Thermomechanical Behavior of SaturatedGeosynthetic Clay Liners

Hossam M. Abuel-Naga1 and Abdelmalek Bouazza2

Abstract: The aim of this study is to assess experimentally the volume change behavior of geosynthetic clay liners (GCLs) under elevatedtemperatures. Such elevated temperatures can be generated in landfills as the result of biological decomposition of organic matter in municipalsolidwaste.Amodifiedconsolidometer capableof handling temperatures up to60�Cwas utilized for this purpose. A series of thermomechanicalconsolidation tests were performed on two different GCLs at different elevated temperatures, varying from 20 to 60�C, and different verticalstress levels (50, 100, and 150 kPa). The results show that the thermally induced volume change is stress dependent. A preliminary conceptualexplanation for this behavior is introduced and discussed in this study. DOI: 10.1061/(ASCE)GT.1943-5606.0000799. © 2013 AmericanSociety of Civil Engineers.

CE Database subject headings: Landfills; Temperature effects; Soil consolidation; Hydraulic conductivity; Clay liners; Geosynthetics;Thermal factors; Mechanical properties.

Author keywords: Geosynthetic clay liner (GCL); Landfill; Temperature; Consolidation; Hydraulic conductivity.

Introduction

Geosynthetic clay liners (GCLs) are thin manufactured hydraulicbarriers comprised of a layer of bentonite bonded to layers of geo-textiles and/or geomembranes. A typical GCL thickness variesbetween 5 and 10 mm. The primary differences between GCLs arethe mineralogy and form of bentonite (e.g., natural sodium versussodium-activated calcium bentonite, powder versus granular forms,etc.) used in the GCL, the type of geotextile (e.g., woven versusnonwoven geotextiles), the addition of a geomembrane, and thereinforcement methods.

Over the past two decades, GCLs have become integral com-ponents of modern municipal solid waste landfill lining systems.They are increasingly used as an alternative to compacted clays incover and bottom lining systems of waste containment facilitiesbecause they often have very low hydraulic conductivity to water(, 103 10210 m/s) and because of their relatively low cost and easeof installation. As a result, they have been subjected to considerableresearch, pertaining especially to their hydraulic and diffusionproperties, chemical compatibility, mechanical behavior, durability,and gas migration (Bouazza 2002; Bouazza et al. 2006, 2008;Bouazza and Rahman 2007; McCartney et al. 2009; Gassner 2009;Guyonnet et al. 2009; Abuel-Naga and Bouazza 2009, 2010; Gateset al. 2009;Benson et al. 2010a, b; Shackelford et al. 2010;Gates andBouazza 2010; Bouazza and Bowders 2010; Rowe et al. 2010;Scalia and Benson 2011; Rayhani et al. 2011).

Heat generated in landfills, as the result of the biological de-composition of waste, has been identified as a factor that can impactthe service life of liners, including geosynthetic clay liners (Rowe2005; Southen and Rowe 2005). The available data indicate thatlandfill liner temperatures can be expected to reach up to 60�Cunder normal landfill operations (Yesiller et al. 2005; Rowe 2005;Koerner and Koerner 2006; Bouazza et al. 2011). Even highertemperatures, up to 70�C, may occur at the base of landfills if thereis a significant leachate mound within the landfill (Yoshida et al.1996). These results highlight the fact that GCLs, if used as part oflandfill barrier systems, may be subjected to high temperaturevariations and thermal gradients that may impact their long-termhydro-mechanical performance. Therefore, an understanding of thecoupled thermomechanical behavior of GCLs is required for ef-fective and safe design of landfill liner systems.

The objective of this study was to examine the effects ofthermal loading on the volume change of GCLs. For this purpose,a modified consolidometer capable of applying temperatures upto 60�C was utilized to test two different GCLs. The metho-dology, procedures, and analysis are presented with the aim ofadvancing our understanding of key variables and processesinfluencing the volume change behavior GCLs at elevatedtemperatures.

Tested Material and Specimen Preparation

Two different types of GCLs, referred to as GCL 1 and GCL 2,were investigated in this study. Their properties are listed inTable 1. The moisture absorption test [DIN 18132; DeutschesInstitut Fur Normung E.V. (DIN) 1995] was used to assess in-directly the difference in the surface area between the bentonitecomponent of GCL 1 and GCL 2. The moisture absorption testprocedure requires the placement of 1 g of dried bentonite ona glass filter within a cylinder. The cylinder is funneled intoa graduated capillary tube filled with water. The bentonite absorbsthe water through the filter, causing a reduction of the waterlevel in the tube. The change in water volume after 24 h and the

1Senior Lecturer, Dept. of Civil and Environmental Engineering, Univ.of Auckland, Private Bag 92019, Auckland 1142, New Zealand (corre-sponding author). E-mail: h.naga@auckland.ac.nz

2Professor, Dept. of Civil Engineering, Monash Univ., Melbourne, VIC3800, Australia. E-mail: malek.bouazza@monash.edu

Note. This manuscript was submitted on March 30, 2011; approved onJune 22, 2012; published online on August 1, 2012. Discussion period openuntil September 1, 2013; separate discussions must be submitted for in-dividual papers. This paper is part of the Journal of Geotechnical andGeoenvironmental Engineering, Vol. 139, No. 4, April 1, 2013. ©ASCE,ISSN 1090-0241/2013/4-539–547/$25.00.

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corresponding weight of water is recorded as a percentage of theoriginal weight of bentonite. The result of this test is defined by anEnslin-Neff value (EN) as follows:

EN ¼ absorbed water weight at 24 hDry bentonite weight

� 100 ð1Þ

The results of the moisture absorption test (DIN 18132) show thatEN values are similar for the bentonites fromGCL 1 and GCL 2, aslisted in Table 1. Consequently, we can consider that their surfaceareas are roughly similar, as they absorbed similar amounts ofwater.

GCL specimens were cut from larger GCL sheets, using a sharputility knife, to A4 size paper. A circular stainless steel cutting ringwith an inner diameter of 76 mm was used to cut the GCL speci-mens. Each GCL specimen was placed between a cutting ring andplywood, which was used as a cutting base. The GCL was thenplaced on the platen of a compression machine for cutting. Toachieve an effective cut, a force of e15e20 kN was applied. Thecutting ring containing a GCL specimen was weighed to determinethe mass per unit area. The initial unhydrated GCL height wasmeasured under an applied normal stress of e2 kPa using a high-gauge vernier. The GCL was then gently pushed from the cuttingring directly into the consolidometer ring. A plastic disc witha diameter slightly smaller than the inner diameter of the ring wasused to support the GCL during extraction from the cutting ring.Care was taken not to lose any bentonite from the outer edge of theGCL sample.

Test Apparatus

A conventional oedometer apparatus was modified to include a ringheater attached to the outer oedometer ring, a K-type thermocouple,

a water tank, and a thermocontroller unit with an accuracy60.1�C,as depicted in Fig. 1(a). The temperature of the GCL specimenundergoing consolidation is increased indirectly by heating thewater in the annular space between the outer ring of the oedometerand the specimen ring. The thermocouple was placed in theoedometer annulus space to avoid specimen disturbance and wasused for temperature measurements and to provide the feedbacksignal for the thermocontroller unit. The water tank was used tocompensate for the evaporated water from the oedometer cell. Acalibration test with two thermocouples placed in the oedometerannulus space and in the center of the test specimen showed that thetime to attain the required temperature (i.e., the temperature of thewater being maintained in the annulus) at the center of the GCLspecimen undergoing consolidation was e20 min. Furthermore, thethermal gradient between the water being maintained in the annulusspace and the center of the test specimen was less than 1�C. Theaverage thermal vertical deformation of the oedometer based on threeheating/cooling cycles is shown in Fig. 1(b). Vertical deformationwasmeasured using aLVDT.Thesemeasurementsweremadewherethe top cap of the oedometer was directly placed over the saturatedporous stone discs. The results show almost reversible expansion ofthe oedometer after having been subjected to heating/cooling cycles.The calibration results were used to correct the measured readingsunder nonisothermal conditions during actual tests.

Table 1. Properties of Tested Material

Property GCL 1 GCL 2

GCL mass per unitarea (kg/m2)

5.0–5.5 6.0–6.4

As-received GCLthickness (mm)

8–9 5–6

Mineral compositionof bentonite

90% smectite,4% cristobalite,3% quartz,3% Ca-albite

88% smectite,7% anorthite,3% quartz,2% calcite

Atterberg limits LL 5 405% LL 5 540%PI 5 351% PI 5 502%

Bentonite form Powder GranularDensity of the bentonitelayer (kg/m3)

940–1,175 1,567–1,757

Bentonite mass per unitarea (kg/m2)

4.7 5.8

Thickness of bentonitelayer (mm)

4.0–5.0 3.3–3.7

Cover geotextile type;mass per unit area (kg/m2)

Nonwovenpolypropylene; 0.27

Slit-film wovengeotextile; 0.15

Carrier geotextile;mass per unit area (kg/m2)

Nonwoven geotextilereinforced by a wovengeotextile; 0.38

Nonwovenpolypropylenegeotextile; 0.30

Bonding method Needle-punchedfibers, heat bonded

Needle-punchedfibers

Swelling pressure (kPa) 175 101Enslin-Neff value (%) 715 715

Fig. 1. Test apparatus: (a) modified consolidometer test apparatus(not to scale); (b) thermal vertical displacement of the modifiedconsolidometer

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The lateral thermal expansion of the stainless steel oedometerring could affect the accuracy of the one dimension test conditionand consequently the thermally induced volume change mea-surements. To evaluate the error induced by this process, a constantvolume assumption was used where the diametrical expansion canbe related to an equivalent vertical deformation change, as follows:

DhðDTÞho

¼ 22

�DdðDTÞ

do

�¼ 22aeDT ð2Þ

whereDhðDTÞ5 vertical deformation of test specimen as a result ofthe change in oedometer ring diameter DdðDTÞ at temperaturechange DT ; ho and do 5 height of test specimen and oedometer ringdiameter at room temperature (do 5 75 mm), respectively; andae 5diametrical thermal expansion coefficient of the oedometer ring.Abuel-Naga et al. (2006) reported that for a stainless steel ring,ae 5 1:6373 1025�C21.

According to Burghignoli et al. (1995), the thermally inducedvolumetric strain error generated by the lateral thermal deflectionsof an oedometer cell during heating, DɛT , can be calculated by takinginto account the effect of the thermal expansion of clay particles, asfollows:

DɛT ¼�ðDhðDTÞ=hoÞ2asDT

1 þ asDT

�ð3Þ

where as 5 coefficient of volumetric thermal expansion of clayparticles (as 5 3:53 1025�C21, from Campanella and Mitchell1968). Substituting the vertical strain induced by the thermal ex-pansion of the oedometer ring, determined using Eq. (2), into Eq. (3)(for typical values relevant to this study: DT5 40�C) yieldsDɛT 529:023 1025, which is very insignificant and therefore canbe neglected. A similar conclusion was reported by Towhata et al.(1995) and Romero et al. (2003) for their nonisothermal oedometricconsolidation tests.

Experimental Program

The experimental program involved measuring the thermally in-duced volume change of 75-mm-diameter GCL specimens at dif-ferent vertical pressures (50, 100, and 150 kPa) under changingtemperatures (20 to 60 to 20�C). An incremental, drained heating/cooling approach was adopted (Towhata et al. 1993; Delage et al.2000; Abuel-Naga et al. 2006). This approach involves raising/decreasing the temperature incrementally (10�C) once the volumechange is stabilized. It is to be noted that this study only investigatesthe thermomechanical behavior of water (deaired) saturated GCLs.However, it should be mentioned that the effect of temperature onthe behavior of GCLs in contact with leachate could be different as itinvolves a chemo-thermomechanical (CTM) multicoupled process.

The experimental testing program for GCL 1 and GCL 2 isdepicted in Fig. 2. After installing the dry GCL specimen in theoedometer ring, a vertical stress of 50 kPa was applied to it, and itsvertical displacement was monitored until it stabilized. Then, theGCL specimen was hydrated under 50-kPa vertical stress by fillingthe annular space between the oedometer outer and inner ring withdistilled deaired water. The hydration heave of the GCL specimenwas observed until equilibrium was achieved, i.e., when the changein heave was less than 0.005 mm over a 24-h period. At the end ofthe hydration stage, the GCL specimen was subjected to the des-ignated vertical consolidation pressures and temperatures, as shownin Fig. 2. The heating phase of a GCL specimen was started after

finishing the mechanical consolidation stage. Vertical deformationreadings were recorded during the heating and cooling phase. Toconfirm the observed volume change behavior under the elevatedtemperatures, duplicate GCL specimens of each GCL type (GCL 1,GCL 2) were tested.

Test Results and Discussion

The heave of GCL specimens (GCL 1, GCL 2) during the hydrationprocess under 50 kPa vertical stress is shown in Fig. 3. Theobserved slight difference in the heave behavior of the two identicalspecimens of eachGCL type (GCL 1,GCL 2) can be attributed to thepossible difference in the density and initial moisture content of thebentonite component. The results show that GCL 2 had a higherswelling capacity than GCL 1. The average heave strains of GCL 1and GCL 2 specimens are 12.5 and 29.3%, respectively. Becausethe bentonite clay mineralogy of GCL 1 and GCL 2 are almostsimilar, as listed in Table 1, the difference in their swelling capacitycan be attributed to the difference in density among the bentonite layers,as GCL 2 has a higher initial density, as listed in Table 1. However, itshould also be mentioned that any possible difference in the densityof the needle-punched fibers could affect the swelling capacity ofGCLs (Petrov et al. 1997; Lake and Rowe 2000; Fratalocchi 2005).

Typical results for thermal consolidation corrected for the tem-perature effect on the testing apparatus are shown in Fig. 4. The ob-served initial expansion behavior could be attributed to the thermalexpansion of the apparatus that controls the early time results, as thethermally induced volume change of the test apparatus parts (metal)reaches thermal equilibrium faster than the GCL specimen.

Fig. 5 shows the thickness changes of the tested GCL specimensunder the conduced thermomechanical consolidation path. Theresults indicate that two identical specimens from each GCL typeused in this study (GCL 1, GCL 2) have similar thickness changemodes (expansion or contraction) but different thickness changemagnitudes under the imposed thermomechanical consolidationpath. The differences in thickness change magnitudes could berelated to themanufacture quality control of the GCL (mass per area,density in needle punching, etc.). Fig. 6 depicts the thermallyinduced volumetric strains of GCL 1 and GCL 2 specimens, re-spectively, under different vertical stress levels. Fig. 6(a) indicatesthat GCL 1 specimens undergo irreversible thermally inducedvolumetric contraction upon subjection to a heating/cooling cycle(20 to 60 to 20�C). Generally, during each heating increment,the specimen was observed to contract more with increasingtemperature. Further, additional contractile strain was observedduring cooling. Moreover, the magnitude of the irreversiblethermally induced volumetric contraction strain, upon subjection

Fig. 2. Experimental program

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to a heating/cooling cycle, increases as the vertical stress levelincreases.

The test results of GCL 2 under vertical stresses of 100 and 150kPa are shown in Fig. 6(b). The results look similar to the observedbehavior of GCL 1 specimens shown in Fig. 6(a). The specimensshow irreversible thermally induced volumetric contraction uponsubjection to a heating/cooling cycle. However, under 50 kPavertical stress, the GCL 2 specimens show irreversible thermalvolumetric expansion, as illustrated in Fig. 6(b). Based on theresults shown in Fig. 6, it can be concluded that the magnitude andmode (expansion/contraction) of the thermally induced volumechange of GCL is dependent on the level of stress applied.

Conceptual Explanation for the Temperature Effecton Volume Change Behavior of GCL

Thermomechanical models that are able to reproduce most of theobserved volume change behavior of saturated clays at increasedtemperatures have been developed by several researchers (Hueckeland Borsetto 1990; Cui et al. 2000; Laloui and Cekerevac 2003;Abuel-Naga et al. 2007); however, these models cannot be used forGCLs, as they do not incorporate the influences of layered geo-materials. The volume change behavior of GCLs depends on the

engineering properties of the bentonite and geotextile componentsof the GCL as well as on the composite structure configuration, inparticular, the properties of the reinforcement fibers (Petrov et al.1997; Fratalocchi 2005). However, under a constant vertical stress, itcan be assumed that subjecting the GCL to a heating/cooling cyclewill induce a very small and insignificant change in the thicknessof the GCL geotextile layers. Therefore, the thermally inducedthickness change of the hydrated GCL is assumed to be attributedmainly to a change in the thickness of the GCL bentonite layer.

At volume equilibrium, the GCL bentonite layer is subjected toa net stress (snet), which is the resultant of three stress components,namely, the applied vertical stress (sv), the swelling pressure ofthe bentonite (Pbent

s ), and the mobilized restricted pressure induced bythe reinforcing fibers of GCL (sf ), as depicted in Fig. 7. However, itshould be mentioned that the role of the reinforcing fibers (sf ) willnot be active if the applied vertical stress is greater than the swellingpressure of bentonite. Therefore, the thermally induced volumechange of GCL under constant applied vertical stress (sv) couldbe conceptually explained by considering the temperature effects onsf and Pbent

s . Thus, during heating, if the net stress (snet) increases,the GCL will contract, and if it decreases, the GCL will expand.

To the authors’ knowledge, no direct published information isavailable regarding the temperature effect on the engineeringproperties of the reinforcing fibers of GCLs. However, based on theexperimental results reported by Thomas (2002), it could be in-directly concluded that the fibers would expand irreversibly underheating conditions. Furthermore, similar conclusions can also beobtained from the concept of the stepped isothermal method used totest the creep behavior of geotextiles using time-temperature su-perposition methods (Zornberg et al. 2004). Therefore, if sf isactive, it will decrease irreversibly as the temperature increases.

Several studies have shown that the swelling pressure of differentclays decreases as temperature increases (Karnland et al. 1994;Lingnau et al. 1996; Romero et al. 2003; Villar and Lloret 2004;Villar et al. 2010). This behavior was explained in light of thethermally induced lattice contraction caused by dehydration of theinterlamellar space (Pusch et al. 1990). This contraction can lead toirreversible aggregation of particles that may decrease the specificsurface area and consequently irreversibly reduce the swellingcapacity (Al-Homoud et al. 1995).

Therefore, in summary, the proposed explanation for the ob-served thermally induced volume change behavior of GCLsreported in this study is based on two hypotheses. First, the bentoniteswelling pressure (Pbent

s ) has an irreversible temperature-dependent

Fig. 3. Swelling behavior of tested GCL specimens under 50-kPavertical stress, where Ho and Hf are the initial and final thickness ofGCL specimen, respectively: (a) GCL 1; (b) GCL 2

Fig. 4. Typical thermal consolidation test results; GCL 1 (1) specimen

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behavior: it decreases as the temperature increases. Second, themobilized restricted pressure (sf ) induced by the GCL bondingfibers also has an irreversible temperature-dependent behavior; italso decreases as the temperature increases.

The proposed conceptual explanation decomposes the thermallyinduced volumetric strain, ɛv, into two components, namely: a re-versible component of strain, ɛev, and an irreversible component ofstrain, ɛpv . The reversible component can be explained in terms of themineral thermal expansion phenomena (Hueckel and Borsetto 1990;Cui et al. 2000; Laloui andCekerevac 2003;Abuel-Naga et al. 2007),whereas the irreversible component can be attributed to the tem-perature effect on bothsf , andPbent

s . In the following sections, twomainstress cases that control the thermally induced volume change behaviorof GCLs will be explained; then, the observed experimental results ofGCL1andGCL2will bediscussed in lightof these twomainstresscases.

Case 1: sv >Pbents

Under this condition, sf is not active. Therefore, as the temperatureincreases from To to T1 under constant sv, Pbent

s will decrease andsnet will increase, as depicted in Fig. 8(a) on the left-hand side.Consequently, an irreversible contraction volume change, ɛpv , will beinduced, as shown in Fig. 8(a) on the right-hand side. Moreover,

a reversible expansion volume change, ɛev, will also be generated asthe result ofmineral thermal expansion behavior, as shown inFig. 8(a)on the right-hand side. The magnitude of the irreversible, thermallyinduced volume change is a function of the applied vertical stress, as itcontrols the value of Pbent

s , and the rate of Pbents changes with tem-

perature (Villar et al. 2010).

Case 2: sv < Pbents

Under this condition,sf is active. Therefore, upon heating, in additiontomineral thermal reversible expansion behavior, bothsf andPbent

s willdecrease, and an irreversible volume change could be expected. In fact,under this condition, the magnitude and the mode (expansion/contraction) of the thermally induced volume change could be con-trolled by the relation between the rate atwhichsf andPbent

s changewithtemperature. Therefore, three subcases can be considered, as follows:

Case 2a:

����∂Psbent

∂T

����5����∂sf

∂T

����In this case, the temperature effect on sf and Pbent

s is equal. Con-sequently,

Ps stays constant until sf becomes equal to zero at

T 5 Tf , as shown in Fig. 8(b) on the left-hand side. Within thistemperature range (To to Tf ), only reversible expansion volumechange as the result of mineral thermal expansion behavior is

Fig. 5. GCL thickness change under dry loading, hydration, and mechanical and thermal consolidation

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Fig. 6. Thermally induced volumetric strain under different vertical stress levels: (a) GCL 1; (b) GCL 2

Fig. 7. GCL stress system

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expected, as shown in Fig. 8(b) on the right-hand side. Beyond thistemperature range, sf will not be active, and snet will increase as theresult of the decrease in Pbent

s as temperature increases, as depicted inFig. 8(b) on the left-hand side.Consequently, an irreversible contractionvolume change can occur, as shown in Fig. 8(b) on the right-hand side.

Case 2b:

����∂Psbent

∂T

���� <����∂sf

∂T

����In this case, because the temperature hasmore of an effect onsf thanon Pbent

s , snet will decrease, as shown in Fig. 8(c) on the left-handside. Therefore, under a heating path, an irreversible expansion

Fig. 8. Qualitative conceptual explanation for temperature effect on GCL volume change: (a) Case 1; (b) Cases 2a and 2c; (c) Case 2b

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volume change, in addition to the reversible expansion volumechange, as the result of mineral thermal expansion behavior canoccur, as shown in Fig. 8(c) on the right-hand side.

Case 2c:

����∂Psbent

∂T

����>����∂sf

∂T

����Becausesf ismobilized byPbent

s , and the temperature hasmore effecton Pbent

s , the change in sf as temperature increases will be equal tothe change in Pbent

s as temperature increases. Therefore, this case issimilar to Case 2a, as shown in Fig. 8(b).

Behavior of GCL 1 and GCL 2

Upon hydrating the GCL specimens under sv 5 50 kPa verticalstress, sf will be activated as the GCL specimens swell. At thehydration equilibrium, the value of sf will be equal to the diff-erence between Pbent

s and sv; therefore, Pbents .sv. However, it

should be mentioned that the value Pbents at this condition is less than

the GCL swelling pressure under the constant volume conditionlisted in Table 1. As Pbent

s .sv, subjecting this hydrated GCLspecimens to a heating/cooling cycle should generate a thermallyinduced volume change pattern similar to one of the subcases ofCase 2. In fact, the thermally induced volume change results ofGCL 1 and GCL 2 under sv 5 50 kPa, as shown in Fig. 6, confirmthe validity of this explanation, as the results of GCL 1 follows Case2a, whereas the results of GCL 2 follows Case 2b. The differencebetween Cases 2a and 2b can be attributed to the difference in thereduction rate of Pbent

s and sf as the temperature increases, asexplained earlier.

After subjecting the GCL specimens under sv 5 50 kPa toa heating/cooling cycle, the value of Pbent

s decreases irreversibly.However, upon increasing the vertical stress from 50 to 100 kPa atambient temperature, in accord with the experimental programshown in Fig. 2, the value of Pbent

s will increase as the bentonite drydensity increases. Nevertheless, the new value of Pbent

s is expected tobe less than the applied vertical stress (sv 5 100 kPa). Therefore, asPbents ,sv, the thermally induced volume change behavior of the

GCL specimens should follow the pattern of Case 1, in which ir-reversible, thermally induced volume change is expected. The ex-perimental results of GCL 1 and GCL 2, under sv 5 100 kPa(Fig. 6), support this explanation.

Defined by the thermomechanical path shown in Fig. 2, at theend of the heating/cooling cycle under sv 5 100 kPa the verti-cal stress is increased from 100 to 150 kPa. As a result, Pbent

salso increases, but it is still less than the applied vertical stress(sv 5 150 kPa). Therefore, the thermally induced volume changepattern of Case 1 again is expected. The experimental results ofGCL 1 and GCL 2, under sv 5 150 kPa, shown in Fig. 6, alsosupport this explanation. The observed difference in the magni-tude of the irreversible contraction thermally induced volume, atsv 5 100 and 150 kPa, could be attributed to the increase in thebentonite dry density as sv increases. Villar et al. (2010) found that,upon heating, the reduction in the swelling pressure increases as theclay dry density increases. Consequently, the heat will have moreinfluence on the swelling pressure, Pbent

s , at sv 5 150 kPa, anda larger contraction thermally induced volume change is expected,as shown in Fig. 6.

Conclusions

A conceptual framework was proposed in this study to explain thethermally induced volume changes of GCLs. The hypothesis of theframework is that the thermally induced volume change of GCLs isa function of three parameters: (1) the swelling pressure of bentonite,

(2) the applied vertical pressure, and (3) the restricted swellingpressure induced by the GCL bonding fibers. The conceptualframework was found to provide a reasonable interpretation of thethermally induced volume change of GCLs measured experimen-tally in a temperature-controlled oedometer. Although the basisof the conceptual framework is supported by experimental ob-servations for bentonite clays, additional experimental evidence isrequired to confirm the validity of the proposed concept and todevelop a quantitative model.

Acknowledgments

The current study was financially supported by a large grant fromthe Australian Research Council. Our sincere appreciation is ex-tended to the council. The anonymous reviewers made many con-structive comments and valuable suggestions. These commentsand efforts associated with the review are greatly appreciated bythe authors.

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