
Concrete
Bond Skirt of Vent to Liner
*•?— Gas Flow
Figure 4-24. Miscellaneous details of a gas collector system.
74
-------
5. SECURING A COMPLETED LANDFILL
Introduction
This chapter describes the elements in a closure or
cap system of a completed landfill, including flexible
membrane caps, surface water collection and
removal systems, gas control layers, biotic barriers,
and vegetative top covers. It also discusses
infiltration, erosion control, and long-term aesthetic
concerns associated with securing a completed
landfill.
Figure 5-1 shows a typical landfill profile designed to
meet EPA's proposed minimum technology guidance
(MTG) requirements. The upper subprofile comprises
the cap, or cover, and includes the required 2-foot
vegetative top cover, 1-foot lateral drainage layer,
and low permeability cap of barrier soil (clay), which
must be more than 2 feet thick. This three-tier
system also includes an optional flexible membrane
cap and an optional gas control layer. The guidance
originally required a 20-mil thick flexible membrane
cap, but EPA currently is proposing a 40 mil
minimum.
Flexible Membrane Caps
Flexible membrane caps (FMCs) are placed over the
low permeable clay cap and beneath the surface
water collection and removal (SWCR) system. FMCs
function primarily in keeping surface water off the
landfill and increasing the efficiency of the drainage
layer. EPA leaves operators with the option of
choosing the synthetic material for the FMC that
will be most effective for site-specific conditions. In
selecting materials, operators should keep in mind
several distinctions between flexible membrane
liners (FMLs) and FMCs. Unlike a FML, a FMC
usually is not exposed to leachate, so chemical
compatibility is not an issue. Membrane caps also
have low normal stresses acting on them in
comparison with FMLs, which generally carry the
weight of the landfill. An advantage FMCs have over
liners is that they are much easier to repair, because
their proximity to the surface of the facility makes
them more accessible. FMCs will, however, be
subject to greater strains than FMLs due to
settlement of the waste.
Surface Water Collection and Removal
(SWCR) Systems
The SWCR system is built on top of the flexible
membrane cap. The purpose of the SWCR system is
to prevent infiltration of surface water into the
landfill by containing and systematically removing
any liquid that collects within it. Actual design
levels of surface water infiltration into the drainage
layer can be calculated using the water balance
equation or the Hydrologic Evaluation of Landfill
Performance (HELP) model. (A more detailed
discussion of HELP is contained in Chapter Four.)
Figure 5-2 shows the results of two verification
studies of the HELP model published by EPA.
Errors in grading the perimeter of the cap often
integrates (or cross-connects) the SWCR system with
the secondary leak detection and removal system,
resulting in a significant amount of water
infiltrating the secondary detection system. This
situation should be remedied as soon as possible if it
occurs. Infiltration of surface water is a particular
concern in nuclear and hazardous waste facilities,
where gas vent stacks are found. A containment
system should be designed to prevent water from
entering the system through these vents.
In designing a SWCR system above a FMC, three
issues must be considered: (1) cover stability, (2)
puncture resistance, and (3) the ability of the closure
system to withstand considerable stresses due to the
impact of settlement. Figure 5-3 illustrates the
effects of these phenomena.
Cover Stability
The stability of the FMC supporting the SWCR
system can be affected by the materials used to
construct the drainage layer and by the slope of the
site. In some new facilities, the drainage layer is a
geonet placed on top of the flexible membrane cap,
75
-------
'•Primary Anchor Trench
I I
I
Secondary Anchor Trench
Cap Anchor Trench
Figure 5-1. Typical geosynthetic cell profile.
with the coefficient of friction between those two
elements being as low as 8 to 10 degrees. Such low
friction could allow the cover to slide. One facility at
the Meadowlands in New Jersey is constructed on a
high mound having side slopes steeper than 2:1. In
order to ensure adhesion of the membrane to the side
slopes of the facility, a nonwoven geotextile was
bonded to both sides of the FMC. Figures 5-4 and 5-5
give example problems that evaluate the sliding
stability of a SWCR system in terms of shear
capacities and tensile stress.
Puncture Resistance
Flexible membrane caps must resist penetration by
construction equipment, rocks, roots, and other
natural phenomena. Traffic by operational
equipment can cause serious tearing. A geotextile
placed on top of or beneath a membrane increases its
puncture resistance by three or four times. Figure 5-
6 shows the results of puncture tests on several
common geotextile/membrane combinations.
Remember, however, that a geotextile placed
berieath the FMC and the clay layer will destroy the
composite action between the two. This will lead to
increased infiltration through penetrations in the
FMC.
Impact of Settlement
The impact of settlement is a major concern in the
design of the SWCR system. A number of facilities
have settled 6 feet in a single year, and 40 feet or
more over a period of years. The Meadowlands site in
New Jersey, for example, was built at a height of 95
feet, settled to 40 feet, and then was rebuilt to 135
feet. Uniform settlement can actually be beneficial
by compressing the length of the; FMC and reducing
tensile strains. However, if waste does not settle
uniformly it can be caused by interior berms that
separate waste cells.
In one current closure site in California, a waste
transfer facility with an 18-foot wall is being built
within a 30-foot trench on top of a 130-foot high
landfill. The waste transfer facility will settle faster
than the adjacent area, causing tension at the edge of
the trench. Electronic extensometers are proposed at
the tension points to check cracking strains in the
clay cap and FMC.
Settlements can be estimated, although the margin
for error is large. Secure commercial hazardous
waste landfills have the smallest displacement, less
than 1.5 percent. Displacements at new larger solid
waste landfills can be estimated at 15 percent, while
older, unregulated facilities with mixed wastes have
settlements of up to 50 percent. Figure 5-7 -gives an
example problem showing how to verify the
durability of a FMC under long-term settlement
compression. :
Gas Control Layer
Gas collector systems are installed directly beneath
the low permeability clay cap in a hazardous waste
landfill. Landfills dedicated to receiving only
hazardous wastes are relatively new and gas has
never been detected in these systems. It may take 40
years or more for gas to develop in a closed secure
hazardous waste landfill facility. Because the long-
term effects of gas generation are not known, and
76
-------
, i*. 5^ JL
Placing FMC at edge of cap.
costs are minimal, EPA strongly recommends the
use of gas collector systems.
Figure 5-8 shows details from a gas vent pipe system.
The two details at the left of the illustration show
closeups of the boot seal and flange seals located
directly at the interface of the SWCR system with
the flexible membrane cap. To keep the vent
operating properly, the slope of. the closure system
should never be less than 2 percent; 5 to 7 percent is
preferable. A potential problem with gas collector
systems is that a gas venting pipe, if not properly
maintained, can allow surface water to drain directly
into the landfill waste.
Figure 5-9 illustrates two moisture control options in
gas collector systems. Gas collector systems will
tolerate a large amount of moisture before air
transmissivity is affected. Figure 5-10 shows air and
water transmissivity in a needle-punched nonwoven
geotextile. Condensates from the gas collector layer
that form beneath the clay and flexible membrane
77
-------
OC
I
CO
O
Legend
i Help Simulation
1 Field Measurement
• Cover Stability
= 125-j
0 100-
Jan
1973
Jan
1974
Jan
1975
Date
Jan
1976
Jan
1977
Figure 5-2.
Cumulative comparison of HELP simulation and
field measurements, University of Wisconsin,
Madison, uncovered cell.
cap also can be taken back into the waste, since most
hazardous wastes are deposited very dry.
Biotic Barriers
A biotic barrier is a gravel and rock layer designed to
prevent the intrusion of burrowing animals into the
landfill area. This protection is primarily necessary
around the cap but, in some cases, may also be
needed at the bottom of the liner. Animals cannot
generally penetrate a FMC, but they can widen an
existing hole or tear the material where it has
wrinkled.
Figure 5-11 shows the gravel filter and cobblestone
components of the biotic barrier and their placement
in the landfill system. The proposed 1-meter
thickness for a biotic barrier should effectively
prevent penetration by all but the smallest insects.
Note that the biotic barrier also serves as the surface
water collection/drainage layer. Biotic barriers used
Puncture Resistance
Wheel
Impact of Settlement
^- Interior Berms •
Figure 5-3. SWCR systems considerations.
in nuclear caps may be up to 14-feet thick with rocks
several feet in diameter. These barriers are designed
to prevent disruption of the landfill by humans both
now and in the future.
Vegetative Layer
The top layer in the landfill profile is the vegetative
layer. In the short term, this layer prevents wind and
water erosion, minimizes the percolation of surface
water into the waste layer, and maximizes
evapotranspiration, the loss of water from soil by
evaporation and transpiration. Tfre vegetative layer
also functions in the long term to enhance aesthetics
and to promote a self-sustaining ecosystem on top of
the landfill. The latter is of primary importance
because facilities may not be maintained for an
78
-------
Meadow/lands test of slip-resistant FMC.
indefinite period of time by either government or
industry.
Erosion can seriously affect a landfill closure by
disrupting the functioning of drainage layers and
surface water and leachate collection and removal
systems. Heavy erosion could lead to the exposure of
the waste itself. For this reason, it is important to
predict the amount of erosion that will occur at a site
and reinforce the facility accordingly. The Universal
Soil Loss Equation shown below can be used to
determine soil loss from water erosion:
X = RKSLCP
where X = soil loss
R = rainfall erosion index
K = soil erodibility index
S = slope gradient factor
L = slope length factor
C = crop management factor
P = erosion control practice
Figure 5-12 can be used to find the soil-loss ratio due
to the slope of the site as used in the Universal Soil
Loss Equation. Loss from wind erosion can be
determined by the following equation:
X' = I'K'C'L'V
where X' = annual wind erosion
I' = field roughness factor
K' = soil erodibility index
79
-------
Cell Component: 5 uRFAee WATER COUEC.-POM/ REMOVAL
Con^idontion' ^>HEAP FAILURE
£«VE A Soil. A^UO
Required Material Properties
* SW^R T* TMC. £L
Sni*,*. StfllU^tH °f ^^^^^^Aii^vj
frifl UIC UniTxn
EVALUATE ^LIOIU^ srAO
QCSiqu RATIO AqAiuir
Range
S^
Analysis Procedure:
+~s-
P"~ -t«jofi> ^ ' —
^O^AL'diM' SMSAA. SlBtis xaooe f O6i.»" 5
/ L~16'1
•TALV.U,
N ••*• SM,M
Design Ratio:
^^TIR 4i.ioiMq 0*^^ 2.O
ivocR SHIAR on> if.o
Test
U.PS U'PTH
5-rwEM
SrtEAC?
Standard
Z«
oPE af cToVER
W^R SY- (1184)
Example:
- SUJ<:PS -r» FMC .2.5*
•cT0vER -Son. DcprH - 4* ;
•SHEAR SJ-RC^TM SwcR= I5lt'/iu't j
i»u *.n .
^Zil2o~ 45° P^F
i5)^AL<:LjLATe PE^ICJM f?A.Tio r<»« "Su^R •SHEAR
l^> x 144
(Example No. 5.Z
Figure 5-4. Shear failure for surface water collection and removal system.
C' = climate factor
L' = field length factor
V = vegetative cover factor
There are many problems in maintaining an
agricultural layer on top of a landfill site, especially
in arid or semiarid regions. An agricultural layer
built on a surface water collection and removal
system composed of well-drained stone and synthetic
material may have trouble supporting crops of any
kind because the soil moisture is removed. In arid
regions, a continuous sprinkler system may be
needed to maintain growth on top of the cap, even if
the soil is sufficiently deep and fertile. A final
problem involves landfills built on slopes greater
than 3:1. Equipment necessary to plant and
maintain crops cannot operate on steeper slopes.
Operators should contact their local agricultural
extension agent or State Department of
Transportation to find out what kinds of vegetation
will grow under the conditions at the site. The
impact of the SWCR system on the soil layer also
should be studied before vegetation is chosen. Native
grasses usually are the best choice because they
already are adapted to the surrounding
environment. Sometimes vegetation can overcome
adverse conditions, however. At one site in the New
Jersey Meadowlands, plants responded to excess
surface water by anchoring to the Underlying waste
through holes in a FMC, creating a sturdy bond
between surface plants and underlying material.
For sites on very arid land or on steep slopes, an
armoring system, or hardened cap, may be more
effective than a vegetative layer for securing a
landfill. Operators should not depend on an
agricultural layer for protection in areas where
vegetation cannot survive. Many States allow
80
-------
Cell Component: SURF/US L]A
IU 5*4 £4. 0 -TAP*.
TEK ^oUE
—»
^^
. ST^tig^TH of SUJtT R
L
l&c
References:
Example*
*~ C"oVt** ^"U To "SKJcR " 4O°
~ "SkO£(? T> Ft~4£ • 26*
•SuoPt AAJc-jLe. s.i *
* Teuiite Sr«£M
-------
1500f
6 osy Geotextile
EDPM.75 mm PUC.75 mm
CPE.75 mm HDPE.75 mm
• Geomembrane Alone
0 Geotextile Both Sides
B Geotextile Front
B Geotextile Back
EDPM.75 mm PUC.75 mm
CPE.75 mm HDPE.75 mm
12 osy Geotextile
EDPM.75 mm PUC.75 mm
CPE.75 mm HDPE.75 mm
' EDPM.75 mm PUC.75 mm
CPE.75 mm HDPE.75 mm
• Geomembrane Alone
O Geotextile Both Sides
B Geotextile Front
3 Geotextile Back
| 1111 I 18 osy Geotextile
EDPM.75 mm PUC.75 mm
CPE.75 mm HDPE.75 mm
1 N = .225 Ib.
a. Puncture Resistance
(Koerner, 1986)
' EDPM.75 mm PUC.75
CPE.75 mm
1 J = .738 ft-lb
mm
HDPE.75 mm
b. Impact Resistance
Figure 5-6. Puncture and impact resistance of common FMLs.
82
-------
Cell Component: fLe*i»tE. He.MBg*uE
Consideration: •SETTLE. ME.MT : VERIFY J.IHUTY •"• Fs-»£. T- *
5tTTLtMluT Ke*ULTHJ<5 FROM L*uej-~TCRM UJA*T£
5iTTx*Meur nATJ*ts MAY ee &IA«IAU OA uui*.xi*
Required Material Properties
- YltLO OTAIM r»«
Range
Test
Standard
ASTM 045=15
Analysis Procedure:
ou 7»«tiTi.tMeuT * UA-STE
(Sj.Ti0
e.i o
SETTLEMENT RATIO, S/2t
PE' *<*•/<„
Design Ratio:
References:
KMIP-^H IE.LD
Example:
• MIUIMIJM UJlPTH OF ^C LL "" 5O FT
• DEPTH =F UA»T£ ' eo rr
.' ^*OMPo»irE
(l ^ E^T'MATE
OF 5fcTTt£MEUT F"EA
EMT PERTH » S'/.*io - Z
ETTLEt->EUT RATIO , SR
«
2
l»
|
o
2 s
D
CltcuUI Tfouih Model
^^
U_
^
OK
0 0.1 0.1 0.)
SETTLEMENT RATIO, S/21
[Example No. 5.4
Figure 5-7. The effects of settlement on a flexible membrane cap.
-------
Steel Clamp
. Mastic
Boot Seal at FMC
Gasket
Flange Seal at FMC
Figure 5-8. Details of a gas vent pipe system.
::;. .;'.. Perforated Pipe .'.- .'.;. • Gas Vent
Operational Cover
84
-------
Gas Flow
Condensate Drain to Collector
Gas Flow
• Gas Well
Condensate Drain to Well
• Gas Flow
• Water Flow
Moisture Control
(after Rovers, et al)
100%
100%
Figure 5-9. Water traps in a gas collector system.
0.2 0.4
0.6 0.8 1.0
Normalized Pressure Ratio
Figure 5-10. Air and water transmissivity in a needle-punched
nonwoven geotextile.
Vegetation
Topsoil
(60 cm)
Gravel Filter
(30 cm)
Cobblestone
(70 cm)
Biotic
Barrier
' .'- • Protctive Layer
=^==, FMC
Compacted Soil
(90 cm)
Gas Vent
'.' ' • (30 cm)
Waste
Figure 5-11. Optional biotic barrier layer.
85
-------
=5
100
Figure 5-12. Soil erosion due to slope.
200
300
400
500
600
700
800
Slope Length (Feet)
86
-------
Figure 5-13. Regional depth of frost penetration in inches.
Extrusion Weld
\
FMC
Extrusion Weld
Figure 5-14. Geosynthetic cell profile with extrusion welds at FML and FMC junctures.
87
-------
-------
6. Construction, Quality Assurance, and Control:
Construction of Clay Liners
Introduction
This chapter focuses on construction criteria for clay
liners, including important variables in soil
compaction, excavation and placement of liner
materials, and protection of liners after construction.
The chapter concludes with a discussion of
construction quality assurance, and of test fills and
their incorporation into the design and construction
quality assurance plan for liners.
Compaction Variables
The most important variables in the construction of
soil liners are the compaction variables: soil water
content, type of compaction, compactive effort, size of
soil clods, and bonding between lifts. Of these
variables, soil water content is the most critical
parameter.
So/7 Water Content
Figure 6-1 shows the influence of molding water
content (moisture content of the soil at the time of
molding or compaction) on hydraulic conductivity of
the soil. The lower half of the diagram is a
compaction curve and shows the relationship
between dry unit weight, or dry density of the soil,
and water content of the soil. A water content called
the optimum moisture content is related to a peak
value of dry density, called a maximum dry density.
Maximum dry density is achieved at the optimum
moisture content.
The smallest hydraulic conductivity of the
compacted clay soil usually occurs when the soil is
molded at a moisture content slightly higher than
the optimum moisture content. That minimum
hydraulic conductivity value can occur anywhere in
the range of 1 to 7 percent wet of optimum water
content. Ideally, the liner should be constructed
when the water content of the soil is wet of optimum.
Uncompacted clay soils that are dry of their optimum
water content contain dry hard clods that are not
easily broken down during compaction. After
Hydraulic
Conductivity
Dry Unit
Weight
Figure 6-1.
Molding Water Content
Hydraulic conductivity and dry unit weight as a
function of molding water content.
compaction, large, highly permeable pores are left
between the clods. In contrast, the clods in wet
uncompacted soil are soft and weak. Upon
compaction, the clods are remolded into a
homogeneous relatively impermeable mass of soil.
Low hydraulic conductivity is the single most
important factor in constructing soil liners. In order
to achieve that low value in compacted soil, the large
voids or pores between the clods must be destroyed.
Soils are compacted while wet because the clods can
best be broken down in that condition.
Type of Compaction
The method used to compact the soil is another
important factor in achieving low hydraulic
conductivity. Static compaction is a method by which
soil packed in a mold is squeezed with a piston to
89
-------
compress the soil. In kneading compaction, a probe or
pie-shaped metal piece is pushed repeatedly into the
soil. The kneading action remolds the soil much like
kneading bread dough. The kneading method is
generally more successful in breaking down clods
than is the static compacting method (see Figure 6-
2).
ID'5
ID'6
1
10-7
10-8
Optimum w —
Static
Compaction
Kneading Compaction
15
19 23
Molding w (%)
27
Figure 6-2. Efficiencies of kneading compaction and static
compaction.
The best type of field compaction equipment is a
drum roller (called a sheepsfoot roller) with rods, or
feet, sticking out from the drum that penetrate the
soil, remolding it and destroying the clods.
Compactive Effort
A third compaction variable to consider is
compactive effort. The lower half of the diagram in
Figure 6-3 shows that increased compactive effort
results in increased maximum density of the soil. In
general, increased compactive effort also reduces
hydraulic conductivity. For this reason, it is
advantageous to use heavy compaction equipment
when building the soil liner.
Two samples of soils with the same water content
and similar densities can have vastly different
hydraulic conductivities when compacted with
different energies. The extra compaction energy may
12
16 20
Molding Water Content (%)
Figure 6-3. Effects of compactive effort on maximum density
and hydraulic conductivity.
not make the soil more dense, but it breaks up the
clods and molds them together more thoroughly.
The compaction equipment also must pass over the
soil liner a sufficient number of times to maximize
compaction. Generally, 5 to 20 passes of the
equipment over a given lift of soil ensures that the
liner has been compacted properly.
A set of data for clay with a plasticity index of 41
percent used in a trial pad in Houston illustrates two
commonly used compaction methods. Figure 6-4
shows the significantly different compaction curves
produced by standard Proctor and modified Proctor
compaction procedures. The modified Proctor
compaction technique uses about five times more
compaction energy than the standard Proctor.
Figure 6-4 shows the hydraulic conductivities of the
soil molded at 12 percent water content to be less
than 10-!0 cm/sec, using the modified Proctor, and
10-3 cm/sec, using the standard Proctor. The different
levels of compaction energy produced a seven order of
magnitude difference in hydraulic conductivity.
Apparently, modified Proctor compaction provided
enough energy to destroy the clods and produce low
90
-------
hydraulic conductivity, whereas standard Proctor
compaction did not.
Figure 6-4 also shows that at 20 percent molding
water content, the soil's water content and density
are virtually the same when packed with either
modified or standard Proctor equipment. Modified
Proctor compaction, however, still gave one order of
'magnitude lower hydraulic conductivity. In this
case, additional compaction energy produced
significantly lower hydraulic conductivity without
producing greater density.
130
10 15 20
Molding Water Content (%)
25
moisture content between the soil that was crushed
and passed through a No. 4 sieve and the soil that
merely was passed through a 0.75-inch sieve. .The
implication for laboratory testing is that the
conditions of compaction in the field must be
simulated as closely as possible in the laboratory to
ensure reliable results.
c
135
125
115
105
95
85
0.2-in. Clods
0.75-in. Clods
10 15 20
Molding Water Content (%)
25
Figure 6-5. Effects of clod size on hydraulic conductivity.
Figure 6-4. Effects of modified Proctor versus standard
Proctor compaction procedures.
Size of Clods
Another factor that affects hydraulic conductivity is
the size of soil clods. Figure 6-5 shows the results of
processing the same highly plastic soil from a
Houston site in two ways. In one, the soil was passed
through the openings of a 0.75-inch sieve, and in the
other, the soil was ground and crushed to pass
through a 0.2-inch (No. 4) sieve. Most geotechnical
laboratories performing standard Proctor
compaction tests first air dry, crush, and pulverize
the soil to pass through a No. 4 sieve, then moisten
the soil to various water contents before compaction.
Figure 6-5 shows a 3 percent difference in optimum
Table 6-1 summarizes data concerning the influence
of clod size on hydraulic conductivity for the same
soil. At a molding water content of 12 percent,
hydraulic conductivity of the sample with the
smaller (0.2-inch) clods is about four orders of
magnitude lower than the hydraulic conductivity for
the sample with larger (0.75-inch) clods. Apparently,
at 12 percent water content, the clods are strong, but
when reduced in size, become weak enough to be
broken down by the compaction process. For the wet
soils with a molding water content of 20 percent, clod
size appears to have little influence on hydraulic
conductivity, because the soft, wet clods are easily
remolded and packed more tightly together. Clod size
then is an important factor in dry, hard soil, but less
important in wet, soft soil, where the clods are easily
remolded.
_
91
-------
Table 6-1. Influence of Clod Size on Hydraulic Conductivity
Fully-Penetrating Feet:
Molding
W.C. (%)
12
16
18
20
Hydraulic
0.2-in. Clods
2x 10-8
2x10-9
1 x 10-9
2x10-9
Conductivity (cm/s)
0.75-in. Clods
4x 10-4
1 x 10-3
8 x 10-'Q
7x 10-'°
Bonding between Lifts
A final important compaction variable is the extent
of bonding between lifts of soil. Eliminating highly
permeable zones between lifts cuts off liquid flow
from one lift to another. If defects in one lift can be
made discontinuous with defects in another lift, then
hydraulic continuity between lifts can be destroyed
(see Figure 6-6).
I
Figure 6-6. Conductivity between lifts.
Compaction equipment with rollers that knead or
remold the soil can help destroy hydraulic connection
between defects. Footed rollers are the best kind to
use for this purpose. The two kinds of footed rollers
generally used for soil compaction are those with
long, thin feet that fully penetrate the soil and those
with feet that only partially penetrate the soil (see
Figure 6-7). The roller with fully penetrating feet,
typically called a sheepsfoot roller, has shafts about 9
inches long. Because the lift thickness of a clay liner
is typically 8 to 9 inches before compaction and 6
inches after compaction, the shaft of the sheepsfoot
roller can push through an entire lift of soil.
The fully penetrating feet go all the way through the
loose lift of soil to compact the bottom of the lift
directly into the top of the previously compacted lift,
blending the new lift in with the old. The partly
penetrating foot, or padfoot, cannot blend the new lift
Loose Lift
of Soil
/ / X / X
Compacted Lift
/ s / / / /
Partly-Penetrating Feet:
Loose Lift
of Soil
/ X X /^
Compacted Lift
Figure 6-7. Two kinds of footed rollers on compaction
equipment.
to the old, since its shorter shafts do not go
completely through the lift.
Another way to blend the new lift with the old lift is
to make the surface of the previously compacted lift
very rough. Commonly in the construction of soil
liners, the finished surface of a completed lift is
compacted with a smooth, steel drum roller to seal
the surface of the completed lift. The smooth soil
surface of the completed lift minimizes desiccation,
helps prevent erosion caused by runoff from heavy
rains, and helps in quality control testing. The soil,
however, must be broken up with a disc before a new
lift of soil can be placed over it.
In below-ground disposal pits, it is sometimes
necessary to construct a soil liner on the side slopes.
This sloping clay liner component can be constructed
either with lifts parallel to the face of the soil or with
horizontal lifts (see Figure 6-8). Horizontal lifts must
be at least the width of one construction vehicle, of
about 12 feet.
Horizontal lifts can be constructed on almost any
slope, even one that is almost vertical. Parallel lifts,
however, cannot be constructed on slopes at angles
steeper than about 2.5 horizontal to 1 vertical (a
92
-------
Parallel Lifts
Sandy Soil
Horizontal Lifts
Figure 6-8. Liner construction on side slopes with horizontal
and parallel lifts.
Horizontal Lifts
Sandy Soil
Figure 6-9.
Effect of sandy soil zone on liners with parallel
and horizontal lifts.
slope angle of about 22 degrees), because the
compaction equipment cannot operate on them.
On surfaces without steep slopes, soil liners with
parallel lifts are less sensitive to some of the defects
that might occur during construction than those
built with horizontal lifts. Figure 6-9 shows a liner
containing a quantity of sandy material. With
parallel lifts, the sandy zone is surrounded by zones
of good soil, and so has little influence. But with
horizontal lifts, a window through the soil liner could
allow greater permeability to waste leachate if it
were to occur on the bottom in a sump area.
Compaction Equipment
In addition to increasing bonding between lifts (as
discussed in the previous section), the equipment
used to compact soil liners should maximize
compactive energy and the remolding capability of
the soil. The type of roller, weight of the roller, and
number of passes the equipment makes over the
surface of the soil are all important factors. The
heaviest rollers available weigh between 50,000 and
70,000 pounds. The Caterpillar 825 is an example of
one of the heaviest, weighing more than 50,000
pounds and having long, penetrating feet. A medium
weight roller weighs between 30,000 and 50,000
pounds and a relatively light roller weighs 15,000 to
30,000 pounds.
The best way to compare one roller to another is to
examine weight per linear foot along the drum
surface. A very lightweight roller will typically
weigh about 500 pounds per linear foot along the
drum surfaces, while a very heavy roller weighs
3,000 to 5,000 pounds per linear foot.
Vibratory rollers, weighing typically 20,000 to
30,000 pounds static weight may not be effective for
clay compaction. A piece of vibration equipment
inside the drum gives the vibratory roller its name.
The drums of static rollers are filled with liquid,
making them very heavy. The vibratory equipment
inside the drum of the vibratory roller, however,
prevents it from being filled with water, so the total
weight is in the drum itself. This kind of roller works
well for compacting granular base materials beneath
pavements, so contractors frequently have them
available. However, there is no evidence that the
high frequency of vibration is effective in compacting
clay.
Vibratory rollers are not good rollers for compacting
clay liner materials for several reasons. First, the
padfoot (only about 3 inches long) does not fully
penetrate the soil. Second, the area of the foot is
fairly large. Because the weight is spread over a
large area, the stresses are smaller and the soil is not
compacted as effectively. The smaller the area of the
foot, the more effective in remolding the soil clods.
Third, the roller is relatively lightweight, weighing
93
-------
only 20,000 to 30,000 pounds. In addition,
approximately half the rollers' weight goes to the
rear axle and the rubber tires, leaving only about
15,000 pounds or less to be delivered to the drum.
The feet of a classic sheepsfoot roller, in contrast to
those of the vibratory roller, are about 9 inches long.
The area of the foot is relatively small so that the
compact stress on the tip typically ranges from 200 to
700 pounds per square inch. The drum normally is
filled with liquid so that great weights are achieved
directly on the drum. Manufacturers make very few
sheepsfoot rollers now, despite the fact that they are
the most effective roller for clay compaction. The
Caterpillar 815 and 825 are two of the few sheepsfoot
rollers currently being produced.
The Construction Process
Table 6-2 outlines the major steps in the construction
process for clay liners. First, a source of soil to be
used in constructing the liner must be found. Then
the soil is excavated at this location from a pit called
a "borrow pit." (Excavated soil is referred to as
"borrow soil.") Digging tests pits in the borrow area
helps determine the stratification of the soil before
beginning excavation of the borrow pit itself.
Table 6-2. Steps in the Construction Process
1. Location of Borrow Source
— Boreholes, Test Pits
— Laboratory Tests
2. Excavation of Borrow Soil
3. Preliminary Moisture Adjustment; Amendments; Pulverization
4. Stockpile; Hydration; Other
5. Transport to Construction Area; Surface Preparation
6. Spreading in Lifts; Breakdown of Clods
7. Final Moisture Adjustment; Mixing; Hydration
8. Compaction; Smoothing of Surface
9. Construction Quality Assurance Testing
10. Further Compaction, If Necessary
The borrow soil is mixed and blended as it is
excavated to produce as homogeneous a soil as
possible. Scrapers are useful for excavating soils
from borrow areas, because the soil is mixed up in
the scraper pan by the action of the scraper. The soil
also can be sieved and processed through a rock
crusher to grind down hard clods. Cutting across
zones of horizontal stratification also will help mix
up the soil as it is excavated. Using some of these
methods, the excavation process can be designed to
maximize soil mixing without significantly
increasing excavation costs.
The next step is to moisten or dry the soil as needed.
If the required change in water content is only 1 to 2
percent, the adjustment in moisture content can be
made after the soil is put in place and before it is
compacted. However, if a substantial change in soil
moisture content is necessary, it should be performed
slowly so moistening occurs uniformly throughout
the soil. To change the soil moisture content, the soil
is spread evenly in a layer, moistened, and then
covered for several days, if possible, while the
moisture softens the soil clods. A jdisc or a rototiller
passed through the soil periodically speeds up the
process.
If soil moisture content is too high, the soil should be
spread in lifts and allowed to dry. Mixing the soil
during the drying process will prevent a dry crust of
soil from forming on th.e top with wet soil
underneath.
When the moisture adjustments have been made, the
soil is transported to the construction area. Then the
soil is spread in lifts by bulldozer or scraper, and a
disc or rototiller is used to break down soil clods
further. A pulvermixer, a piece of equipment widely
used for reclaiming asphaltic concrete pavement,
also works well. These machines can pulverize and
mix a lift of soil as much as 24 inches deep.
Once the soil is in place and prior to compaction,
minor adjustments in moisture content again can be
made. No large changes in water content should be
made at this time, however.
In the next step, the soil is compacted. Afterwards,
the surface of the soil may be smoothed by a smooth
steel drum roller before the construction quality
control inspector performs the moisture density test.
If the test indicates that the soil has been compacted
adequately, the next lift is placed on top of it. If the
compaction has not been performed properly, the soil
is either compacted further or that section of the
liner is dug up and replaced.
Soil-Bentonite Liners
When there is not enough clay available at a site to
construct a soil liner, the clay can be mixed with
bentonite. The amount of bentonite needed should be
determined in the laboratory and then adjusted to
account for any irregularities occurring during
construction. Dry bentonite is mixed with the soil
first, and water is added only after the mixing
process is complete.
The bentonite can be mixed using a pugmill or by
spreading the soil in lifts and placing the bentonite
over the surface. Passing a heavy-duty pulvermixer
repeatedly through the soil in both directions mixes
the soil with the bentonite. After the bentonite and
clay are mixed, water is added in small amounts,
with the soil mixed well after each addition. When
the appropriate moisture content is reached, the
clay-bentonite soil is compacted.
94
-------
After the construction process is finished, the newly
compacted soil liner, along with the last lift of soil,
must be covered to protect against desiccation or
frost action, which can crack the soil liner.
Construction Quality Assurance (CQA)
Testing
Construction quality assurance (CQA) control tests
must be performed on the finished liner. There are
two categories of CQA tests: tests on the quality of
the material used in construction, and tests on the
completed lift soil to ensure that proper construction
has taken place. These tests include:
Materials Tests:
• Atterberg Limits
• Grain Size Distribution
• Compaction Curve
• Hydraulic Conductivity of Lab-compacted Soil
Tests on Prepared and Compacted Soil:
• Moisture Content
• Dry Density
• Hydraulic Conductivity of "Undisturbed" Sample
Table 6-3 presents recommendations for testing
frequency at municipal solid waste landfills. This
table was developed by the Wisconsin Department of
Natural Resources and is also contained in the EPA's
technical resource document on clay liners.
Tests on prepared and compacted soils often focus on
water content and soil density. Construction
specifications usually require minimums of 95
percent of maximum density with standard Proctor
compaction or 90 percent of maximum density with
modified Proctor compaction. Acceptable
percentages of water content commonly range from 1
to 5 percent higher than optimum moisture content.
Figure 6-10 shows a typical window of acceptable
ranges for percentages of water content and
minimum density. Typical data for water content
and density gathered in the laboratory are plotted in
Figure 6-11. The solid data points represent samples
for which the hydraulic conductivities were less than
10-7 cm/sec. The open symbols correspond to samples
that have hydraulic conductivities greater than 10-7
cm/sec. The window of acceptable moisture contents
and densities includes all of the solid data points and
excludes all of the open data points. Such a window
defines acceptable moisture content/density
combinations with respect to methods of soil
compaction (modified Proctor or standard Proctor).
The window in Figure 6-10 is actually a subset of the
window in Figure 6-11.
Defining a window of acceptable water contents and
densities for hydraulic conductivity is a
recommended first step in developing construction
specifications. Someone designing a clay liner might
choose just a small part of an acceptable range such
as that defined in Figure 6-11. For example,
extremely high water content may be excluded to
establish an upper water content limitation because
of concerns over the strength of the soil. In an arid
region, where wet soil can dry up and desiccate, the
decision might be made to use only materials at the
dry end of the range.
Factors Affecting Construction Quality
Assurance (CQA) Control Testing
Key factors that affect construction quality
assurance control testing include sampling patterns,
testing bias, and outliers, or data that occurs outside
of the normally accepted range.
The first problem in construction quality control
testing is deciding where to sample. Some bias is
likely to be introduced into a sampling pattern
unless a completely random sampling pattern is
used. For random sampling, it is useful to design a
grid pattern with about 10 times as many grids as
samples to be taken. A random number generator,
such as those on many pocket calculators, can be
used to pick the sampling points.
Bias in test results can originate from many areas, so
it is important to include in a CQA plan a procedure
for verifying test results. Nuclear density and
moisture content tests, for example, can err slightly.
The CQA plan should specify that these tests will be
checked with other tests on a prescribed frequency to
cut bias to a minimum. Certain "quick" moisture
content tests, such as tests using a microwave oven
to dry soil, can also be biased. The plan must specify
that these kinds of tests be cross-referenced
periodically to more standard tests.
Clay content and hydraulic conductivity tests on so-
called "undisturbed" samples of soil often give little
useful information. To obtain accurate results, the
conditions of the tested sample must match the field
conditions as closely as possible. In the Houston test
pad, for example, laboratory tests on 3-inch diameter
tube samples gave results that differed by five orders
of magnitude from the field value.
Inevitably, because soil is a variable material, some
data points will be outside the acceptable range. The
percentage of such points, or outliers, that will be
allowed should be determined in advance of testing.
Figure 6-12 shows that if enough data points define a
95
-------
Table 6-3. Recommendations for Construction Documentation of Clay-Lined Landfills by the Wisconsin Department of Natural
Resources
Item
Testing
Frequency
1. Clay borrow source testing
2. Clay liner testing during construction
3. Granular drainage blanket testing
Grain size
Moisture
Atterberg limits (liquid limit and plasticity
index)
Moisture-density curve
Lab permeability (remolded samples)
Density (nuclear or sand cone)
Moisture content
Undisturbed permeability
Dry density (undisturbed sample)
Moisture content (undisturbed sample)
Atterberg limits (liquid limit and plasticity
index)
Grain size (to the 2-micron particle size)
Moisture-density curve (as per clay borrow
requirements)
Grain size (to the No. 200 sieve)
Permeability
1,000 yd3
1,000 yd3
5,000 yd3 ;
5,000 yd3 and ail changes in material
10,000 yd3
5 tests/acre/lift (250 yd?)
5 tests/acre/lift (250 yd3)
1 test/acre/lift (t,500 yd3)
1 tesVacre/lift (1,500 yd3)
1 test/acre/lift (1,500 yd3)
1 tesVacre/lift (1,500 yd3)
1 test/acre/lift (1,500 yd3)
5,000 yd3 and all changes in material
1,500 yd3
3,000 yd3
Source: Gordon, M. E., P. M. Huebner, and P. Kmet. 1984. An evaluation of the performance of four clay-lined landfill^ in Wisconsin.
Proceedings, Seventh Annual Madison Waste Conference, pp. 399-460.
Yd
(Yd)miix
0.95{Yd)max
Zero Air Voids
Acceptable
Range
W,
'opt
W
Figure 6-10. Acceptable range for water content and
minimum densities.
Acceptable
Range
Molding Water Content
Figure 6-11. Molding water contents and densities from
Houston test pad.
normal distribution, allowing for a percentage of
outliers, the acceptable minimum value for a
parameter can be determined.
Test Fills
Test fills simulate the actual conditions of soil liner
construction before the full-sized liner is built.
Generally, they are approximately 3 feet thick, 40 to
80 feet long, and 20 to 40 feet wide. The materials
and construction practices for a test fill should
imitate those proposed for the full-sized liners as
closely as possible. In situ hydraulic conductivity of
the soil at the test pad is required to confirm that the
finished liner will conform to regulations. A sealed
double-ring infiltrometer usually is used for this
96
-------
Mean
Value of Parameter
Minimum
d)
-------
Collection
Pit
Gravel to Load Clay
to Evaluate Effect of
Overburden Stress
Compacted Clay
) V
Collection Pan Lysimeter XI > i inHorHrai:
Underdrain
Geomembrane
To Collection
Pit
Figure 6-13. An Ideal test pad.
98
-------
7. CONSTRUCTION OF FLEXIBLE MEMBRANE LINERS
Introduction
This chapter describes the construction of flexible
membrane liners (FMLs), quality control measures
that should be taken during construction, and EPA's
construction quality assurance (CQA) program. The
CQA program for FMLs is a planned series of
activities performed by the owner of a hazardous
waste facility to ensure that the flexible membrane
liner is constructed as specified in the design. There
are five elements to a successful CQA program: (1)
responsibility and authority, (2) CQA personnel
qualifications, (3) inspection activities, (4) sampling
strategies, and (5) documentation. This chapter
discusses each of these elements.
Responsibility and Authority
A FML may be manufactured by one company,
fabricated by a second company, and installed by a
third company. The FML also may be manufactured,
fabricated, and installed by the same company.
Depending on how the FML is constructed, various
individuals will have responsibilities within the
construction process. These individuals may include
engineers, manufacturers, contractors, and owners.
In general, engineers design the components and
prepare specifications, manufacturers fabricate the
FML, and contractors perform the installation.
Any company that installs a FML should have had
past experience with at least 10 million square feet of
a similar FML material. Supervisors should have
been responsible for installing at least 2 million
square feet of the FML material being installed at
the facility. Caution should be exercised in selecting
firms to install FMLs since many companies have
experienced dramatic growth in the last several
years and do not have a sufficient number of
experienced senior supervisors.
A qualified auditor should be employed to review two
key documents: (1) a checklist of requirements for
facilities, which will help ensure that all facility
requirements are met; and (2) a CQA plan, which
will be used during construction to guide
observation, inspection, and testing.
Designers are responsible for drawing up general
design specifications. These specifications indicate
the type of raw polymer and manufactured sheet to
be used, as well as the limitations on delivery,
storage, installation, and sampling. Some specific
high density polyethylene (HOPE) raw polymer and
manufactured sheet specifications are:
Raw Polymer Specifications
• Density (ASTM D1505)
• Melt index (ASTM D1238)
• Carbon black (ASTM D1603)
• Thermogravimetric analysis (TGA) or differ-
ential scanning calorimetry (DSC)
Manufactured Sheet Specifications
• Thickness (ASTM D1593)
• Tensile properties (ASTM D638)
• Tear resistance (ASTM D1004)
• Carbon black content (ASTM D1603)
• Carbon black disp. (ASTM D3015)
• Dimensional stability (ASTM D1204)
• Stress crack resistance (ASTM D1693)
Both the design specifications and the CQA plan are
reviewed during a preconstruction CQA meeting.
This meeting is the most important part of a CQA
program.
The preconstruction meeting also is the time to
define criteria for "seam acceptance." Seams are the
most difficult aspect of field construction. What
constitutes an acceptable seam should be defined
before the installation gets under way. One
technique is to define seam acceptance and verify the
99
-------
qualifications of the personnel installing the seams
at the same time. The installer's seamers produce
samples of welds during the preconstruction CQA
meeting that are then tested to determine seam
acceptability. Samples of "acceptable" seams are
retained by both the owner and the installer in case
of disputes later on. Agreement on the most
appropriate repair method also should be made
during the preconstruction CQA meeting. Various
repair methods may be used, including capstripping
or grinding and re welding.
CQA Personnel Qualifications
EPA requires that the CQA officer be a professional
engineer (PE), or the equivalent, with sufficient
practical, technical, and managerial experience.
Beyond these basic criteria, the CQA officer must
understand the assumptions made in the design of
the facility and the installation requirements of the
geosynthetics. Finding personnel with the requisite
qualifications and actual field experience can be
somewhat difficult. To develop field expertise in
landfill CQA, some consulting firms routinely assign
an inexperienced engineer to work with trained CQA
people on a job site and not bill for the inexperienced
engineer receiving training. This enables companies
to build up a reservoir of experience in a short period
of time.
Inspection Activities
Because handling and work in the field can damage
the manufactured sheets, care must be taken when
shipping, storing, and placing FMLs. At every step,
the material should be carefully checked for signs of
damage and defects.
Shipping and Storage Considerations
FML panels frequently are fabricated in the factory,
rather than on site. The panels must be shipped and
stored carefully. High crystalline FML, for example,
should not be folded for shipment. White lines, which
indicate stress failure, will develop if this material is
folded. Flexible membrane liners that can be folded
should be placed on pallets when being shipped to the
field. All FMLs should be covered during shipment.
Each shipping roll should be identified properly with
name of manufacturer/fabricator, product type and
thickness, manufacturer batch code, date of
manufacture, physical dimensions, panel number,
and directions for unfolding.
Proper onsite storage also must be provided for these
materials. All FMLs should be stored in a secure
area, away from dirt, dust, water, and extreme heat.
In addition, they should be placed where people and
animals cannot disturb them. Proper storage
prevents heat-induced bonding of the rolled
membrane (blocking), and loss of plasticizer or
curing of the polymer, which could cause
embrittlement of the membrane and subsequent
seaming problems.
Bedding Considerations
Before placing the membrane, bedding preparations
must be completed. Adequate compaction (90 percent
by modified proctor equipment; 95 percent by
standard proctor equipment) is a must. The landfill
surface must be free of rocks, roots, and water. The
subgrades should be rolled smooth and should be free
from desiccation cracks. The use of herbicides can
also affect bedding. Only chemically compatible
herbicides should be used, particularly in surface
impoundments. Many herbicides have hydrocarbon
carriers that will react with the membranes and
destroy them.
FML Panel Placement
Prior to unfolding or unrolling, each panel should be
inspected carefully for defects. If no defects are
found, the panels may be unrolled. The delivery
ticket should describe how to unroll each panel.
Starting with the unrolling process, care should be
taken to minimize sliding of the panel. A proper
overlap for welding should be allowed as each panel
is placed. The amount of panel placed should be
limited to that which can be seamed in 1 day.
Seaming and Seam Repair
After the panels have been inspected for defects, they
must be seamed by a qualified seamer. The
membrane must be clean for the seaming process and
there must be a firm foundation beneath the seam.
Figure 7-1 shows the configuration of several types of
seams.
The most important seam repair criterion is that any
defective seam must be bounded by areas that pass
fitness structure tests. Everything between such
areas must be repaired. The repair method should be
determined and agreed upon in advance, and
following a repair, a careful visual inspection should
be performed to ensure the repair is successful.
Weather and Anchorage Criteria
Weather is an additional consideration when
installing a FML. From the seaming standpoint, it is
important not to expose the liner materials to rain or
dust. Any time the temperature drops below 50°F,
the installer should take precautions for
temperature. For example, preheaters with the
chambers around them may be used in cold weather
to keep the FML warm. There also should be no
excessive wind, because it is very difficult to weld
under windy conditions.
In addition, FML panels should be anchored as soon
as possible. The anchor trench may remain open for
100
-------
Lap Seam
Lap Seam with Gum Tape
Adhesive
Gum Tape
Tongue and Groove Splice
Factory
Vulcanized \JS5S!
Extrusion Weld Lap Seam
Fillet Weld Lap Seam
Tape
Double Hot Air or Wedge Seam
Figure 7-1. Configurations of field geomembrane seams.
several days after installation of a panel. However,
the anchor trench must be filled when the panel is at
its coolest temperature and is, therefore, shortest in
length. This will occur early in the morning.
Additional Polymer Components
Polymer components, such as.geotextiles, geonets,
and geogrids, must be carefully inspected, as there is
no CQA program for these components. Chapter
Four discusses polymer components in more detail.
To date, CQA activities have focused on FMLs, and
there is no way to "fingerprint" other materials to
determine their characteristic properties over the
long term. Fingerprinting refers to the evaluation of
the molecular structure of the polymer. For example,
some geonets sold on the market use air-entrained
polymers to create "foamed" geonets with greater
thicknesses. Over time, however, the air -in the
entrained bubbles diffuses through the polymer and
the drainage net goes flat. When loads are left on
these geonets for testing purposes, it is possible to
observe orders-of-magnitude reductions in the
capacity of these materials by the 30th day of testing.
Geotextiles, geogrids, and geonets all should be
purchased from companies that have instituted
quality control procedures at their plants and
understand the liabilities, the risks, and the
problems associated with landfill liner failure.
Sampling Strategies
In a CQA program, there are three sampling
frequency criteria: (1) continuous (100 percent), (2)
judgmental, and (3) statistical. Every FML seam
should be tested over 100 percent of its length. Any
time a seaming operation begins, a sample should be
cut for testing. A sample also should be taken any
time a seaming operation is significantly modified
(by using a new seamer or a new factory extrusion
rod, or by making a major adjustment to the
equipment).
Continuous (100 Percent) Testing
There are three types of continuous tests: visual,
destruct (DT), and nondestruct (NDT). Visual
inspection must be done on all seams, and DT tests
must be done on all startup seams.
There are several types of nondestruct (NDT) seam
tests (see Table 7-1). The actual NDT test depends on
the seam type and membrane polymer. An air lance
(a. low pressure blast of air focused on the edge of the
seam) can be used on polyvinyl chloride (PVC),
chlorinated polyethylene (CPE), and other flexible
liner materials. If there is a loose bond, the air lance
will pop the seam open.
In a mechanical point stress test, a screwdriver or a
pick is pressed into the edge of the seam to detect a
weak bond location. In a vacuum chamber test, the
worker applies soapy water to the seam. The vacuum
chamber is then moved over the seam. If there is a
hole, the vacuum draws air from beneath the
membrane, causing a bubble to occur. The chamber
should not be moved too quickly across the seam. To
be effective, the vacuum box should remain on each
portion of the seam at least 15 seconds before it is
moved. Otherwise, it may not detect any leaks.
The pressurized dual seam test checks air retention
under pressure. This test is used with double hot air
or wedge seams that have two parallel welds with an
air space between them, so that air pressure can .be
applied between the welds. Approximately 30 psi is
applied for 5 minutes with a successful seam losing
no more than 1 psi in that time. This seam cannot be
used in sumps or areas in which there is limited
space for the equipment to operate.
Ultrasonic equipment also may be used in a variety of
seam tests. This equipment measures the energy
transfer across a seam using two rollers: one that
transmits a high frequency signal, and one that
receives it. An oscilloscope shows the signal being
received. An anomaly in the signal indicates some
change in properties, typically a void (caused by the
101
-------
Overgrind of an extruded seam.
presence of water). Ultrasonic equipment, however,
will not detect a tacked, low-strength seam or dirt
contamination, and the tests are very operator-
dependent.
Judgmental Testing
Judgmental testing involves a reasonable
assessment of seam strength by a trained operator or
CQA inspector. Judgmental testing is required when
a visual inspection detects factors such as apparent
dirt, debris, grinding, or moisture that may affect
seam quality.
Statistical Testing
True statistical testing is not used in evaluating
seams; however, a minimum of one DT every 500 feet,
of seam, with a minimum of one test per seam, is
required. Sumps or ramps, however, may have seams
that are very short, and samples should not be cut
from these seams unless they appear defective. In
102
-------
Table 7-1. Overview of Nondestructive Geomembrane Seam Tests
Nondestructive Test Primary User
1,
2.
3.
4.
5.
6.
7.
Method contrac-
tor
Air lance Yes
Mechanical Yes
point (pick)
stress
Vacuum Yes
chamber
(negative
pressure)
Dual seam Yes
(positive
pressure)
Ultrasonic pulse
echo
Ultrasonic
impedance
Ultrasonic
shadow
Engr.
Insp.
--
--
Yes
Yes
Yes
Yes
Yes
Third Cos'fbf-y
Party Equipment
Inspector ($)
200
Nil
1000
200
Yes 5000
Yes 7000
Yes 5000
Speed of-
Fast
Fast
'Slow
Fast
Mod.
'Mod.
Mod.
General
Cost of
Tests
Nil
Nil
V. high
Mod.
High
High
High
Comments
Type of
Result
Yes-No
Yes-No
Yes-No
Yes-No
Yes-No
Qualitative
Qualitative
Recording
Method
Manual
Manual
Manual
Manual
Automatic
Automatic
Automatic
Operator
Dependency
V. high
V. high
High
Low
Moderate
Unknown
Low
Source: Koerner, R. M. and G. N. Richardson. 1987. Design of geosynthetic systems for waste disposal. ASCE-GT Specialty Conference,
Geotechnical Practices for Waste Disposal, Ann Arbor, Michigan.
addition, a minimum of one DT test should be done
per shift.
There are no outlier criteria for statistical testing of
seams. In other words, no failure is acceptable.
Typically two tests, a shear test and a peel test, are
performed on a DT sample (Figure 7-2). The shear
test measures the continuity of tensile strength in a
membrane. It is not, however, a good indicator of
seam quality. The peel test provides a good
indication of the quality of a weld because it works
on one face of a weld. A poor quality weld will fail
very quickly in a peel test.
In a shear test, pulling occurs in the plane of the
weld. This is comparable to grabbing onto the
formica on a desk top and trying to pull the formica
off horizontally. The bond is being sheared. The peel
test, on the other hand, is a true test of bond quality.
This test is comparable to getting beneath the
formica at one corner of a desk top and peeling up.
Documentation
Documentation is a very important part of the CQA
process. Documents must be maintained throughout
FML placement, inspection, and testing. A FML
panel placement log (Figure 7-3), which details the
panel identity, subgrade conditions, panel
conditions, and seam details, should be kept for every
panel that is placed. This form is filled out on site
and typically carries three signatures: the
engineer's, the installer's, and the_ regulatory
agency's onsite coordinator's (if appropriate).
In addition, all inspection documents (e.g.,
information on repairs, test sites, etc.) must be
carefully maintained. Every repair must be logged
(Figure 7-4). Permits should never be issued to a
facility whose records do not clearly document all
repairs.
During testing, samples must be identified by seam
number and location along the seam. A
geomembrane seam test log is depicted in Figure 7-5.
This log indicates the seam number and length, the
test methods performed, the location and date of the
test, and the person who performed the test.
At the completion of a FML construction, an as-built
record of the landfill construction should be produced
that provides reviewers with an idea of the quality of
work performed in the construction, as well as where
problems occurred. This record should contain true
panel dimensions, location of repairs, and location of
penetrations.
103
-------
Dirt within an extruded seam.
104
-------
Shear Test
Figure 7-2. Seam strength tests.
Peel Test
Owner: _
Project:
Date/Time:
Panel Placement Log
- Panel Number ---
Weather:
Temperature:
Wind:
-Subgrade Conditions •
Line & Grade:
Surface Compaction:
Protrusions:
Ponded Water:
Dessication
Panel Conditions
Transport Equipment:
Visual Panel Inspection:
Temporary Loading:
Temp. Welds/Bonds:
Temperature:
Damages:
Seam Details
Seam Nos.:.
Seaming Crews:
Seam Crew Testing:.
Notes:.
Figure 7-3. Panel placement log.
105
-------
Geomembrane Repair Log
Date
Seam
Panels
Location
Material
Type
Description of Damage
|
Type of
Repair
Repair Test
Type
i
1
Tested By,
Figure 7-4. Geomembrane repair log.
106
-------
Geomembrane Seam Test Log
Continuous Testing Destructive Test
Seam
No.
Seam
Length
Visual
Inspect
Air
Temp.
Test
Method
Pressure
Init/Final
Peel
Test
Shear
Test
Location
Date
Tested
By
•
Figure 7-5. Geomembrane seam test log.
107
-------
-------
8. LINER COMPATIBILITY WITH WASTES
Introduction
This chapter discusses chemical compatibility
(resistance) of geosynthetic and natural liner
materials with wastes and leachates. Even in a
relatively inert environment, certain materials
deteriorate over time when exposed to chemicals
contained in both hazardous and nonhazardous
leachate. It is important to anticipate the kind and
quality of leachate a site will generate and select
liner materials accordingly. The chemical resistance
of any flexible membrane liner (FML) materials,
geonets, geotextiles, and pipe should be evaluated
before installation.
Chemical compatibility tests using EPA Method
9090 should always be performed for hazardous
waste sites, but some municipal waste sites also
contain hazardous, nondegradable materials. EPA
conducted a 5-year study of the impact of municipal
refuse on commercially available liner materials and
found no evidence of deterioration within that
period. However, in a current study of leachate
quality in municipal landfills, the Agency has
discovered some organic chemical .constituents
normally found in hazardous waste landfill facilities.
Apparently small quantities of household hazardous
waste enter municipal sites or are disposed of as
small quantity generator wastes. As a result of these
findings, EPA is developing a position on the need for
chemical compatibility tests for thousands of
municipal waste disposal sites.
In general, cover materials, including membranes
and geosynthetics, do not need to be checked for
chemical compatibility since these materials do not
encounter leachates. Research data indicate that the
predominant gases coming from municipal sites are
methane, hydrogen, and carbon dioxide, although a
few others may be emitted from household hazardous
waste. These gases pass through cover materials by
diffusion and evidence to date indicates that they
have caused no deterioration of membranes. Also,
chemical compatibility of cover materials with gases
has not been a major problem at hazardous waste
facilities.
A primary objective of chemical compatibility testing
is to ensure that liner materials will remain intact
not just during a landfill's operation but also through
the post-closure period, and preferably longer. It is
difficult, however, to predict future chemical
impacts. There is no guarantee that liner materials
selected for a site today will be the same as materials
manufactured 20 years from now. For example, the
quality of basic resins has improved considerably
over the last few years.
The wastes themselves also change over time. Tests
should be performed to ensure that landfill leachate
will not permeate the liner layer. EPA recommends a
variety of physical property degradation tests,
including a fingerprint program of thermo-
gravimetric analysis, differential scanning
calorimetric tests, and infrared analysis.
Fingerprinting involves analyzing the molecular
structure of the leachate components. Sometimes a
particularly aggressive leachate component can be
identified by evaluating the fingerprint analysis
tests after exposure of the membrane to the leachate.
Exposure Chamber
The first area of concern in chemical compatibility
testing is the exposure chamber used to hold the
leachate and membranes being tested. The exposure
chamber tank can be made of stainless steel,
polyethylene, glass, or a variety of other materials.
Any geosynthetic liner material being considered
must be tested for chemical compatibility with the
leachate. Some leachates have caused rusting and
deterioration of stainless steel tanks in the past, and
if polyethylene is being evaluated, the tank should
be of another type of material to prevent competition
between the tank material and the test specimen for
aggressive agents in the leachate.
The conditions under which the material is tested
are crucial. The top of the exposure chamber must be
109
-------
sealed and the tank should contain no free air space.
A stirring mechanism in the tank keeps the leachate
mixture homogeneous and a heater block keeps it at
an elevated temperature as required for the test.
Stress conditions of the material in the field also
should be simulated as closely as possible. The
original EPA Method 9090 test included a rack to
hold specimens under stress conditions but was
revised when some materials shrank in the leachate.
Due to the hazardous nature of the material, testing
should be performed in a contained environment and
safety procedures should be rigorously followed.
In some cases a sump at the waste management
facility can be used as an exposure chamber if it is
large enough. The designer of a new landfill site can
design a slightly larger sump especially for this
purpose. However, since the temperature of a sump
is colder than room temperature (55°F instead of
72°F), the geosynthetics need to be exposed for a
longer period of time. Instead of 120 days, the test
might take 6 months to a year or longer.
Representative Leachate
It is important that the sample being, tested is
representative of the leachate in the landfill.
Leachate sampled directly from a sump is usually
representative, but care must be taken not to mix it
during removal. This will disturb the sample's
homogeneity and may result in components
separating out. Another problem is that municipal
solid waste landfill leachate will start to oxidize as
soon as it leaves the sump and probably should be
sampled under an inert atmosphere.
A sampler should be familiar with the source of all
the leachate at a site before removing a sample. If
radioactive materials are present, extra care must be
taken.
At some existing waste management facilities,
operators have placed coupons of geosynthetic
materials into sump areas to monitor leachate
effects. Information gathered from this monitoring
procedure provides an excellent data base. Regular
recording of data allows the operator to discover
compatibility problems as they develop, rather than
waiting until a landfill liner fails. If the coupon
shows early signs of deterioration, the operator can
respond immediately to potential problems in the
facility.
When planning construction of a new site, an
operator first assesses the market to determine the
quantity and quality of waste the landfill will
receive. Representative leachate is then formulated
based on the operator's assessment.
The Permit Applicant's Guidance Manual for
Treatment, Storage, and Disposal (TSD) facilities
contains additional information on leachate'
representativeness (see Chapter 5, pp. 15-17;
Chapter 6, pp. 18-21; and Chapter 8, pp. 13-16).
Compatibility Testing of Components
Geosynthetics
EPA's Method 9090 can be used to evaluate all:
geosynthetic materials used in liner and leachate
collection and removal systems! currently being
designed. Method 9090 is used to predict the effects
of leachate under field conditions and has been
verified with limited field data. The test is performed
by immersing a geosynthetic in a chemical
environment for 120 days at two different
temperatures, room and elevated. Every 30 days,
samples are removed and evaluated for changes in
physical properties. Tests performed on FMLs are
listed in Table 8-1. The results of any test should be
cross-referenced to a second, corollary test to avoid
errors due to the test itself or to the laboratory
personnel.
i
Table 8-1. Chemical Compatibility Tests for FMLs
• Hardness
• Melt Index ;
• Extractives
• Volatile Loss
• Peel Adhesion
• Tear Resistance
• Specific Gravity ;
• Low Temperature
• Water Absorption
• Puncture Resistance
• Dimensional Stability
• Modulus of Elasticity
• Bonded Seam Strength
• Hydrostatic Resistance
• Carbon Black Dispersion
• Thickness, Length, Width
• Tensile at Yield and Break
• Environmental Stress Crack
• Elongation at Yield and Break
Physical property tests on geotextiles and geonets
must be designed to assess different uses, weights,
and thicknesses of these materials, as well as
construction methods used in the field. EPA has a
limited data base on chemical compatibility with
geotextiles. Some tests for geonets and geotextiles
recommended by EPA are listed in Table 8-2. The
ASTM D35 Committee should be consulted for
information on the latest testing procedures.
110
-------
Table 8-2. Chemical Compatibility Tests for
Geonets/Geotextiles
• Puncture
• Thickness
• Permittivity
• Transmissivity
• Mass/Unit Area
• Burst Strength
• Abrasive Resistant
• Percent Open Area
• Ultraviolet Resistivity
• Grab Tensile/Elongation
• Equivalent Opening Size
• Hydrostatic Bursting Strength
• Tearing Strength (Trapezoidal)
• Compression Behavior/Crush Strength
Until recently, EPA recommended using 1 1/2 times
the expected overburden pressure for inplane
transmissivity tests. Laboratory research, however,
has revealed that creep and intrusion cause a loss of
transmissivity, so the Agency has amended its
recommendation to 2 to 3 times the overburden
pressure. EPA also recommends that the geotextile
or geonet be aged in leachate, but that the actual test
be performed with water. Performing the test with
the leachate creates too great a risk of contamination
to test equipment and personnel. The transmissivity
test should be run for a minimum of 100 hours. The
test apparatus should be designed to simulate the
field conditions of the actual cross section as closely
as possible.
Pipes
The crushing strength of pipes also should be tested.
There have been examples where pipes in landfills
have actually collapsed, and thus forced the site to
stop operating. The ASTM D2412 is used to measure
the strength of pipe materials.
Natural Drainage Materials
Natural drainage materials should be tested to
ensure that they will not dissolve in the leachate or
form a precipitant that might clog the system. ASTM
D2434 will evaluate the ability of the materials to
retain permeability characteristics, while ASTM
D1883 tests for bearing ratio, or the ability of the
material to support the waste unit.
Blanket Approvals
EPA does not grant "blanket approvals" for any
landfill construction materials. The quality of liner
materials varies considerably, depending on
quantities produced and on the manufacturer. Even
high density polyethylene (HOPE) does not receive
blanket approval. The Agency, together with a group
of; chemists, biologists, and researchers from the
liner manufacturing industry, determined that
HDPE varies slightly in its composition among
manufacturers. Because different calendaring aids,
processing aids, or stabilizer packages can change
the overall characteristics of a product, each
material should be individually tested.
Landfill designers should select materials on the
basis of site-specific conditions, as the composition of
specific leachates will vary from site to site. A
designer working with the operator determines in
advance of construction what materials will be most
effective. In recent years, EPA has restricted certain
wastes, including liquids, from land disposal. These
regulations have expanded the number of potential
candidate materials, thus allowing more flexibility
to landfill designers.
Interpreting Data
When liner material test data show the rate of
change of the material to be nil over a period of time,
then the membrane is probably not undergoing any
chemical change. There have been instances,
however, in which a material was tested for a year
without change and then suddenly collapsed. For
this reason, the longer the testing process can
continue, the more reliable the data will be. When
test data reveal a continuous rate of change, then the
material is reacting with the leachate in some way. If
the data show an initial continuous rate of change
that then tapers off, new leachate may need to be
added more often. In any case, the situation should
be studied in more detail.
A designer should consult with experts to interpret
data from chemical compatibility tests. To meet this
need, EPA developed a software system called
Flexible Membrane Liner Advisory Expert System
(FLEX) to assist in evaluating test data. FLEX is an
expert system that is based on data from many
chemical compatibility tests and contains
interpretations from experts in the field.
Ill
-------
-------
9. LONG-TERM CONSIDERATIONS: PROBLEM AREAS AND UNKNOWNS
Introduction
This chapter presents an overview of long-term
considerations regarding the liner and collection
systems of hazardous waste landfills, surface
impoundments, and waste piles. Included in the
discussion are flexible membrane liner and clay liner
durability, potential problems in leachate collection
and removal systems, and disturbance and aesthetic
concerns in caps and closures.
In judging the impact of any facility, site-specific
conditions such as geology and stratigraphy;
seismicity; ground-water location and quality;
population density; facility size; leachate quantity
and quality; and nontechnical political, social, and
economic factors must be taken into account. Table
9-1 summarizes areas of concern in various landfill
materials and components.
One of the most important considerations in
planning a waste facility is estimating the length of
time the facility is expected to operate. Some
recommended time frames for different kinds of
facilities are:
• Heap leach pads 1-5 years
• Waste piles 5-10 years
• Surface impoundments 5-25 years
• Solid waste landfills 30-100 years
• Radioactive waste landfills 100-1000+years
None of these time frames are set forth in
regulations, however. The only time frame regulated
by EPA is the 30-year post-closure care period for all
hazardous waste landfill facilities.
Flexible Membrane Liners
The major long-term consideration in any
synthetically lined facility is the durability of
flexible membrane liners (FMLs). In the absence of
sunlight, oxygen, or stresses of any kind, a properly
formulated, compounded, and manufactured FML
will stay intact indefinitely. This is because stable
polymers in isolation have an equilibrium molecular
structure that prevents aging. The usual indicators
of stress are changes in density, p, and glass
transition temperature, Tg. The glass transition
temperature is the temperature below which the
amorphous region of a crystalline polymer is rigid
and brittle (glossy) and above which it is rubbery or
fluidlike.
Polymers in the field, however, are subject to many
external influences and stresses. Experiments done
on FML materials attempt to simulate the long-term
in situ aging processes. One approach is to place a
sample of material in a test chamber at room
temperature (approximately 70°F), and another
sample in a chamber heated to 120° to 160°F. The
activation energy for the two FMLs is evaluated.
Then a model based on the Arrhenius equation is
used to determine the length of time it would take
the sample kept at 70°F to degrade to the same
extent as the high temperature sample. This
procedure, however, assumes that temperature
increase is physically equivalent to the time of
exposure in the field, an assumption with which
many chemists disagree.
Therefore, in the absence of a direct measurement
method for FML lifetime, it becomes necessary to
evaluate all of the possible degradation mechanisms
that may contribute to aging.
Degradation Concerns
A number of mechanisms contribute to degradation
in the field, many of which can be controlled with
proper design and construction. A FML can be
weakened by various individual physical,
mechanical, and chemical phenomena or by the
synergistic effects of two or more of these
phenomena. Polymeric materials have an extremely
elongated molecular structure. Degradation cuts
across the length of this structure in a process known
as chain scission. The more chain breakages that
occur, the more the polymer is degraded by loss of
113
-------
Table 9-1. Long-Term Concerns In Landfill Mechanisms (Y = yes; N = no)
uap uap - HVVUH LCR
1.
2.
3.
4.
5.
6.
Mechanism
Movement of
Subsoil
Subsidence of
Waste
Aging
Degradation
Clogging
Disturbance
FML
N
Y
Y
Y
N
Y
Clay
N
Y
Y
N
N
Y
Nat.
N
Y
Y
N
Y
Y
SYN
N
Y
Y
Y
Y
Y
P-FML
Y
N
Y
Y
N
N
Nat.
N
N
Y
N
Y
N
SYN
N
N
Y
Y
Y
N
Sec.
FML
Y
N
Y
Y
N
N
Liner
Clay
Y
N
N
N
N
N ;
LDCR
Nat.
N
N
N
N
Y
N
SYN .
N
N
Y
Y
Y
N
where:
FML = geomembrane liner
LCR » leachate collection and removal system
SWCR s surface water collection and removal system
Nat a made from natural soil materials
Syn. - made from synthetic polymeric materials
strength and loss of elongation. Each of the processes
involved in chain scission will be discussed in the
following sections.
Oxidation Degradation
Oxidation is a major source of polymer degradation
leading to a loss of mechanical properties and
embrittlement. The steps in this process are as
follows:
Heat liberates free radicals.
Oxygen uptake occurs.
Hydroperoxides accelerate uptake.
Hydrogen ions attach to tertiary carbons which
are most vulnerable.
• Subsequent bond scission occurs.
At high temperatures (over 200°F) oxidation occurs
very rapidly. Consequently, oxygen will create
serious problems for FMLs built near furnaces,
incinerators, or in extremely hot climates. The
impact of oxidation is greatly reduced at ambient
temperatures.
One can minimize oxidative degradation of FMLs by
designing facilities with FMLs buried to avoid
contact with oxygen and to dissipate heat generated
by direct rays of the sun. Oxygen degradation is a
very serious problem with materials used for surface
impoundments, however, where the FML cannot be
buried or covered.
Ultraviolet Degradation
All polymers degrade when exposed to ultraviolet
light via the process of photooxidation. The part of
the ultraviolet spectrum responsible for the bulk of
polymer degradation is the wavelength UV-B (315-
380 nm). ASTM D4355 uses Xenon Arc apparatus for
assessing the effects of this wavelength on polymeric
test specimens. The Xenon Arc apparatus is
essentially a weatherometer capable of replicating
the effects of sunlight under laboratory conditions.
Blocking or screening agents, such as carbon black,
are commonly used to retard ultraviolet degradation.
For this reason, FMLs are manufactured with
approximately 2 to 3 percent carbon black. Even that
small amount effectively retards degradation by
ultraviolet rays. Although the addition of carbon
black retards degradation, it does not stop it
completely. Ultraviolet degradation, however, can be
prevented by burying the material beneath 6 to 12
inches of soil. FMLs should be buried within 6 to 8
weeks of the time of construction, geonets within 3 to
6 weeks, and geotextiles within 1 to 3 weeks, i.e., the
higher the surface area of the material, the more
rapidly the geosynthetic must be covered.
In surface impoundments where FMLs cannot be
buried, ultraviolet degradation also contributes to
the oxidation degradation. An attempt still should be
made to cover the FML, even though sloughing of the
cover soils will occur unless the site has very gentle
slopes. Various other covering strategies are being
evaluated, such as bonding geotextiles to FML,
surfaces.
High Energy Radiation '
Radiation is a serious problem, ajs evidenced by the
Nuclear Regulatory Commission and U.S.
Department of Energy's concerns with transuranic
and high level nuclear wastes. High energy radiation
breaks the molecular chains of polymers and gives
off various products of disintegration.
As of 1992, low-level radioactive wastes, such as
those from hospitals and testing laboratories, must
114
-------
be contained in landfills. The effects of low-level
radiation associated with these waste materials on
polymers still needs to be evaluated.
Chemical Degradation: pH Effects
All polymers swell to a certain extent when placed in
contact with water (pH = 7) because they accept
water and/or water vapor into their molecular
structure. Degrees of swelling for some common
polymers are listed below:
• Polyvinyl chloride (PVC)
• Polyamide (PA)
• Polypropylene (PP)
• Polyethylene (PE)
• Polyester (PET)
10 percent
4 to 4.5 percent
3 percent
0.5 to 2.0 percent
0.4 to 0.8 percent
Polymer swelling, however, does not necessarily
prevent a material from functioning properly. The
U.S. Bureau of Reclamation has observed polyvinyl
chloride liners functioning adequately in water
canals for 20. to 25 years despite relatively large
increases in the thickness of the material.
In very acidic environments (pH < 3), some
polymers, such as polyamides, (i.e., Kevlar and
nylon) begin to degrade. On the other end of the
spectrum, certain polyesters degrade in extremely
alkaline environments (pH > 12). High
temperatures generally accelerate the chemical
degradation process. Most landfill leachates are not
acidic or basic enough to cause concern. However, in
certain kinds of landfills, such as. those used for ash
disposal, the pH of the leachate might be quite
alkaline and needs to be taken into account when
choosing liner materials.
Chemical Degradation: Leachate
EPA's Method 9090 is a test procedure used to
evaluate leachate degradation of FML materials. As
described in Chapter Eight, the FML must be
immersed in the site-specific chemical environment
for at least 120 days at two different temperatures.
Physical and mechanical properties of the tested
material are then compared to those of the original
material every 30 days. Assessing subtle property
changes can be difficult. Flexible Liner Evaluation
Expert (FLEX), a software system designed to assist
in the Resource Conservation Recovery Act (RCRA)
permitting process, can help in evaluating EPA
Method 9090 test data.
Biological Degradation
Neither laboratory nor field tests have demonstrated
significant evidence of biological degradation.
Degradation by fungi or bacteria cannot take place
unless the microorganisms attach themselves to the
polymer and find the end of a molecular chain, an
extremely unlikely event. Chemical companies have
been unable to manufacture biological additives
capable of destroying high-molecular weight
polymers, like those used in FMLs and related
geosynthetic materials. Microbiologists have tried
unsuccessfully to make usable biodegradable plastic
trash bags for many years. The polymers in FMLs
have 10,000 times the molecular weight of these
materials, thus are very unlikely to biodegrade from
microorganisms.
There also is little evidence that insects or burrowing
animals destroy polymer liners or cover materials. In
tests done with rats placed in lined boxes, none of the
animals were able to chew their way through the
FMLs. Thus, degradation from a wide spectrum of
biological sources seems highly unlikely.
Other Degradation Processes
Other possible sources of degradation include
thermal processes, ozone, extraction via diffusion,
and delamination. Freeze-thaw cycling, or the
process by which a material undergoes alternating
rapid extremes of temperature, has proven to have
an insignificant effect on polymer strength or FML
seam strength. Polymeric materials experience some
stress due to warming, thereby slightly decreasing
their strength, but within the range of 0° to 160°F,
there is no loss of integrity.
Ozone is related to ultraviolet degradation in the
photooxidation process, and, therefore, creates a
more serious problem for polymeric materials.
However, ozone effects can be essentially eliminated
by covering the geosynthetic materials within the
time frames previously mentioned.
In the extraction via diffusion mechanism,
plasticizers leach out of polymers leaving a tacky
substance on the surface of the material. The FML
becomes less flexible, but not, apparently, weaker
nor less durable.
Delamination of scrim-reinforced material was a
problem until 15 years ago when manufacturers
began using large polymer calendar presses. The
presses thoroughly incorporate the scrim
reinforcement, so that delamination rarely occurs
today.
Stress-induced Mechanisms
Freeze-thaw, abrasion, creep, and stress cracking are
all stress mechanisms that can affect polymers. The
first two, freeze-thaw and abrasion, are not likely to
be problems if the material is buried sufficiently
deep. Soil burying will eliminate temperature
extremes. Abrasion is a consideration only in surface
115
-------
impoundments in which water waves come into
direct contact with the FML.
Creep
Creep refers to the deformation of the FML over a
prolonged period of time under constant stress. It can
occur at side slopes, at anchor trenches, sumps,
protrusions, settlement locations, folds, and creases.
A typical creep test for a FML, or any other
geosynthetic material, involves suspending a load
from an 8-inch wide tensile test specimen. Initially
an elastic elongation of the material occurs. The
material should quickly reach an equilibrium state,
however, and experience no further elongation over
time. This is shown in the stabilized lower curve in
Figure 9-1. The second and third curves in Figure 9-1
show test specimens undergoing states of constant
creep elongation and creep failure, respectively.
@ o = "x"% oy
Creep Failure
Constant Creep
No Creep
Time
Figure 9-1. Typical results of a sustained load (creep) test.
One can use the design-by-function concept to
minimize creep by selecting materials in which the
allowable stress compared with the actual stress
gives a high factor of safety. For semicrystalline
FMLs, such as polyethylenes, the actual stress must
be significantly less than the yield stress. For scrim-
reinforced FMLs such as chlorosulfonated
polyethylene (CSPE), or reinforced ethylene
propylene diene monomer (EPDM), the actual stress
must be significantly less than the breaking strength
of the scrim. Finally, for nonreinforced plastics such
as PVC or nonreinforced chlorinated polyethylene
(CPE), the actual stress must be less than the
allowable stress at 20 percent elongation. In all
cases, one should maintain a factor of safety of 3 or
higher on values for these materials.
Stress Cracking
Stress cracking is a potential problem with
semicrystalline FMLs. The higher the crystalline
portion of the molecular structure, the lower the
portion of the amorphous phase and the greater the
possibility of stress cracking. High density
polyethylene (HOPE) is the primary material of
concern.
ASTM defines stress cracking as a brittle failure that
occurs at a stress value less than a material's short-
term strength. The test usually applied to FMLs is
the "Bent Strip Test," D1693. The bent strip test is a
constant strain test that depends on the type and
thickness of the material being tested. In performing
the test, a specimen of the FML 0.5 inch wide by 1.5
inches long is prepared by notching its surface
approximately 10 mils deep. Then the specimen is
bent 180 degrees and placed within the flanges of a
small channel holder as shown in Figure 9-2.
Approximately 10 replicate specimens can be placed
in the holder simultaneously. The assembly is then
placed in a glass tube containing a surface active
wetting agent and kept in a constant temperature
bath at 122°F. The notch tips are observed for
cracking and/or crazing. Most specifications call for
stress-crack' free results for 500 or 1,000 hours.
Commercial HDPE sheet usually performs very well
in this particular test.
There are two things, however, that the D1693 test
does not allow: a constant stress testing of the
material and an evaluation of the seams. The D1693
process bends the test specimen initially, but then
allows it to relax into its original state. Furthermore,
the notch cannot be made to span over separate
sheets at seam locations.
ASTM D2552, "Environmental Stress Rupture
Under Tensile Load," tests HDPE materials under
constant stress conditions. In this particular test,
dogbone-shaped specimens under constant load are
immersed in a surface active agent at 122°F (see
Figure 9-3). The test specimens eventually enter
elastic, plastic, or cracked states. Commercially
available HDPE sheet material performs very well
in this test, resulting in negligible (= 1 percent)
stress cracking. The test specimens are generally
elastic for stresses less than 50 percent yield and
plastic for stress levels greater than 50 percent yield.
This apparatus can be readily modified to test HDPE
seams (see Figure 9-4). Long dogbone-shaped
specimens with a constant-width cross section are
taken from the seamed region. The same test
procedure is followed and the test results should be
the same. If cracking does occur, the cracks go
through the fillet portion of the extrusion fillet-
seamed samples or through the FML sheet material
on either side of the fillet. For other types of seams,
the cracking goes through the parent sheet on one or
the other side of the seamed region. Results on a wide
range of field-collected HDPE seams have shown a
relatively high incidence of cracking. This
phenomenon is currently being evaluated with
replicate tests; carefully prepared seams; and
116
-------
r
[
A
i
-1
— *i F r*""
G
C J
-*•
*
*
^r
.- D
B.k
Test Sample
J
,
N
j
^
r
*
)
j
)
J
A
A
^- J
E
**}**'
^J
<^i£j1
L,
'CL.
5i
!*=:
»» —
•^
^~
s
>s^_
.
3 )
^- X— ^
Test
Spec men Holder Assembly
(B) (C)
Figure 9-2. Details of ASTM D1693 on "Environmental Stress-Cracking of Ethylene Plastics."
Micro Switch to
Timers
Pin
r-3:1 MAn/ Joints
J
Shot
Can
s
%
CO
76.20 cm
30"
20 Positions
T^
I
h
rn
„• •*•
-Specimen
p=o=-
Side View
• Tray Moved Up and
Down on Rack and
Pinion Arrangement
Front View
Figure 9-3. Environmental stress rupture test for HOPE sheet ASTM D2552.
117
-------
variations of temperature, pressure, and other
seaming controls.
\
\
O
o
Front View
Edge View
Edge View
Figure 9-4. Details of test specimens from ASTM D2552
modified test for HOPE seams.
The test is sometimes criticized as being
nonrepresentative of field conditions since the
wetting agent exposes all sides of the seamed region
to liquid, which does not happen in the field. The
specimen also is too narrow to simulate real life
wide-width action. Also there is no confinement on
the surfaces of the material. While these criticisms
do challenge the test as being nonrepresentative,
there have been three field failures that mimicked
the laboratory cracking exactly. The first was. in a
surface impoundment lined with 80-mil HDPE that
was an extrusion fillet seam. The problem was not
picked up by construction quality assurance work,
but instead occurred after the facility was closed. The
second case involved extensive FML seam failures at
a site in the Southwest. The seam used there was a
flat extrudate. Cracking originated in several areas
due to high localized stresses and exposure to wide
temperature fluctuations. The third failure occurred
at a site in the Northeast that used 60 mil HDPE and
a hot wedge seam. In this case a stress crack 18 to 20
inches long formed at the outer track of the wedge.
Of all the degradation processes reviewed, stress
cracking in field seams is of the greatest current
concern. It appears as though the field problems only
occur in the exposed FML seam areas of surface
impoundments. Work is ongoing to investigate the
phenomenon further and to understand the
mechanisms involved. An emphasis on carefully
constructed and monitored field seams will certainly
be part of the final recommendations.
Clay Liners
Clay has a long history of use as a liner material.
Data is available on the effects of leachate on clay
liners over periods of 10 years or more, and the
results generally have been satisfactory. While clays
do not experience degradation or stress cracking,
they can have problems with moisture content and
clods. High concentrations of organic solvents, and
severe volume changes and desiccation also cause
concern at specific sites. The rapid freezing and
thawing of clay liner materials also affects their
integrity, but freeze-thaw can usually be alleviated
with proper design and construction considerations.
For a more complete discussion of clay liner
durability, see Chapter Two.
Leachate Collection and Removal
Systems
The leachate collection and removal system includes
all materials involved in the primary leachate
collection system and the leak detection collection
system. For the proper functioning of these systems,
all materials being used must be chemically
resistant to the leachates that they are handling. Of
the natural soil materials,'gravel and sand are
generally quite resistant to leachates, with the
possible exception of freshly ground limestone. With
this material, a solution containing calcium,
magnesium, or other ion deposits can develop as the
flow rate decreases in low gradient areas. A form of
weak bonding called "limerock" has been known to
occur. Its occurrence would be disastrous in terms of
leachate collection and removal. Regarding
geotextiles and geonets, there are no established test
protocols for chemical resistivity evaluation like the
EPA Method 9090 test for FMLs. Table 9-2 suggests
some tests that should be considered.
Table 9-2. Suggested Test Methods for Assessing Chemical
Resistance of Geosynthetics Used in Leachate
Collection Systems (Y = yes; N = no)
Test Type
Geotextile Geonet Geocomposite
Thickness
Mass/unit area
Grab tensile
Wide width tensile
Puncture (pin)
Puncture (GBR)
Trapezoidal tear
Burst
Y
Y
Y
Y
N
Y
Y
Y
Y
Y
N
Y
N
Y
N
N
Y
Y
N
N
N
Y
N
N
The system for collecting and removing the leachate
must function continuously over the anticipated
lifetime of the facility and its post-closure care
period. During operation, leachate is removed
regularly depending on the amount of liquid in the
incoming waste and natural precipitation entering
118
-------
the site. Leachate, however, continues to be
generated long after landfill closure. During the first
few post-closure years the rate of leachate removal is
almost 100 percent of that during construction.
Approximately 2 to 5 years after closure, leachate
generally levels off to a low-level constant cap leak
rate or, in a very tight, nonleaking closure, falls to
zero (see Figure 9-5).
Clogging is the primary cause of concern for the long-
term performance of leachate collection and removal
systems. Particulate clogging can occur in a number
of locations. First, the sand filter itself can clog the
drainage gravel. Second, the solid material within
the leachate can clog the drainage gravel or geonet.
Third, and most likely, the solid suspended material
within.the leachate can clog the sand filter or
geotextile filter. The following breakdown of
particulate concentration in leachate at 18 landfills
shows the potential for particulate clogging:
• Total solids 0-59,200 mg/L
• Total dissolved solids 584 - 44,900 mg/L
• Total suspended solids 10-700 mg/L
Salts precipitating from high pH leachate, iron
pcher, sulfides, and carbonates can all contribute to
particulate clogging.
The potential for clogging of a filter or drainage
system can be evaluated by modeling the system in
the laboratory. This modeling requires an exact
replica system of the proposed components, i.e., cover
soil, geotextile, geonet, etc. Flow rate plotted as a
function of time will decrease in the beginning, but
eventually should level off to a horizontal line at a
constant value. It may take more than 1,000 hours
for this leveling to occur. Zero slope, at a constant
value, indicates an equlibrium (or nonclogging)
situation. A continuing negative slope is evidence of
clogging. As yet, there is no formula or criteria that
can be substituted for this type of a long-term
laboratory flow test.
Biological clogging can arise from many sources
including slime and sheath formation, biomass
formation, ochering, sulfide deposition, and
carbonate deposition. Ocher is the precipitate left
when biological activity moves from one zone to
another. It is an iron or sulfide deposit, and is most
likely to occur in the smallest apertures of filter
materials. Sand filters and geotextile filters are most
likely to clog, with gravel, geonets, and
geocomposites next in order from most to least likely.
The biological oxygen demand (BOD) value of the
leachate is a good indicator of biological clogging
potential; the higher the number, the more viable
bacteria are present in the leachate. Bacterial
clogging is more likely to be a problem in municipal
landfills than at hazardous waste facilities because
hazardous leachates probably would be fatal to most
bacteria. Currently, an EPA contractor is monitoring
six municipal landfills for evidence of aerobic and
anaerobic clogging. The results should be available
in 1989.
The most effective method for relieving particulate
and/or biological clogging is creating a high-pressure
water flush to clean out the filter and/or the drain. In
cases of high biological growth, a biocide may also
need to be introduced. Alternatively, a biocide might
be introduced into the materials during their
manufacture. Geotextiles and geonets can include a
time-release biocide on their surface, or within their
structure. Work by a number of geotextile and
geonet manufacturers is currently ongoing in this
area. Measures to remedy clogging must be
considered in the design stage.
The final factor to be considered in leachate
collection and removal systems is extrusion and
intrusion of materials into the leak detection system.
In a composite primary liner system, clay can readily
extrude through a geotextile into a geonet if the
geotextile has continuous open spaces, i.e., percent
open area (POA) > 1 percent. Therefore, relatively
heavy nonwoven geotextiles ,are recommended.
Elasticity and creep can cause geotextiles to intrude
into geonets from composite primary liners as well. A
FML above or below a geonet can also intrude into
the geonet due to elasticity and creep. Design-by-
function and laboratory evaluations that simulate
field conditions should alert designers to these
potential problems. For all geosynthetics, a high
design ratio value or factor of safety for strength
should be chosen.
Cap/Closure Systems
Water and wind erosion, lack of vegetation, excessive
sunlight, and disturbance by soil-dwelling animals
(or by people) all are potential problems for landfill
closure systems. The effects of rain, hail, snow,
freeze-thaw, and wind are discussed in Chapter Five.
Healthy vegetation growing over the cap minimizes
the erosion of soil on the slopes by these natural
elements.
The effects of animals and of sunlight (ultraviolet
rays and ozone) can be minimized by adequately
burying the cap/closure facility. Soil depths over
FMLs in covers range from 3 to 6 feet in thickness.
Large rocks above the FML cover can also thwart the
intrusion of animals into the area. Human intrusion,
either accidental or intentional, can usually be
prevented by posting signs and erecting fences.
The final long-term consideration related to cap and
closure systems is aesthetic. The New Jersey
Meadowlands Commission is planning for the final
119
-------
1
•^ 50
CD
Q.
o
s?
n
^^
Conl-iS. j
Weekly 1 Monthly
X^ Cap Leakage
i ' ** ^~~ — __. - * No Leakaae
To
30
Years after Closure
Figure 9-5. Approximate amount of leachate withdrawal after closure.
closure of 54 landfills in the northeastern part of the
state, all but 3 of which are already closed and ready
to be capped. A graphic artist was hired to design an
attractive landscape out of one facility along a
heavily traveled automobile and rail route. The
design included looping pipes for methane recovery,
a solar lake, and an 8-foot concrete sphere all
contributing to a visually pleasing lunar theme.
The performance of a capped and closed waste
facility is critically important. If a breach should
occur many years after closure, there is a high
likelihood that maintenance forces would be
unavailable. In that event, surface water could enter
the facility with largely unknown consequences.
Thus the design stage must be carefully thought out
with long-term considerations in mind.
120
-------
10. LEAK RESPONSE ACTION PLANS
This final chapter reviews proposed requirements for
Response Action Plans, or RAPs, that are contained
in the proposed leak detection rule issued in May,
1987. It focuses on the concepts behind the RAPs and
the preliminary, technical calculations used in
developing them. The main topics of discussion will
be the technical basis for the two response action
triggers, action leakage rate (ALR) and rapid and
large leakage (RLL) rate; the RAPs themselves; and
the RAP submittal process.
Background
In the Hazardous and Solid Waste Amendments
(HSWA) of 1984, Congress required that leaks from
new land disposal facilities be detected at the earliest
practical time. However, HSWA did not require or
specify actions to be taken once a leak is detected in
the leak detection system. Therefore, EPA proposed
requirements for response action plans to deal with
leaks detected in the leak detection system between
the two liners. EPA realizes that even with a good
construction quality assurance plan, flexible
membrane liners (FMLs) will allow some liquid
transmission either through water vapor permeation
of an intact FML, or through small pinholes or tears
in a slightly flawed FML. Leakage rates resulting
from these mechanisms can range from less than 1 to
300 gallons per acre per day (gal/acre/day). If
unchecked, these leak rates may result in increased
hydraulic heads acting on the bottom liner and
potential subsequent damage to the liner system.
The idea behind the RAP is to be prepared for any
leaks or clogging of the drainage layer in the leak
detection system that may occur during \ the active
life or post-closure care period of a waste facility. The
first step is to identify the top liner leak rates that
would require response actions. Therefore, in the
proposed leak detection rule of May 29, 1987, EPA
established two triggers for response actions: the
Action Leakage Rate (ALR) and the Rapid and Large
Leakage (RLL) rate. The ALR is a low-level leak rate
that would indicate the presence of a small hole or
defect in the top liner. The RLL is indicative of a
severe breach or large tear in the top liner. A
different level of responsiveness would be required
for leakage rates above these two triggers. RAPs
developed by owners or operators may have more
than two triggers as appropriate to cover the range of
leak rates expected for a landfill unit. In addition to
triggers, the proposed rule also defines the elements
of a RAP, gives an example of one, and discusses the
procedures for submitting and reviewing a RAP.
Action Leakage Rate (ALR)
EPA has historically used the term de minimus
leakage when referring to leaks resulting from
permeation of an intact FML. Action leakage rate
(ALR) was developed to distinguish leak rates due to
holes from mere permeation of an intact FML, and to
initiate early interaction between the owner/oper-
ator of the unit and the Agency. The ALR essentially
defines top liner leakage in a landfill, and the
proposed value is based on calculated leak rates
through a 1 to 2 mm hole in a FML subject to low
hydraulic heads on the order of 1 inch. The proposed
ALR, therefore, is representative of well-designed
and operated landfills, although, as proposed, it
would also apply to surface impoundments and waste
piles.
Because EPA is considering setting a single ALR
value applicable to landfills, surface impoundments,
and waste piles, the Agency calculated top liner leak
rates for different sizes of holes and for different
hydraulic heads. In addition, EPA compared leak
rates for a FML top liner with that for a composite
top liner, since many new facilities have double
composite liner systems. Table 10-1 shows the
results of these calculations for FML and composite
top liners. Even for FMLs with very small holes (i.e.,
1 to 2 mm in diameter), leak rates can be significant
depending on the hydraulic head acting on the top
liner. The addition of the compacted low
permeability soil layer to the FML significantly
reduces these leak rates to less than 10 gal/acre/day,
even for large hydraulic heads that are common in
surface impoundments. These results indicate that,
121
-------
at least for deep surface impoundments with large
hydraulic heads, double composite liner systems may
be the key to reducing the leak rates to de minimus
levels that are below the proposed ALR.
Table 10-1. Calculated Leakage Rates through FML and
Composite Liners (gal/acre/day)
FML Alone
Hydraulic Head, ft
Leakage Mechanism
Small Hole (1-2 mm)
Standard Hole (1 cm2)
0.1
30
300
1
100
1,000
10
300
3,000
Composite Liner (good contact)
Hydraulic Head, ft
Leakage Mechanism
0.1
1
10
Small Hole (1-2 mm) 0.01
Standard Hole (1 cm2) 0.01
0.1
0.2
Source: U.S. EPA. 1987. Background document on proposed liner
and leak detection rule. EPA/530-SW-87-015.
EPA's proposed rule sets the ALR at 5 to 20
gal/acre/day, a difficult range to achieve with a
primary FML alone (especially for surface impound-
ments). The proposed rule also enables the
owner/operator to use a site-specific ALR value that
would take into account meteorological and
hydrogeological factors, as well as design factors that
might result in leak rates that would frequently
exceed the ALR value. Using these factors, a surface
impoundment that meets the minimum
technological requirements of a FML top liner could
conceivably apply for a site-specific ALR value.
Daily leakage rates through top liners can vary by 10
to 20 percent or more, even in the absence of major
precipitation events. Because of these variations,
EPA may allow the landfill owner/operator to
average daily readings over a 30-day period, as long
as the leakage rate does not exceed 50 gal/acre/day
on any 1 day. If the average daily leak rate does not
exceed the ALR, then the owner/operator does not
have to implement a RAP.
Rapid and Large Leakage (RLL)
The Rapid and Large Leakage (RLL) rate is the high-
level trigger that indicates a serious malfunction of
system components in the double-lined unit and that
warrants immediate action. In developing the
proposed rule, EPA defined the RLL as the
maximum design leakage rate that the leak
detection system can accept. In other words, the RLL
is exceeded when the fluid head is greater than the
thickness of the secondary leachate collection and
removal system (LCRS) drainage layer. The visible
expression of RLL leakage in surface impoundments
is the creation of bubbles, or "whales," as the FML is
lifted up under the fluid pressure. See Chapter Three
for further discussion of "whales".
Because the RLL is highly dependent on the design
of the leak detection system, EPA's proposed rule
requires that owners/operators calculate their own
site-specific RLL values. EPA also proposes to
require that owners/operators submit a RAP for
leakage rates exceeding that value prior to
beginning operation of a unit. The EPA Regional
Administrator must approve the RAP before a
facility can receive wastes.
The following equations represent EPA's
preliminary attempt to define a range of potential
RLL values for a hypothetical leak detection system,
which consists of a 1-foot granular drainage layer
with 1 cm/sec hydraulic conductivity. These
calculations are for two-dimensional rather than
three-dimensional flow. In addition, the equations
apply to flow from a single defect in the FML, rather
than multiple defects. Therefore, results from this
analysis are only preliminary ones, and the EPA will
develop guidance on calculating RLL values in the
near future.
RLL values can be calculated using the following
equation:
where:
h =
h
Qd
B
kd
10) (I)'
hydraulic head
flow rate entering into the
drainage layer
width of the drainage layer
hydraulic conductivity of the
drainage layer
slope of the drainage layer
perpendicular to, and in the plane
of, flow toward the collection pipe
When the value for h exceeds the thickness of the
drainage layer (1 foot in this example), the leakage
rate is greater than the RLL value for the unit.
In reality, a leak from an isolated source, i.e., a tear
or a hole in the FML, results in a discreet zone of
saturation as the liquids flow toward the collection
pipe (see Figure 10-1). The appropriate variable
representing the width of flow, then, is not really B,
the entire width of the drainage layer perpendicular
to flow, but b, the width of saturated flow pe'rpen-
dicular to the flow direction. If b were known, the
equation could be solved. But to date, the data has
122
-------
not been available to quantify b for all drainage
layers and leakage scenarios.
Leak
acre, or in units of m2; N = l/4,000m2. Substituting
this value into Equation 3:
h = 4000q/(bkdtan|3)
(4)
Where q is in units of liters/1,000 m2/day (Ltd),
Equation 4 can be written as follows:
h = 4.6 x 10-8q/(bkdtanj3)
(5)
The proposed rule requires leak detection systems to
have a minimum bottom slope of 2 percent (tang) and
minimum hydraulic conductivity of 10-2 m/sec (kd).
Substituting these values into Equation 5:
h = 2.3 x 10-4 q/b
(6)
• Leak
Flow
Direction
\ 7
High Edge
(Upgradient)
Lower Edge
(Downgradient)
Collector Pipe —'
Cross Section A - A1
Figure 10-1. Plan view of a leak detection system with a large
leak flowing over a width b.
From Equation 1, one can make substitutions for
variables B and Qd and give values for the other
variables kd and tang. If N represents the frequency
of leaks in a well-designed and installed unit, then Q,
the flow rate in the drainage layer (m3/s) is directly
related to q, the leakage rate per unit area (m/sec):
Q = NQ or Q = q/N
(2)
Combining Equations 1 and 2 and substituting b for
B, and q for Q:
h = q/(Nbkdtanp-)
(3)
Equation 3 now can be used to define the leakage
rate (q) that exceeds the leak detection system
capacity. All that is needed are the values for the
other variables (N, kd, tang). For a well-designed and
installed unit, the frequency of leaks (N) is 1 hole per
where h is in units of m, q is in units of Ltd, and b is
in units of m. For the purposes of these calculations,
it is assumed that Ltd is equivalent to about 1
gal/acre/day. The final results were derived by using
three different values for b (the unknown variable)
and determining what values of q between 100 and
10,000 gal/acre/day (Ltd) result in hydraulic heads
exceeding the 1-foot thickness of the drainage layer
(h).
Table 10-2 shows the results of these preliminary
calculations. For values of q between 100 and 10,000
gal/acre/day and values of b between 3 and 6 foot, the
hydraulic head exceeds 1 foot when leak rates are in
the range of 2,000 to 10,000 gal/acre/day. Therefore,
RLL values for leak detection systems consisting of
granular drainage layer are expected to be in the
range of 2,000 to 10,000 gal/acre/day. Clogging of the
drainage layer would decrease the design capacity of
the leak detection system, and hence the RLL value,
over time. With respect to the variables described
above, clogging of the drainage layer could be
represented using smaller values for b, the width of
saturated flow, since clogging would result in a
reduced width of saturated flow. As shown in Table
10-2, smaller values of b reduce the minimum
leakage rate, q, needed to generate heads exceeding
the 1-foot thickness. EPA plans to issue guidance on
estimating the effect of clogging on RLL values.
Table 10-2. Results of Preliminary Studies Defining Ranges
of RLL Values
Width (b)
ft
Flow (q)
gal/acre/day
3.3
5.0
6.6
1,000 - 2,000
2,000 - 5,000
5,000 - 10,000
123
-------
Response Action Plans (RAPs)
According to the proposed leak detection rule, the
key elements of a RAP are:
• General description of unit.
• Description of waste constituents.
• Description of all events that may cause leakage.
• Discussion of factors affecting amounts of leakage
entering LCRS.
• Design and operational mechanisms to prevent
leakage of hazardous constituents.
• Assessment of effectiveness of possible response
actions.
In developing a RAP, owners/operators of landfills
should gather information from Part B of the permit
application, available operational records, leachate
analysis results for existing facilities, and the
construction quality assurance report. The
construction quality assurance report is very
important because it helps define where potential
leaks are likely to occur in the unit.
Sources of Liquids Other than Leachate
Depending on the unit design and location, other
liquids besides leachate could accumulate in the leak
detection system and result in apparent leak rates
that exceed the ALR value. For example,
precipitation may pass through a tear in the FML
that is located above the waste elevation (e.g. a tear
in the FML at a pipe penetration point). The liquids
entering the leak detection system under this
scenario may not have contacted any wastes and
hence would not be considered to-be hazardous
leachate. In addition, rainwater can become trapped
in the drainage layer during construction and
installation of the leak detection system, but these
construction waters are typically flushed through
the system early on in the active life of the facility. In
the case of a composite top liner, moisture from the
compacted soil component may be squeezed but over
time and also contribute to liquids collected in the
leak detection sump. These sources of nonhazardous
liquids can add significant quantities of liquids to a
leak detection system and might result in an ALR
being exceeded. Therefore, these other sources of
liquids should also be considered when developing a
RAP, and steps to verify that certain liquids are not
hazardous should be outlined in the plan.
Ground-water permeation is one other possible
source of nonhazardous liquids in the leak detection
system that can occur when the water table elevation
is above the bottom of the unit. The ability of ground
water to enter the leak detection sump, however,
raises serious questions about the integrity of the
bottom liner, which is the backup system in a double-
lined unit. If ground water is being collected in the
leak detection system, then hazardous constituents
could conceivably migrate out of the landfill and into
the environment when the water table elevation
drops below the bottom of the unit, e.g., in the case of
dry weather conditions. As a result, while ground-
water permeation is another source of liquids, it is
not a source that would ordinarily be used by the
owner/operator to justify ALR exceedances.
Preparing and Submitting the RAP
Response action plans must be developed for two
basic ranges: (1) leakage rates that exceed the RLL
and (2) leakage rates that equal or exceed the ALR
but are less than the RLL. In submitting a RAP, a
facility owner/operator has two choices. First, the
owner/operator can submit a plan to EPA before the,
facility opens that describes all measures to be taken
for "every possible leakage scenario. The major
drawback to this option is that the RAP may have to
be modified as specific leak incidents occur, because
there are several variables that affect the selection of
suitable response actions. One variable is the time at
which the leak occurs. For example, if a leak is
discovered at the beginning of operation, the best
response might be to locate and repair the leak, since
there would be little waste in the unit and the tear or
hole may be easy to fix. If, however, a leak is
discovered 6 months before a facility is scheduled to
close, it would probably make sense to close the unit
immediately to minimize infiltration. If the
owner/operator chooses to develop and submit one
RAP before the unit begins operation, he or she must
develop suitable response actions for different leak
rates and for different stages during the active life
and post-closure care period of the unit.
The second choice an owner/operator has is to submit
the RAP in two phases: one RAP for the first range,
serious RLL leakage, that would be submitted before
the start of operation; and another for the second
range of leakage rates (exceeding the ALR but less
than the RLL) that would be submitted after a leak
has been detected.
EPA developed three generic types of response
actions that the owner/operator must consider when
developing a RAP for leakage rates greater than or
equal to the RLL. The three responses for very
serious leakage are straightforward:
• Stop receiving waste and close the unit, or close
part of the unit.
• Repair the leak or retrofit the top liner.
• Institute operational changes.
These three response actions also would apply to
leakage rates less than RLL, although, as moderate
to serious responses, they would apply to leakage
124
-------
rates in the moderate to serious range, i.e., 500 to
2,000 gal/acre/day. For most landfills, 500
gal/acre/day leak rates would be considered fairly
serious, even though they may not exceed the RLL..
In addition, clogging of the leak detection system
could also result in serious leakage scenarios at rates
less than 2,000 gal/acre/day. For lower leak rates just
above the ALR, the best response would be promptly
to increase the liquids removal frequency to
minimize head 'on the bottom liner, analyze the
liquids, and follow up with progress reports.
Another key step in developing RAPs is to set up
leakage bands, with each band representing a
specific range of leakage rates that requires a
specific response or set of responses. Table 10-3
shows an example of a RAP developed for three
specific leakage bands. The number and range of
leakage bands should be site-specific and take into
account the type of unit (i.e., surface impoundment,
landfill, waste pile), unit design, and operational
factors.
Table 10-3. Sample RAP for Leakage < RLL
ALR = 20 gal/acre/day and RLL = 2,500 gal/acre/day
Leakage Band
(gal/acre/day)
20
20-250
Generic Response Action
Notify RA and identify sources of liquids.
Increase pumping and analyze liquids in
250-2,500
sump.
Implement operational changes.
The RAP submittal requirements proposed by EPA
differ for permitted facilities and interim status
facilities. For newly permitted facilities, the RAP for
RLL must be submitted along with Part B of the
permit application. For existing facilities, the RAP
for RLL must be submitted as a request for permit
modification. Facilities in interim status must
submit RAPs for RLL 120 days prior to the receipt of
waste.
If the RAP for low to moderate leakage (greater than
ALR but less than RLL) has not been submitted
before operation, EPA has proposed that it must be
submitted within 90 days of detecting a leak. In any
case, the EPA Regional Administrator's approval
would be required before that RAP can be
implemented.
Requirements for Reporting a Leak
Once a leak has been detected, the proposed
procedure is similar for both ALR and RLL leakage
scenarios. The owner/operator would need to notify
the EPA Regional Administrator in writing within 7
days of the date the ALR or RLL was determined to
be exceeded. The RAP should be implemented if it
has been approved (as in the case for RLL leaks), or
submitted within 90 days for approval if not already
submitted. Regardless of whether the RAP for the
leak incident is approved, the owner/operator would
be required to collect and remove liquids from the
leak detection sump. Examples of the liquids should
be analyzed for leachate quality parameters, as
specified by the Regional Administrator in an
approved RAP. Both the need for analysis and the
parameters would be determined by the Regional
Administrator.
In addition to the leachate sampling, the EPA
Regional Administrator would also specify a
schedule for follow-up reporting, once the ALR or
RLL is exceeded. According to the proposed rule, this
follow-up reporting will include a discussion of the
response actions taken and the change in leak rates
over time. The first progress report would be
submitted within 60 days of RAP implementation,
arid then periodically or annually, thereafter, as
specified in an approved RAP. Additional reporting
would also be required within 45 days of detecting a
significant increase in the leak rate (an amount
specified in the RAP). This significant increase in
leak rate indicates a failure in the response actions
taken and, therefore, may require modifications of
the RAP and the implementation of other response
actions. These additional reporting and monitoring
requirements would be part of the RAP
implementation to be completed only when the
resulting leak rate drops below the ALR.
Summary
Although the overall containment system consisting
of two liners and two LCRSs may achieve the
performance objective of preventing hazardous
constituent migration out of the unit for a period of
about 30 to 50 years, the individual components may
at some point malfunction. Liners may leak or
LCRS/leak detection systems may clog during the
active life or post-closure care period. Therefore,
EPA has developed and proposed requirements for
early response actions to be taken upon detecting a
malfunction of the top liner or leak detection system.
These requirements, once finalized, will ensure
maximum protection of human health and the
environment.
125
-------
-------
Abbreviations
ALR = Action Leakage Rate
ASTM = American Society for Testing and Materials
BOD = biological oxygen demand
°C = degrees Centigrade
cm/sec = centimeters per second
CPE = chlorinated polyethylene
CQA = Construction Quality Assurance
CSPE = chlorylsulfonated polyethylene
D • = dielectric constant
DR = design ratio
DSC = differential scanning calorimetry
DT .= destruct tests
EPA = U.S. Environmental Protection Agency
EPDM = ethylene propylene diene monomer
°F = degrees Fahrenheit
FLEX = Flexible Liner Evaluation Expert
FMC = flexible membrane caps
FML = flexible membrane liner
FS = factor of safety
ft = feet
ft2 = square foot
ft3 = cubic foot ]
gal/acre/day = gallon per acre per day
HOPE = high density polyethylene
HELP = Hydrologic Evaluation Landfill Performance Model
HSWA = Hazardous and Solid Waste Amendment
LCRS = leachate collection and removal system
LDCR = leak detection, collection, and removal
LLDPE = linear low density polyethylene
Ltd = liters/1,000 m2/day
m2 = square meters
m2/sec = square meters per second
min = minute
mm = millimeters
MTG = minimum technology guidance
MTR = minimum technological requirements
NOT = nondestruct tests
nm = nanometer
oz/yd2 = ounces per square yard
PA = polyamide
POA = percent open space •
PE = polyethylene
PE = professional engineer
PET = polyester
ph = hydrogen ion concentration
PI = plasticity index
PLCR = primary leachate collection and removal
PP = polypropylene
psf = pounds per square foot
psi = pounds per square inch
PVC = polyvinyl chloride
RAP = Response Action Plans
RCRA = Resource Conservation Recovery Act
RLL = Rapid and Large Leakage Rate
SLCR = secondary leachate collection and removal
SWCR = surface water collection and removal
TGA = thermogravimetric analysis
TOT = time of travel
USDA = U.S. Department of Agriculture
127
*US GOVERNMENT PRINTING OFFICE 1993-750-002/ 80286
-------
-------
-------
-------
-------
&EPA
------- |