Geosynthetic Clay Liners (GCLs) in Landfill Covers

Preprint of Paper Presented at The Thirty-First Annua! Solid Waste Exposition, The .'
San Jose, California, August 2-5, 1993.
EPA/600/A-93/283
Geosynthetic Clay Liners (GCLs) In Landfill Covers
by David E. Daniel
Professor of Civil Engineering
University of Texas
Austin, TX 78712
ABSTRACT
Low-permeability, compacted clay liners are commonly required as a barrier to water
infiltration in landfill covers. A relatively new material, known as geosynthetic clay liner (GCL),
has been proposed as an alternative to a compacted clay liner. A GCL has the practical
advantages of relatively low cost (approximately $0.50 to $0.60 per square foot for a landfill
cover, installed), rapid installation with light-weight equipment, and ease of repair. A GCL also
has several technical advantages, including greater tolerance for differential settlement and better
self-healing characteristics under wet-dry and freeze-thaw conditions. A potentially important
disadvantage of the GCL is that, because it is thin, it is more vulnerable to damage from puncture
than a compacted clay liner. However, compacted clay liners are not without their problems, too,
and designers, as well as regulators, of final landfill covers are encouraged weigh the advantages
and disadvantages of the various materials before reaching a decision about the best material to
use for a particular landfill.
Most regulatory agencies require that compacted clay, or the equivalent, be used as a
barrier to water infiltration in final covers. Typically, a 1- to 2-ft-thick layer of compacted clay
having a hydraulic conductivity (coefficient of permeability) < 1 x 10~7 cm/s is required. To
achieve regulatory approval, an applicant who proposes to use a GCL rather than a compacted
clay liner may be required to demonstrate that the GCL will perform in an equivalent manner to a
compacted clay liner. If the GCL can be shown to be equivalent in terms of meeting
performance objectives, a basis for regulatory approval is established.
The objectives of this paper are: (1) to provide an introduction to GCLs for those who
may be unfamiliar with this lining material; (2) to summarize the potential applications of GCLs
to landfill covers; (3) to examine the relative advantages and disadvantages of GCLs compared to
compacted clay liners; and (4) to provide a generic assessment of performance equivalency of
GCLs compared to low-permeability, compacted clay barriers. The fourth item will comprise the
bulk of the paper. The conclusion is drawn that geosynthetic clay liners can be shown to provide
equivalent performance to low-permeability, compacted clay liners for many landfill sites. The
key issues concerning equivalency are ability to limit per™,!,t^n fhrnnah the barrier,
permeability to gas, slope stability, and puncture resistance
INTRODUCTION TO GEOSYNTHETIC CLAY LINERS
The Material
Geosynthetic clay liners (GCLs) are thin "blankets" of bentonite clay attached to one or
more geosynthetic materials (e.g., geotextile or geomembrane). Bentonite is a unique clay
mineral with very high swelling potential and water absorption capacity. When wetted, bentonite
is the least permeable of all naturally-occurring, soil-like minerals. Bentonite is also a
chemically stable mineral that has undergone complete weathering and will last, in effect,
forever.

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Geosynthetic clay liners are manufactured by laying down a layer of dry bentonite,
approximately 1/4-inch thick, on a geosynthetic material and attaching the bentonite to the
geosynthetic material. Two general configurations are currently employed in commercial
processes: bentonite sandwiched between two geotextiles (Fig. la) or bentonite glued to a
geomembrane (Fig. lb). The primary purpose of the geosynthetic component or components is
to hold the bentonite together in a uniform layer and permit transportation and installation of the
material without losing bentonite or altering the thickness of the bentonite. However, the
geosynthetic components may serve other important purposes, as well, such as adding tensile or
shear strength to the material.
(A) Bentonite Sandwiched Between Two Geotextiles

Geotextile

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benfoni component of a manufactured GCL is essentially dry, and there are open
vr:es vn 1 >niic granules in the manufactured material. When the bentonite is hydrated
¦ water (for unnpiC, by imbibing water from underlying or overlying soils), the bentonite
:¦;iIs and the. voids between bentonite granules close. The swelling action of bentonite is
crucial to attainment, of low permeability.
Geosynthetic clay liners contain approximately 1 pound per square foot of high-quality
sodium bentonite that has a hydraulic conductivity (coefficient of permeability) of approximately
1 x 109 cm/s or less. Continuous gravity percolation under unit hydraulic gradient through a
material with a hydraulic conductivity of 1 x 10"9 cm/s would result in an infiltration rate of 0.01
inches per year, or approximately 1 inch every 100 years. For landfill covers, an intact GCL may
be considered essentially impermeable to water.
Geosynthetic clay liners were first manufactured in the early 1980's and were initially
used for foundation water proofing and for sealing water retention structures. Geosynthetic clay
liners were first used for landfill liners in 1986. Since 1986, geosynthetic clay liners have been
used for a variety of lining applications and also in several final cover systems for hazardous
wastes, radioactive wastes, and non-hazardous solid wastes.
Commercial Products
Four geosynthetic clay liners are currently manufactured: Bentofix®, Bentomat®,
Claymax®, and Gundseal®. The GCLs fall into the broad categories shown in Fig. 1 as follows:
• Bentonite sandwiched between two geotextiles: Bentofix®, Bentomat®, and Claymax®
• Bentonite mixed with an adhesive and glued to a geomembrane: Gundseal®.
The GCLs are sketched in Fig. 2. Bentofix® and Bentomat® consist of bentonite
sandwiched between a woven and non-woven geotextile that are needle-punched together.
Claymax® 200R consists of bentonite mixed with glue and sandwiched between two woven
geotextiles. Claymax® 500SP consists of bentonite mixed with glue and sandwiched between
two woven geotextiles that are sewn together. The purpose of stitching the two geotextiles
together is to provide additional internal reinforcement and greater shear strength. With all the
geotextile-encased GCLs, special geotextiles can be selected to "custom design" the GCL to a
particular application. Gundseal® is made by mixing bentonite with an adhesive and attaching
the bentonite layer to a polyethylene geomembrane. Gundseal® can be supplied with high
density polyethylene (HDPE) or very low density polyethylene (VLDPE), and the geomembrane
can be either smooth or textured.
All GCLs are manufactured in panels with widths of approximately 13 to 17 ft and
lengths of approximately 75 to 200 ft. The panels are placed on rolls at the factory and are
unrolled at the time of installation. The weight of the roll varies, depending on size and
materials, from about 1,400 to 4,000 pounds.
The panels are typically overlapped 3 to 12 in. during installation and are said to be "self
sealing" at the overlap. A sketch of the overlapped zones is shown in Fig. 3. With geotextile-
encased, needle-punched GCLs, sodium bentonite is placed along the overlap (Fig. 3a) at a rate
of approximately 0.25 lb/ft. The bentonite penetrates the pores of the geotextiles and is said by
the manufacturers to cause the materials to self seam when the bentonite hydrates. With
geotextile-encased, adhesive-bonded GCLs, no additional bentonite is needed (Fig. 3b). The
material is said to self seal upon hydration at the overlaps through expansion and "oozing" of
bentonite out through the openings of the geotextile in the overlap area.
3
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Woven Geotextile
Bentofix® and Bentomat®
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4
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A. Geotextile-Encased, Needle-Punched GCLs
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Figure 3. Overlapped Zone of Geosynthetic Clay Liners.
5
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With GCLs containing a geomembrane, the GCL can be placed with the bentonite facing
upward (Fig. 2) or, as shown in Fig. 3c and 3d, downward. If the GCL will be used by itself as a
composite geomembrane-clay liner, the geomembrane would face upward. If a separate
geomembrane is to be placed on the GCL, the bentonite would face upward. The material is said
to be self sealing at overlaps with no need for any mechanical seam at the overlap (Fig. 3c).
However, if one wants to form a continuous geomembrane out of the geomembrane component
of the GCL, a cap strip can be welded over the overlap (Fig. 3d).
Potential Uses of Geosvnthetic Clav Liners in Final Cover Systems
Geosynthetic clay liners can be used in final cover systems in several ways, as shown in
Fig. 4. One choice (Fig. 4a) is to use the GCL by itself as a barrier to water infiltration. The
GCL would be buried below a layer of protective soil. As indicated earlier, the bentonite
component is expected to be essentially impermeable to water after it is has been hydrated,
assuming that the GCL withstands the potentially damaging effects of wet-dry cycles and
differential settlement (discussed later). One possible problem with using a GCL by itself as a
barrier layer is that the dry bentonite is initially highly permeable to landfill gas — the bentonite
would have to absorb water, hydrate, and swell before the bentonite becomes an effective barrier
to gas migration, and the bentonite could not be allowed to dry out because the bentonite would
again become permeable to landfill gas. At extremely arid sites, there may not be adequate water
available to hydrate the bentonite to the extent that is necessary in order for the GCL to have a
low permeability to gas. However, for those GCLs that contain a geomembrane, the
geomembrane itself provides a barrier to gas migration. In addition, a barrier to gas migration
within the final cover may or may not be a design consideration, depending on site-specific
considerations.
The second potential use of a geosynthetic clay liner in a final cover system is in
conjunction with a geomembrane (Fig. 4b) to form a composite geomembrane/GCL liner. The
composite could either be formed by using a GCL that contains a geomembrane or by separately
constructing a geomembrane on top of a GCL. By placing clay under the geomembrane, the clay
serves to seal off any imperfections in the geomembrane, e.g., pinholes or defects in seams, and
to help in providing an extremely effective composite barrier to infiltration of water. The
geomembrane would protect the underlying GCL from wet-dry cycles and would serve as a gas
barrier for those periods when the bentonite component of the GCL is relatively dry. The main
advantages of a separately-constructed geomembrane are that a separate polyethylene
geomembrane liner could be seamed with the most advanced welding equipment available,
which is microprocessor-controlled, dual-track, hot wedge welding equipment, or that some other
type of geomembrane besides polyethylene could be used, if desired. If a bentonite-polyethylene
composite GCL is used and the polyethylene components are to be seamed at overlaps, a cap
strip is typically placed over the overlapped region and the edges of the cap strip are welded with
fillet extrusion welding apparatus (Fig. 3d). However, because water flow through non-welded
seams is expected to be negligible, the author encourages designers not to use cap strips over
overlapped panels unless there is a good reason to do so.
A third option is to sandwich the GCL between two geomembranes (Fig. 4c). One or
both geomembranes would be separately installed, depending upon the GCL material employed.
The advantage of this design is that even less percolation of water through the barrier would
occur. In fact, the bentonite component would become wetted only around minor imperfections
in a geomembrane or its seams, where the bentonite would serve to seal off the leakage through
the imperfection. This type of design approach, with a triple-composite liner, has rarely (if ever)
been used for final covers over solid waste landfills and would be considered an extreme design
for those facilities requiring extraordinary protection from water percolation or gas migration
through the final cover.
6
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(A)
GCL used for Barrier Layer by Itself

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(B) GCL Used with Geomembrane
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7
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A fourth option is to place the GCL on top of a low-permeability, compacted soil liner
(Fig. 4d), possibly with a geomembrane placed on top of the GCL (Fig. 4d). This design adds
redundancy of materials and enables one to provide a very high degree of protection in the final
cover system. In such cases, the GCL may replace part of a conventional compacted clay liner,
or the low-permeability soil component may have a hydraulic conductivity that is greater than the
usual 1 x 10~7 cm/s (i.e., use of the GCL lessens the need for extremely low permeability in the
underlying soil barrier layer).
A fifth option is to place the GCL on top of a low-permeability, re-used waste material
(Fig. 4e), possibly with a geomembrane placed on top of the GCL (Fig. 4e). This design adds
redundancy of materials and enables one to make productive use of waste materials. An example
of a waste material that might be considered is paper industry sludges (Maltby and Eppstein,
1993).
ENGINEERING PROPERTIES OF GCLs
Hydraulic Conductivity
In general, the hydraulic conductivity of the bentonite component of GCLs varies
between about 1 x 10"10 and 1 x 10"8 cm/s, depending on the confining stress. The higher the
compressive stress, the lower the hydraulic conductivity. There are some differences between
the hydraulic conductivities of the various GCLs, but, except for bentonite-geomembrane
composite GCLs (for which the geomembrane will significantly reduce the overall hydraulic
conductivity), the differences do not appear to be very large. The available data are summarized
by Schubert (1987), Daniel and Estornell (1990), Scheu et al. (1990), Daniel (1991), Eith et al.
(1991), Shan and Daniel (1991), Estornell and Daniel (1992), Grube (1992), Daniel et al. (1993),
and Daniel and Boardman (1993).
For a final cover system, a confining stress on the order of 200 psf to 600 psf is a
reasonable range. Laboratory hydraulic conductivity tests performed on backpressure-saturated
test specimens in flexible-wall permeameters indicate that the hydraulic conductivity of the
bentonite component of GCLs in this range of compressive stress is approximately 1 to 4 x 10"9
cm/s. Estornell and Daniel (1992) measured the hydraulic conductivity of GCLs in large tanks.
The tests were specifically set up to simulate conditions of low overburden stress that are typical
of final cover systems and to test very large specimens with overlaps. Of the 10 tests for which
hydraulic conductivities were measured, the average value was 4.6 x 10-9 cm/s (normal
averaging) or 2.2 x 109 (logarithmic averaging). Based on all the data, a reasonable assumption
is that a GCL can be supplied with a hydraulic conductivity for a landfill cover application less
than 1 to 5 x 10 9 cm/s.
Studies of the hydraulic properties of overlapped seams performed by Estornell and
Daniel (1992) indicate that the overlapped seams in GCLs self seam in the manner described by
the manufacturers. For geotextile-encased, needle-punched GCLs with additional bentonite
along the overlap, the bentonite appears to swell upon hydration and plug voids in the geotextiles
present in the overlap. For the geotextile-encased, adhesive-bonded GCLs that have been tested,
the bentonite within the GCL appears to ooze out through the openings in the geotextile and to
allow the material to self seal. For bentonite-geomembrane composite GCLs, the bentonite
swells upon hydration, seals at the bentonite-polyethylene interface, and effects self-seaming at
the overlap. TTius, based on the available data, it is reasonable to assume that with proper quality
control in the field, seams can be installed that will self-seal.
8
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Strength
Internal Shear Strength. The internal shear strength of GCLs has been determined by the
manufacturers and various organizations and testing laboratories. "Internal shear strength" refers
to the strength of the material when sheared through the mid-plane of the bentonite. The author
and his students at the University of Texas have performed independent tests, which are
described below.
Direct shear tests were performed on square specimens that measured approximately 2.5
in. in length and width. Test specimens were cut from parent material, set up in a direct shear
apparatus, and subjected to the desired normal load. For tests on water-saturated specimens, the
specimens were then soaked with water and allowed to equilibrate; about 3 weeks were required
before swelling ceased. Test specimens were sheared very slowly with failure occurring in 3 to 7
days. Results on water-saturated GCLs are summarized in Figure 5.
• Bentomat j r
¦ Claymax j _J L
A Gundseal j | i..
Normal Stress (psi)
Figure 5. Results of Direct Shear Tests on Fully Hydrated GCLs.
9
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The failure envelopes shown in this figure were determined from linear regression
analysis, which yielded the following results:
Geosvnthetic Clav Liner Effective Cohesion fpsi') Angle of Internal Friction (Degrees')
Bentomat® 4.4 29
Claymax® 0.6 9
Gundseal® 1.2 8
The reader is reminded that these results are for completely water-saturated bentonite — if the
bentonite is encased between two geomembranes, it is unlikely that the bentonite will become
saturated throughout.
Careful examination of the low-normal-stress region shows that the failure envelope is
distinctly curved. This curvature is significant because it means that the materials are stronger at
low compressive stresses (such as experienced in final covers) than other situations. In studies
recently completed at the University of Texas, tilt-table tests were performed. Samples of GCL
materials that measured 12 in. by 12 in. were set up on a tilt table, loaded with a steel plate,
placed in a water bath, and allowed to fully hydrate. Then the table was slowly tilted over a
period of several weeks until sliding occurred. The tilt table and direct shear data for one GCL
(Gundseal®) are shown in Fig. 6. The failure envelope is obviously curved. Figure 7 presents
the relationship between angle of internal friction and normal stress. For landfill covers, a
typical range of normal stress is approximately 200 to 600 psf. Although the data are presented
for only one GCL, similar trends are expected for other GCLs. Designers should exercise care in
evaluation of shear strength data to ensure that the proper parameters for the conditions expected
in the field are utilized in design.
Dry bentonite is much stronger than water-saturated bentonite. For dry GCLs or slightly
damp GCLs, the angle of internal friction (even for the materials that are not internally
reinforced) is approximately 35°. It is only if the material is hydrated that bentonite becomes
weaker.
For those GCLs that are needle-punched or sewn together, the internal reinforcement of
the GCL makes the material's internal shear strength much less sensitive to the strength of the
bentonite contained between the attached geotextiles. However, the reader is cautioned that for
landfill covers, the GCL may be exposed to prolonged shearing stresses for periods of years,
decades, or even centuries, and that the long-term shearing resistance should be carefully
considered.
Interfacial Shear Strength. "Interfacial shear strength" refers to the shearing strength
between two adjacent components of a liner or cover system. The GCL may be placed against
soil, a geomembrane, or a geotextile. Because the range of possible materials at an interface is
unlimited, the actual interfacial shearing properties are usually determined on a project-specific
basis. It is the author's experience that the internal shear strength will often govern the design
because, with proper selection of materials, relatively high interfacial strengths can usually be
obtained.
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• Direct Shear
¦ Tilt Table
Normal Stress (psi)
Figure 6. Failure Envelope for One Water-Saturated GCL Including Results of Tilt Table Tests.
Normal Stress (psf)
Figure 7. Influence of Normal Stress on Internal Shear Strength of One Water-Saturated GCL.
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Tensile Strength
The tensile strength of a GCL is derived almost exclusively from the tensile strength of
the geosynthetic components. For those GCLs that are constructed from unmodified
geosynthetics (i.e., no needle-punching or other alteration of the parent geosynthetic material),
the tensile strength of the GCL may be taken as the tensile strength of the geosynthetic
components. For those GCLs whose geosynthetic components have been altered during the
manufacturing of the GCL (i.e., needle-punched or sewn GCLs), tensile strength can be
measured by performing a wide-width tensile test on the GCL material itself. Data on tensile
properties of GCLs is available from the manufacturers.
Durability
Puncture Resistance. Shan and Daniel (1991) studied the effects of punctures on a
geotextile-encased, adhesive-bonded GCL. The manufacturers of other GCL products have
developed similar data for their particular products. The effects of punctures on the hydraulic
conductivity of the GCL were studied by drilling or cutting circular holes into the dry GCL,
setting the punctured GCL up in flexible-wall permeameters, and permeating the GCL slowly
until steady flow was achieved. Results are summarized in the following table:
Diameter of Puncture Hydraulic Conductivity (cm/si
No Punctures
2 x 10-9
0.5 in.
3 x 10-9
1 in.
5 x 10"9
3 in.
> 1 x 10-*
Small (< 1 in. diameter) punctures made in the dry material self-sealed upon hydration of
the bentonite. These tests illustrate the self-healing capability of bentonite. Each particular GCL
has a different capacity to self-heal punctures. However, all GCLs are capable of self healing
small punctures in the dry GCL when the bentonite is hydrated. It should be emphasized that
these tests were performed under carefully controlled conditions in which no material other than
bentonite was allowed to fill the puncture. In the field, other materials may fill large punctures.
Although GCLs have some capability to self-seal if punctured, there are clearly limitations in the
size of puncture that could self seal in the field.
Desiccation. Concern has been expressed that the bentonite component of a GCL may
swell when hydrated but may later dry out, shrink, crack, and lose its impermeability. Shan and
Daniel (1991) investigated the healing capability of one geotextile-encased, adhesive-bonded
GCL that was subject to wet-dry cycles. Samples of the GCL were permeated in a flexible-wall
permeameter, removed from the permeameter, and allowed to air dry with a small vertical stress
applied to the specimens. All specimens exhibited severe cracking upon drying. The specimens
were then set back up in a flexible-wall permeameter, slowly rehydrated, and then repermeated.
There was no change in hydraulic conductivity from the initial value of 2 x 10"9 cm/s, even after
three wet/dry cycles. These tests reinforce the fully reversible shrink/swell nature of bentonite
and suggest that any desiccation cracks will self-heal when the bentonite is hydrated.
12
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In research recently completed at the University of Texas (Boardman, 1993), large
samples of GCLs (with and without overlaps) were buried under 2 ft of gravel and subjected to a
wet-dry cycle that simulates severe conditions that might occur in a final cover for a landfill.
The GCLs were set up in the tanks, hydrated with water until a steady hydraulic conductivity was
measured, and then severely desiccated by draining away the water on top of the GCL and
circulating heated air into the gravel that was placed over the GCL. The heated air caused severe
desiccation cracking in the GCLs. However, when the GCLs were rehydrated, the bentonite
quickly swelled and the hydraulic conductivity eventually returned to the original, extremely low
value. Thus, it appears from the available data that GCLs have an excellent capacity to self seal
from desiccation-induced cracking. Geosynthetic clay liners probably possess much greater
ability to self seal than conventional compacted clay liners.
Freeze/Thaw. Compacted clay liners are known to be vulnerable to damage from
freezing. When water in soil freezes, the water expands, and when the water thaws, the water
contracts. This expansion and contraction causes small cracks to appear in the soil and causes
other alterations in the soil structure that tend to increase hydraulic conductivity.
Shan and Daniel (1991) subjected a geotextile-encased, adhesive-bonded GCL to
freeze/thaw. A test specimen was set up in a flexible-wall permeameter, hydrated with water,
and permeated until a steady hydraulic conductivity was obtained. Then the specimen was
removed from the flexible-wall permeameter and subjected to five freeze/thaw cycles at constant
water content. The specimen was repermeated, and it was found that the hydraulic conductivity
did not change. Similar results have been obtained by commercial testing laboratories for other
GCL products. Available data indicate that the high shrink-swell capability of bentonite gives
bentonite the ability to self-heal if any alteration occurs from freeze/thaw cycles. Geosynthetic
clay liners appear to have a much better capacity to remain undamaged after freeze-thaw than
conventional compacted clay liners.
PERFORMANCE ASSESSMENT
Many regulatory agencies have traditionally required a low-permeability, compacted clay
liner (or the equivalent) as the primary hydraulic barrier within landfill covers. The thickness of
a compacted clay liner typically ranges from 1 to 2 ft (occasionally up to 3 to 4 ft), and the
maximum allowable hydraulic conductivity is typically 10"7 cm/s. If one wishes to substitute a
GCL for a compacted clay liner, one must usually demonstrate that the GCL will be equivalent in
terms of meeting performance.objectives. Neither federal nor state regulations mention the
criteria by which equivalency should be evaluated. At the present time equivalency must be
evaluated on a case-by-case basis using criteria that are not very well defined. The lack of
accepted criteria is perhaps the single greatest problem that the landfill designer and owner face
in seeking regulatory approval for substitution of a GCL for a compacted clay liner.
One should not really think of a geosynthetic clay liner as being equivalent to a
compacted clay liner. Indeed, a 1/4-in.-thick layer of bentonite could not possibly be equivalent
to a much thicker layer of compacted clay in all respects. The critical issue is whether
substitution of an alternative material such as a GCL for the more traditional compacted clay
liner in a landfill cover will meet or exceed the performance objectives of the compacted clay
liner. If the GCL will meet or exceed the performance objectives, then it should be considered
that equivalency has been established.
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Differences Between CCLs and GCLs
Some of the differences between compacted clay liners and geosynthetic clay liners are
listed in Table 1.
Table 1 - Differences Between GCLs and Compacted Clay Liners.
Characteristic
Geosynthetic Clay Liner
Compacted Clay Liner
Materials
Bentonite, Adhesives,
Geotextiles, and
Geomembranes
Native Soils or Blend of Soil
and Bentonite
Thickness
Approximately 1/2 inch
Typically 1 to 2 ft
Hydraulic Conductivity
< 1 to 5 x 10"9 cm/s
< 1 x 10"7 cm/s
Speed and Ease of
Construction
Rapid, Simple Installation
Slow, Complicated
Construction
Ease of Quality Assurance
(QA)
Relatively Simple, Straight-
Forward, Common-Sense
Procedures
Complex QA Procedures
Requiring Highly Skilled and
Knowledgeable People
Vulnerability to Damage
During Construction as a
Result of Desiccation
GCLs Are Essentially Dry;
GCLs Cannot Desiccate
during Construction
Compacted Clay Liners Are
Nearly Saturated; Can
Desiccate during Construction
Availability of Materials
Materials Easily Shipped to
Any Site
Suitable Materials Not
Available at All Sites
Cost
Typically $0.50 to $0.60 per
Square Foot for a Large Site
Highly Variable -- Estimated
Range: $0.50 to $5.00 per
Square Foot
Experience
Limited Due to Newness
Has Been Used for Many
Years
Some of the potentially important (depending upon specific application) relative
advantages of CCLs and GCLs may be summarized as follows:
• Key advantages of compacted clay liners (CCLs):
• Many regulatory agencies require CCLs -- use of another type of liner may
require time-consuming demonstration of equivalency to a CCL;
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A CCL is a logical choice if large quantities of suitable clay are available locally;
• The large thickness of CCLs makes them virtually puncture proof;
• The large thickness of CCLs and the fact that they are constructed of multiple
layers makes them relatively insensitive to small imperfections in any one layer;
• There is a long history of use of CCLs;
• Quality assurance procedures are reasonably well established for CCLs.
• Key advantages of geosynthetic clay liners (GCLs):
Small thickness of GCLs leads to low consumption of landfill space;
Construction of GCLs is rapid and simple;
• GCLs can be shipped to any location -- their use is not dependent upon local
availability of materials;
Heavy equipment is not needed to install a GCL, which is very helpful for final
covers underlain by compressible waste (where compaction with heavy equipment
is difficult);
• Installation of a GCL requires less vehicular traffic and less energy use than
placement and compaction of a CCL — this also leads to less air pollution with a
GCL;
Some inclement weather delays (e.g., freezing temperatures) that stop
construction of CCLs are not a problem with GCLs;
• Construction water is not needed with a GCL, which can be critical in arid areas
where water resources are scarce;
• Because a GCL is a manufactured material, a consistent and uniform material can
be produced;
• Because GCLs are manufactured materials, specialized performance properties
can be determined and need not be repeatedly re-determined;
• GCLs can accommodate large differential settlement;
• Quality assurance is simpler for a GCL compared to a CCL;
• GCLs are more easily repaired than CCLs;
• GCLs can probably better withstand freeze/thaw and wet/dry cycles than CCLs;
• GCLs are not vulnerable to desiccation damage during construction.
Criteria for Performance Assessment and Equivalency Analysis
Three broad issues may be addressed when one considers the equivalency of a GCL to a
CCL:
15
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1. Hydraulic issues;
2. Physical/mechanical issues;
3. Construction issues.
The specific technical issues that might have to be addressed for a particular site are listed in
Table 2. For completeness, the issues are identified for both bottom liners and final covers.
Only final covers are considered in the succeeding discussion.
Table 2 - Potential Equivalency Issues.

Possiblv Relevant for:
Category
Criterion for Evaluation
Liners
Covers
Hydraulic
Steady Flux of Water
X
X
Issues
Steady Solute Flux
X

Chemical Adsorption Capacity
X

Breakout Time:

-Water
X
X

-Solute
X

Production of Consolidation
X
X

Water

Permeability to Gas
X
X
Physical/
Freeze-Thaw
X1
X
Mechanical
Wet-Dry

X
Issues
Total Settlement
X2
X

Differential Settlement
X2
X

Slope Stability
X3
X

Erosion

Bearing Capacity
X
X
Construction
Puncture Resistance
X
X
Issues
Subgrade Condition
X
X

Ease of Placement
X
X

. Speed of Construction
X
X

Availability of Materials
X
X

Requirements for Water
X
X

Air Pollution Effects
X
X

Weather Constraints
X
X

Quality Assurance
X
X
Relevant only until liner is covered sufficiently to prevent freezing
•^Settlement of liners usually of concern only in certain circumstances, e.g., vertical expansions
3Stability of liner may not be relevant after filling, if no permanent slope remains
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Hydraulic Issues. Hydraulic issues are the easiest to quantify. The criteria, which are
discussed separately, include steady water flux, time to initiate release of water from the base of
the liner ("breakout time"), production of consolidation water, and air permeability.
1. Steady Flux of Water
Water flux is defined as the volume of flow across a unit area in a unit time. For a barrier
in a final cover system, water flux is equal to the rate of percolation of water through the barrier
layer.
Water flux is usually analyzed based on the long-term, steady state water flux. The flux
of water (v) through an individual layer of porous material is defined from Darcy's law as:
where k is the hydraulic conductivity, H is the depth of liquid ponded on the liner, and T is the
thickness of the liner. The water pressure on the base of the liner is assumed to be atmospheric
pressure in Eq. 1.
Equation 1 is applicable only for flow through the bentonite component of a GCL; if the
GCL contains a geomembrane, water flux will be controlled by water vapor diffusion through the
geomembrane component. The geomembrane component, if present, should be considered in the
equivalency analysis and in computation of water flux. The simplest way to do this is to adjust
the hydraulic conductivity of the GCL to reflect the presence of a geomembrane. (Note: such a
simplification does not mimic reality because water flows through a geomembrane via diffusion,
and Darcy's law is not applicable to diffusion. Nevertheless, as a matter of computational
convenience, one may make estimates of water flux by using appropriate values of equivalent
hydraulic conductivity.) Also, Eq. 1 applies to a CCL or GCL liner alone and not to composite
liners involving one or more separate geomembrane components. Composite action with a
geomembrane is considered later.
The flux ratio for water, Fw, is defined as the flux through the GCL divided by the flux
through the compacted clay liner (CCL):
Fw = vgcl/vccl (2)
If the flux ratio is < 1, then the GCL is equivalent to the CCL in terms of steady water flux. For
example, for a situation with H = 1 ft (0.3 m) and a GCL with:
(1)
or:
• _ kpcL Tccl H+TGcl
w kccL Tgcl H + 1'ccl
(3)
kccL = 1 x 10-9cm/s = lx l0-nm/s
Tgcl = 7 mm = 0.007 m
and a compacted clay liner (CCL) with:
kccL = 1 x 10"7 cm/s = 1 x 10-9 m/s
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Tccl = 2 ft = 0.6 m
then Fw from Eqs 3 equals 0.3, which means that there would be less water percolation through
the GCL than a compacted clay liner -- equivalency is established for these conditions.
Alternatively, one can assume that water flux through the GCL is equal to the water flux
through a CCL (i.e., Fw = 1):
VGCL = VCCL (4)
and compute the required hydraulic conductivity of the GCL by substitution in Eq. 4:
H + Tqcl _ ^ H + TCcl
k0CL Tccl " kcCL "TccT" <5)
to obtain:
(kGCL)Required = kCCL H + TqCL (6)
Equation 6 may be used to determined the hydraulic conductivity of the GCL necessary
to establish equivalency. So long as the job specifications require that the actual hydraulic
conductivity be less than the value computed from Eq. 6, equivalency in terms of steady water
flux is theoretically guaranteed. The required hydraulic conductivity of the compacted clay liner
(kccL) is almost universally established as 1 x 10"7 cm/s by regulatory agencies in the U.S. The
thickness of GCLs (Tgcl) varies from product to product, but is typically about 7 mm after
hydration at low overburden stress. The head of liquid on the barrier layer is expected to be low
in a final cover system; evapotranspiration and the nature of rainfall events makes the buildup of
head on the barrier layer much less likely in final covers than in landfill liners. For illustrative
purposes, three values of head of water (H) on the CCL or GCL are assumed: 0, 1 inch, and 1
foot. The required hydraulic conductivity of the GCL for equivalent performance to a compacted
clay liner in terms of steady flux of water through the liner is computed as follows:
For a 1-ft-thick compacted clay liner:
* (kGCL)Required = 1 X 10"7 Cm/s for
* (kGCL)Required = 2 X 10"8 Cm/s for
* (kGCL)Required = 4 X 10"9 Cm/s for
For a 2-ft-thick compacted clay liner:
a negligibly small head of water on the liner
a water head of 1 inch on the liner
a water head of 12 inches on the liner
(kGCL)Required = 1 x 10"7 cm/s for a negligibly small head of water on the liner
• (kGCL)Required = 2 x 10"8 cm/s for a water head of 1 inch on the liner
• (kGCL)Required = 3 x 10"9 cm/s for a water head of 12 inches on the liner
As discussed earlier, the hydraulic conductivity of the bentonite component of
commercially-produced GCLs is typically < 1 to 5 x 10"9 cm/s. Thus, it is clear that equivalency
18
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of a GCL to a CCL, in terms of the amount of water that passes through a GCL under conditions
of steady seepage, can be established for most, if not all, landfill covers.
A GCL can also be used in conjunction with a layer of compacted soil as shown in Fig.
4d. In such cases, the compacted soil will tend to be thinner or be of higher hydraulic
conductivity compared to the minimum requirements for compacted clay liners usually
established by regulatory agencies. If the compacted soil liner were neither thinner nor more
permeable than required by regulation, there would be no motivation to use a GCL, other than to
provide redundancy.
By employing a GCL and a compacted soil liner (CSL) of hydraulic conductivity kcsL>
which is greater than the usual requirement for a compacted clay liner, one may be able to
achieve an acceptable alternative to a conventional compacted clay liner. The equivalent
hydraulic conductivity (keq)of the composite GCL-CSL may be computed from the following
equation:
Tgcl + Tcsl
Tgcl + Tqsl
kccL kcsL
i, _ icjcl T /-7s
Keq " TGcl , TCSL {,)
For example, if compacted soil liner has kcsL = 1 * 10"6 cm/s and Tcsl = 1 ft. ar)d the GCL is 7-
mm-thick with a hydraulic conductivity of 1 x 10~9 cm/s, then the equivalent hydraulic
conductivity (keq) is 4 x 10"10 cm/s, or roughly half the hydraulic conductivity of the GCL alone.
The idea of combining GCLs with native soils is very appealing not only based on theoretical
considerations but also because of the redundancy that the combination provides and the fact that
a relatively low-permeability, native soil material is backing up the GCL. The situation depicted
in Figure 4d and described in this paragraph is presented primarily to illustrate the options
available to the designer in trying to meet regulatory agency concerns and yet use non-standard
materials or designs.
A composite liner consists of a geomembrane placed in contact with a low-permeability
soil. A geomembrane/GCL composite may be considered as an alternate to a geomembrane/CCL
composite. If so, flow through the composite should be analyzed. The rate of flow through a
flaw in a geomembrane in a composite liner depends on the size of the flaw, the hydraulic
conductivity of the underlying clay component, the hydraulic gradient across the clay
component, the hydraulic contact between the geomembrane and the clay component, and the
presence of a geomembrane within the GCL. No equations have been published for explicit
purpose of computing flow rates through a defect in a geomembrane component of a
geomembrane/GCL composite liner. The presence of a geotextile between the geomembrane
and bentonite may influence overall performance. This is a topic of current research. However,
it is likely that equivalency can be demonstrated with reasonable assurance for some or all GCLs
that are used with geomembranes to form composite liners.
2. Time to Initiate Discharge of Water from Base of Liner f"Breakout Time"')
Geosynthetic clay liners and compacted clay liners are initially unsaturated with water.
Geosynthetic clay liners contain essentially dry bentonite, but compacted clay liners are often
very close to saturation at the time of construction. When liquid first enters the upper surface of
an unsaturated liner, no liquid discharges from the base of the liner until the liner absorbs enough
water to reach field capacity at the base.
A GCL might be compared to a CCL in terms of time to discharge of water from the
bottom of the liner on the assumption that leachate production would not begin until water is
19
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discharged from the base of the barrier layer. However, many people would consider the
"breakout time" of water from the barrier layer to be essentially irrelevant because over the long
term, the time to initiate discharge of water from the barrier layer is not important. Over the long
term, the flux of water through the barrier layer (which controls the amount of leachate
produced) is the important issue. As stated earlier, a liner with a hydraulic conductivity of 1 x
10~9 cm/s allows only about 0.01 inch of water to percolate through it per year under continuous
exposure to a water source and unit hydraulic gradient. Again, for those GCLs that contain a
geomembrane, the presence of the geomembrane should be taken into account in evaluation of
breakout time.
The time to discharge water from the base of the liner is difficult to analyze in a simple
way. For CCLs, the time depends greatly upon the hydraulic conductivity, initial water content,
tendency to swell, and rate of water infiltration into the top of the liner. For GCLs, the time to
initiate discharge of water from the base is usually fairly short (a few weeks) if the liner is
continuously flooded with water or may be extremely long if water is slowly absorbed by the
bentonite. For GCLs that contain a geomembrane, the time may be much greater. A comparison
of time to initiate discharge of water from the base of the liner would have to be performed on a
site and product specific basis.
In general, it is not believed that breakout time should be an important issue in an
equivalency assessment. Other factors seem far more important.
3. Production of Consolidation Water
Application of load to a compacted clay liner tends to squeeze water out of the clay. If
this were to occur in a cover, the water might eventually become leachate. Dry GCLs have no
capacity to produce consolidation water loading upon loading. In general, the GCL should be
viewed as superior to a CCL in terms of minimizing production of consolidation water.
However, because the applied loads in final covers are so small, the entire issue of production of
consolidation water is usually moot for final covers. This issue is far more important for clay
liners located above leak detection layers in bottom liner systems for landfills.
4. Air Permeability
The permeability of a barrier layer to gas may be very important if the barrier layer is
expected to restrict the movement of gas through the cover. For porous materials, the air
permeability is extremely sensitive to the water content of the soil. Dry materials are highly
permeable to air, but water-saturated porous materials are practically impermeable to air.
Compacted clay liners are compacted at a water content that is wet of optimum. Any air
present in the CCL tends to be present as isolated bubbles and not in continuous channels. Thus,
the air permeability of CCLs tends to be very low. The air permeability of GCLs depends
greatly on whether or not a geomembrane is present and how much moisture has been absorbed
by the bentonite. The air permeability is high for dry bentonite that is sandwiched between two
geotextiles. For GCLs that contain a geomembrane, the geomembrane dominates the material's
air permeability and gives it a very low permeability to air. Equivalency in terms of air
permeability probably can be demonstrated for GCLs that contain a geomembrane or for GCLs
that are sufficiently hydrated to attain a low permeability to air. The bentonite in the GCL can be
forced to hydrate quickly either by placing the GCL in contact with a moist soil or by applying
water to the overlying soil after the GCL is placed and covered. Laboratory tests indicate that
absorption of water by the bentonite occurs within a few weeks (Daniel et al., 1993) -- the
hydration of the bentonite can be forced to occur if air permeability is a critical issue.
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Phvsical/Mechanical Issues
The physical/mechanical issues that might be considered in an equivalency analysis
include freeze/thaw effects, wet/dry effects, response to total settlement, response to differential
settlement, stability on slopes, vulnerability to erosion, and bearing capacity.
1. Freeze/Thaw Resistance
Compacted clay liners are known to be vulnerable to large increases in hydraulic
conductivity from freeze/thaw (e.g., Kim and Daniel, 1992, and the references therein), although
compacted soil-bentonite mixtures may not be as vulnerable to damage. As discussed earlier,
limited laboratory data indicate that GCLs do not undergo increases in hydraulic conductivity as
a result of freeze/thaw. Thus, from the available data, GCLs appear to be superior to CCLs in
terms of freeze/thaw resistance.
2. Wet/Drv Effects
Wetting and drying of CCLs and GCLs can cause either type of clay liner to swell or
shrink. The main concern with clay liners is that desiccation can lead to cracking and to an
increase in hydraulic conductivity.
As discussed earlier, available laboratory data indicate that desiccation of wet GCLs does
cause cracking, but rehydration of the GCL causes the bentonite to swell and the material to self
heal. Thus, GCLs appear to be superior to CCLs in terms of ability to self-heal if the material is
wetted, dried, and then rewetted.
3. Response to Total Settlement
Total settlement refers to block-like settlement without significant bending or distortion.
It is believed that GCLs and CCLs would both respond similarly to total settlement and that
neither would be damaged if there is no bending or distortion.
4. Response to Differential Settlement
LaGatta (1992) studied the effects of differential settlement on the hydraulic conductivity
of GCLs. LaGatta placed a water-filled bladder in a "false bottom" located beneath the GCL.
The GCL was placed over the bladder and was then covered with 2 ft of gravel to simulate cover
material. The GCL was flooded with 1 ft of water, and water draining out the bottom of the
experimental apparatus was collected for 2 to 4 months, until the flow rate became steady. Then
the bladder was incrementally deflated to produce differential settlement. Boardman (1993)
performed similar tests but subjected dry (rather than hydrated) GCLs to differential settlement;
the GCLs were hydrated and permeated after the distoration took place in the dry material. The
extreme differential settlement caused by the deflated bladders did not produce large increases in
hydraulic conductivity for most of the GCLs tested.
Distortion is defined as the differential settlement, A, divided by the horizontal distance
over which that settlement occurs, L, as shown in Fig. 8. Distortion produces tension, which can
lead to cracking. It appears from LaGatta's and Boardman's tests that many GCLs can withstand
large distortion (a/L up to 0.5) and tensile strain (up to 10 to 15%) without undergoing
significant increases in hydraulic conductivity. This finding is in sharp contrast to the results for
compacted clay, which are summarized in Table 3 compiled by LaGatta (1992). Normal
compacted clay materials cannot withstand tensile strains greater than approximately 0.85%
without failing (cracking). Pure bentonite, on the other hand, is reported to have a tensile strain
at failure of 3.4%, but LaGatta measured much greater tensile strains without cracking in many
21
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GCLs, probably due to the beneficial reinforcing effects from the geotextiles or geomembrane in
the GCLs. In any case, the available data indicate that GCLs can withstand much greater tensile
deformation than normal compacted soils without cracking, which is a very favorable
characteristic for final covers. Geosynthetic clay liners are considered to be superior to
compacted clay liners in terms of resistance to damage from differential settlement.
Area of Differential
Settlement \
Final Cover

Figure 8. Definition of Distoration (a/L).
Table 3. Data on Tensile Strain at Failure for Compacted Clay (from LaGatta, 1992).
TvDe or Source of Soil
Water Content (%)
Plasticitv Index (%)
Failure Tensile Strain
Natural Clayey Soil
19.9
7
0.80%
Bentonite
101
487
3.4%
Elite
31.5
34
0.84%
Kaolinite
37.6
38
0.16%
Portland Dam
16.3
8
0.14%
Rector Creek Dam
19.8
16
0.16%
Woodcrest Dam
10.2
Non-plastic
0.18%
Shell Oil Dam
11.2
Non-plastic
0.07%
Willard Test Embankment
16.4
11
0.20%
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5. Stability on Slopes
The shear strength of GCLs is very sensitive to the water content and type of GCL (Shan
and Daniel, 1991; and Daniel et al., 1993). Water-saturated GCLs that contain unreinforced,
adhesive-bonded bentonite have angles of internal friction for consolidated-drained conditions of
approximately 10 degrees. Dry or damp materials are 2 to 3 times as strong as water-saturated
GCLs. Also, needle-punched and stitch-bonded GCLs tend to have higher strengths, at least in
the short term. The shear strength of CCLs varies widely, depending on materials, water content,
and compaction conditions.
In stability analyses, one often must consider not only internal shear failure but interfacial
shear with an adjacent layer, e.g., a geomembrane. No general statement can be made about
equivalency of a GCL to a CCL in terms of shear strength because the assessment depends on
specific materials, the degree to which the bentonite can wet, slope angle, and other site-specific
conditions.
6. Vulnerability to Erosion
Erosion resistance may be of concern in final covers if inadequate cover soil is present.
With a well-designed and properly maintained cover system, the barrier layer should never be
subjected to forces of erosion after the construction phase is over and equivalency should not be
an issue. In some cases, however, there may be insufficient cover soil to guarantee that the
barrier layer will not be exposed. Because of the presence of erosion-resistant geosynthetic
materials in GCLs, most GCLs can potentially be more resistant to erosion than CCLs.
However, if the clay liner is exposed to erosive forces, the bentonite may be washed out of some
GCL materials. Thus, equivalency depends upon the specific materials being considered. For
many sites, erosion will not be of any concern, e.g., for a GCL underlying a geomembrane or a
cover with adequate cover soil.
7. Bearing Capacity
A clay liner must have adequate bearing capacity to support loads, e.g., wheel loads from
construction or maintenance equipment. The clay liner must not thin or pump clay into adjacent
layers under static or dynamic (e.g., traffic) loads.
Hydrated bentonite is not as strong as most materials used in constructing CCLs.
However, under most circumstances, both a GCL and a CCL will provide adequate foundation
bearing capacity, particularly if the GCL or CCL is buried under sufficient soil overburden.
Equivalency is heavily dependent upon site-specific conditions.
Construction Issues
The construction issues that might be considered in an equivalency analysis include
puncture resistance, effect of subgrade condition on constructability, ease of placement, speed of
construction, availability of materials, requirements for water, air pollution effects, weather
constraints, and quality assurance requirements.
1. Puncture Resistance
Geosynthetic clay liners are thin and, like all thin liner materials, are vulnerable to
damage from accidental puncture during or after construction. Thick CCLs cannot be
accidentally punctured. Some GCLs have the capability to self-seal around certain punctures,
e.g., penetration of the GCL with a sharp object such as a nail. The swelling capacity of
bentonite gives GCLs this self-healing capability. Of greater concern than penetration of the
23
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GCL by an object after construction is accidental puncture during construction. For example, if
the blade of a bulldozer accidentally punctures the GCL during spreading of cover material, the
GCL would probably not self seal at the puncture.
The puncture resistance of GCLs will generally not be equivalent to that of CCLs.
However, this does not mean that a GCL cannot meet or exceed the performance objectives of a
compacted clay liner. Quality assurance and quality control procedures can be established and
implemented to make the probability of puncture during construction extremely low. In final
covers, one or two accidental punctures would probably not have a major impact on the overall
performance of the barrier layer. In a bottom liner system subjected to a continuous head of
liquid, a different conclusion might be drawn about the significance of undetected and unrepaired
damage to a GCL from puncture. Ultimately, site-specific conditions and quality assurance
procedures will be critical in dealing with the issue of puncture and in establishing equivalency
of a GCL to a CCL for a particular project.
2. Effect of Subgrade Condition
Compacted clay liners are constructed with heavy equipment. If the subgrade is
compressible (e.g, solid waste), the GCL, which can be installed with lightweight equipment,
will be easier to construct. On the other hand, stones and rocks can puncture a GCL but not a
CCL; if the subgrade contains stones or rocks, the integrity of the GCL may be compromised.
Also, in order for the overlapped seams in a GCL to self seal properly, the overlapped panels
must be placed on a reasonably smooth and even subgrade. Thus, equivalency of a GCL to a
CCL in terms of the effect of subgrade depends on the condition of the subgrade and will have to
be evaluated on a site-specific basis.
3. Ease of Placement or Construction
A GCL will generally be easier to place than a CCL, except under rainy conditions —
both GCLs and CCLs are difficult or impossible to construct in heavy rain. In general, GCLs are
superior to CCLs in terms of ease of placement or construction.
4. Speed of Construction
Geosynthetic clay liners can be placed much more quickly than CCLs. Geosynthetic clay
liners are superior to compacted clay liners in terms of speed of construction.
5. Availability of Materials
Suitable clays for construction of a CCL may or may not be available locally, depending
on the site. Because GCLs are a manufactured material, they are readily available and can be
shipped to a site quickly. The cost of shipment is usually not a large percentage of the total cost
of a GCL. Thus, GCLs will always be at least equivalent to CCLs in terms of availability of
materials and will be superior to CCLs at sites lacking local sources of suitable clay.
6. Requirements for Water
Construction water is necessary for many compacted clay soils, which must usually be
placed at a moisture content wet of optimum to achieve the desired low hydraulic conductivity.
The total amount of water required to moisten a clay liner can be very large. For example, if a 2-
ft-thick compacted clay liner were to be constructed over a 10-acre site, and the water content of
the soil had to be increased 5% to achieve the required moisture conditions, the total amount of
water used would be approximately 600,000 gal. In arid regions, this water may represent a
valuable resource, and in some remote locations, it may be very expensive to provide the water.
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Geosynthetic clay liners do not require construction water and are superior to CCLs in
this regard.
7. Air Pollution Effects
Air pollution is a subject of great concern in some areas. Construction of compacted clay
liners tends to be an energy intensive activity with heavy equipment excavating the soil, hauling
the soil, processing the soil, spreading the soil, and compacting the soil with repeated passes of
heavy compactors. All of this activity adds to air pollution in terms of hydrocarbon emissions
from the equipment and air-borne particulate matter (dust). Geosynthetic clay liners are shipped
to the site, moved into position by machinery, and then unrolled (sometimes by hand).
Relatively speaking, the impacts to air quality are less with a GCL than a CCL.
8. Weather Constraints
Compacted clay liners are difficult to construct when soils are wet, heavy precipitation is
occurring, the weather is extremely dry (clay desiccates), the soil is frozen, or the temperature is
below freezing. Geosynthetic clay liners are difficult to construct during precipitation. Weather
constraints generally favor GCLs.
Some, if not all, GCLs must be covered before they hydrate. If a geomembrane will be
placed over the GCL, the GCL must be covered almost immediately with the geomembrane.
Additional weather constraints, e.g., wind speed, may apply to the geomembrane and, indirectly,
influence the GCL. The fact that many GCLs must be covered before they are hydrated can be a
significant weather constraint for GCLs. However, CCLs have weather constraints, too: CCLs
must not be allowed to freeze or desiccate, and wet weather often brings construction of
compacted clay liners to a halt. GCLs cannot desiccate during construction because they are dry,
and dry GCLs are unaffected by freezing temperatures.
Equivalency in terms of weather constraints must be considered on a site-specific basis,
but weather constraints generally favor GCLs over CCLs.
9. Ease of Quality Assurance
The proper construction of a low-permeability, compacted clay liner is a very challenging
task. Careful control must exist over materials, moisture conditions, clod size, maximum particle
size, surface preparation for a lift of soil, lift thickness, compaction coverage and energy, and
protection of each completed lift. Comparatively, quality assurance (QA) requirements are much
less extensive for GCLs compared to CCLs, but no less critical. In general, while QA for a
compacted clay liner requires a number of relatively sophisticated tests and points of control by
very experienced and capable personnel, QA for GCLs is more nearly the application of common
sense. Far fewer things can go wrong with the installation of a GCL compared to placement and
compaction of a CCL. However, testing procedures and observational techniques are well
established for CCLs but are not for GCLs. Many people are working to establish testing
methods for GCLs. While it would appear that GCLs are superior to CCLs in terms of ease of
quality control, more work needs to be done to establish standard test methods for GCLs.
Summary of Equivalency Issues
Table 4 summarizes the preceding discussion of equivalency. Equivalency can be
demonstrated generically in many categories. In several areas, geosynthetic clay liners (GCLs)
are clearly superior to compacted clay liners. However, in one category, equivalency probably
cannot be demonstrated: thin GCLs do not have the same resistance to puncture as much thicker
compacted clay liners. Although thin GCLs can be punctured during construction, careful QA
25
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should be capable of addressing this potential problem. Further, for final covers, an occasional
small puncture may be of little consequence. Indeed, puncture is probably of much greater
concern for a bottom liner than a final cover. Also, if puncture is of concern, a layer of relatively
low permeability soil or waste material may be placed below the GCL to provide a back-up
should puncture occur at an isolated location. In any case, the GCL enjoys several important
advantages over a compacted clay liner which may more than offset greater vulnerability to
puncture.
As suggested by Table 4, many equivalency issues depend on the GCL product and the
particular conditions unique to a given site. Equivalency will have to be evaluated on a case-by-
case basis. The most important site-specific issues are likely to be permeability to gas and slope
stability. It may be difficult to provide adequate factors of safety against slope failure on
relatively steeply sloping final covers that contain GCLs, but designers have a variety of
reinforcement materials (such as geogrids) available for use, if necessary.
Table 4 - Potential Equivalency Issues.
Category
Criterion for Evaluation

Eauivalencv of GCL to CCL
GCL Is
Probably
Superior
GCL Is
GCL Is Probably
Probably Not
Equivalent Equivalent
Equivalency
Depends on
Site or Product
Hydraulic
Steady Flux of Water

Issues
Breakout Time of Water

Production of
X

Consolidation Water

Permeability to Gas

X
Physical/
Freeze-Thaw
X

Mechanical
Wet-Dry
X

Issues
Total Settlement

Differential Settlement
X

Slope Stability

Erosion

Bearing Capacity

X
Construction
Puncture Resistance

Issues
Subgrade Condition

Ease of Placement
X

Speed of Construction
X

Availability of Materials
X

Requirements for Water
X

Air Pollution Effects
X

Weather Constraints

Ease of Quality Assurance
X

26
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;clusions
In this paper the characteristics of geosynthetic clay liners (GCLs) have been described
and potential applications of GCLs in final covers for landfills have been discussed. Current
regulations typically require that a final cover contain a compacted clay liner (CCL) with a
thickness of 1 to 2 ft and a maximum hydraulic conductivity of 1 x 10"7'cm/s. The issue is
whether it is sensible to replace all or part of the compacted clay liner with a GCL in final covers
at some landfill sites.
There are several advantages of GCLs over CCLs, including better resistance to freeze-
thaw, better self healing characteristics in wet-dry conditions, less vulnerability to damage from
differential settlement, less consumption of landfill space, easier placement, faster placement,
lack of need for local clay materials, less requirement for construction water (relevant for arid
areas), and easier quality assurance. Geosynthetic clay liners will probably cost less than
compacted clay liners for many, and perhaps most, sites. The major draw-backs of GCLs are
greater vulnerability to damage from puncture, concern over shear strength on slopes, high
permeability of dry bentonite to landfill gas if the GCL remains dry (e.g., in an extremely arid
location), and lack of explicit endorsement of GCLs by regulatory agencies.
A framework has been established in this paper for evaluating whether or not a GCL can
meet the same performance objectives as a compacted clay liner used in a landfill cover. Three
main criteria were established: hydraulic performance, physical and mechanical performance,
and construction issues (including quality assurance). For landfill covers, geosynthetic clay
liners can be shown to provide equivalent or superior performance to compacted clay liners in
many respects. However, some performance considerations (e.g., slope stability) depend on site
and product specific considerations. Thus, no generic conclusion can be reached about
equivalency of a GCL to a CCL at all sites — an equivalency assessment is needed on a project-
specific basis. It is expected that GCLs can be shown to provide superior or equivalent
performance at many landfill sites.
Although GCLs are not without limitations, their favorable properties are sufficiently
advantageous that landfill owners, designers, and regulatory officials should give serious
consideration to expanded use of GCLs in landfill covers. There is a need to reach agreement
about the criteria upon which GCLs will be evaluated, and it is hoped that this paper will help to
initiate a dialogue that will ultimately lead to establishment of appropriate criteria.
ACKNOWLEDGMENTS
The work performed at the University of Texas on geosynthetic clay liners has been
sponsored by several organizations. The author expresses his appreciation to the Gulf Coast
Hazardous Substance Research Center, the U.S. Environmental Protection Agency, Gundle
Lining Systems, Inc., and the James Clem Corporation for their support. Except for the U.S.
EPA, this article has not been submitted to these organizations for review, and the contents of
this article do not necessarily reflect the views of any of these organizations.
The information in this document has been funded wholly or in part by the United States
Environmental Proection Agency under cooperative agrteement CR-815546 with The University
of Texas. It has been subjected to the Agency's peer and administrative review, and it has been
approved for publication. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
27
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The author expresses his sincere appreciation to many individuals involved in
geosynthetic clay liners who have shared data and ideas with the author, but particular thanks are
extended to James Anderson, David Carson, Richard Carriker, Kenneth Dotson, Walter Grube,
Robert Landreth, Martin Simpson, and Fred Struve, who have been particularly helpful in
sharing their thoughts and ideas about use of GCLs in landfills. The graduate students at the
University of Texas who developed much of the data described in this paper are thanked -- these
include David Lutz, Hsin-yu Shan, Paula Estornell, Mark LaGatta, and Tom Boardman. Finally,
the author gratefully acknowledges Prof. Robert M. Koerner, who has generously given many
hours of time to share research findings and ideas concerning criteria for comparing geosynthetic
clay liners with compacted clay liners.
REFERENCES
Boardman, B.T. (1993), "The Potential Use of Geosynthetic Clay Liners as Final Covers in Arid
Regions," M.S. Thesis, University of Texas, Austin, Texas, 109 p.
Daniel, D.E. (1991), "Geosynthetic Clay Liners," Geotechnical News, Vol. 9, No. 4, p. 28-33
Daniel, D.E., and P.M. Estornell (1990), "Compilation of Information on Alternative Barriers for
Liner and Cover Systems," U. S. Environmental Protection Agency, EPA 600/2-91/002,
Cincinnati, Ohio, 81 p.
Daniel, D.E., Shan, H.Y., and Anderson, J. (1993), "Effects of Partial Wetting on Strength and
Hydrocarbon Permeability of a Geosynthetic Clay Liner," Proceedings, Geosynthetics '93,
International Fabrics Association International, Vol. 3, pp. 1483-1496..
Daniel, D.E., and Boardman, B.T. (1993), "Report on Workshop on Geosynthetic Clay Liners,"
U.S. Environmental Protection Agency, Cincinnati, Ohio, in review.
Eith, A.W., Boschuk, J., and R.M. Koerner (1991), "Prefabricated Bentonite Clay Liners,"
Landfill Closures: Geosynthetics, Interface Friction, and New Developments, R.M. Koerner
(Ed.), Elsevier Applied Science, New York, pp. 193-218.
Estornell, P. and D.E. Daniel (1992), "Hydraulic Conductivity of Three Geosynthetic Clay
Liners," Journal of Geotechnical Engineering, Vol. 118, No. 10, pp. 1592-1606.
Grube, W.E. (1992), "Geosynthetic Liners Offer Cover Option," Environmental Protection, May,
pp. 29-33.
Kim, W. H., and D. E. Daniel, "Effects of Freezing on the Hydraulic Conductivity of a
Compacted Clay," Journal of Geotechnical Engineering, Vol. 118, No.7, 1992, pp. 1083-
1097.
LaGatta, M. D. (1992), "Hydraulic Conductivity Tests on Geosynthetic Clay Liners Subjected to
Differential Settlement," M.S. Thesis, University of Texas, Austin, Texas.
Maltby, V., and L.K. Eppstein (1993), "A Field-Scale Study of the Use of Paper Industry Sludges
as Hydraulic Barriers in Landfill Cover Systems," Hydraulic Conductivity and Waste
Contaminant Transport in Soils, ASTM STP 1142, D. E. Daniel and S. J. Trautwein (Eds.),
American Society for Testing and Materials, Philadelphia, in press.
Scheu, C., Johannben, K., and F. Saathoff (1990), "Non-Woven Bentonite Fabrics - A New Fibre
Reinforced Mineral Liner System," Geotextiles, Geomembranes and Related Products, D.
Hoet (Ed.), Balkema Publishing, Rotterdam, pp. 467-472.
28
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Schubert, W.R. (1987), "Bentonite Matting in Composite Lining Systems," Geoiechnical
Practice for Waste Disposal '87, American Society of Civil Engineers, New York, pp. 784-
796.
Shan, H.Y., and D.E. Daniel (1991), "Results of Laboratory Tests on a Geotextile/Bentonite
Liner Material," Proceedings, Geosynthetics '91, Industrial Fabrics Association International,
St. Paul, MN, Vol. 2, pp. 517-535.
29
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TECHNICAL REPORT DATA
(Please read Ins auctions on the reverse before complelim

1. REPORT NO. 2.
EPA/600/A-93/283
3. RE
4. TITLE ANO SUBTITLE
Geosynthetic Clay Liners (GCLs) In Landfill Covers
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
David E. Daniel
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME ANO ADDRESS
University of Texas
Austin, Texas 78712
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
CR 815546
12. SPONSORING AGENCY NAME ANO AOORESS
Risk Reduction Engineering Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOO COVERED
Conference Proceedings
14. SPONSORING AGENCY CODE
EPA/600/14
is. supplementary notes project Officer = Michael Roulier (513) 569-7796; The Thirty-First
Annual Solid Waste Exposition, San Jose, CA, 8/2-5/93, p:l-29
i6.abstractLow-permeabi1ity, compacted clay liners are commonly required as a barrier to
water infiltration in landfill covers. A relatively new material, known as geosynthetic
clay liner (GCL), has been proposed as an alternative to a compacted clay liner. A GCL
has the practical advantages of relatively low cost (approximately $0.50 to $0.60 per
square foot for a landfill cover, installed), rapid installation with light-weight equip-
ment, and ease of repair. A-.GCL also has several technical advantages, including greater
tolerance for differential settlement and better self-healing characteristics under wet-
dry and freeze-thaw conditions. A potentially important disadvantage of the GCL is that,
because it is thin, it is more vulnerable to damage from puncture than a compacted clay,
liner. The objectives of this paper are: (1) to provide an introduction to GCLs for those
who may be unfamiliar with this lining material; (2) to summarize the potential applica-
tions of GCLs to landfill covers; (3) to examine the relative advantages and disadvantage:
of GCLs compared to compacted clay liners; and (4);to provide a generic assessment of per-
formance equivalency of GCLs compared to:low-permeability, compacted clay barriers. The
fourth item will comprise the bulk of the paper. The conclusion is drawn that geosyn-
thetic clay liners can be shown to provide equivalent performance to low-permeability,
compacted clay liners for many landfill sites. The key issues concerning equivalency are
ability to limit percolation of water through the barrier, permeability to gas, slope
stability, and puncture resistance.
17. KEY WORDS ANO DOCUMENT ANALYSIS
a. DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATi Field/Group
Landfi11
Waste
Barrier
Membranes
Cover
Liner
Clay

18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS fTlus Report)
UNCLASSIFIED
21. NO. OF PAGES
30
20. SECURITY CLASS (This page 1
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (R»v. 4-77) previous edition is obsolete
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