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Evaluation of the water retention curve for geosynthetic clay liners
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Geotextiles and Geomembranes 25 (2007) 2–9 www.elsevier.com/locate/geotexmem
Evaluation of the water retention curve for geosynthetic clay liners J.M. Southen, R. Kerry Rowe Department of Civil Engineering, GeoEngineering Centre at Queen’s-RMC, Queen’s University, Kingston, Ont., Canada K7L 3N6 Received 22 May 2005; received in revised form 1 September 2006; accepted 16 October 2006 Available online 5 December 2006
Abstract Geosynthetic clay liners (GCLs) are frequently used as part of composite base liners for landfills. Heat generated by the waste can potentially cause drying of the hydrated GCL. In order to better understand the performance of GCLs under such conditions, an assessment of the water retention characteristics under drying conditions is required. To date, such an investigation has not been undertaken for GCLs, although some data exists for compacted bentonite. This paper presents the results of laboratory testing aimed at obtaining water retention curves (WRCs) for two GCL materials representative of those commonly used in practise. Pressure plate and pressure membrane extractors were used to assess the relationship between the degree of saturation and the applied capillary pressure (suction) for a range of capillary pressures between 10 and 10 000 kPa. The effect of overburden pressure was investigated, as was the relationship between capillary pressure and bulk GCL void ratio. Volume change behaviour was found to have a significant effect on the WRCs. Representative parameters were obtained by fitting the predictions of three WRC models to the experimental data. Reasonable fits were achieved and the parameters obtained may be used for numerical modelling. r 2006 Elsevier Ltd. All rights reserved. Keywords: GCL; Unsaturated soil; Soil–water characteristic curve; Water retention curve; Geosynthetic clay liners
1. Introduction An understanding of the behaviour of geosynthetic clay liners (GCLs) is of importance in many applications (Barroso et al., 2006a; Bouazza and Vangpaisal, 2006; Dickinson and Brachman, 2006; Touze-Foltz et al., 2006). One aspect of particular interest is the potential for desiccation of GCLs used in composite base liners for municipal solid waste landfills due to heat generated by the degradation of the waste (Rowe, 2005; Southen and Rowe, 2005a). To address this issue it is necessary to have an understanding of the behaviour of GCLs under unsaturated conditions. The relationship between capillary pressure (suction) within the soil and the volumetric water content or degree of saturation of the soil is termed the water retention curve (WRC) and is a fundamental constitutive relationship governing the behaviour of unsaturated soils (Barbour, 1998; Fredlund and Rahardjo, Corresponding author. Tel.: +1 613 533 6933; fax: +1 613 533 2128, +613 533 6934. E-mail address: [email protected] (R. Kerry Rowe).
0266-1144/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2006.10.002
1993). Numerous techniques are available to evaluate the WRCs of soil, such as the use of tensiometers, pressure plate extractors, thermal conductivity sensors and thermocouple psychrometers (e.g. Fredlund and Rahardjo, 1993). A review of the literature indicates that the only published data relating to the WRC for geosynthetic clay liners was very recently published by Barroso et al. (2006b), although data for compacted bentonite exists (e.g. Marcial et al., 2002; Villar and Lloret, 2004). An investigation into the water retention characteristics of GCLs would improve the understanding of the behaviour of these materials under the unsaturated conditions which prevail in many common applications (e.g. landfill liner and covers). Also, it is necessary to experimentally obtain parameters that describe the WRC of GCLs in order to numerically model their unsaturated behaviour. This paper forms part of a broader study of potential desiccation of GCLs used in composite landfill base liners. As such it focuses on the drying curve since the GCL will typically be hydrated before waste generated temperature induces potential drying. Thus the objective of this paper is to presents the results of a series of laboratory
ARTICLE IN PRESS J.M. Southen, R. Kerry Rowe / Geotextiles and Geomembranes 25 (2007) 2–9
tests aimed at obtaining representative parameters describing the WRCs in drying for two geosynthetic clay liners that are typical of materials used in a lining system for a municipal solid waste landfill. 2. Materials Two GCLs were studied. One GCL is commonly used in landfill basal liner or final cover applications in North America while the other is similarly used in Europe. The first, denoted herein as G1 (Bentofix NSTM) is comprised of a granular sodium bentonite core sandwiched between a slit-film polypropylene woven carrier geotextile and a polypropylene virgin staple fibre nonwoven cover geotextile. The GCL is reinforced by needlepunching and has thermally treated needlepunched fibres (thermal locking). The second GCL, G2 (Bentofix BFG 5000TM) differs from the first in that the core is a powdered sodium bentonite and the cover nonwoven geotextile is impregnated with 800 g/m2 of bentonite. Pertinent material properties for the two GCLs are given in Table 1.
3
3.2. Testing procedure A 100 bar pressure membrane extractor (Soilmoisture Equipment Corp., 2000) was used to test the GCL samples. A schematic of the apparatus is given in Fig. 2. The pressure membrane extractor procedure for evaluating the WRC is based on the axis translation approach, which generates capillary pressure (suction) within the geomaterial by applying air pressure to the samples while maintaining water pressure at the atmospheric level. Further discussion of this approach may be found in Fredlund and Rahardjo (1993). The GCL samples rest on a cellulose membrane within the chamber of the extractor. The very
3. WRC testing 3.1. Sample preparation Sections of GCL approximately 30 cm 30 cm were cut from a roll of each material. These sections were placed in a bath of distilled water and allowed to hydrate, while the change in GCL mass over time was monitored. The GCL mass typically reached a maximum at 10 days, with little variation seen thereafter. At this point, the GCL section was removed from the water bath. Four circular samples approximately 53 mm in diameter were cut from each of the GCL sections using a cutting shoe and hydraulic press. The cut samples were weighed, placed into stainless steel rings and transferred to the pressure extractor. A photograph of a GCL sample at the time of placement within the extractor is shown in Fig. 1.
Fig. 1. GCL specimen within steel ring at time of placement within extractor.
Table 1 Geosynthetic clay liner properties Property GCL G1 Nominal mass/unit area Bentonite mass/unit area Carrier (lower) geotextile Cover (upper) geotextile Construction As-manufactured water content GCL G2 Nominal mass/unit area Bentonite mass/unit area Carrier (lower) geotextile Cover (upper) geotextile Construction As-manufactured water content
Method
Value
ASTM ASTM ASTM ASTM
4.65 kg/m2 4.34 kg/m2 Polypropylene slit-film woven 105 g/m2 Polypropylene virgin staple fibre nonwoven 200 g/m2 Needlepunched 7%
D5993 D5993 D5261 D5261
ASTM D2216 ASTM ASTM ASTM ASTM
D5993 D5993 D5261 D5261
ASTM D2216
5.50 kg/m2 5.00 kg/m2 (incl. 800 g/m2 in NW GT) Polypropylene slit-film woven 200 g/m2 Polypropylene nonwoven 300 g/m2 (impregnated with 800 g/m2 bentonite) Needlepunched 9%
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Handle Clamping Bolt & Nut Top Plate Exhaust Value
Inlet Pressure Stem O-Ring Seals Bottom Plate
Screen Drain Plate Outflow Stem Cross Section of O-Ring
Legs Plate Support Plug
Outflow Stem
Fig. 2. Cross section of pressure membrane extractor (from SoilMoisture Equipment Corp., 2000).
small pore diameter of the cellulose membrane (24 A˚) results in an air entry value exceeding 100 bar, which allows the membrane to remain saturated under high air pressures. When the air pressure is increased within the chamber, water flows from the saturated GCL samples through the cellulose membrane and out through the outflow stem to a burette. The cellulose membrane was saturated in a water bath for a minimum of 15 min, as indicated in the equipment instruction manual. The membrane was then placed in the chamber and the eight GCL samples, contained within stainless steel rings, were placed on the cellulose membrane. To investigate the effects of overburden pressure on the WRC relationship, different stresses were applied to the samples. For each GCL, two samples were subjected to a low confining stress (0.5 or 3 kPa) by means of weighted discs placed on top of the samples. The remaining two samples were subjected to a 100 kPa vertical stress. The samples were allowed to consolidate before determination of the WRC. Once consolidation was complete, the air pressure was increased until the desired capillary pressure was reached. The water level in the outflow burette was monitored to observe when equilibrium had been obtained. In addition to the pressure membrane tests, one series of tests was conducted for each of the G1 and G2 GCLs using a pressure plate extractor (Soilmoisture Equipment Corp., 2002) in an effort to evaluate the behaviour of the materials under lower capillary pressures. The pressure plate apparatus is very similar to the pressure membrane apparatus, except for the use of a ceramic plate in place of the cellulose membrane. Due to the nature of the testing chamber, the samples for this series of tests were not subjected to any loading. The sample preparation
techniques and experimental procedure followed those used in the pressure membrane tests. 3.3. Results Five sets of tests were run within the pressure membrane extractor for each GCL type at applied capillary pressures of 10, 1500, 2000, 4500 and 9000 kPa. As well, tests were conducted at a range of capillary pressures between 10 and 1000 kPa using the pressure plate extractor. The results of the tests for the G1 material are plotted in Fig. 3. Good agreement (715% variation) is generally noted between the two duplicate samples for each capillary pressure tested in the pressure membrane extractor. The air entry value appears to fall within the range of 50–200 kPa, depending on the applied load. The samples under 100 kPa loading tended to have higher degrees of saturation than those under 3 kPa loading. This may be attributed to reduction in void ratio caused by the loading. After testing, the samples under 100 kPa load were 4–8 mm thick or 2–4 mm thinner than those under the lower pressure. The reduction in volume under load is likely to reduce the effective pore size, which would tend to make the GCL more resistant to dewatering. A detailed discussion of the variation in void ratio is presented in Section 3.4. The measured degree of saturation for the specimens with 3 kPa applied load at 10 kPa capillary pressure are lower than would be expected. This may be the result of the samples not being completely saturated prior to the start of the test. This effect would not be as pronounced for samples under load, since consolidation would reduce the air-filled void space and increase saturation. The results for the G2 material are shown in Fig. 4. This material shows greater resistance to dewatering
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1.0
Degree of Saturation
0.8
?
0.6
0.4
0.2
Membrane - 3 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
0.0 1
10
100 1000 10000 Capillary Pressure (kPa)
100000
Fig. 3. Water retention curve for G1 obtained using pressure membrane and pressure plate extractors under an applied load. Fig. 5. GCL specimens following testing under a capillary pressure of 4500 kPa, specimen on the left with applied load of 0.5 kPa, specimen on the right with applied load of 100 kPa. 1.0
3.4. Volume change behaviour
Degree of Saturation
0.8
0.6
0.4
0.2
Membrane - 0.5 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
0.0 1
10
100 1000 10000 Capillary Pressure (kPa)
100000
Fig. 4. Water retention curve for G2 obtained using pressure membrane and pressure plate extractors under an applied load.
than the G1 product, with saturation remaining greater than 0.80 for capillary pressures up to 1000 kPa. The air entry pressure falls within the range of 200–1000 kPa. The trend of higher degrees of saturation for samples tested under higher loads noted for the G1 product is not observed for the G2 material. This may be due to poor contact between the GCL and the cellulose membrane resulting from the lower confining stresses (0.5 kPa) and/or the heavier carrier geotextile used with this GCL. A photo of two GCL specimens taken at the completion of the test using the G2 GCL conducted under a capillary pressure of 4500 kPa is shown in Fig. 5. The sample on the left was tested under an applied load of 0.5 kPa and had an average final thickness of 9.5 mm, while the sample on the right was tested under an applied load of 100 kPa and had an average final thickness of 5.7 mm.
The calculation of the degree of saturation for the GCL specimens reported in Section 3.3 was based on measurements of mass and volume taken at the end of each test. The gravimetric water content obtained through weighing the GCL samples before and after oven drying was converted to volumetric water content and degree of saturation based on the bulk GCL void ratio. Petrov and Rowe (1997) introduced the concept of bulk GCL void ratio to normalize the effects of variable mass of bentonite on GCL height. Bulk GCL void ratio, eB, is defined as the ratio of the volume of voids within the geotextile and bentonite components of the GCL to the volume of solids within the GCL. Further details on the computations may be found in Petrov and Rowe (1997). Fig. 6 shows the variation in bulk GCL void ratio for the tests conducted on the G1 specimens. Fig. 7 gives the similar data for the G2 specimens. It may be seen that the bulk GCL void ratio is highly dependant on the magnitude of both the capillary pressure and applied stress. For the range of capillary pressure examined, the trend of decreasing void ratio with increased capillary pressure appears to be bilinear. The increase in slope noted in Figs. 6 and 7 approximately corresponds with the air entry value noted in the WRCs (Figs. 3 and 4). The slope of the capillary pressure–void ratio curve appears to be flatter when the specimen is subjected to a vertical stress. This implies a coupling effect between the effects of applied stress and capillary pressure. 4. Comparison with bentonite WRCs Data on the water retention behaviour of compacted samples and slurries of bentonite have been reported. Villar and Lloret (2004) give results for FEBEX bentonite, a
ARTICLE IN PRESS J.M. Southen, R. Kerry Rowe / Geotextiles and Geomembranes 25 (2007) 2–9
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4 Gravimetric Water Content
5
Bulk GCL Void Ratio
4
3
2
1
Membrane - 3 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
10
100 1000 10000 Capillary Pressure (kPa)
100000
Fig. 6. Bulk GCL void ratio calculated at termination of pressure plate and pressure membrane tests for the G1 GCL.
5
Bulk GCL Void Ratio
4
3
2
1
Membrane - 0.5 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
0 1
10
100 1000 Capillary Pressure (kPa)
3
e = 0.9 - 2.7 e = 2.1 - 4.6 e = 1.2 - 2.5 e = 2.4 - 4.5 e = 0.6 e = 0.2 - 9.0 e = 0.2 - 7.2 e = unreported e = unreported
Scatter attributed to poor contact with membrane
2
1
0
0 1
G2 (100 kPa Load) G2 (0.5/0 kPa Load) G1 (100 kPa Load) G2 (3.0/0 kPa Load) Villar and Lloret (2004) Marcial et al. (2002) MX-80 Marcial et al. (2002) Kunigel JNC (2000) MX-80 JNC (2000) Kunigel
10000
100000
Fig. 7. Bulk GCL void ratio calculated at termination of pressure plate and pressure membrane tests for the G2 GCL.
predominantly calcium bentonite (42% Ca, 23% Na), compacted to a dry density of 1.65 g/cm3. Marcial et al. (2002) present WRCs obtained from slurries of two sodium bentonites, Kunigel and MX-80. The Japanese Nuclear Cycle Development Institute (JNC) (2000) report WRCs for the same Kunigel and MX-80 materials using powdered samples and a thermocouple psychrometer. Fig. 8 shows the WRCs obtained for the bentonite samples identified above and the two GCL products in terms of gravimetric water content. Included in the inset to Fig. 8 is the range of void ratios for the samples tested, where available. The water contents of the G2 GCL tested under a load of 0.5 kPa are significantly higher than observed water contents in the other GCL tests and in the bentonite data, supporting the belief that intimate contact was not achieved during the tests under low applied load. This may be related to the fact that at these low stresses the geotextile component of the GCL may not be sufficiently compressed to bring bentonite into contact with the
1
10
100 1000 10000 Capillary Pressure (kPa)
100000 1000000
Fig. 8. Comparison of water retention curves obtained for GCLs (solid symbols) and bentonite (open symbols).
apparatus in a uniform manner. The remaining GCL samples give water contents that are reasonably consistent with reported results for bentonite samples in the range of 100–10 000 kPa. The GCL samples exhibit water contents at lower capillary pressures (o100 kPa) that are lower than those observed for bentonite samples. This is attributed to the reinforcing properties of the needlepunched fibres linking the upper and lower geotextiles of the GCL. These fibres restrict swelling at lower capillary pressures, resulting in lower void ratios than were observed for the bentonite samples. As an example, the G2 product under a 0.5 kPa load and 10 kPa capillary pressure had a bulk void ratio of 2.5. The MX-80 bentonite slurry used by Marcial et al. (2002) had a void ratio of 9 at a capillary pressure of 20 kPa. At higher capillary pressures (4100 kPa), the GCL water contents tend to be somewhat higher than those of the bentonite samples, although the discrepancy is not as great as at lower capillary pressures. The void ratio of the GCL specimens tends to be higher in this range of capillary pressure than bentonite samples, which may again be related to the reinforcing nature of the geotextiles and needlepunched fibres resisting shrinkage. Contributing to this may be the presence of a geotextile between the bentonite core of the GCL and the cellulose membrane. Geotextiles exhibit water retention characteristics similar to that of coarse grained soil materials, i.e. rapid dewatering under low applied capillary pressures (e.g. Iryo and Rowe, 2003). It is postulated that the carrier geotextile acts as a capillary barrier, restricting the outward flow of water from the bentonite core of the GCL due to the low unsaturated hydraulic conductivity of the unsaturated geotextile. 5. Model fit to experimental data In order to use the experimental data as a basis for numerical modelling, it is necessary to obtain representative parameters that numerically describe the WRC.
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ys yr ; ð1 þ jacjn Þm
yr pypys ,
100 kPa Load
0.8
(1)
where y is the volumetric water content (m3/m3), yr the residual water content (m3/m3), ys the saturated water content (m3/m3), c the capillary pressure (suction) head (m), a the van Genuchten–Mualem fitting parameter (1/m), n the van Genuchten-Mualem fitting parameter, m the van Genuchten–Mualem fitting parameter ( ¼ 11/n). The model of Zhou and Rowe (2003) uses an expression for volumetric water content expressed as a function of capillary pressure and net mean stress, based on the work of Lloret and Alonso (1985) e ½a0 ðb0 c0 sÞ tanhðd 0 pc Þ, y¼ (2) 1þe where y is the volumetric water content (m3/m3), e the void ratio, s the net mean stress (Pa), pc the capillary pressure (suction) (Pa), a0 , b0 , c0 , d0 are fitting parameters. In addition to these two equations, the model of Fredlund and Xing (1994) was used to fit the experimental data. The Fredlund and Xing equation is given by #mf " lnð1 þ c=cr Þ 1 , y ¼ ys 1 (3) lnð1 þ 106 =cr Þ ln e þ ðc=af Þnf where y is the volumetric water content (m3/m3), ys the saturated water content (m3/m3), c the capillary pressure (suction) (kPa), cr the residual capillary pressure (suction) (kPa), af the fitting parameter (kPa), nf the fitting parameter, mf the fitting parameter. Best-fit curves using these equations were developed for each of the two GCL materials. Fig. 9 gives the curves obtained for the G1 GCL product tested under an applied stress of 100 kPa. Fig. 10 gives the same results for the G2 product. Reasonable fits are achieved to the experimental data using all three models within the range of capillary pressures examined (10–10 000 kPa). The best fit with the model of Lloret and Alonso levels off to a residual degree of saturation at capillary pressures above 20 000 kPa, while above this pressure the degree of saturation calculated using the other two models continues to decrease, approaching much lower degrees of saturation at high capillary pressures. It has been shown that at high capillary pressures (4106 kPa), only absorbed double layer water remains within a soil, resulting in a predicted degree of saturation of essentially zero (Fredlund and Rahardjo, 1993). With this consideration in mind, care should be taken in applying the model of Lloret and Alonso at high
0.6
0.4
Experimental van Genuchten - Mualem Lloret and Alonso Fredlund and Xing
0.2
0.0 10
100 1000 Capillary Pressure (kPa)
10000
Fig. 9. Model predictions for WRC of G1 compared with experimental data under 100 kPa load.
1.0 100 kPa Load
0.8 Degree of Saturation
yðc; T 0 Þ ¼ yr þ
1.0
Degree of Saturation
The numerical models of Do¨ll (1996, 1997) and Zhou and Rowe (2003) have been used by the authors to investigate the nonisothermal behaviour of GCLs in landfill applications (Southen and Rowe, 2005a, b). These models use two different equations to describe the WRC. The Do¨ll model uses the traditional van Genuchten–Mualem equation (van Genuchten, 1980; Mualem, 1976), where volumetric water content is expressed as a function of capillary pressure head by
7
0.6
0.4
Experimental van Genuchten - Mualem Lloret and Alonso Fredlund and Xing
0.2
0.0 10
100 1000 Capillary Pressure (kPa)
10000
Fig. 10. Model predictions for WRC of G2 compared with experimental data under 100 kPa load.
capillary pressures. The parameters used in the equations to achieve the best fit to the experimental data are given in Table 2. 6. Conclusions The results of an experimental investigation into the water retention characteristics in drying of two geosynthetic clay liners used in lining systems for municipal solid waste landfills have been presented. The WRCs defining the relationship between capillary pressure (suction) and the degree of saturation of these materials were established using the axis translation technique with pressure plate and pressure membrane extractors. Significant changes in void ratio were noted during the testing of the GCL materials, due both to the capillary pressure and applied vertical stress. The volume change behaviour resulted in the magnitude of the applied load having an effect on the WRCs of the GCLs to varying degrees. In addition,
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Table 2 Fitting parameters for WRC equations Material
G1 G2
van Genuchten–Mualem
Lloret and Alonso
ys
yr
a
N
a0
b0
c0
d0
0.74 0.73
0.05 0.008
0.0002 0.00006
1.45 1.55
1 1
0.93 0.90
1.55 106 2.5 106
1.70 107 1.80 107
Fredlund and Xing
G1 G2
a
n
m
cr
800 2000
1.1 1.3
1 1
106 106
the applied load plays a key role in ensuring intimate contact between the GCL and the cellulose membrane, which is necessary to obtain representative results. Comparison with reported WRCs of bentonite highlight the differences between the two materials. At low capillary pressures, GCL water contents are lower than bentonite water contents due to lower GCL void ratios caused by the reinforcement effect of needlepunched fibres. At capillary pressures above approximately 100 kPa, the GCL water content becomes higher than observed bentonite water contents, due perhaps to the capillary barrier effect of the carrier geotextile. The difference in observed behaviour suggests that results obtained using compacted bentonite or bentonite slurries cannot be directly applied to GCLs. Values for parameters used to represent the WRC in the models of Do¨ll (1996, 1997) and Zhou and Rowe (2003) were established, and good agreement with the experimental data was achieved using the van Genuchten– Mualem and Lloret and Alonso (1985) functions as well as the equation proposed by Fredlund and Xing (1994). These parameters will be used in future studies by the authors involving the numerical modelling of the behaviour of landfill lining systems under nonisothermal conditions. This paper has focussed on the WRC in drying. It is noted that WRCs are usually hysteretic and hence these curves may not be appropriate to the hydration phase of a GCL. Given the evidence that the WRCs for these GCLs differed from each other and from bentonite, it appears that the method of manufacture of the GCL can impact on the WRC. Thus more research is required to asses the effects of different methods of GCL construction on the WRC of GCLs and the results presented herein may not be applicable to products other than those tested. There is also a need to examine the WRC in wetting as well as drying to fully characterize the unsaturated behaviour of GCLs. Acknowledgements The research presented in this paper was funded by Terrafix Geosynthetics Inc., the Centre for Research in Earth and Space Technology (CRESTech) and the Natural Sciences and Engineering Research Council of Canada
(NSERC). In addition, the value of discussions with Mr. K. von Maubeuge of Naue Fasertechnik and Mr. B. Herlin of Terrafix Geosynthetics Inc. is gratefully acknowledged.
References Barbour, S.L., 1998. Nineteenth Canadian geotechnical colloquium: the soil–water characteristic curve: a historical perspective. Canadian Geotechnical Journal 35 (5), 873–894. Barroso, M., Touze-Foltz, N., von Maubeuge, K., Pierson, P., 2006a. Laboratory investigation of flow rate through composite liners consisting of a geomembrane, a GCL and a soil liner. Geotextiles and Geomembranes 24 (3), 139–155. Barroso, M., Touze-Foltz, N. and Saidi, F.K., 2006b. Validation of the use of filter paper suction measurements for the determination of GCL water retention curves. In: Proceedings of the Eighth International Conference on Geosynthetics, Yokohama, pp. 171–174. Bouazza, A., Vangpaisal, T., 2006. Laboratory investigation of gas leakage rate through a GM/GCL composite liner due to a circular defect in the geomembrane. Geotextiles and Geomembranes 24 (2), 110–115. Dickinson, S., Brachman, R.W.I., 2006. Deformations of a geosynthetic clay liner beneath a geomembrane wrinkle and coarse gravel. Geotextiles and Geomembranes 24 (5), 285–298. Do¨ll, P., 1996. Modeling of moisture movement under the influence of temperature gradients: desiccation of mineral liners below landfills. Ph.D. Thesis, Technical University of Berlin, Germany. Do¨ll, P., 1997. Desiccation of mineral liners below landfills with heat generation. Journal of Geotechnical and Geoenvironmental Engineering 123, 1001–1009. Fredlund, D.G., Rahardjo, H., 1993. Soil Mechanics for Unsaturated Soils. Wiley, New York. Fredlund, D.G., Xing, A., 1994. Equations for the soil–water characteristic curve. Canadian Geotechnical Journal 31, 533–546. Iryo, T., Rowe, R.K., 2003. On the unsaturated behaviour of unsaturated nonwoven geotextiles. Geotextiles and Geomembranes 21 (6), 381–404. Japanese Nuclear Cycle Development Institute (JNC), 2000. H12: project to establish the scientific and technical basis for HLW disposal in Japan, Supporting Report 2: repository design and engineering technology, JNC TN1410 2000-001. Available online at: /http:// www.jnc.go.jp/kaihatu/tisou/zh12/h12/index.htmlS. Lloret, A., Alonso, E.E., 1985. State surface for partially saturated soils. In: Proceedings of the 11th ICSMFE, San Fanscisco, 2, pp. 557–562. Marcial, D., Delage, P., Cui, Y.J., 2002. On the high stress compression of bentonites. Canadian Geotechnical Journal 39, 812–820. Mualem, Y., 1976. A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resources Research 12 (3), 513–522.
ARTICLE IN PRESS J.M. Southen, R. Kerry Rowe / Geotextiles and Geomembranes 25 (2007) 2–9 Petrov, R.J., Rowe, R.K., 1997. Geosynthetic clay liner (GCL)—chemical compatibility by hydraulic conductivity testing and factors impacting its performance. Canadian Geotechnical Journal 34 (6), 863–885. Rowe, R.K., 2005. Long-term performance of contaminant barrier systems. 45th Rankine Lecture. Geotechnique 55 (9), 631–678. SoilMoisture Equipment Corp., 2000. 100 bar Pressure Membrane Extractor Operating Instructions, 0898-1020.P65. Santa Barbara, CA, August. SoilMoisture Equipment Corp., 2002. 15 bar Pressure Plate Extractor Operating Instructions, 0898-1500.P65. Santa Barbara, CA, March. Southen, J.M., Rowe, R.K., 2005a. Modeling of thermally induced desiccation of geosynthetic clay liners. Geotextiles and Geomembranes 23 (5), 425–442. Southen, J.M. and Rowe, R.K., 2005b. Thermally induced desiccation of geosynthetic clay liners in landfill basal liner applications. In:
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GeoFrontiers 2005, Session GSP 142—Waste Containment and Remediation, Austin, TX, CD_ROM. Touze-Foltz, N., Duquennoi, C., Gaget, E., 2006. Hydraulic and mechanical behavior of GCLs in contact with leachate as part of a composite liner. Geotextiles and Geomembranes 24 (3), 188–197. van Genuchten, M.T.H., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal 44 (5), 892–898. Villar, M.V., Lloret, A., 2004. Influence of temperature on the hydromechanical behaviour of a compacted bentonite. Applied Clay Science 26, 337–350. Zhou, Y., Rowe, R.K., 2003. Development of a technique for modelling clay liner desiccation. International Journal for Numerical and Analytical Methods in Geomechanics 27, 473–493.
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Evaluation of the water retention curve for geosynthetic clay liners J.M. Southen, R. Kerry Rowe Department of Civil Engineering, GeoEngineering Centre at Queen’s-RMC, Queen’s University, Kingston, Ont., Canada K7L 3N6 Received 22 May 2005; received in revised form 1 September 2006; accepted 16 October 2006 Available online 5 December 2006
Abstract Geosynthetic clay liners (GCLs) are frequently used as part of composite base liners for landfills. Heat generated by the waste can potentially cause drying of the hydrated GCL. In order to better understand the performance of GCLs under such conditions, an assessment of the water retention characteristics under drying conditions is required. To date, such an investigation has not been undertaken for GCLs, although some data exists for compacted bentonite. This paper presents the results of laboratory testing aimed at obtaining water retention curves (WRCs) for two GCL materials representative of those commonly used in practise. Pressure plate and pressure membrane extractors were used to assess the relationship between the degree of saturation and the applied capillary pressure (suction) for a range of capillary pressures between 10 and 10 000 kPa. The effect of overburden pressure was investigated, as was the relationship between capillary pressure and bulk GCL void ratio. Volume change behaviour was found to have a significant effect on the WRCs. Representative parameters were obtained by fitting the predictions of three WRC models to the experimental data. Reasonable fits were achieved and the parameters obtained may be used for numerical modelling. r 2006 Elsevier Ltd. All rights reserved. Keywords: GCL; Unsaturated soil; Soil–water characteristic curve; Water retention curve; Geosynthetic clay liners
1. Introduction An understanding of the behaviour of geosynthetic clay liners (GCLs) is of importance in many applications (Barroso et al., 2006a; Bouazza and Vangpaisal, 2006; Dickinson and Brachman, 2006; Touze-Foltz et al., 2006). One aspect of particular interest is the potential for desiccation of GCLs used in composite base liners for municipal solid waste landfills due to heat generated by the degradation of the waste (Rowe, 2005; Southen and Rowe, 2005a). To address this issue it is necessary to have an understanding of the behaviour of GCLs under unsaturated conditions. The relationship between capillary pressure (suction) within the soil and the volumetric water content or degree of saturation of the soil is termed the water retention curve (WRC) and is a fundamental constitutive relationship governing the behaviour of unsaturated soils (Barbour, 1998; Fredlund and Rahardjo, Corresponding author. Tel.: +1 613 533 6933; fax: +1 613 533 2128, +613 533 6934. E-mail address: [email protected] (R. Kerry Rowe).
0266-1144/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2006.10.002
1993). Numerous techniques are available to evaluate the WRCs of soil, such as the use of tensiometers, pressure plate extractors, thermal conductivity sensors and thermocouple psychrometers (e.g. Fredlund and Rahardjo, 1993). A review of the literature indicates that the only published data relating to the WRC for geosynthetic clay liners was very recently published by Barroso et al. (2006b), although data for compacted bentonite exists (e.g. Marcial et al., 2002; Villar and Lloret, 2004). An investigation into the water retention characteristics of GCLs would improve the understanding of the behaviour of these materials under the unsaturated conditions which prevail in many common applications (e.g. landfill liner and covers). Also, it is necessary to experimentally obtain parameters that describe the WRC of GCLs in order to numerically model their unsaturated behaviour. This paper forms part of a broader study of potential desiccation of GCLs used in composite landfill base liners. As such it focuses on the drying curve since the GCL will typically be hydrated before waste generated temperature induces potential drying. Thus the objective of this paper is to presents the results of a series of laboratory
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tests aimed at obtaining representative parameters describing the WRCs in drying for two geosynthetic clay liners that are typical of materials used in a lining system for a municipal solid waste landfill. 2. Materials Two GCLs were studied. One GCL is commonly used in landfill basal liner or final cover applications in North America while the other is similarly used in Europe. The first, denoted herein as G1 (Bentofix NSTM) is comprised of a granular sodium bentonite core sandwiched between a slit-film polypropylene woven carrier geotextile and a polypropylene virgin staple fibre nonwoven cover geotextile. The GCL is reinforced by needlepunching and has thermally treated needlepunched fibres (thermal locking). The second GCL, G2 (Bentofix BFG 5000TM) differs from the first in that the core is a powdered sodium bentonite and the cover nonwoven geotextile is impregnated with 800 g/m2 of bentonite. Pertinent material properties for the two GCLs are given in Table 1.
3
3.2. Testing procedure A 100 bar pressure membrane extractor (Soilmoisture Equipment Corp., 2000) was used to test the GCL samples. A schematic of the apparatus is given in Fig. 2. The pressure membrane extractor procedure for evaluating the WRC is based on the axis translation approach, which generates capillary pressure (suction) within the geomaterial by applying air pressure to the samples while maintaining water pressure at the atmospheric level. Further discussion of this approach may be found in Fredlund and Rahardjo (1993). The GCL samples rest on a cellulose membrane within the chamber of the extractor. The very
3. WRC testing 3.1. Sample preparation Sections of GCL approximately 30 cm 30 cm were cut from a roll of each material. These sections were placed in a bath of distilled water and allowed to hydrate, while the change in GCL mass over time was monitored. The GCL mass typically reached a maximum at 10 days, with little variation seen thereafter. At this point, the GCL section was removed from the water bath. Four circular samples approximately 53 mm in diameter were cut from each of the GCL sections using a cutting shoe and hydraulic press. The cut samples were weighed, placed into stainless steel rings and transferred to the pressure extractor. A photograph of a GCL sample at the time of placement within the extractor is shown in Fig. 1.
Fig. 1. GCL specimen within steel ring at time of placement within extractor.
Table 1 Geosynthetic clay liner properties Property GCL G1 Nominal mass/unit area Bentonite mass/unit area Carrier (lower) geotextile Cover (upper) geotextile Construction As-manufactured water content GCL G2 Nominal mass/unit area Bentonite mass/unit area Carrier (lower) geotextile Cover (upper) geotextile Construction As-manufactured water content
Method
Value
ASTM ASTM ASTM ASTM
4.65 kg/m2 4.34 kg/m2 Polypropylene slit-film woven 105 g/m2 Polypropylene virgin staple fibre nonwoven 200 g/m2 Needlepunched 7%
D5993 D5993 D5261 D5261
ASTM D2216 ASTM ASTM ASTM ASTM
D5993 D5993 D5261 D5261
ASTM D2216
5.50 kg/m2 5.00 kg/m2 (incl. 800 g/m2 in NW GT) Polypropylene slit-film woven 200 g/m2 Polypropylene nonwoven 300 g/m2 (impregnated with 800 g/m2 bentonite) Needlepunched 9%
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Handle Clamping Bolt & Nut Top Plate Exhaust Value
Inlet Pressure Stem O-Ring Seals Bottom Plate
Screen Drain Plate Outflow Stem Cross Section of O-Ring
Legs Plate Support Plug
Outflow Stem
Fig. 2. Cross section of pressure membrane extractor (from SoilMoisture Equipment Corp., 2000).
small pore diameter of the cellulose membrane (24 A˚) results in an air entry value exceeding 100 bar, which allows the membrane to remain saturated under high air pressures. When the air pressure is increased within the chamber, water flows from the saturated GCL samples through the cellulose membrane and out through the outflow stem to a burette. The cellulose membrane was saturated in a water bath for a minimum of 15 min, as indicated in the equipment instruction manual. The membrane was then placed in the chamber and the eight GCL samples, contained within stainless steel rings, were placed on the cellulose membrane. To investigate the effects of overburden pressure on the WRC relationship, different stresses were applied to the samples. For each GCL, two samples were subjected to a low confining stress (0.5 or 3 kPa) by means of weighted discs placed on top of the samples. The remaining two samples were subjected to a 100 kPa vertical stress. The samples were allowed to consolidate before determination of the WRC. Once consolidation was complete, the air pressure was increased until the desired capillary pressure was reached. The water level in the outflow burette was monitored to observe when equilibrium had been obtained. In addition to the pressure membrane tests, one series of tests was conducted for each of the G1 and G2 GCLs using a pressure plate extractor (Soilmoisture Equipment Corp., 2002) in an effort to evaluate the behaviour of the materials under lower capillary pressures. The pressure plate apparatus is very similar to the pressure membrane apparatus, except for the use of a ceramic plate in place of the cellulose membrane. Due to the nature of the testing chamber, the samples for this series of tests were not subjected to any loading. The sample preparation
techniques and experimental procedure followed those used in the pressure membrane tests. 3.3. Results Five sets of tests were run within the pressure membrane extractor for each GCL type at applied capillary pressures of 10, 1500, 2000, 4500 and 9000 kPa. As well, tests were conducted at a range of capillary pressures between 10 and 1000 kPa using the pressure plate extractor. The results of the tests for the G1 material are plotted in Fig. 3. Good agreement (715% variation) is generally noted between the two duplicate samples for each capillary pressure tested in the pressure membrane extractor. The air entry value appears to fall within the range of 50–200 kPa, depending on the applied load. The samples under 100 kPa loading tended to have higher degrees of saturation than those under 3 kPa loading. This may be attributed to reduction in void ratio caused by the loading. After testing, the samples under 100 kPa load were 4–8 mm thick or 2–4 mm thinner than those under the lower pressure. The reduction in volume under load is likely to reduce the effective pore size, which would tend to make the GCL more resistant to dewatering. A detailed discussion of the variation in void ratio is presented in Section 3.4. The measured degree of saturation for the specimens with 3 kPa applied load at 10 kPa capillary pressure are lower than would be expected. This may be the result of the samples not being completely saturated prior to the start of the test. This effect would not be as pronounced for samples under load, since consolidation would reduce the air-filled void space and increase saturation. The results for the G2 material are shown in Fig. 4. This material shows greater resistance to dewatering
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5
1.0
Degree of Saturation
0.8
?
0.6
0.4
0.2
Membrane - 3 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
0.0 1
10
100 1000 10000 Capillary Pressure (kPa)
100000
Fig. 3. Water retention curve for G1 obtained using pressure membrane and pressure plate extractors under an applied load. Fig. 5. GCL specimens following testing under a capillary pressure of 4500 kPa, specimen on the left with applied load of 0.5 kPa, specimen on the right with applied load of 100 kPa. 1.0
3.4. Volume change behaviour
Degree of Saturation
0.8
0.6
0.4
0.2
Membrane - 0.5 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
0.0 1
10
100 1000 10000 Capillary Pressure (kPa)
100000
Fig. 4. Water retention curve for G2 obtained using pressure membrane and pressure plate extractors under an applied load.
than the G1 product, with saturation remaining greater than 0.80 for capillary pressures up to 1000 kPa. The air entry pressure falls within the range of 200–1000 kPa. The trend of higher degrees of saturation for samples tested under higher loads noted for the G1 product is not observed for the G2 material. This may be due to poor contact between the GCL and the cellulose membrane resulting from the lower confining stresses (0.5 kPa) and/or the heavier carrier geotextile used with this GCL. A photo of two GCL specimens taken at the completion of the test using the G2 GCL conducted under a capillary pressure of 4500 kPa is shown in Fig. 5. The sample on the left was tested under an applied load of 0.5 kPa and had an average final thickness of 9.5 mm, while the sample on the right was tested under an applied load of 100 kPa and had an average final thickness of 5.7 mm.
The calculation of the degree of saturation for the GCL specimens reported in Section 3.3 was based on measurements of mass and volume taken at the end of each test. The gravimetric water content obtained through weighing the GCL samples before and after oven drying was converted to volumetric water content and degree of saturation based on the bulk GCL void ratio. Petrov and Rowe (1997) introduced the concept of bulk GCL void ratio to normalize the effects of variable mass of bentonite on GCL height. Bulk GCL void ratio, eB, is defined as the ratio of the volume of voids within the geotextile and bentonite components of the GCL to the volume of solids within the GCL. Further details on the computations may be found in Petrov and Rowe (1997). Fig. 6 shows the variation in bulk GCL void ratio for the tests conducted on the G1 specimens. Fig. 7 gives the similar data for the G2 specimens. It may be seen that the bulk GCL void ratio is highly dependant on the magnitude of both the capillary pressure and applied stress. For the range of capillary pressure examined, the trend of decreasing void ratio with increased capillary pressure appears to be bilinear. The increase in slope noted in Figs. 6 and 7 approximately corresponds with the air entry value noted in the WRCs (Figs. 3 and 4). The slope of the capillary pressure–void ratio curve appears to be flatter when the specimen is subjected to a vertical stress. This implies a coupling effect between the effects of applied stress and capillary pressure. 4. Comparison with bentonite WRCs Data on the water retention behaviour of compacted samples and slurries of bentonite have been reported. Villar and Lloret (2004) give results for FEBEX bentonite, a
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6
4 Gravimetric Water Content
5
Bulk GCL Void Ratio
4
3
2
1
Membrane - 3 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
10
100 1000 10000 Capillary Pressure (kPa)
100000
Fig. 6. Bulk GCL void ratio calculated at termination of pressure plate and pressure membrane tests for the G1 GCL.
5
Bulk GCL Void Ratio
4
3
2
1
Membrane - 0.5 kPa stress Membrane - 100 kPa stress Plate - 0 kPa stress
0 1
10
100 1000 Capillary Pressure (kPa)
3
e = 0.9 - 2.7 e = 2.1 - 4.6 e = 1.2 - 2.5 e = 2.4 - 4.5 e = 0.6 e = 0.2 - 9.0 e = 0.2 - 7.2 e = unreported e = unreported
Scatter attributed to poor contact with membrane
2
1
0
0 1
G2 (100 kPa Load) G2 (0.5/0 kPa Load) G1 (100 kPa Load) G2 (3.0/0 kPa Load) Villar and Lloret (2004) Marcial et al. (2002) MX-80 Marcial et al. (2002) Kunigel JNC (2000) MX-80 JNC (2000) Kunigel
10000
100000
Fig. 7. Bulk GCL void ratio calculated at termination of pressure plate and pressure membrane tests for the G2 GCL.
predominantly calcium bentonite (42% Ca, 23% Na), compacted to a dry density of 1.65 g/cm3. Marcial et al. (2002) present WRCs obtained from slurries of two sodium bentonites, Kunigel and MX-80. The Japanese Nuclear Cycle Development Institute (JNC) (2000) report WRCs for the same Kunigel and MX-80 materials using powdered samples and a thermocouple psychrometer. Fig. 8 shows the WRCs obtained for the bentonite samples identified above and the two GCL products in terms of gravimetric water content. Included in the inset to Fig. 8 is the range of void ratios for the samples tested, where available. The water contents of the G2 GCL tested under a load of 0.5 kPa are significantly higher than observed water contents in the other GCL tests and in the bentonite data, supporting the belief that intimate contact was not achieved during the tests under low applied load. This may be related to the fact that at these low stresses the geotextile component of the GCL may not be sufficiently compressed to bring bentonite into contact with the
1
10
100 1000 10000 Capillary Pressure (kPa)
100000 1000000
Fig. 8. Comparison of water retention curves obtained for GCLs (solid symbols) and bentonite (open symbols).
apparatus in a uniform manner. The remaining GCL samples give water contents that are reasonably consistent with reported results for bentonite samples in the range of 100–10 000 kPa. The GCL samples exhibit water contents at lower capillary pressures (o100 kPa) that are lower than those observed for bentonite samples. This is attributed to the reinforcing properties of the needlepunched fibres linking the upper and lower geotextiles of the GCL. These fibres restrict swelling at lower capillary pressures, resulting in lower void ratios than were observed for the bentonite samples. As an example, the G2 product under a 0.5 kPa load and 10 kPa capillary pressure had a bulk void ratio of 2.5. The MX-80 bentonite slurry used by Marcial et al. (2002) had a void ratio of 9 at a capillary pressure of 20 kPa. At higher capillary pressures (4100 kPa), the GCL water contents tend to be somewhat higher than those of the bentonite samples, although the discrepancy is not as great as at lower capillary pressures. The void ratio of the GCL specimens tends to be higher in this range of capillary pressure than bentonite samples, which may again be related to the reinforcing nature of the geotextiles and needlepunched fibres resisting shrinkage. Contributing to this may be the presence of a geotextile between the bentonite core of the GCL and the cellulose membrane. Geotextiles exhibit water retention characteristics similar to that of coarse grained soil materials, i.e. rapid dewatering under low applied capillary pressures (e.g. Iryo and Rowe, 2003). It is postulated that the carrier geotextile acts as a capillary barrier, restricting the outward flow of water from the bentonite core of the GCL due to the low unsaturated hydraulic conductivity of the unsaturated geotextile. 5. Model fit to experimental data In order to use the experimental data as a basis for numerical modelling, it is necessary to obtain representative parameters that numerically describe the WRC.
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ys yr ; ð1 þ jacjn Þm
yr pypys ,
100 kPa Load
0.8
(1)
where y is the volumetric water content (m3/m3), yr the residual water content (m3/m3), ys the saturated water content (m3/m3), c the capillary pressure (suction) head (m), a the van Genuchten–Mualem fitting parameter (1/m), n the van Genuchten-Mualem fitting parameter, m the van Genuchten–Mualem fitting parameter ( ¼ 11/n). The model of Zhou and Rowe (2003) uses an expression for volumetric water content expressed as a function of capillary pressure and net mean stress, based on the work of Lloret and Alonso (1985) e ½a0 ðb0 c0 sÞ tanhðd 0 pc Þ, y¼ (2) 1þe where y is the volumetric water content (m3/m3), e the void ratio, s the net mean stress (Pa), pc the capillary pressure (suction) (Pa), a0 , b0 , c0 , d0 are fitting parameters. In addition to these two equations, the model of Fredlund and Xing (1994) was used to fit the experimental data. The Fredlund and Xing equation is given by #mf " lnð1 þ c=cr Þ 1 , y ¼ ys 1 (3) lnð1 þ 106 =cr Þ ln e þ ðc=af Þnf where y is the volumetric water content (m3/m3), ys the saturated water content (m3/m3), c the capillary pressure (suction) (kPa), cr the residual capillary pressure (suction) (kPa), af the fitting parameter (kPa), nf the fitting parameter, mf the fitting parameter. Best-fit curves using these equations were developed for each of the two GCL materials. Fig. 9 gives the curves obtained for the G1 GCL product tested under an applied stress of 100 kPa. Fig. 10 gives the same results for the G2 product. Reasonable fits are achieved to the experimental data using all three models within the range of capillary pressures examined (10–10 000 kPa). The best fit with the model of Lloret and Alonso levels off to a residual degree of saturation at capillary pressures above 20 000 kPa, while above this pressure the degree of saturation calculated using the other two models continues to decrease, approaching much lower degrees of saturation at high capillary pressures. It has been shown that at high capillary pressures (4106 kPa), only absorbed double layer water remains within a soil, resulting in a predicted degree of saturation of essentially zero (Fredlund and Rahardjo, 1993). With this consideration in mind, care should be taken in applying the model of Lloret and Alonso at high
0.6
0.4
Experimental van Genuchten - Mualem Lloret and Alonso Fredlund and Xing
0.2
0.0 10
100 1000 Capillary Pressure (kPa)
10000
Fig. 9. Model predictions for WRC of G1 compared with experimental data under 100 kPa load.
1.0 100 kPa Load
0.8 Degree of Saturation
yðc; T 0 Þ ¼ yr þ
1.0
Degree of Saturation
The numerical models of Do¨ll (1996, 1997) and Zhou and Rowe (2003) have been used by the authors to investigate the nonisothermal behaviour of GCLs in landfill applications (Southen and Rowe, 2005a, b). These models use two different equations to describe the WRC. The Do¨ll model uses the traditional van Genuchten–Mualem equation (van Genuchten, 1980; Mualem, 1976), where volumetric water content is expressed as a function of capillary pressure head by
7
0.6
0.4
Experimental van Genuchten - Mualem Lloret and Alonso Fredlund and Xing
0.2
0.0 10
100 1000 Capillary Pressure (kPa)
10000
Fig. 10. Model predictions for WRC of G2 compared with experimental data under 100 kPa load.
capillary pressures. The parameters used in the equations to achieve the best fit to the experimental data are given in Table 2. 6. Conclusions The results of an experimental investigation into the water retention characteristics in drying of two geosynthetic clay liners used in lining systems for municipal solid waste landfills have been presented. The WRCs defining the relationship between capillary pressure (suction) and the degree of saturation of these materials were established using the axis translation technique with pressure plate and pressure membrane extractors. Significant changes in void ratio were noted during the testing of the GCL materials, due both to the capillary pressure and applied vertical stress. The volume change behaviour resulted in the magnitude of the applied load having an effect on the WRCs of the GCLs to varying degrees. In addition,
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Table 2 Fitting parameters for WRC equations Material
G1 G2
van Genuchten–Mualem
Lloret and Alonso
ys
yr
a
N
a0
b0
c0
d0
0.74 0.73
0.05 0.008
0.0002 0.00006
1.45 1.55
1 1
0.93 0.90
1.55 106 2.5 106
1.70 107 1.80 107
Fredlund and Xing
G1 G2
a
n
m
cr
800 2000
1.1 1.3
1 1
106 106
the applied load plays a key role in ensuring intimate contact between the GCL and the cellulose membrane, which is necessary to obtain representative results. Comparison with reported WRCs of bentonite highlight the differences between the two materials. At low capillary pressures, GCL water contents are lower than bentonite water contents due to lower GCL void ratios caused by the reinforcement effect of needlepunched fibres. At capillary pressures above approximately 100 kPa, the GCL water content becomes higher than observed bentonite water contents, due perhaps to the capillary barrier effect of the carrier geotextile. The difference in observed behaviour suggests that results obtained using compacted bentonite or bentonite slurries cannot be directly applied to GCLs. Values for parameters used to represent the WRC in the models of Do¨ll (1996, 1997) and Zhou and Rowe (2003) were established, and good agreement with the experimental data was achieved using the van Genuchten– Mualem and Lloret and Alonso (1985) functions as well as the equation proposed by Fredlund and Xing (1994). These parameters will be used in future studies by the authors involving the numerical modelling of the behaviour of landfill lining systems under nonisothermal conditions. This paper has focussed on the WRC in drying. It is noted that WRCs are usually hysteretic and hence these curves may not be appropriate to the hydration phase of a GCL. Given the evidence that the WRCs for these GCLs differed from each other and from bentonite, it appears that the method of manufacture of the GCL can impact on the WRC. Thus more research is required to asses the effects of different methods of GCL construction on the WRC of GCLs and the results presented herein may not be applicable to products other than those tested. There is also a need to examine the WRC in wetting as well as drying to fully characterize the unsaturated behaviour of GCLs. Acknowledgements The research presented in this paper was funded by Terrafix Geosynthetics Inc., the Centre for Research in Earth and Space Technology (CRESTech) and the Natural Sciences and Engineering Research Council of Canada
(NSERC). In addition, the value of discussions with Mr. K. von Maubeuge of Naue Fasertechnik and Mr. B. Herlin of Terrafix Geosynthetics Inc. is gratefully acknowledged.
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