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Case Study
Hydraulic and Geochemical Characteristics of a
Geosynthetic Clay Liner Exhumed from an
Exposed Composite Liner
Thomas R. Williams, A.M.ASCE1; Craig H. Benson, F.ASCE2; Kuo Tian, A.M.ASCE3;
Nazli Yeşiller, A.M.ASCE4; and James L. Hanson, M.ASCE5
Abstract: A geosynthetic clay liner (GCL) was exhumed from a composite liner (geomembrane over GCL containing granular sodium
bentonite) in the base of a landfill cell after 12 years of atmospheric exposure. The GCL was altered appreciably during exposure, despite
being overlain by a geomembrane. Water content, hydration, and bentonite erosion varied considerably along the slope. GCL samples
from the top of the slope were dry, with visible bentonite granules comparable to those in a virgin GCL as if the bentonite had not hydrated.
No substantial change in exchange complex or swell index (SI) occurred in samples from the top of the slope, and hydraulic conductivity
of these samples to water was low (≈10−11 m=s). GCL samples from the toe of the slope varied considerably, from moist and soft to dry
and cracked, but all had undergone hydration, and had low SI, a small mole fraction of monovalent cations in the exchange complex, and
high hydraulic conductivity (10−8m=s or higher). Bentonite in GCL samples from midslope had a cracked structure commonly observed
in GCLs that have undergone wet–dry cycling. Midslope GCLs typically had hydraulic conductivity greater than 10−7m=s. A sample
from the top of the slope from a location near a GCL panel separation was comparable to other samples from the top of the slope. GCL
samples from the anchor trench differed considerably, being very permeable or having low hydraulic conductivity. Hydraulic conductivity
was strongly correlated with SI; GCL samples with SI < 15 mL=2g were highly permeable, and those with SI >15 mL=2gwerecom-
parable to a virgin GCL. The variation in SI was related directly to replacement of sodium by calcium in the exchange complex of the
bentonite. Hydraulic conductivity also was affected by the thinning of the GCL. DOI: 10.1061/JGGEFK.GTENG-12219.© 2024
American Society of Civil Engineers.
Author keywords: Composite liner; Geosynthetic clay liner (GCL); Bentonite; Hydraulic conductivity; Desiccation; Cation exchange;
Swell index (SI); Bentonite loss.
Introduction
Geosynthetic clay liners (GCLs) are thin (≈10 mm) factory-
manufactured hydraulic barriers consisting of air-dry granular or
powdered bentonite clay sandwiched between two geotextiles
bonded together by needlepunching or stitching. GCLs are used
widely in waste containment systems due to their ease of deploy-
ment, minimal consumption of air space, and very low hydraulic
conductivity to water (≈10−11 m=s) (Shackelford et al. 2000;
Yeşiller and Shackelford 2011;Rowe 2020). The in-service hy-
draulic conductivity of GCLs can vary considerably depending on
the geochemical and moisture state when the GCL hydrates, the
chemical characteristics of the liquid permeating the GCL, and the
physical environment to which the GCL is exposed (Ruhl and
Daniel 1997;Meer and Benson 2007;Benson et al. 2010,2013;
Scalia and Benson 2010,2011;Bradshaw et al. 2013;Bradshaw
and Benson 2014;Scalia et al. 2017;Rowe 2020).
The low hydraulic conductivity of GCLs is attributed to benton-
ite swelling during hydration, which closes intergranular pores and
constricts the pore space through which water can flow (Jo et al.
2001;Kolstad et al. 2004;Scalia and Benson 2011;Tian et al. 2016;
Rowe 2020;Hou et al. 2023). When sufficient swelling of the ben-
tonite occurs, the pores through which liquid flows are small and
tortuous, yielding very low hydraulic conductivity (Hou et al.
2023). Swelling of bentonite and the hydraulic conductivity of
GCLs is affected by the relative abundance of monovalent and
polyvalent cations in the exchange complex of the bentonite
(i.e., the collection of cations bound to the negatively charged
mineral surface to provide charge balance) and the ionic strength
of the hydrating solution. The bentonite used in most GCLs is
sodium (Na) bentonite, with Naþas the predominant cation in
the exchange complex (Scalia et al. 2018). Na-bentonites swell
appreciably and have low hydraulic conductivity when hydrated
or permeated with dilute solutions that do not induce significant
exchange of Naþfor higher-valence cations during hydration or
permeation (Jo et al. 2001;Kolstad et al. 2004;Chen et al. 2018;
Scalia et al. 2018).
1Project Engineer, SCS Engineers, 15521 Midlothian Tpke #305,
Midlothian, VA 23113. Email: twilliams@scsengineers.com
2Wisconsin Distinguished Professor Emeritus, Dept. of Civil and
Environmental Engineering, Univ. of Wisconsin-Madison, Madison, WI
53706 (corresponding author). ORCID: https://orcid.org/0000-0001-8871
-382X. Email: chbenson@wisc.edu
3Assistant Professor, Sid and Reva Dewberry Dept. of Civil, Environ-
mental, and Infrastructure Engineering, George Mason Univ., Fairfax,
VA 22030. Email: ktian@gmu.edu
4Director, Global Waste Research Institute, California Polytechnic State
Univ., San Luis Obispo, CA 93407. ORCID: https://orcid.org/0000-0001
-8673-0212. Email: nyesille@calpoly.edu
5Professor, Dept. of Civil and Environmental Engineering, California
Polytechnic State Univ., San Luis Obispo, CA 93407. Email: jahanson@
calpoly.edu
Note. This manuscript was submitted on August 16, 2023; approved on
November 21, 2023; published online on February 28, 2024. Discussion
period open until July 28, 2024; separate discussions must be submitted
for individual papers. This paper is part of the Journal of Geotechnical
and Geoenvironmental Engineering, © ASCE, ISSN 1090-0241.
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Cation exchange processes can alter the relative abundance of
monovalent and polyvalent cations in the exchange complex of the
bentonite during hydration or while the GCL is in service (Benson
et al. 2007,2013;Meer and Benson 2007;Scalia and Benson 2011;
Bradshaw et al. 2013,2015;Bradshaw and Benson 2014), altering
the swelling characteristics of the bentonite and the hydraulic con-
ductivity of the GCL. Cation exchange can occur in response to
exchange reactions with liquid permeating the GCL, hydration with
moisture from adjacent soil after deployment, or from ions dissolv-
ing from secondary minerals in the bentonite (e.g., calcite) (James
et al. 1997;Bradshaw et al. 2013,2015;Benson et al. 2013;
Bradshaw and Benson 2014;Rowe 2020). If the exchange complex
remains predominantly monovalent, the hydraulic conductivity of
the GCL will be low and the GCL will be resistant to environmental
stresses such as freeze–thaw and wet–dry cycling (Scalia and
Benson 2011;Makusa et al. 2014;Scalia et al. 2017). If the ex-
change complex becomes predominantly polyvalent, the hydraulic
conductivity can be higher and the GCL can be much more vulner-
able to environmental stresses, especially if the GCL undergoes
wet–dry cycling (Lin and Benson 2000;Meer and Benson 2007;
Benson et al. 2007;Benson and Meer 2009). For example, Lin and
Benson (2000) demonstrated that desiccation cracks that form
in bentonite when GCLs dry do not swell shut during subsequent
hydration if polyvalent cations are predominant in the exchange
complex of the bentonite. In most environments, exchange of na-
tive Naþcations on the bentonite surface for polyvalent cations in
solution is thermodynamically favorable (McBride 1994).
Several research studies indicated that GCLs in composite liners
that are left exposed (i.e., no leachate collection system, waste, or
cover soils are placed over the liner after construction) are suscep-
tible to moisture accumulating at the interface of the geomembrane
(GM) and underlying GCL in response to thermal gradients and
condensation (Azad et al. 2011;Rowe et al. 2016;Rowe and
Hamdan 2021;Fan and Rowe 2023;Rowe et al. 2023). This mois-
ture flow induces erosion of bentonite within the GCL as water
flows within and on the surface of the GCL along the slope in
response to the downslope hydraulic gradient. Bentonite eroded
from the GCL migrates downslope with the flowing water
(Brachman et al. 2014;Rowe et al. 2014;Take et al. 2015). If
bentonite erosion is significant, preferential pathways may form,
rendering the GCL much more permeable regardless of the propen-
sity for the bentonite to swell (Rowe and Orsini 2003). The wet–dry
cycling associated with thermal cycling also can create desicca-
tion cracks in the GCL that may not swell shut on rehydration
in a manner similar to that described by Lin and Benson (2000).
These phenomena associated with thermal cycling have been ob-
served primarily in research studies, whereas examples from actual
liners are limited.
This paper describes an examination of the GCL component
of the composite liner (geomembrane over GCL) exhumed and
examined in the case study reported by Hanson and Yeşiller (2020).
The composite liner was composed of a high-density polyethylene
(HDPE) geomembrane placed directly over the GCL and deployed
on a compacted silty sand subgrade. The liner had been exposed to
the atmosphere for 12 years in an unused cell within a municipal
solid waste (MSW) landfill at a coastal location in central California.
The leachate collection system, buffer layer, and waste were not
placed on the composite liner due operational changes at the landfill
that occurred after construction of the liner, leaving the composite
liner exposed to the atmosphere (Hanson and Yeşiller 2020). This
paper describes the properties of the GCL exhumed from the ex-
posed liner. The properties of the geomembrane were described by
Tian et al. (2019).
Field Conditions
The composite liner was constructed on a 2H:1V side slope with
slope lengths ranging from 24 m (south slope) to 30 m (east slope)
(Fig. 1). The geomembrane was 1.5-mm-thick black HDPE with
texturing on the lower side and a smooth surface on the upper
(exposed) side. The GCL contained granular bentonite encased
between two nonwoven geotextiles bonded by needlepunching.
The upper geotextile was black, and the lower geotextile was gray.
The GCL was placed directly on the underlying subgrade, and the
geomembrane was placed directly on top of the GCL. No other
materials were placed above the geomembrane.
The composite liner was constructed in 2004 and remained
uncovered through June 2016 (≈12 years), when the liner was ex-
humed. During the exposure period (2004–2016), the annual mean
temperature at the site varied between 14.8°C and 17.3°C, with an
average of 15.8°C. The mean minimum temperature was 9.1°C and
the mean maximum temperature was 22.5°C. The average annual
precipitation during the exposure period was 415 mm=year. These
atmospheric conditions are typical of the region, which is a tem-
perate climate zone [Csb, which is characterized by temperate,
warm, and dry summers (Beck et al. 2018)].
Thermal Monitoring
Prior to exhumation, two sets of Type K thermocouples were in-
stalled in the liner profile on the east and south slopes (Fig. 1) ap-
proximately 10 m from top of slope. Each set consisted of five
thermocouples at the following locations: geomembrane surface,
GM–GCL interface, GCL–subgrade interface, 75 mm below the
subgrade surface (BSS), and 150 mm BSS. Hourly air temperatures
were obtained from a nearby weather station operated by the
US National Oceanic and Atmospheric Administration (NOAA)
(Station USW00093206, San Luis Obispo McChesney Field).
The thermocouples were inserted through a small hole in the geo-
membrane, which was sealed using an extrusion welder. The
thermocouple on the geomembrane surface was set with a small
dab of extrudate. The deeper thermocouples were installed through
a 15-mm-diameter vertical hole drilled through the geosynthetics
into the subgrade. The thermocouples at the GM–GCL and GCL–
subgrade interfaces were extended laterally from the drilled hole
approximately 25 mm to anchor the thermocouples into position
at the respective interfaces. Data were collected from April 14 to
May 2, 2016, just prior to removal of the geosynthetics. Measure-
ments were recorded using a datalogger every 3 min.
Temperature data from the east slope over the initial 10 days of
monitoring are shown in Fig. 2. The temperature on the surface of
the geomembrane consistently was higher than the air temperature
obtained from NOAA, which is expected given the black color of
the geomembrane and solar exposure on the slopes. Over the meas-
urement period, the minimum air temperature was 5°C and the
maximum was 31°C. The maximum temperature of the geomem-
brane surface was 51°C on the south slope and 56°C on the east
slope during the monitoring period. The east slope had higher tem-
peratures due to longer solar exposure in the afternoons. Temper-
atures on the surface of the GCL followed a similar temporal trend
as the surface of the geomembrane, but were lower than temper-
atures on the surface of the geomembrane during daylight hours
and nearly identical to temperatures on the geomembrane other-
wise. Temperatures at greater depths in the subgrade soil had less
daily variation and a phase lag that increased with depth. The
upward thermal gradient across the GCL, computed using the tem-
peratures measured on the upper and lower GCL surfaces and
assuming a 10-mm thickness, was as large as 0.34°C=mm (evening
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to early morning) and the downward thermal gradient was as large
as 1.4°C=mm (midafternoon). These thermal gradients induce flow,
subjecting the GCL to wet-dry cycling and dessication (Nassar
et al. 1997;Azad et al. 2011).
Visual Observations
Hanson and Yeşiller (2020) provided a detailed description of the
exhumation process and observations that were made during the
exhumation. Hanson and Yeşiller (2020) indicated that the condi-
tion of the GCL varied substantially over short distances, hydration
of the GCL was highly variable, and downslope erosion of benton-
ite was significant. Areas with no visible bentonite loss and areas
with high loss often were separated by only 100–150 mm. The ben-
tonite in the GCL varied from moist and soft near the toe, to dry and
cracked along the slope, and dry and granular at the top of slope or
in the anchor trench.
Bentonite erosion and migration was extensive, with dry ben-
tonite rivulets visible on the surface of the GCL and significant
accumulation of bentonite occurring along the base of the east slope
(Hanson and Yeşiller 2020). The bentonite accumulation occurred
between the geomembrane and the GCL in the shape of a wedge
(Fig. 1). This wedge-shaped zone had a maximum thickness
of ≈90 mm at the toe of the slope, and extended approximately
90 m in the cross-slope direction. The wedge was composed of
99.8% fines with 58% 2-μm clay content, and appeared to be ben-
tonite that had migrated downslope. A localized mound of eroded
material with a height of 160 mm also was observed between the
geomembrane and the GCL near the southeast slope, extending
approximately 2.6 m upslope (Fig. 1). The mound had a similar
Fig. 1. (Color) Aerial image of (a) exposed geomembrane and unified cell (image by authors); (b) schematic of locations at which GCL samples were
obtained and where soil/bentonite accumulated; and (c) profile of liner showing components. In Fig 1(a), dotted lines indicate locations at which
samples were collected along length of slope. In Fig 1(b), colored squares indicate locations at which GCL was sampled at discrete locations, and the
dashed rectangle indicates the location of the strip sample in the southeast corner.
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particle-size distribution as the subgrade. These features are indica-
tive of bentonite and other solids migrating with water moving
downslope (wedge), and subgrade solids migrating upslope with
seasonal water level fluctuations within the adjacent sump for
the cell (mound) (Hanson and Yeşiller 2020). The sump was not
being pumped regularly, allowing the water level in the sump area
to vary appreciably.
Forty-three GCL panel overlaps were uncovered in the cell.
Panel separation was observed at eight locations, in seven of the
overlaps (there were two distinct gaps in one overlap). Gaps be-
tween the panels ranged from 20 to 220 mm wide and spanned
lengths of 1.7–17 m. Panel separation generally occurred along the
top half of the slope, and the longest gap was observed in the
southeast corner of the cell (Hanson and Yeşiller 2020).
GCL Sampling
The GCL sampling locations are shown in Fig. 1. GCL samples
were collected along three transects corresponding to the east slope
(samples identified as E-#; where # is the distance from top of slope
in meters), the south slope (samples identified as S-#), and the
southeast corner (samples identified as SE-#). For the east and
south slopes, samples were collected near the top of the slope,
the midslope, and the toe of the slope (Fig. 1). A continuous strip
of GCL (width ¼0.3m) was obtained along the entire slope length
in the southeast corner (Fig. 1). The strip was near the longest con-
tinuous panel separation observed during exhumation (a 17-m long
gap in the seam). Samples for testing were obtained from this strip
at 3-m intervals.
Samples were also exhumed from the anchor trench at the top of
slope (samples designated A#), at a location at which the GCL
panel had separated (Sample PS), and from an overlap seam at
the toe of the south slope (Sample S-24-OL) (Fig. 1). All samples
were obtained directly from the liner system. No samples were
archived during installation of the liner, and therefore a direct com-
parison with the as-built liner could not be made. However, tests
were conducted on a sample of the same GCL product from a panel
manufactured circa. 2018. This GCL had similar mass per area as
the GCL installed at the landfill, and the bentonite was from the
same source. Thus, the virgin GCL likely was similar, but not
necessarily identical to the GCL installed in 2004.
Samples of the GCL from the east and south slopes were ob-
tained following the methods in ASTM D6072, Method A (hand
cutting method) (ASTM 2018a) and the procedures described by
Scalia and Benson (2011). The geomembrane over the area of
GCL to be sampled was removed by cutting it with a sharp utility
knife. A GCL sample with dimensions of approximately 300 ×
300 mm then was cut from the GCL panel with the same knife,
gently transferred onto a rigid plastic plate, and sealed in a zip-
top plastic bag. The sampling was conducted as expediently as
practical to minimize exposure of the GCL. The sealed samples
were packed in padded shipping crates and delivered to the labo-
ratory for testing. For the strip, the 27-m-long sample was separated
into three rolls along the length (for ease of handling), wrapped in
plastic, placed in large zip-top bags, and packaged for delivery to
the laboratory. Grab samples of the underlying subgrade were ob-
tained after each GCL sample was removed, sealed in plastic bags,
and delivered to the laboratory for testing.
Laboratory Methods
Water Content
Water content of the subgrade soil and the bentonite in the GCL
was measured using the procedure in ASTM D2216 (ASTM
2018g). The bentonite was removed from the GCL sample when
determining the water content.
0
10
20
30
40
50
60
0246810
Top of G e om e mbr a ne ( G M)
GM/GCL Interface
GCL/Subrade Interface
Subgrade: 75 mm bss
Subgrade: 150 mm bss
Air Temperature
Temperature(
o
C)
Elapsed Time (Days)
East Slope
Fig. 2. (Color) Geomembrane, GCL, and subgrade temperatures measured on the east slope in April 2016.
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Mass per Area
Mass per area of each GCL sample was measured using the pro-
cedure in ASTM D5993 (ASTM 2018e). Square specimens were
cut from the GCL sample using a sharp utility knife. The perimeter
of the specimen was wetted to prevent bentonite loss during trim-
ming. One test specimen was cut per GCL sample.
GCL samples obtained near the toe of the south slope were
covered with bentonite from the overlying wedge of bentonite at
this sampling location. This bentonite was removed from the GCL
samples to the extent practical prior to measuring the mass per area.
This bentonite was bound strongly to the GCL surface, and not all
the bentonite could be removed without damaging the specimen.
Mineralogy
The mineralogy of bentonite from the GCLs and the subgrade soil
was determined by X-ray diffraction (XRD) using the method de-
scribed by Moore and Reynolds (1989). Samples for XRD analysis
were ground with a mortar and pestle, dispersed in dilute sodium
phosphate solution using a sonic probe, vacuum-deposited on
nylon membrane filters, attached to glass slides, and exposed to
ethylene glycol vapor for 24 h. A Rigaku (Tokyo) Ultima IV
XRD was used for analysis. Quantitation of the mineral fraction
was conducted using the Rietveld method as described by Moore
and Reynolds (1989).
Swell Index
The swell index of the bentonite from the GCL samples was mea-
sured on bentonite trimmings from the GCL following the pro-
cedure in ASTM D5890 (ASTM 2018f) using ASTM Type II
deionized (DI) water [ASTM D1193 (ASTM 2018b)] as the hyd-
rating solution.
Soluble Cations, Bound Cations, and Cation Exchange
Capacity
Soluble cations, bound cations, and cation exchange capacity
(CEC) were measured in bentonite from the GCL samples follow-
ing the methods in ASTM D7503 (ASTM 2018d). The bentonite
was oven dried at 110°C and then ground to pass a US No. 10 sieve.
Type II DI water, 1.0MNH
4OAc, and 1.0 M KCl solutions were
used as reagents.
Extracts for soluble and bound cation analysis were analyzed
using an Agilent 7900 (Santa Clara, California) Quadrupole induc-
tively coupled plasma mass spectrometer (ICP-MS) following the
procedure in USEPA Method 6020B (EPA 2014). Extracts were
vacuum filtered (0.45 μm) and acidified with trace metal–grade ni-
tric acid to pH 2 prior to analysis.
CEC was determined from an ammonium extract using the
methods in D7503. The extraction solution was 1.0 M KCl.
NH3-N concentrations in the extract were measured with a Hach
(Loveland, Colorado) DR6000 spectrophotometer using the salicy-
late method.
Hydraulic Conductivity
Hydraulic conductivity of the exhumed GCLs was measured in
flexible-wall permeameters following the procedures in Method B
(falling headwater–constant tailwater) of ASTM D6766 (ASTM
2018c). Test specimens with a diameter of 100 or 150 mm were
trimmed from the GCL samples using a sharp razor knife following
the procedures in Jo et al. (2001). DI water was applied around the
periphery of the specimen to prevent loss of bentonite during
trimming, and bentonite paste prepared with DI water was smeared
around the edge to prevent sidewall leakage during permeation.
The NaCl-CaCl2solution representing typical pore water
(1.3 mM NaCl, 0.8m MCaCl2), as described in ASTM D5084
(ASTM 2018h) and Scalia and Benson (2010), was used as the per-
meant solution. The solution was prepared by dissolving reagent-
grade NaCl and CaCl2salts in ASTM D1193 Type II deionized
water. Testing was conducted at an average effective stress of
21 kPa with flow from bottom to top using an average hydraulic
gradient of approximately 150. This hydraulic gradient is larger
than the gradient anticipated in the field, but has been shown to
have negligible impact on hydraulic conductivity relative to the
effective stress (Petrov et al. 1997;Shackelford et al. 2000). Back-
pressure was not used to simulate the field condition and to avoid
unintended geochemical changes (i.e., Le Chatelier’s principle), as
recommended by Meer and Benson (2007), Scalia and Benson
(2011), and Scalia et al. (2017).
Checks for sidewall leakage were made on all specimens with
hydraulic conductivity ≥10−9m=s. The permeant solution was
spiked with rhodamine WT dye, and the test was discontinued
and disassembled immediately after dye was observed in the efflu-
ent. All tests indicated that flow was through the GCL and not
along the sidewall. Data from all tests are in Williams (2018).
X-Ray Imaging
X-ray images of the GCL samples were obtained to evaluate the
bentonite structure nondestructively. All images were obtained
using a Fuji CR XL-2 digital gray-scale X-ray imaging system
with a resolution of 0.1 mm (Fuji Film, Tokyo). Square speci-
mens (105 ×105 mm) were cut from the strip samples using a
template and razor knife. Edges along the template were wetted
with DI water applied from the nozzle of a squeeze bottle to avoid
loss of bentonite. After trimming, each specimen was placed in a
resealable zip-top plastic bag and arranged on a plastic tray for
analysis. A coin was placed in the corner of the tray as a marker
on the X-ray image to maintain alignment for interpretation of the
images.
Results and Discussion
Bentonite Hydration, Structure, Water Content, and
Mass per Area
The exhumed GCL had varying states of hydration (Fig. 3). GCL
samples exhumed from the anchor trench contained relatively large
aggregates of bentonite that appeared to have hydrated partially
[Fig. 3(a)]. GCL samples exhumed from the top to the middle
of the slope contained small granules of bentonite [Fig. 3(b)]
similar to what commonly is found in virgin GCLs with granular
bentonite, as if the bentonite had never hydrated. Water contents
of samples from the top to middle of the slope were compar-
able to those in a virgin GCL—e.g., 14% for Sample SE-12-A
[Fig. 3(b)]. In contrast, other GCL samples from midslope ap-
peared to have hydrated and then dried and cracked, most likely
from condensate moisture flowing along the interface between
the GCL and geomembrane during thermal cycling [Fig. 3(c)].
Samples collected from the toe of the slope, including the overlap
sample from the south slope (S-24-OL), had either a moist and
soft gel-like structure [Fig. 3(d)] or a gel-like structure that had
dried and cracked, comparable to Sample S-12 from midslope
[Fig. 3(c)].
Similar structures were evident in the X-ray images of the GCL
samples (Fig. 4). Virgin GCLs in unhydrated and hydrated states
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are shown in Figs. 4(a and b), in which DI water was used for
hydration. The image corresponding to the virgin GCL prior
to hydration [Fig. 4(a)] shows pores corresponding to bundles of
needlepunching fibers. No pores are evident in the virgin GCL
hydrated with DI water [Fig. 4(b)], indicating that the bentonite
swelled and filled the pores associated with needlepunching fiber
bundles. Cracks are evident in the X-ray images of the GCLs
collected along the slope, indicative of hydration and swelling
followed by desiccation and shrinkage cracking [Figs. 4(c–e)].
These cracks must swell shut during subsequent hydration for the
GCL to maintain low hydraulic conductivity (Lin and Benson 2000;
Benson et al. 2007).
Some of the GCL samples had lost bentonite due to internal
erosion from water migrating at the interface of the GCL and geo-
membrane, as illustrated by the dashed outlined area in the upper
right-hand corner of the X-ray image for GCL sample SE-6
[Fig. 4(c)]. The GCLs exhumed midslope and downslope had
rivulets of bentonite on the surface (Fig. 5), indicative of water
containing eroded particles moving at the interface of the GCL
and geomembrane (Hanson and Yeşiller 2020). Erosion extended
through the toe on the south slope, and GCL sample S-24 was
heavily eroded. This sample had very low water content and little
bentonite remaining [Figs. 6(a and b) and Table 1].
X-ray images of GCL samples at the toe of slope had a mono-
lithic appearance [Fig. 4(f)] similar to that of the hydrated virgin
GCL in Fig. 4(b). In some cases, the GCL had been covered by
the bentonite and subgrade material that accumulated near the toe
(e.g., SE-27-B), which likely contributed to accumulation and
retention of moisture in the bentonite within the GCL.
The water content of the bentonite in the GCLs varied over
a broad range (2%–107%), which is consistent with the highly
variable hydration state observed during exhumation (Table 1).
However, most of the water contents were between 10% and 39%.
GCL samples near the toe of the slope had the highest water con-
tents and the broadest range of water contents, 2%–107% [Table 1
and Fig. 6(a)]. The highest water contents corresponded to the
moist bentonite in GCL samples from the toe of the slope that were
wetted by moisture migrating downward at the interface between
the geomembrane and GCL.
Very low water contents, characteristic of a virgin GCL in an
air-dry state [≈15% (Table 1)], were measured in GCL samples
upslope of the toe and to the top of the slope [Fig. 6(a)]. These
low water contents varied little along the slope, and corresponded
to the desiccated and cracked structures in Figs. 3(c) and 4(c–e) as
well as to the absence of granule hydration observed near the top of
the slope [Fig. 3(b)]. Subgrade soil sampled beneath the GCL along
the slope in areas outside the toe had very low water content [0.8%–
5.0%, average ¼1.9% (Table 1)], which is consistent with the low
bentonite water contents, the absence of hydrated bentonite, and the
presence of highly desiccated bentonite in the same regions.
GCL samples from the anchor trench [Fig. 3(a)] had higher
water contents {32% and 37% [Table 1and Fig. 6(a)]} than GCL
samples near the top of slope. Moisture in the trench backfill (water
(a) (b)
(c) (d)
(e) (f)
25
mm
Fig. 4. X-ray images of GCLs: (a) virgin GCL prior to hydration;
(b) virgin GCL after hydration with DI water; (c) SE-6, exhumed along
southeast transect between top and midslope, showing cracking and
thinning (upper right corner) from erosion; (d) SE-15, exhumed from
midslope, showing cracking; (e) SE-21, exhumed towards the bottom
of the slope, showing cracking; and (f) SE-27-B, exhumed at the bot-
tom of the slope, which was moist and covered with eroded material.
Fig. 3. (Color) Exhumed GCL with varying water content. The con-
dition of the exhumed bentonite varied between gel-like, desiccated,
and granular.
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content ¼8.1%) appears to have partially hydrated the bentonite,
forming larger bentonite aggregates [Fig. 3(a)]. However, the mois-
ture available in the anchor trench apparently was insufficient to
support swelling that would form a gel of hydrated bentonite while
in the trench.
The GCL water content was related directly to the subgrade
water content (Fig. 7). The highest water contents occurred near
the toe of the slope due to water accumulating at the toe; the lowest
GCL water contents occurred near the top of the slope where the
GCL did not hydrate; and intermediate water contents occurred in
the anchor trench, where the GCL was buried in soil. GCL Sample
S-24 was the only exception to the trend. This GCL had substantial
bentonite loss, and the bentonite that remained was commingled
with other soil-like material, as discussed subsequently). This com-
mingled material had less affinity for water than did the other
samples.
Mass per area of bentonite in the GCL samples is shown versus
distance along the slope in Fig. 6(b). The influence of downslope
erosion of bentonite in the GCL is evident; most GCL samples
in the lower half of the slope had either modest or substantial loss
of bentonite, or substantial accumulation of bentonite. GCL sample
SE-3 near the top of the slope had substantial bentonite loss
[Fig. 6(b)], and three GCL samples distributed throughout the slope
had moderate bentonite loss [Fig. 6(b)]. In contrast, six samples
from the lower half of the slope had substantial bentonite accumu-
lation associated with deposition of eroded material from upslope
locations [Fig. 6(b)]. As indicated previously, the bentonite on the
upper surface of these GCL samples was very difficult to remove,
and likely was responsible for the higher bentonite mass per area of
the six samples from the lower half of the slope.
One GCL sample (S-24) from the lower part of the southern
slope had only 1.6kg=m2of mineral solid remaining. Some of
this mineral solid appeared to be material that had migrated from
the subgrade and into the GCL, because the mineral solid from
the GCL had lower montmorillonite content (57%) than the other
GCL samples [73%–82% (Table 1)]. The mineral solid in GCL
sample S-24 also had a different appearance, that resembled
a mixture of bentonite and the subgrade soil. The sump in the
cell was located directly adjacent to the sampling location for
S-24, and the water level in the sump region varied significantly
with annual precipitation cycles. Hydrodynamic forces acting on
the GCL due to varying water levels over time likely contributed
to bentonite loss as well as to entrainment of subgrade into
the GCL.
020406080100120
0
5
10
15
20
25
30
35
East
South
Southeast
Anchor
Panel Sep.
GCL Bentonite Water Content (%)
Distance from Top of Slope (m)
S-24
Substantial
Bentonite Loss
(a)
012345678
0
5
10
15
20
25
30
35
Bentonite Mass per Area (kg/m
2
)
Distance from Top of Slope (m)
Substantial Loss
Substantial Accumulation
(b)
New GCL
Modest Accumulation orLoss
SE-3
S-24
Fig. 6. (Color) (a) Water content of bentonite in GCL; and (b) bentonite mass per unit area along length of slope.
Fig. 5. (Color) Photograph of surface of GCL showing rivulets of
eroded bentonite that had migrated down slope with flowing water.
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Swell Index and Exchange Complex
The SI of the exhumed GCL varied over a broad range, from
3mL=2g for Sample S-24 near the toe of the slope to a high
of 24 mL=2g at the panel separation at the top of the slope [Table 1
and Fig. 8(a)]. The virgin GCL had a SI of 32 mL=2g, suggesting
that the SI of the bentonite in all GCL samples diminished while
in service. The SI generally decreased with distance downslope,
although the SI varied considerably at any distance along the slope
[Fig. 8(a)]. The mole fraction of monovalent cations (XM) followed
a similar trend with distance from top of slope; the highest XMwas
at the top of the slope, the lowest XMwas at the bottom of the slope,
and XMvaried at midslope [Fig. 8(b)].
The variation in SI is attributed primarily to exchange of poly-
valent cations (e.g., Ca2þand Mg2þ) for the Naþoriginally in the
exchange complex, as illustrated by the strong trend between the SI
and XMin Fig. 9(a). The data in Fig. 9(b) indicate that there was no
systematic variation in the montmorillonite content of the bentonite
along the length of the slope, further suggesting that the variation in
SI along the slope was associated with cation exchange and not
variations in mineralogy. Sample S-24 is an exception; this sample
was highly eroded, and had a very low SI and much lower mont-
morillonite content than all other samples [Fig. 9(b)]. As indicated
previously, the solids in Sample S-24 appeared to be predominantly
subgrade rather than bentonite, which is consistent with the very
low SI and very low montmorillonite content.
The higher SI at the top of the slope, 20–24 mL=2g [Fig. 8(a)],
agrees with the visual observations of these samples, which had
granular appearance as if they had never hydrated. The lower SI
downslope is consistent with wet–dry cycling, in which Ca2þand
Mg2þfrom adjacent soils replaced Naþinitially present in the
exchange complex. Samples removed from the anchor trench
had a lower SI despite being near the top of slope {11–16 mL=2g
[Table 1and Fig. 8(a)]}, and lower XMrelative to other samples
near the top of the slope. The lower SI and XMin the anchor trench
likely were due to the greater availability of water in the trench,
facilitating transfer of Ca2þand Mg2þfrom the trench backfill to
the bentonite as the bentonite hydrated. The SI and XMof the GCL
Table 1. Characteristics of subgrade and exhumed GCL samples outside of strip
GCL sample
IDa
Subgrade
water
content (%)
Bentonite
water
content (%)
GCL
mass/area
(kg=m2)b
Bentonite
montmorillonite
content (%)
Bentonite swell
index (mL=2g)
Mole fraction
bound monovalent
cations (XM)
GCL hydraulic
conductivity
(m=s)
GCL post-test
water content
(%)
New GCL —15 4.2 84 32 0.65 4.5×10−11 151
E-0 5.0 14 4.5 78 20 0.56 3.9×10−11 148
E-15 2.4 14 6.2 75 10 0.15 2.6×10−677
E-30 8.5 39 4.9 79 10 0.03 1.3×10−698
S-0-A 1.1 13 4.4 —22 0.64 2.5×10−11 184
S-0-B 2.1 14 3.5 —20 —6.0×10−11 189
S-12 1.1 11 6.1 —20 0.61 3.1×10−11 151
S-24 4.4 2 1.6 57 3 —2.6×10−6—
S-24-OL 6.5 25 —— 10 0.12 2.1×10−781
SE-0-A 0.8 15 4.5 81 23 0.65 2.9×10−11 163
SE-0-B 1.1 16 4.5 —23 —4.9×10−11 —
SE-3 —16 1.7 —14 —7.5×10−795
SE-6 —16 4.8 —21 —5.2×10−7220
SE-9 —17 5.0 —10 —1.1×10−779
SE-12-A 1.3 14 4.8 73 18 0.56 2.4×10−11 124
SE-12-B —17 4.2 —9—1.4×10−675
SE-15 —18 6.1 —10 —1.1×10−671
SE-16 2.2 16 3.5 —14 —1.1×10−680
SE-18 —17 3.9 —12 —8.8×10−763
SE-21 —17 6.5 —13 —4.1×10−771
SE-24 —55 6.1 —7—8.3×10−9—
SE-27-A 16.0 86 5.8 82 8 0.02 3.1×10−7100
SE-27-B 11.0 107 4.2 —7—1.3×10−8—
A1 8.1 32 5.4 78 11 0.22 1.2×10−691
A2 8.1 37 5.2 —16 0.38 3.9×10−11 133
PS 1.4 10 —— 24 0.66 9.7×10−12 133
Note: “—” indicates not measured.
aE = east slope, S = south slope, SE = southeast slope, A = anchor trench, PS = panel separation, and OL = overlap.
bIncludes geotextiles and bentonite; initial mass per unit area of installed GCL ≈4.2 kg/m2.
0
20
40
60
80
100
120
140
160
0 5 10 15 20
East
South
Southeast
Anchor
Panel Sep.
Exhumed GCL Water Content (%)
Subgrade Water Content (%)
SE-27-B
SE-27-A
SI > 18 mL/2 g
X
M
>0.5
Substantial Bentonite Loss, S-24
Fig. 7. (Color) Exhumed GCL water content versus subgrade water
content.
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samples from the anchor trench also illustrate how the GCL proper-
ties varied significantly over short distances. The anchor trench
samples were collected from locations separated by only 300 mm,
and yet had significantly different SI (11 versus 16 mL=2g) and
XM(0.22 versus 0.38) (Table 1and Fig. 8).
Hydraulic Conductivity
The hydraulic conductivity of the GCL varied by more than five
orders of magnitude along the slope, ranging from 9.7×10−12
to 2.6×10−6m=s, with no systematic variation along the slope
length (Fig. 10). GCL samples with the lowest hydraulic conduc-
tivities were exhumed from the very top of the slope. Two of the
samples collected at midslope also had very low hydraulic conduc-
tivity, ≈10−11 m=s [S-12 and SE-12A (Table 1and Fig. 10)]. The
other samples along the slope had hydraulic conductivities on the
order of 10−8to 10−6m=s; most were between 1×10−7and
2×10−6m=s (Table 1and Fig. 10). One of the samples from the
anchor trench, A2 (Table 1) had very low hydraulic conductivity
(3.9×10−11 m=s) comparable to that of a virgin GCL, whereas
the other, A1, was very permeable [1.2×10−6m=s (Table 1
and Fig. 10)]. GCL samples collected near or at the toe of the
slope had more-varied hydraulic conductivities (8.3×10−9to
2.6×10−6m=s), but all were much more permeable than the GCLs
at the very top of the slope or a virgin GCL. The broad range of
hydraulic conductivities is consistent with the highly variable
condition of the GCL observed during exhumation, and reflects
the range of conditions to which the GCL was exposed while in
service.
Hydraulic conductivity as a function of SI is shown in
Fig. 11(a). GCL samples with bentonite with SI >15 mL=2g
had hydraulic conductivities less than 6.0×10−11 m=s, with one
0 5 10 15 20 25
0
5
10
15
20
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30
35
South
Southeast
Anchor
Panel Sep.
East
Swell Index (mL/2 g)
Distance from Top of Slope (m)
S-24
Substantial
Bentonite
Loss
0.0 0.2 0.4 0.6 0.8 1.0
0
5
10
15
20
25
30
35
East
South
Southeast
Anchor
Panel Sep.
Mole Fraction Monovalent Cations, X
m
Distance from TopofS
lope (m)
(a)
(b)
Fig. 8. (Color) (a) Swell index; and (b) mole fraction of monovalent cations as a function of distance from top of slope.
0
5
10
15
20
25
East
South
Southeast
Anchor
Panel Sep.
Swell Index(mL/2g)
Mole Fraction Monovalent Bound Cations, XM
0
5
10
15
20
25
East
South
Southeast
Anchor
Swell Index (mL/2 g)
GCL Montmorillonite Content (%)
S-24 Substantial
bentonite loss
0.0 0.2 0.4 0.6 0.8 1.0
020406080100
(a)
(b)
Fig. 9. (Color) Swell index of bentonite from exhumed GCLs as a function of (a) mole fraction of monovalent bound cations, XM; and
(b) montmorillonite content.
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exception {SE-6 [Fig. 11(a)]}, whereas those with SI < 15 mL=2g
had hydraulic conductivities exceeding 1.0×10−7m=s, with SE-24
and SE-27-B as exceptions. GCL SE-6 had high hydraulic
conductivity despite having SI ¼21 mL=2g because the bentonite
had thinned substantially in one area (Fig. 12), allowing flow to
occur preferentially through the thin region despite the bentonite
having higher SI.
Samples SE-24 and SE-27-B were obtained beneath the wedge
of material that accumulated near the toe of slope, which may have
contributed to their lower hydraulic conductivity relative to the
other GCLs with SI < 15 mL=2g. Nevertheless, even these GCL
samples were much more permeable than a virgin GCL, with hy-
draulic conductivities exceeding 8×10−9m=s. The high hydraulic
conductivities of GCL samples with SI < 15 mL=2g were due to
flow through desiccation cracks [Fig. 13(a)] or through needle-
punching fibers that did not seal shut during hydration due to the
low SI of the bentonite, despite the absence of desiccation cracks
{SE-27-A [Figs. 13(b and c)]}.
No systematic relationship was identified between the hydraulic
conductivity of the GCL samples and mass per area [Fig. 11(b)].
GCLs with at least 3.5kg=m2mass per area had hydraulic
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
0
5
10
15
20
25
30
35
Southeast
East
South
Anchor
Panel Sep.
Hydraulic Conductivity (m/s)
Distancefrom Top ofSlope(m)
Fig. 10. (Color) Hydraulic conductivity of GCLs as a function of
distance from top of slope.
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
0510152025
East
South
Southeast
Anchor
Panel Sep.
Hydraulic Conductivity (m/s)
Swell Index (mL/2 g)
SE-6
SE-27-B
SE-24
10-11
10-10
10-9
10-8
10-7
10-6
10-5
1234567
East
South
Southeast
Anchor
Hydraulic Conductivity (m/s)
Mass per Area (kg/m2)
SE-3
S-24
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
0 20 40 60 80 100 120 140
East
South
Southeast
Anchor
HydraulicConductivity(m/s)
Swell Index x Mass per Area (mL/2 g - kg/m
2
)
SE-3
S-24
(a)
(b)
(c)
Fig. 11. (Color) Hydraulic conductivity of GCL as a function of (a) mass of bentonite per unit area; (b) swell index; and (c) product of swell index and
mass per unit area.
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conductivities ranging from approximately 1×10−11 m=sto1×
10−6m=s regardless of their mass of bentonite per unit area.
The two GCLs with mass per area less than 3.5kg=m2had very
low mass per area (1.7 and 1.6kg=m2) and very high hydraulic
conductivities (7.5×10−7and 2.6×10−6m=s). However, these
two GCLs also had SI <15 mL=2g, and therefore did not have
sufficient swelling to achieve low hydraulic conductivity regardless
of mass per area. A threshold mass per area likely exists below
which the GCL will not have low hydraulic conductivity regardless
of the SI of the bentonite. However, no threshold mass per area
necessary for low hydraulic conductivity was identified from the
GCL samples evaluated in this study.
Hydraulic conductivity is shown in Fig. 11(c) as a function
of the product of SI and mass per unit area (MPA). The parameter
SI ×MPA represents the combined attributes of sufficient benton-
ite in the GCL and adequate swelling of the bentonite to close
intergranular pores and desiccation cracks. Low hydraulic con-
ductivity is associated with high SI ×MPA, and high hydraulic
conductivity with low SI ×MPA. More-variable hydraulic conduc-
tivity occurs for intermediate SI ×MPA (60–100 mL=2g-kg=m2),
and a threshold of SI ×MPA ¼100 mL=2g-kg=m2may be ap-
propriate to ensure low hydraulic conductivity. Further systematic
testing is needed to assess the efficacy of SI ×MPA as an indicator
of hydraulic conductivity.
Summary and Practical Implications
A composite liner for the base of a landfill comprised of a high-
density polyethylene geomembrane over a needlepunched geosyn-
thetic clay liner containing granular sodium bentonite was exhumed
after 12 years of atmospheric exposure. A leachate collection sys-
tem was never installed, and waste was not placed due to changes
in landfill operations after construction of the liner was completed.
This resulted in continuous atmospheric exposure of the liner.
Two sets of thermocouples were installed prior to exhuming the
liner to understand temperature variations with depth and the direc-
tion and magnitude of the thermal gradient. Thermocouples were
installed on the upper surface of the geomembrane, both sides of
the GCL, and in the subgrade at depths of 75 and 150 mm below the
subgrade surface. Data were collected every 3 min from the thermo-
couple sets for 18 days. These data illustrated that significant tem-
perature gradients existed in the liner system, with a downward
gradient during the daylight hours and an upward gradient at night.
The upward thermal gradient causes moisture from the GCL and
subgrade to migrate upward, with the moisture accumulating as
condensation at the interface of the geomembrane and GCL.
Moisture that accumulated at the interface of the geomembrane
and GCL migrated downslope along the interface, and eroded
the bentonite in the GCL (predominantly) and subgrade material.
The eroded material migrated downslope and collected in a large
wedge-shaped feature near the toe of slope. In addition, a mound
composed mainly of subgrade solids accumulated near the toe of
the slope in a location adjacent to the cell sump, most likely due to
fluctuations in the water level in the sump area, which was not
being pumped.
Samples of the GCL were collected from the top of the slope,
the midslope, and the toe of the slope on transects on the eastern
slope and the southern slope. A series of samples was collected
along a transect spanning the entire length of the slope in the
southeast corner. Two GCL samples were collected from the anchor
trench, one sample was collected from a seam overlap, and one
sample was collected in a region in which the GCL panels had
separated. Hydraulic conductivity tests were conducted on each
of the GCL samples, and water content and swell index of the
bentonite were determined. The composition of the exchange com-
plex, the GCL mass per area, and the montmorillonite content were
measured in bentonite within a subset of the GCL samples.
The following conclusions are drawn based on the findings in
this study:
•Exposure of a geomembrane–GCL composite liner for extended
periods can cause considerable damage to the GCL, including
both loss of bentonite due to erosion, and wet–dry cycling of the
bentonite coupled with cation exchange. Collectively, these
Fig. 13. (Color) Cross sections of GCL samples illustrating different
causes of high hydraulic conductivity: (a) desiccation cracks in E-30
from the bottom of the east slope; (b) uncracked bentonite structure of
SE-27-A; and (c) stained needlepunching fibers in SE-27-A from the
bottom of the slope in the southeast corner.
Thicker
Section
Thinner
Section
Fig. 12. (Color) Photographs from different orientations illustrating
thinner and thicker areas of Specimen SE-6.
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mechanisms can result in large increases in hydraulic conduc-
tivity of the GCL.
•Loss of bentonite and entrainment of overlying or underlying
soils may occur in GCLs at toe-of-slope locations when liquid
levels on the liner vary substantially and little to no overburden
stress is present to maintain firm contact between the liner
components.
•The GCLs with a hydraulic conductivity similar to that of a
virgin GCL had low subgrade and bentonite water contents.
These samples, which were from the top of the slope and from
some midslope locations, likely never hydrated and therefore
did not undergo adverse alterations over the exposure period.
The GCLs with high hydraulic conductivity had both low and
high subgrade and bentonite water content, and were located at
the midslope to the toe of the slope. These GCLs likely under-
went multiple wet–dry cycles along with erosion of bentonite,
which caused cracking of the bentonite and bentonite loss.
•All the GCLs that maintained low hydraulic conductivity had
bentonite swell indexes exceeding 15 mL=2g. The swell index
was related directly to the replacement of Naþin the exchange
complex by divalent cations, such as Ca2þ. The divalent cations
most likely came from the underlying subgrade, migrating
upward in subgrade pore water that was hydrating the GCL.
Loss of swell of the bentonite prevented desiccation cracks
in the bentonite from swelling shut when hydrated, resulting
in high hydraulic conductivity. The potential impacts of cation
exchange due to hydration on a subgrade should be considered
carefully when using GCLs.
•One of the GCL samples had higher hydraulic conductivity
despite having bentonite with a swell index in excess of
15 mL=2g and a mass per area comparable to that of a virgin
GCL. This GCL contained a section in which the bentonite
had thinned, and this thin section allowed flow to pass through
more readily, resulting in higher hydraulic conductivity. Thus,
mechanisms that cause thinning, such as erosion or stress con-
centrations, have the propensity to cause higher hydraulic con-
ductivity even if the bentonite is not altered significantly by
cation exchange. A parameter, the product of swell index and
mass per area of bentonite (SI ×MPA), is suggested as a means
to represent the contributions of bentonite swell and sufficient
mass per area of bentonite.
The findings from this study show that composite liners should
be covered as soon as practical with sufficient overburden to ensure
firm contact between the geomembrane and GCL so as to limit the
magnitude of thermal cycling, moisture migration at geosynthetic
interfaces, erosion of bentonite in the GCL due to thermal cycling,
and alterations of the GCL due to fluctuating water levels near areas
in which water collects or is managed. In addition, if cells with a
composite liner with a GCL are left open, the sump area should be
actively pumped to minimize the potential for large fluctuations in
water level that may affect the lining components over time.
Data Availability Statement
All data that support the findings of this study are available from the
corresponding author on request.
Acknowledgments
Financial support for this study was provided by the US Depart-
ment of Energy (DOE) under cooperative agreement DE-FC01-
06EW07053 (Consortium for Risk Evaluation with Stakeholder
Participation III) and the Global Waste Research Institute at
California Polytechnic State University. Waste Connections Inc.
and Cold Canyon Landfill are acknowledged for allowing site
access and sampling of the liner system. Amro El Badawy, Kyle
O’Hara, John Buringa, Sean Herman, and Spencer Jemes assisted
with sampling. Jonathan Owen, Anthony Trujillo, and Brett Crews
assisted with laboratory testing. Med Stop Urgent Care in San Luis
Obispo, California provided the X-ray imaging for the study.
References
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