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Swelling characteristics of needlepunched, thermally treated geosynthetic clay liners
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Geotextiles and Geomembranes 18 (2000) 77}101
Swelling characteristics of needlepunched, thermally treated geosynthetic clay liners Craig B. Lake, R. Kerry Rowe* Department of Civil and Environmental Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9 Received 4 February 1999; received in revised form 19 July 1999; accepted 21 August 1999
Abstract The swelling characteristics of "ve di!erent geosynthetic clay liners (GCLs) are examined. Results show that two thermally treated GCLs have similar swell properties but a third thermally treated GCL with bentonite impregnated in the cover geotextile swells to higher GCL bulk void ratios at stresses below 100 kPa. This is attributed to the uncon"ned swelling of the bentonite present on the surface of the cover geotextile. A thermally treated needlepunched GCL is shown to have an equilibrium swell height that is 50% smaller than that of a nonthermally treated needlepunched GCL at 6 kPa. Microscopic examination of the thermal and non-thermally treated "bres before and after the 6 kPa swell tests shows that more thermally treated "bres remain visible on the bottom of the geotextile compared to the non-thermally treated "bres. Additional swell tests at 20 kPa and 100 kPa suggest that as the con"ning stress increases on a GCL during hydration, the tendency for pullout of the "bres decreases. Di!usion tests performed on a thermally treated needlepunched GCL and non-thermally treated needlepunched GCL under free swell conditions show that the di!usion coe$cients for Na` and Cl~ are lower for the thermally treated, needlepunched GCL than for the non-thermally treated GCL under otherwise similar conditions because of the higher bulk void ratio of the latter GCL. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: GCL; Needlepunching; Thermal treatment; Swell tests; Di!usion
1. Introduction Geosynthetic clay liners (GCLs) are utilized for many di!erent sealing applications including municipal solid waste (MSW) land"ll covers and liners, lagoon and canal * Correponding author. Tel.: 001-519-661-2126; fax: 001-519-661-3942. E-mail address: [email protected] (R.K. Kerry Rowe) 0266-1144/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 9 9 ) 0 0 0 2 2 - 9
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liners. Depending on the application, GCL design considerations may include: interface shear strength (Scranton, 1996); internal shear strength (Fox, 1998); freeze/thaw and wet/dry cycles (Kraus et al., 1997); hydraulic conductivity and compatibility (Petrov and Rowe, 1997); and di!usion (Rowe et al., 2000). Manufacturers have developed di!erent types of GCLs (Koerner, 1998), but all involve a clay (typically sodium bentonite) core with geosynthetics (geotextiles or geomembranes) either on one or both sides. The geosynthetics and bentonite may be held together by needlepunched "bres, stitching or adhesives with some manufacturers increasing the internal shear resistance of needlepunched "bre GCLs by thermal treatment (heat-bonding, thermal locking). Thermally treated GCLs refer to "bres that are heated on the bottom geotextile after needlepunching. The heating process fuses together groups of individual "bres and sometimes fuses the "bres to the carrier geotextile, minimizing pullout of the "bres from the geotextile when the GCL is hydrated. For non-thermally treated needlepunched "bres, pullout resistance relies mainly on the entanglement of the "bres on the bottom geotextile. These di!erent variations of GCL manufacturing may in#uence the engineering characteristics of a GCL. Richardson (1997) showed that unreinforced GCLs have lower internal shear strengths than needlepunched GCLs (peak friction angle of approximately 83, von Maubeuge and Eberle (1998)). Petrov and Rowe (1997) examined the swelling behavior of a non-thermally treated, needlepunched GCL and found that needlepunching lowers the "nal bulk GCL void ratio relative to the same type of GCL with the needlepunched "bres cut, particularly at the low con"ning stresses (3 kPa to 6 kPa) representative of applications for ponds, lagoons, or covers. It was subsequently shown that in general, lower "nal bulk GCL void ratios corresponded to lower hydraulic conductivity values, all other factors being equal. Other studies have shown that the con"ning stress and type of GCL can reduce the swelling characteristics of GCLs (Shan and Daniel, 1991; USEPA, 1993). Lake and Rowe (2000) show that Na` and Cl~ di!usion coe$cients for a thermally treated, needlepunched GCL increase with increases in the "nal bulk GCL void ratio. However the means by which the thermally treated needlepunched "bres a!ect these variations in "nal bulk GCL void ratio has not been examined. One may hypothesize that there may be relatively less swelling of thermally treated needlepunched GCLs compared to non-thermally treated GCLs at low stresses where the needlepunched "bres are required to resist the majority of the swelling of the bentonite core. Furthermore, one may also hypothesize that peel tests performed on thermally treated GCLs hydrated under low stresses may exhibit higher values compared to a similar non-thermally treated GCL and this may be an indirect evaluation of the pullout resistance of the GCL "bres during swelling. This paper focuses on di!erences of swelling behavior for three types of thermally treated, needlepunched GCLs and two non-thermally treated, needlepunched GCLs. Results of constant stress swell tests, speci"ed volume swell tests and consolidation tests are combined with a microscopic examination of both thermal and non-thermally treated needlepunched GCL "bres to explore the e!ect of the method of GCL manufacturing and the role of needlepunched "bres on the swell characteristics and "nal bulk void ratio of these GCLs. There is a paucity of published information regarding how
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needlepunched "bres actually behave during the swelling process. By understanding how the "bres interact with the GCL, results of other types of GCL testing may be better understood. To illustrate this point, di!usion through otherwise similar thermally treated and non-thermally treated needlepunched GCLs will be examined. This paper also provides a basis for relating the swell characteristics reported herein to the di!usion characteristics of these GCLs reported in the companion paper (Lake and Rowe, 2000).
2. Materials The basic characteristics of the GCLs examined in this study are summarized in Table 1. They include Bento"x NW and NS GCLs (distributed by Terra"x Geosynthetics Inc., based in Toronto, Ontario, Canada and Naue Fasertechnik in Lubbecke, Germany under slightly di!erent product identi"ers Bento"x B4000 and NSP 4900-3) and BFG5000 GCLs (distributed by Naue Fasertechnik in Germany). The generic symbols used to describe these GCLs throughout the rest of the paper are such that the type of manufacturing of each GCL can be ascertained from the symbols (see Table 1). The "rst `NWa or `Wa in the generic name refers to whether the bottom geotextile (called the `carriera geotextile) is nonwoven or woven. The second `NWa
Table 1 Basic characteristics of "ve needlepunched GCLs!
Middle bentonite GCL type layer
Carrier geotextile
Cover geotextile
Minimum mass/area tested (g/m2)
NW"
Granular sodium bentonite
PP nonwoven$
PP nonwoven
5270
NWNWT
NS"
Granular sodium bentonite
PP slit "lm, woven
PP nonwoven
5380
WNWT
BFG5000# Powdered sodium bentonite
PP slit "lm, woven
PP nonwoven
4990
WNWBT
NW
Powdered sodium bentonite
PP nonwoven$
PP nonwoven
N/T
NWNW
NS
Powdered sodium bentonite
PP slit "lm, woven
PP nonwoven
6189
WNW
Generic symbol used in paper
!Notes: PP } Polypropylene; N/T } Tested by Petrov and Rowe (1997), but discussed in this paper. "Thermally treated and needlepunched. #Thermally treated, needlepunched and cover geotextile impregnated with 500 g/m2 of powder bentonite. $Nonwoven scrim reinforced with slit "lm woven geotextile.
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Fig. 1. Schematic of basic types of GCLs examined.
describes the top geotextile (called the `covera geotextile) which was nonwoven for all GCLs tested. The &B' in the generic name WNWBT indicates that the cover geotextile is impregnated with bentonite and the `Ta, if present, refers to the needlepunched "bres being thermally treated. The NWNW and NWNWT GCLs are often used in liner systems with steeper slopes compared to the WNW and WNWT GCLs. The WNWBT GCL has been used in applications to improve the intimate contact between GCLs and geomembranes in composite liner systems, pond applications and underwater installations where seam overlaps are important. Fig. 1 shows a schematic of these three GCLs. Characteristics of the sodium bentonite in these GCLs have been described by Rowe et al. (2000). The bentonite in each GCL is similar and predominantly composed of smectite clay minerals (&90%) with Na` and SO~ as the predominant ions 4 in the pore #uid.
3. Testing 3.1. Constant stress swell (CSS) tests Constant stress swell (CSS) test procedures were performed as described by Petrov et al. (1997). A GCL is hydrated under a constant applied stress while the GCL height is monitored with time until there is no further swelling (Fig. 2). This is similar to swell tests described in the literature (Shan and Daniel, 1991; USEPA, 1993).
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Fig. 2. Schematic of constant stress and speci"ed volume swell tests.
Constant stress swell tests were performed for applied stresses of 6 kPa, 25 kPa, 100 kPa, 200 kPa, 300 kPa and 400 kPa and with distilled water as the hydration #uid. This range of stresses was chosen to represent the typical range of stresses to which a GCL may be subjected in the "eld. The lower range of stress (6 kPa) may represent the use of a GCL in a cover or lagoon while the higher range may represent that expected by a GCL liner in a large land"ll after placement of a substantial thickness of waste. 3.2. Specixed volume swell (SVS) tests Speci"ed volume swell (SVS) tests were performed to evaluate the stresses generated during GCL hydration when the GCL is allowed to swell to a speci"ed height or volume (Fig. 2). Variations of this type of test (ASTM, 1996) have been performed for swelling rocks and sand/bentonite mixtures (Madsen and Muller-Vonmoos, 1989; Dixon et al., 1991). Results from a speci"ed volume swell test permit the di!usion coe$cient obtained at a given "nal bulk GCL void ratio (Lake and Rowe, 2000) to be correlated with the stress generated from the speci"ed volume swell test (provided the permeating #uid is similar). This type of test permits an assessment of how the GCL manufacturing process a!ects the magnitude of swelling stresses generated. The procedure used for this paper involves hydrating the GCL with distilled water and allowing the specimen to swell in a 70 mm diameter oedometer ring to a speci"ed height (5.6 mm, 7.1 mm, 9.1 mm, or 11.0 mm) in a rigid frame equipped with a sti! load cell. Load cell readings are obtained daily and when a constant stress is reached (within 1 kPa over at least 72 hours), the test is terminated. The bentonite moisture content is measured both before and after testing.
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4. Swell test results 4.1. Comparison of NWNWT, WNWT and WNWBT GCLs Fig. 3 shows constant stress swell (CSS) test results for the NWNWT, WNWT, and WNWBT GCLs. The initial heights (height with a nominal 1 kPa to 3 kPa stress applied to the GCL before addition of the applied stress and subsequent hydration), the initial con"ned heights (height after addition of the applied stress but before
Fig. 3. GCL heights } constant stress swell tests.
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Fig. 4. Comparison of constant stress swell tests.
hydration), and the "nal GCL heights (height after the addition of the applied stress and subsequent hydration with distilled water) are shown. There is a trend of decreasing "nal GCL height with increasing con"ning stress. Both the NWNWT and WNWT GCLs exhibit similar equilibrium swelling heights but the WNWBT GCL exhibits much greater equilibrium swelling heights at 6 kPa and 25 kPa stresses. When all the CSS test results are plotted in terms of "nal bulk GCL void ratio on Fig. 4, the WNWBT GCL exhibits higher "nal bulk GCL void ratios than the NWNWT and WNWT GCLs for applied stresses below 100 kPa. This is due to the uncon"ned nature of the powder bentonite impregnated in the cover geotextile. However, bentonite encapsulated between the geotextiles is still restricted from uncon"ned swelling due to the thermally treated, needlepunched "bres through the core. At stresses above 100 kPa, the WNWBT GCL swelled to similar "nal bulk void ratios as the WNWT and NWNWT GCLs. Despite that the NWNWT and WNWT GCLs had di!erent carrier geotextiles, both GCLs swelled to similar "nal bulk GCL void ratios (Fig. 4) for the entire range of stresses tested. Lake and Rowe (2000) showed that for a range of bulk GCL void ratios of 1.0 to 3.5, the NWNWT, WNWT and WNWBT GCLs exhibited a linear relationship between bulk GCL void ratio and Na` and Cl~ di!usion coe$cients (all other factors being equal). Although there was some scatter at the higher bulk void ratios in the relationship, there was no clear trend for any GCL. Even though the WNWBT GCL is di!erent in construction than the other two GCLs, the data for this GCL "t the same linear relationship as the other two GCLs. The results of the SVS tests are shown in Fig. 5 together with results of the CSS tests for each type of GCL. The speci"ed volume swell test data plot along the same trend line as the constant stress swell test data, con"rming that the stresses generated by a speci"ed volume swell test are similar to the constant stresses applied to generate
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Fig. 5. Comparison of speci"ed volume swell tests and constant stress swell tests.
that same height, provided both tests are performed with the same hydrating #uid. For both test methods, the stress is higher for higher "nal bulk GCL void ratios with a linear trend on a linear-log scale. However the WNWBT GCL seems to exhibit a steeper trend line for the lower stresses. Results for all the SVS tests are also shown in Fig. 6. The WNWT and NWNWT GCLs exert very similar swelling stresses when hydrated, but the WNWBT GCL exerts slightly higher stresses for the same bulk GCL void ratio perhaps due to the uncon"ned nature of the bentonite impregnated in the cover geotextile.
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Fig. 6. Comparison of speci"ed volume swell tests results.
4.2. Post-conxnement stress application GCLs may be subjected to a variety of hydration conditions in the "eld. In most cases the GCL may be purposely hydrated after a cover soil is placed, hydrated by precipitation or hydrated from the moisture of underlying soil (Daniel et al., 1993). These hydration conditions may result in the GCL being hydrated under relatively low stresses (5-20 kPa) and then being subsequently consolidated (post-con"nement stress application) such as when overlying waste is added to a land"ll. Post-con"nement stress applications were applied to the NWNWT, WNWT and WNWBT GCLs to allow direct comparison with previous results reported by Petrov et al. (1997) for non-thermally treated needlepunched GCLs. The procedure for post-con"nement stress application has been described in detail by Petrov et al. (1997) and will not be repeated here. Essentially, the GCL is hydrated with distilled water in an oedometer under a con"ning stress of 6 kPa until swelling equilibrium is reached. Loading increments of 12.5 kPa, 25 kPa, 50 kPa, 100 kPa, 200 kPa, 400 kPa and 800 kPa are then applied to the sample in the same manner as a typical consolidation test. Unloading of the sample proceeds in the same fashion to 400 kPa, 100 kPa, and 10 kPa. Results for the NWNWT, WNWT, and WNWBT GCLs on Fig. 7 show that at stresses below approximately 400 kPa, the WNWBT GCL has higher bulk GCL void ratios compared to the NWNWT and WNWT GCLs because of swelling of the uncon"ned bentonite on the surface of the upper geotextile when hydrated at 6 kPa. Above 400 kPa, the GCLs approach similar bulk void ratios, indicating that at higher stresses the type of GCL does not have as much in#uence on the bulk GCL void ratio as the vertically applied stress. When unloaded from the maximum 800 kPa stress, the NWNWT and WNWT GCLs swell to similar bulk
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Fig. 7. Comparison of post-con"nement versus pre-con"nement stress application.
GCL void ratios while the WNWBT GCL swells to a higher bulk GCL void ratio at the "nal 10 kPa stress level. Hydraulic conductivity testing (Petrov and Rowe, 1997) and di!usion testing (Lake and Rowe, 2000) have shown that it is the bulk GCL void ratio at the time of testing that controls the hydraulic conductivity and di!usion coe$cient regardless of its previous stress history (for engineering purposes). This implies that if the postcon"nement stress level reaches 300 kPa to 400 kPa, the resultant hydraulic conductivity and di!usion coe$cients are likely to be similar for all three GCLs.
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Apparent preconsolidation pressures of&80 kPa for the NWNWT GCL,&80 kPa for the WNWT GCL and &25 kPa for the WNWBT GCL are evident from the curves in Fig. 7 (using the Casagrande method). Previous stress history, desiccation, and the in#uence of needle punched "bres may be responsible for this apparent preconsolidation pressure. As the bentonite in the GCL hydrates, it puts the needlepunched "bres in tension, applying a stress back to the bentonite. This stress being applied by the "bres to the bentonite under a con"ning stress of 6 kPa acts like a `stress historya to the sample as the stress levels are increased during the tests. When the swell data from the previous section are plotted on Fig. 7, they plot on top of the rebound line from the consolidation tests indicating the rebound line is a lower bound for the swell tests performed, at least at higher stresses. This is consistent with the "ndings of Petrov and Rowe (1997). Also, when a sample is con"ned by an applied stress before hydration (see Fig. 7), the resultant void ratio is lower compared to the same post-con"nement stress application, probably as the result of a more oriented soil fabric. 4.3. Ewect of thermally treated needlepunched xbres on swelling behavior 4.3.1. Comparison of non-thermally treated and thermally treated GCLs Since Petrov et al. (1997) used similar swell test procedures for testing the NWNW GCL as described in Table 1, it is useful to compare their NWNW swell test results with the NWNWT, WNWT, and WNWBT GCLs examined so far. Petrov et al. (1997) de"ned the con"ned swell pressure in distilled water, p@ , as the 41 stress above which there is no swelling of the GCL greater than its initial height (H ). 0 They found a p@ of 160 kPa for the non-thermally treated NWNW GCL they tested 41 (Table 1). The p@ values found for the NWNWT, WNWT, and WNWBT GCLs on 41
Fig. 8. E!ects of "bres and thermal treatment on "nal bulk GCL void ratio for constant stress swell tests.
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Fig. 9. Comparison of post-con"nement stress application results of a non-thermally treated GCL and a thermally treated GCL.
Fig. 3 are &100 kPa, &100 kPa, and &135 kPa, respectively. These are lower con"ned swell pressures values than obtained by Petrov et al. (1997) which suggests that less con"ning stress is required to restrict swelling of the thermally treated GCLs above their initial height relative to non-thermally treated GCLs. Hence it may be hypothesized that the thermally treated "bres are resisting more of the swelling stresses than the non-thermally treated "bres. Fig. 8 compares constant stress swell test results for the NWNWT and WNWT GCLs in this study with the NWNW GCL (Table 1) tested by Petrov et al. (1997). The "bre-free NWNW GCL in Fig. 8 refers to an NWNW GCL with no needlepunched "bres. As described by Petrov et al. (1997), this was achieved by cutting the "bres that attach the cover and carrier geotextile and removing the bentonite from the GCL. The GCL was then reassembled by adding a predetermined mass of bentonite onto the lower geotextile and replacing the top geotextile. It is apparent from Fig. 8 that this "bre-free GCL plots above all the GCL results in this study. At stresses lower than 100 kPa, the non-thermally treated needlepunched GCL (NWNW) plots above the two thermally treated needlepunched GCLs in this study. The di!erence in "nal bulk GCL void ratio at the 6 kPa stress level for the four GCLs in Fig. 8 corresponds to a situation where the "bres would be required to carry the majority of the swelling induced stresses during hydration. This low stress range may occur in liquid containment systems, land"ll cover systems, or in land"ll base liner systems where the GCL hydrates just after installation or before the addition of a signi"cant amount of waste. A signi"cant reduction in "nal bulk GCL void ratio is provided by needlepunching of the "bres, and for stresses below approximately 100 kPa, thermal treatment of the
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needlepunched "bres results in a further reduction in "nal bulk GCL void ratio compared to that of no thermal treatment. This suggests that if GCLs are hydrated under small stresses (&6 kPa), the thermally treated GCLs may swell to much lower "nal bulk GCL void ratios, and possibly have lower hydraulic conductivity or di!usion coe$cients than similar non-thermally treated GCLs. The post-con"nement stress application results of the non-thermally treated NWNW GCL of Petrov and Rowe (1997) are reproduced on Fig. 9 with the NWNWT GCL post-con"nement stress application results. As was evident from previous swell test results at low stresses, the di!erence in "nal bulk GCL void ratio between the two GCLs is pronounced at post-con"nement stresses lower than 100 kPa. As the stress increases, the bulk void ratios of the two GCLs converge as the e!ect of thermal treatment is overshadowed by the applied vertical stress.
4.4. Microscopic examination of thermally treated, needlepunched xbres To investigate the means by which thermally treated, needlepunched GCL "bres in#uence the "nal bulk GCL void ratio during GCL hydration, constant stress swell tests were compared for two types of GCLs (WNWT and WNW; see Table 1 for a description). The objective of the comparative swell tests was to observe the height to which each GCL swelled and to examine the needlepunched "bres before and after the swell tests. Constant stress swell tests were performed at 6 kPa, 20 kPa, and 100 kPa. As discussed earlier, at the 6 kPa stress, the GCL "bres should be resisting the majority of the swelling stresses generated during hydration. The 20 kPa stress represents approximately 1 m of cover soil while the 100 kPa stress represents a medium stress level that may be exerted on a GCL in a land"ll. All GCLs tested had similar mass per unit areas (ranging from 6189 g/m2 to 6315 g/m2), polypropylene, slit-"lm, woven carrier geotextiles (100 g/m2) and polypropylene, nonwoven cover geotextiles. The only di!erence between the two types of GCLs was that the WNWT GCL had thermally treated needlepunched "bres and the WNW GCL had no thermal treatment of the needlepunched "bres. GCLs were placed in an oedometer ring and a 16 cm2 area in the centre of each GCL carrier geotextile was numbered and marked with a waterproof felt-tip marker into individual 1 cm2 squares. For the 20 kPa and 100 kPa swell tests, only two, 1 cm2 sections were used for observations as comparison to the 6 kPa stress level. The GCLs were then transferred, still in their oedometer rings, to an optical stereo microscope equipped with a digital camera. Oblique lighting was applied at an angle of approximately 103 from the horizontal to improve the quality of digital images. Each square was examined under the optical microscope. Each hole made by an individual needle punching through the carrier geotextile had several "bres that encompassed the hole left by the needle (herein de"ned as a `bundlea). The number of "bre `bundlesa was counted for each grid and an image was transferred from the optical microscope to a computer. Care was taken during the examination to prevent disturbance of the "bres and prevent a loss of bentonite from the GCL.
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Fig. 10. Comparison of thermal versus non-thermally treated GCLs (constant stress swell tests).
After preliminary observations, the GCLs were transferred from the optical microscope to an oedometer pot and a constant stress swell test was performed on each sample with distilled water. Once equilibrium occurred, the GCLs were transferred from the oedometer pot to the optical microscope. The photographic procedure was repeated on the GCLs to provide a visual before and after comparison of the "bre `bundlesa on the carrier geotextile. A non-thermal "bre `bundlea was considered to be remaining on the bottom of the GCL after the swell test if the density of the "bre `bundlea was approximately 50% to 100% of that before the swell test (even if the length had decreased). Since the resistance to pullout for these types of GCLs relies on entanglement of the "bres, the density of the individual `bundlesa is one measure of remaining e!ectiveness of the "bres. For the thermally treated "bres, the remaining "bre `bundlesa that had been fused together were either present or absent on the carrier geotextile after the swell test. Fig. 10 shows the swell test results for the WNWT and WNW GCLs. For the 6 kPa stress level, after 35 days the thermally treated GCL had essentially reached equilibrium, while the non-thermally treated GCL appeared to be still gradually swelling. During this time, the WNWT GCL swelled to a height of 11.54 mm, while the WNW GCL swelled to a height of 17.09 mm. This is almost a 50% increase in height for the non-thermally treated, needlepunched GCL and is signi"cant considering that there was essentially no di!erence between the GCLs except thermal treatment of the "bres. Table 2 shows a comparison of the number of "bre `bundlesa visible by the optical microscope before and after the 6 kPa swell tests. There are more "bre `bundlesa visible on the thermally treated GCL (69%) compared to the non-thermally treated
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Table 2 Number of visible "bre `bundlesa for thermal and non-thermally treated GCLs, before and after constant stress swell tests (6 kPa)
Grid number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Totals
Non-thermally treated GCL
Thermally treated GCL
Before
Before
After
% remaining
After
% remaining
6 8 7 10 7 6 5 7 7 9 6 4 9 9 10 6
0 1 1 0 1 2 2 0 0 2 2 0 1 1 1 0
0 13 14 0 14 33 40 0 0 22 33 0 11 11 10 0
10 8 8 8 12 11 11 10 7 7 6 5 9 10 9 10
10 5 5 6 5 9 8 7 4 4 4 3 8 6 5 8
100 63 63 75 42 82 73 70 57 57 67 60 89 60 56 80
116
14
12
141
97
69
GCL (12%) after completion of the swell tests. A statistical t-test was performed on the data shown in Table 2 and it was shown that there was a signi"cant di!erence when comparing the means of the non-thermally treated and thermally treated GCLs (two tailed p-value of 3]10~12). It would appear that the "bre `bundlesa that are not visible have been pulled out from the carrier woven geotextile and are no longer participating in the restriction of swelling of the GCL. When "rst observed under the microscope, the non-thermally treated "bres were punched through the carrier geotextile and entangled. After the 6 kPa swell tests, some individual "bres were still visible on the bottom of the geotextile where bundles had one been, however the density of the "bre `bundlesa through the holes had decreased greatly (Fig. 11). Fig. 12 provides visual evidence of how some of the thermally treated "bres have remained attached to the carrier geotextile, providing restriction to the swelling of the GCL. This was emphasized when the carrier geotextiles were compared after the tests. The WNWT GCL exhibited dimples on the bottom of the GCL, showing how some of the thermally treated needlepunching had remained in place during the swelling process. This was in contrast to the #at surface observed for the carrier geotextile of the WNW GCL. The results of the 20 kPa and 100 kPa swell test results are compared with the 6 kPa swell test results on Fig. 10. At the 20 kPa and 100 kPa stress levels, the non-thermally treated needlepunched GCLs swelled to a greater GCL thickness than
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Fig. 11. Comparison of non-thermally treated GCL before and after swell tests (6 kPa).
the respective thermally treated GCL. Also, the relative di!erence in "nal GCL height between the non-thermally and thermally treated GCLs at 6 kPa is greater than that at 100 kPa implying that as the stress level increases, the in#uence of the thermally treated "bres decreases. This is con"rmed by comparing Figs. 13 and 14 as well as
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Fig. 12. Comparison of thermally treated GCL before and after swell tests (6 kPa).
Figs. 15 and 16. At 100 kPa, the "bres of both the non-thermal and thermally treated GCLs are still visible in the carrier geotextile while at 20 kPa it appears that the non-thermally treated "bre `bundlesa have decreased substantially compared to the thermally treated "bres for the two grids examined. Since only two grids were
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Fig. 13. Comparison of non-thermally treated GCL before and after swell tests (20 kPa).
examined for the 20 kPa and 100 kPa stresses (Tables 3 and 4), no statistical conclusions can be drawn as the di!erences between the means for these two samples. Regardless of the stress level tested, the thermally treated "bres appear to be at least as
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95
Fig. 14. Comparison of thermally treated GCL before and after swell tests (20 kPa).
or more e!ective at restricting the GCL swell height, probably due to the resistance of the pullout as shown in Figs. 11 and 12. These results may only be applicable for the type of GCL tested (WNW and WNWT) and should not be extended for other types of GCLs without further testing.
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Table 3 Number of visible "bre `bundlesa for thermal and non-thermally treated GCLs, before and after constant stress swell tests (20 kPa)
Grid number 1 2 Totals
Non-thermally treated GCL
Thermally treated GCL
Before
Before
After
% remaining
After
% remaining
8 5
1 0
13 0
7 9
4 8
57 89
13
1
8
16
12
75
Table 4 Number of visible "bres for thermal and non-thermally treated GCLs, before and after constant stress swell tests (100 kPa)
Grid number 1 2 Totals
Non-thermally treated GCL
Thermally treated GCL
Before
Before
After
% remaining
After
% remaining
6 7
5 7
83 100
9 8
9 8
100 100
13
12
92
17
17
100
Factors such as the density of the needlepunching, the mass per unit area of the carrier geotextile, the mass of bentonite in the GCL, and the type of thermal treatment manufacturing process may e!ect the number of "bres pulled out from the carrier geotextile and the relative di!erence in GCL swell height between a non-thermal and thermally treated GCL. However, these results do show that these simple swell tests perhaps in combination with peel test results can be an e!ective index test if "bre pullout must be evaluated. These observations are applicable to situations where a GCL may be hydrated under low to medium stress conditions. The question remains as to whether there will be any signi"cant di!erence in GCL di!usion coe$cients at low stresses when comparing the WNW and WNWT GCLs.
5. E4ect of thermal treatment on Na` and Cl~ di4usion coe7cients To examine the e!ect of GCL thermal treatment on Na` and Cl~ di!usion coe$cients under free swell conditions (3 kPa), constant stress di!usion (CSD) tests were performed on the WNW and WNWT GCLs (Table 1). Details regarding the test procedures and methods of data analysis for the CSD test are given in Rowe et al.
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Fig. 15. Comparison of non-thermally treated GCL before and after swell tests (100 kPa).
(2000). Both GCL samples were hydrated with deionized, deaired water (DDW) from the bottom of the sample. The water uptake was recorded using a burette attached to the bottom of the GCL di!usion testing apparatus. When water uptake ceased, 1 cm of DDW was added to the source compartment and the sample was left until chemical
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Fig. 16. Comparison of thermally treated GCL before and after swell tests (100 kPa).
equilibrium was reached in the source and receptor compartments. A di!usion test was then initiated by replacing the source #uid in the upper chamber by a 4.6 g/L (0.08 M) NaCl solution. Changes in GCL heights during hydration were recorded and 3 mL samples were taken daily from the source and receptor reservoirs during
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99
di!usion testing. Experimental concentrations obtained from each di!usion test were modelled with a contaminant transport program, POLLUTE (Rowe and Booker, 1999) as described by Rowe et al. (2000). The results for the two tests showed that under free swell conditions, the thermally treated GCL swelled to an equilibrium height of 13.57 mm (bulk GCL void ratio of 4.7) compared to the non-thermally treated, needlepunched GCL equilibrium height of 22.07 mm (bulk GCL void ratio of 8.2). This is over a 60% increase in swell height and the trend is consistent with that for the swell tests performed at 6 kPa described earlier. Post-di!usion test inspection of each GCL carrier geotextile revealed a `dimpleda bottom for the thermally treated GCL and a very smooth bottom for the non-thermally treated GCL indicating the "bres of the thermally treated GCL were restricting the swelling of bentonite GCL core. A chloride di!usion coe$cient of 3.5]10~10 m2/s and sodium di!usion coe$cient 5.1]10~10 m2/s for the thermally treated GCL compared to a chloride di!usion coe$cient of 4.1]10~10 m2/s and sodium di!usion coe$cient of 5.9]10~10 m2/s for the non-thermally treated needlepunched GCL suggest that thermal treatment may be bene"cial for minimizing di!usion at low con"ning stresses. These results suggest that if it is desired to minimize di!usion through GCLs at low con"ning stresses, thermal treatment may be bene"cial compared to non-thermally treated "bres because the thermally treated needlepunched "bres appear to be more e!ective at restricting swelling at lower con"ning stresses during hydration. These results are also consistent with those reported in the companion paper (Lake and Rowe, 2000) that have shown that when comparing similar testing conditions, lower bulk GCL void ratios result in lower di!usion coe$cients.
6. Conclusions Swell tests were performed on several di!erent types of needlepunched GCLs to examine any di!erences in swelling behavior for a range of stress and hydration conditions. Generally there was not much di!erence in swelling behavior for the GCLs examined at higher stresses but as the stress level decreased, the method of manufacturing appeared to exhibit more control over the swelling behavior. The following conclusions can be drawn from the results presented herein: (1) At stresses below 100 kPa, the GCL with bentonite impregnated in the cover geotextile (denoted here as WNWBT) swelled to equilibrium heights and bulk GCL void ratios up to 2.25 times higher than that of two other needlepunched, thermally treated GCLs (denoted NWNWT, WNWT herein). Given that the other needlepunched GCLs swelled to similar but smaller void ratios and that one of these GCLs (WNWT) had a very similar construction to the GCL with the powdered bentonite in the cover (WNWBT) except for the surface bentonite, implies that the di!erence in swelling behavior is due to the uncon"ned nature of the bentonite in the cover geotextile. Since the bentonite core in all surface bentonite GCLs can be expected to be similar, this implies that at low con"ning
GEGE=1244=KCT=VVC=BG 100
(2) (3)
(4)
(5)
(6)
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stresses the GCL with powdered bentonite in the cover geotextile will have a high contrast between the void ratio of the surface bentonite and that in the core (i.e. that needlepunched between the cover and carrier geotextile). A comparison of the results for the three GCLs also con"rms that needlepunching of the GCL signi"cantly reduces the swelling of the bentonite core at con"ning stresses below 100 kPa. At con"ning stresses above 100 kPa, each of the needlepunched GCLs swelled to similar bulk GCL void ratios for a particular stress. When the three thermally treated needlepunched GCLs (NWNWT, WNWT and WNWBT) were hydrated under a 6 kPa con"ning stress and then subsequently consolidated to a higher stress level, all three GCLs converged to similar void ratios at stresses greater than about 400 kPa. This suggests that the type of GCL manufacturing process has a smaller e!ect on the bulk GCL void ratio when the stress is higher if a GCL has been initially hydrated under low stresses. A comparison of constant stress swell test data suggests that thermal treatment of needlepunched GCL "bres restricts the bulk void ratio compared to a similar non-thermally treated GCL at low stresses. A more detailed examination of two similar GCL samples subjected to constant stress swell tests at 6 kPa showed that a non-thermally treated GCL swelled to almost a 50% greater thickness than a similar thermally treated GCL. Microscopic examination of each GCL before and after the swell tests revealed that 69% of the thermally treated, needlepunched GCL "bre `bundlesa were still visible by microscope compared to 12% of the non-thermally treated "bre `bundlesa. Comparison of thermal and non-thermally treated GCLs at higher stresses of 20 kPa and 100 kPa showed that the e!ect of the type of needlepunching begins to decrease as the stress level increases above 100 kPa for the GCLs examined herein. These results may depend on the type of carrier geotextile, the density of needlepunching and the mass per unit area of bentonite in the GCL and therefore should not be generalized. Regardless of the stress level, the thermally treated "bres appear to be as or more e!ective at restricting the GCL swell height. Di!usion tests on a thermally treated, needlepunched GCL and a non-thermally treated GCL showed that under free swell conditions, the thermally treated GCL swelled to a much lower bulk CGL void and exhibited lower di!usion coe$cients. By being more e!ective at controlling the swelling of the GCL at low con"ning stresses, the thermally treated needlepunched "bres appears to be more e!ective limiting the di!usive #ux through the GCLs at these low stresses.
Acknowledgements The research reported in this paper was funded by Terra"x Geosynthetics, the National Research Council of Canada (IRAP), and the Natural Sciences and Engineering Research Council of Canada. The authors very gratefully acknowledge the discussions with Messrs. Don Stewart and Cal Reaume of Terra"x and Mr. Kent von
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Maubeuge of Naue Fasertechnik during the preparation of this paper. The information in this paper should not be used without independent examination and veri"cation of its suitability for any particular project.
References ASTM, 1996. One dimensional swell or settlement potential of cohesive soils. Annual book of ASTM Standards, ASTM D4546-96, Vol. 4.08, Soil and Rock; Dimension Stone; Geosynthetics. Daniel, D.E., Shan, H.Y., Anderson, J.D., 1993. E!ects of partial wetting on the performance of the bentonite component of a geosynthetic clay liner. In: Proceedings Geosynthetics '93, Vancouver, B.C., Canada,. IFAI, St. Paul, MN, USA, pp. 1483}1496. Dixon, D.A., Wan, A.W.L., Graham, J., Kjartarson, B.H., 1991. Assessing pressure-volume equilibrium in bentonite based materials. In Proceedings of the 44th Canadian Geotechnical Conference, Calgary, Alberta, 2: Paper 61. Fox, P.J., 1998. Research on geosynthetic clay liners at Purdue University. Geotechnical News 16 (1), 35}40. Koerner, R.M., 1998. Designing with Geosynthetics, 4th edition. Prentice-Hall, Englewood Cli!s, N.J. Kraus, J.F., Benson, C.H., Erickson, A.E., Chamberlain, E.J., 1997. Freeze thaw cycling and hydraulic conductivity of bentonite barriers. ASCE Journal of Geotechnical and Geoenvironmental Engineering 123 (3), 229}238. Lake, C.B., Rowe, R.K., 2000. Di!usion of sodium and chloride through geosynthetic clay liners. Geotextiles and Geomembranes 18 (2}4), 103}131. Madsen, F.T., Muller-Vonmoos, 1989. The swelling behavior of clays. Applied Clay Science 4, 143}156. Petrov, R.J., Rowe, R.K., 1997. GCL } chemical compatibility by hydraulic conductivity testing and factors impacting its performance. Canadian Geotechnical Journal 34 (6), 863}885. Petrov, R.J., Rowe, R.K., Quigley, R.M., 1997. Selected factors in#uencing GCL hydraulic conductivity. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 123, 683}695. Richardson, G.N., 1997. GCL internal shear strength requirements. Geotechnical Fabrics Report 15 (2), 20}25. Rowe, R.K., Lake, C.B., Petrov, R.J., 2000. Apparatus and procedures for assessing inorganic di!usion coe$cients through geosynthetic clay liners. ASTM Geotechnical Testing J. (in press). Rowe, R.K., Booker, J.R., 1999. POLLUTE v.6.4 } 1-D pollutant migration through a non-homogeneous soil(. Distributed by GAEA Environmental Engineering Ltd., 44 Canadian Oakes Dr., Whitby, Ontario, L1N 6W8. Scranton, H.B., 1996. Field performance of sloping test plots containing geosynthetic clay liners. Master of Science in Engineering Thesis. University of Texas at Austin. Shan, H.Y., Daniel, D.E., 1991. Results of laboratory tests on a geotextile/bentonite material. In: Geosynthetics '91, Atlanta. IFAI, St. Paul, MN, USA, pp. 517}535. USEPA, 1993. Report of workshop on geosynthetic clay liners. EPA/600/R-93/171. Risk Reduction Engineering Laboratory, Cincinnati, Ohio, 106p. von Maubeuge, K.P., Eberle, M.A., 1998. Can geosynthetic clay liners be used on slopes to achieve long term stability? Proceedings of the Third International Congress on Environmental Geotechnics, Lisboa, Portugal, 1998, pp. 375}380.
Geotextiles and Geomembranes 18 (2000) 77}101
Swelling characteristics of needlepunched, thermally treated geosynthetic clay liners Craig B. Lake, R. Kerry Rowe* Department of Civil and Environmental Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9 Received 4 February 1999; received in revised form 19 July 1999; accepted 21 August 1999
Abstract The swelling characteristics of "ve di!erent geosynthetic clay liners (GCLs) are examined. Results show that two thermally treated GCLs have similar swell properties but a third thermally treated GCL with bentonite impregnated in the cover geotextile swells to higher GCL bulk void ratios at stresses below 100 kPa. This is attributed to the uncon"ned swelling of the bentonite present on the surface of the cover geotextile. A thermally treated needlepunched GCL is shown to have an equilibrium swell height that is 50% smaller than that of a nonthermally treated needlepunched GCL at 6 kPa. Microscopic examination of the thermal and non-thermally treated "bres before and after the 6 kPa swell tests shows that more thermally treated "bres remain visible on the bottom of the geotextile compared to the non-thermally treated "bres. Additional swell tests at 20 kPa and 100 kPa suggest that as the con"ning stress increases on a GCL during hydration, the tendency for pullout of the "bres decreases. Di!usion tests performed on a thermally treated needlepunched GCL and non-thermally treated needlepunched GCL under free swell conditions show that the di!usion coe$cients for Na` and Cl~ are lower for the thermally treated, needlepunched GCL than for the non-thermally treated GCL under otherwise similar conditions because of the higher bulk void ratio of the latter GCL. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: GCL; Needlepunching; Thermal treatment; Swell tests; Di!usion
1. Introduction Geosynthetic clay liners (GCLs) are utilized for many di!erent sealing applications including municipal solid waste (MSW) land"ll covers and liners, lagoon and canal * Correponding author. Tel.: 001-519-661-2126; fax: 001-519-661-3942. E-mail address: [email protected] (R.K. Kerry Rowe) 0266-1144/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 6 - 1 1 4 4 ( 9 9 ) 0 0 0 2 2 - 9
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liners. Depending on the application, GCL design considerations may include: interface shear strength (Scranton, 1996); internal shear strength (Fox, 1998); freeze/thaw and wet/dry cycles (Kraus et al., 1997); hydraulic conductivity and compatibility (Petrov and Rowe, 1997); and di!usion (Rowe et al., 2000). Manufacturers have developed di!erent types of GCLs (Koerner, 1998), but all involve a clay (typically sodium bentonite) core with geosynthetics (geotextiles or geomembranes) either on one or both sides. The geosynthetics and bentonite may be held together by needlepunched "bres, stitching or adhesives with some manufacturers increasing the internal shear resistance of needlepunched "bre GCLs by thermal treatment (heat-bonding, thermal locking). Thermally treated GCLs refer to "bres that are heated on the bottom geotextile after needlepunching. The heating process fuses together groups of individual "bres and sometimes fuses the "bres to the carrier geotextile, minimizing pullout of the "bres from the geotextile when the GCL is hydrated. For non-thermally treated needlepunched "bres, pullout resistance relies mainly on the entanglement of the "bres on the bottom geotextile. These di!erent variations of GCL manufacturing may in#uence the engineering characteristics of a GCL. Richardson (1997) showed that unreinforced GCLs have lower internal shear strengths than needlepunched GCLs (peak friction angle of approximately 83, von Maubeuge and Eberle (1998)). Petrov and Rowe (1997) examined the swelling behavior of a non-thermally treated, needlepunched GCL and found that needlepunching lowers the "nal bulk GCL void ratio relative to the same type of GCL with the needlepunched "bres cut, particularly at the low con"ning stresses (3 kPa to 6 kPa) representative of applications for ponds, lagoons, or covers. It was subsequently shown that in general, lower "nal bulk GCL void ratios corresponded to lower hydraulic conductivity values, all other factors being equal. Other studies have shown that the con"ning stress and type of GCL can reduce the swelling characteristics of GCLs (Shan and Daniel, 1991; USEPA, 1993). Lake and Rowe (2000) show that Na` and Cl~ di!usion coe$cients for a thermally treated, needlepunched GCL increase with increases in the "nal bulk GCL void ratio. However the means by which the thermally treated needlepunched "bres a!ect these variations in "nal bulk GCL void ratio has not been examined. One may hypothesize that there may be relatively less swelling of thermally treated needlepunched GCLs compared to non-thermally treated GCLs at low stresses where the needlepunched "bres are required to resist the majority of the swelling of the bentonite core. Furthermore, one may also hypothesize that peel tests performed on thermally treated GCLs hydrated under low stresses may exhibit higher values compared to a similar non-thermally treated GCL and this may be an indirect evaluation of the pullout resistance of the GCL "bres during swelling. This paper focuses on di!erences of swelling behavior for three types of thermally treated, needlepunched GCLs and two non-thermally treated, needlepunched GCLs. Results of constant stress swell tests, speci"ed volume swell tests and consolidation tests are combined with a microscopic examination of both thermal and non-thermally treated needlepunched GCL "bres to explore the e!ect of the method of GCL manufacturing and the role of needlepunched "bres on the swell characteristics and "nal bulk void ratio of these GCLs. There is a paucity of published information regarding how
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79
needlepunched "bres actually behave during the swelling process. By understanding how the "bres interact with the GCL, results of other types of GCL testing may be better understood. To illustrate this point, di!usion through otherwise similar thermally treated and non-thermally treated needlepunched GCLs will be examined. This paper also provides a basis for relating the swell characteristics reported herein to the di!usion characteristics of these GCLs reported in the companion paper (Lake and Rowe, 2000).
2. Materials The basic characteristics of the GCLs examined in this study are summarized in Table 1. They include Bento"x NW and NS GCLs (distributed by Terra"x Geosynthetics Inc., based in Toronto, Ontario, Canada and Naue Fasertechnik in Lubbecke, Germany under slightly di!erent product identi"ers Bento"x B4000 and NSP 4900-3) and BFG5000 GCLs (distributed by Naue Fasertechnik in Germany). The generic symbols used to describe these GCLs throughout the rest of the paper are such that the type of manufacturing of each GCL can be ascertained from the symbols (see Table 1). The "rst `NWa or `Wa in the generic name refers to whether the bottom geotextile (called the `carriera geotextile) is nonwoven or woven. The second `NWa
Table 1 Basic characteristics of "ve needlepunched GCLs!
Middle bentonite GCL type layer
Carrier geotextile
Cover geotextile
Minimum mass/area tested (g/m2)
NW"
Granular sodium bentonite
PP nonwoven$
PP nonwoven
5270
NWNWT
NS"
Granular sodium bentonite
PP slit "lm, woven
PP nonwoven
5380
WNWT
BFG5000# Powdered sodium bentonite
PP slit "lm, woven
PP nonwoven
4990
WNWBT
NW
Powdered sodium bentonite
PP nonwoven$
PP nonwoven
N/T
NWNW
NS
Powdered sodium bentonite
PP slit "lm, woven
PP nonwoven
6189
WNW
Generic symbol used in paper
!Notes: PP } Polypropylene; N/T } Tested by Petrov and Rowe (1997), but discussed in this paper. "Thermally treated and needlepunched. #Thermally treated, needlepunched and cover geotextile impregnated with 500 g/m2 of powder bentonite. $Nonwoven scrim reinforced with slit "lm woven geotextile.
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Fig. 1. Schematic of basic types of GCLs examined.
describes the top geotextile (called the `covera geotextile) which was nonwoven for all GCLs tested. The &B' in the generic name WNWBT indicates that the cover geotextile is impregnated with bentonite and the `Ta, if present, refers to the needlepunched "bres being thermally treated. The NWNW and NWNWT GCLs are often used in liner systems with steeper slopes compared to the WNW and WNWT GCLs. The WNWBT GCL has been used in applications to improve the intimate contact between GCLs and geomembranes in composite liner systems, pond applications and underwater installations where seam overlaps are important. Fig. 1 shows a schematic of these three GCLs. Characteristics of the sodium bentonite in these GCLs have been described by Rowe et al. (2000). The bentonite in each GCL is similar and predominantly composed of smectite clay minerals (&90%) with Na` and SO~ as the predominant ions 4 in the pore #uid.
3. Testing 3.1. Constant stress swell (CSS) tests Constant stress swell (CSS) test procedures were performed as described by Petrov et al. (1997). A GCL is hydrated under a constant applied stress while the GCL height is monitored with time until there is no further swelling (Fig. 2). This is similar to swell tests described in the literature (Shan and Daniel, 1991; USEPA, 1993).
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81
Fig. 2. Schematic of constant stress and speci"ed volume swell tests.
Constant stress swell tests were performed for applied stresses of 6 kPa, 25 kPa, 100 kPa, 200 kPa, 300 kPa and 400 kPa and with distilled water as the hydration #uid. This range of stresses was chosen to represent the typical range of stresses to which a GCL may be subjected in the "eld. The lower range of stress (6 kPa) may represent the use of a GCL in a cover or lagoon while the higher range may represent that expected by a GCL liner in a large land"ll after placement of a substantial thickness of waste. 3.2. Specixed volume swell (SVS) tests Speci"ed volume swell (SVS) tests were performed to evaluate the stresses generated during GCL hydration when the GCL is allowed to swell to a speci"ed height or volume (Fig. 2). Variations of this type of test (ASTM, 1996) have been performed for swelling rocks and sand/bentonite mixtures (Madsen and Muller-Vonmoos, 1989; Dixon et al., 1991). Results from a speci"ed volume swell test permit the di!usion coe$cient obtained at a given "nal bulk GCL void ratio (Lake and Rowe, 2000) to be correlated with the stress generated from the speci"ed volume swell test (provided the permeating #uid is similar). This type of test permits an assessment of how the GCL manufacturing process a!ects the magnitude of swelling stresses generated. The procedure used for this paper involves hydrating the GCL with distilled water and allowing the specimen to swell in a 70 mm diameter oedometer ring to a speci"ed height (5.6 mm, 7.1 mm, 9.1 mm, or 11.0 mm) in a rigid frame equipped with a sti! load cell. Load cell readings are obtained daily and when a constant stress is reached (within 1 kPa over at least 72 hours), the test is terminated. The bentonite moisture content is measured both before and after testing.
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4. Swell test results 4.1. Comparison of NWNWT, WNWT and WNWBT GCLs Fig. 3 shows constant stress swell (CSS) test results for the NWNWT, WNWT, and WNWBT GCLs. The initial heights (height with a nominal 1 kPa to 3 kPa stress applied to the GCL before addition of the applied stress and subsequent hydration), the initial con"ned heights (height after addition of the applied stress but before
Fig. 3. GCL heights } constant stress swell tests.
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83
Fig. 4. Comparison of constant stress swell tests.
hydration), and the "nal GCL heights (height after the addition of the applied stress and subsequent hydration with distilled water) are shown. There is a trend of decreasing "nal GCL height with increasing con"ning stress. Both the NWNWT and WNWT GCLs exhibit similar equilibrium swelling heights but the WNWBT GCL exhibits much greater equilibrium swelling heights at 6 kPa and 25 kPa stresses. When all the CSS test results are plotted in terms of "nal bulk GCL void ratio on Fig. 4, the WNWBT GCL exhibits higher "nal bulk GCL void ratios than the NWNWT and WNWT GCLs for applied stresses below 100 kPa. This is due to the uncon"ned nature of the powder bentonite impregnated in the cover geotextile. However, bentonite encapsulated between the geotextiles is still restricted from uncon"ned swelling due to the thermally treated, needlepunched "bres through the core. At stresses above 100 kPa, the WNWBT GCL swelled to similar "nal bulk void ratios as the WNWT and NWNWT GCLs. Despite that the NWNWT and WNWT GCLs had di!erent carrier geotextiles, both GCLs swelled to similar "nal bulk GCL void ratios (Fig. 4) for the entire range of stresses tested. Lake and Rowe (2000) showed that for a range of bulk GCL void ratios of 1.0 to 3.5, the NWNWT, WNWT and WNWBT GCLs exhibited a linear relationship between bulk GCL void ratio and Na` and Cl~ di!usion coe$cients (all other factors being equal). Although there was some scatter at the higher bulk void ratios in the relationship, there was no clear trend for any GCL. Even though the WNWBT GCL is di!erent in construction than the other two GCLs, the data for this GCL "t the same linear relationship as the other two GCLs. The results of the SVS tests are shown in Fig. 5 together with results of the CSS tests for each type of GCL. The speci"ed volume swell test data plot along the same trend line as the constant stress swell test data, con"rming that the stresses generated by a speci"ed volume swell test are similar to the constant stresses applied to generate
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Fig. 5. Comparison of speci"ed volume swell tests and constant stress swell tests.
that same height, provided both tests are performed with the same hydrating #uid. For both test methods, the stress is higher for higher "nal bulk GCL void ratios with a linear trend on a linear-log scale. However the WNWBT GCL seems to exhibit a steeper trend line for the lower stresses. Results for all the SVS tests are also shown in Fig. 6. The WNWT and NWNWT GCLs exert very similar swelling stresses when hydrated, but the WNWBT GCL exerts slightly higher stresses for the same bulk GCL void ratio perhaps due to the uncon"ned nature of the bentonite impregnated in the cover geotextile.
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85
Fig. 6. Comparison of speci"ed volume swell tests results.
4.2. Post-conxnement stress application GCLs may be subjected to a variety of hydration conditions in the "eld. In most cases the GCL may be purposely hydrated after a cover soil is placed, hydrated by precipitation or hydrated from the moisture of underlying soil (Daniel et al., 1993). These hydration conditions may result in the GCL being hydrated under relatively low stresses (5-20 kPa) and then being subsequently consolidated (post-con"nement stress application) such as when overlying waste is added to a land"ll. Post-con"nement stress applications were applied to the NWNWT, WNWT and WNWBT GCLs to allow direct comparison with previous results reported by Petrov et al. (1997) for non-thermally treated needlepunched GCLs. The procedure for post-con"nement stress application has been described in detail by Petrov et al. (1997) and will not be repeated here. Essentially, the GCL is hydrated with distilled water in an oedometer under a con"ning stress of 6 kPa until swelling equilibrium is reached. Loading increments of 12.5 kPa, 25 kPa, 50 kPa, 100 kPa, 200 kPa, 400 kPa and 800 kPa are then applied to the sample in the same manner as a typical consolidation test. Unloading of the sample proceeds in the same fashion to 400 kPa, 100 kPa, and 10 kPa. Results for the NWNWT, WNWT, and WNWBT GCLs on Fig. 7 show that at stresses below approximately 400 kPa, the WNWBT GCL has higher bulk GCL void ratios compared to the NWNWT and WNWT GCLs because of swelling of the uncon"ned bentonite on the surface of the upper geotextile when hydrated at 6 kPa. Above 400 kPa, the GCLs approach similar bulk void ratios, indicating that at higher stresses the type of GCL does not have as much in#uence on the bulk GCL void ratio as the vertically applied stress. When unloaded from the maximum 800 kPa stress, the NWNWT and WNWT GCLs swell to similar bulk
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Fig. 7. Comparison of post-con"nement versus pre-con"nement stress application.
GCL void ratios while the WNWBT GCL swells to a higher bulk GCL void ratio at the "nal 10 kPa stress level. Hydraulic conductivity testing (Petrov and Rowe, 1997) and di!usion testing (Lake and Rowe, 2000) have shown that it is the bulk GCL void ratio at the time of testing that controls the hydraulic conductivity and di!usion coe$cient regardless of its previous stress history (for engineering purposes). This implies that if the postcon"nement stress level reaches 300 kPa to 400 kPa, the resultant hydraulic conductivity and di!usion coe$cients are likely to be similar for all three GCLs.
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Apparent preconsolidation pressures of&80 kPa for the NWNWT GCL,&80 kPa for the WNWT GCL and &25 kPa for the WNWBT GCL are evident from the curves in Fig. 7 (using the Casagrande method). Previous stress history, desiccation, and the in#uence of needle punched "bres may be responsible for this apparent preconsolidation pressure. As the bentonite in the GCL hydrates, it puts the needlepunched "bres in tension, applying a stress back to the bentonite. This stress being applied by the "bres to the bentonite under a con"ning stress of 6 kPa acts like a `stress historya to the sample as the stress levels are increased during the tests. When the swell data from the previous section are plotted on Fig. 7, they plot on top of the rebound line from the consolidation tests indicating the rebound line is a lower bound for the swell tests performed, at least at higher stresses. This is consistent with the "ndings of Petrov and Rowe (1997). Also, when a sample is con"ned by an applied stress before hydration (see Fig. 7), the resultant void ratio is lower compared to the same post-con"nement stress application, probably as the result of a more oriented soil fabric. 4.3. Ewect of thermally treated needlepunched xbres on swelling behavior 4.3.1. Comparison of non-thermally treated and thermally treated GCLs Since Petrov et al. (1997) used similar swell test procedures for testing the NWNW GCL as described in Table 1, it is useful to compare their NWNW swell test results with the NWNWT, WNWT, and WNWBT GCLs examined so far. Petrov et al. (1997) de"ned the con"ned swell pressure in distilled water, p@ , as the 41 stress above which there is no swelling of the GCL greater than its initial height (H ). 0 They found a p@ of 160 kPa for the non-thermally treated NWNW GCL they tested 41 (Table 1). The p@ values found for the NWNWT, WNWT, and WNWBT GCLs on 41
Fig. 8. E!ects of "bres and thermal treatment on "nal bulk GCL void ratio for constant stress swell tests.
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Fig. 9. Comparison of post-con"nement stress application results of a non-thermally treated GCL and a thermally treated GCL.
Fig. 3 are &100 kPa, &100 kPa, and &135 kPa, respectively. These are lower con"ned swell pressures values than obtained by Petrov et al. (1997) which suggests that less con"ning stress is required to restrict swelling of the thermally treated GCLs above their initial height relative to non-thermally treated GCLs. Hence it may be hypothesized that the thermally treated "bres are resisting more of the swelling stresses than the non-thermally treated "bres. Fig. 8 compares constant stress swell test results for the NWNWT and WNWT GCLs in this study with the NWNW GCL (Table 1) tested by Petrov et al. (1997). The "bre-free NWNW GCL in Fig. 8 refers to an NWNW GCL with no needlepunched "bres. As described by Petrov et al. (1997), this was achieved by cutting the "bres that attach the cover and carrier geotextile and removing the bentonite from the GCL. The GCL was then reassembled by adding a predetermined mass of bentonite onto the lower geotextile and replacing the top geotextile. It is apparent from Fig. 8 that this "bre-free GCL plots above all the GCL results in this study. At stresses lower than 100 kPa, the non-thermally treated needlepunched GCL (NWNW) plots above the two thermally treated needlepunched GCLs in this study. The di!erence in "nal bulk GCL void ratio at the 6 kPa stress level for the four GCLs in Fig. 8 corresponds to a situation where the "bres would be required to carry the majority of the swelling induced stresses during hydration. This low stress range may occur in liquid containment systems, land"ll cover systems, or in land"ll base liner systems where the GCL hydrates just after installation or before the addition of a signi"cant amount of waste. A signi"cant reduction in "nal bulk GCL void ratio is provided by needlepunching of the "bres, and for stresses below approximately 100 kPa, thermal treatment of the
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needlepunched "bres results in a further reduction in "nal bulk GCL void ratio compared to that of no thermal treatment. This suggests that if GCLs are hydrated under small stresses (&6 kPa), the thermally treated GCLs may swell to much lower "nal bulk GCL void ratios, and possibly have lower hydraulic conductivity or di!usion coe$cients than similar non-thermally treated GCLs. The post-con"nement stress application results of the non-thermally treated NWNW GCL of Petrov and Rowe (1997) are reproduced on Fig. 9 with the NWNWT GCL post-con"nement stress application results. As was evident from previous swell test results at low stresses, the di!erence in "nal bulk GCL void ratio between the two GCLs is pronounced at post-con"nement stresses lower than 100 kPa. As the stress increases, the bulk void ratios of the two GCLs converge as the e!ect of thermal treatment is overshadowed by the applied vertical stress.
4.4. Microscopic examination of thermally treated, needlepunched xbres To investigate the means by which thermally treated, needlepunched GCL "bres in#uence the "nal bulk GCL void ratio during GCL hydration, constant stress swell tests were compared for two types of GCLs (WNWT and WNW; see Table 1 for a description). The objective of the comparative swell tests was to observe the height to which each GCL swelled and to examine the needlepunched "bres before and after the swell tests. Constant stress swell tests were performed at 6 kPa, 20 kPa, and 100 kPa. As discussed earlier, at the 6 kPa stress, the GCL "bres should be resisting the majority of the swelling stresses generated during hydration. The 20 kPa stress represents approximately 1 m of cover soil while the 100 kPa stress represents a medium stress level that may be exerted on a GCL in a land"ll. All GCLs tested had similar mass per unit areas (ranging from 6189 g/m2 to 6315 g/m2), polypropylene, slit-"lm, woven carrier geotextiles (100 g/m2) and polypropylene, nonwoven cover geotextiles. The only di!erence between the two types of GCLs was that the WNWT GCL had thermally treated needlepunched "bres and the WNW GCL had no thermal treatment of the needlepunched "bres. GCLs were placed in an oedometer ring and a 16 cm2 area in the centre of each GCL carrier geotextile was numbered and marked with a waterproof felt-tip marker into individual 1 cm2 squares. For the 20 kPa and 100 kPa swell tests, only two, 1 cm2 sections were used for observations as comparison to the 6 kPa stress level. The GCLs were then transferred, still in their oedometer rings, to an optical stereo microscope equipped with a digital camera. Oblique lighting was applied at an angle of approximately 103 from the horizontal to improve the quality of digital images. Each square was examined under the optical microscope. Each hole made by an individual needle punching through the carrier geotextile had several "bres that encompassed the hole left by the needle (herein de"ned as a `bundlea). The number of "bre `bundlesa was counted for each grid and an image was transferred from the optical microscope to a computer. Care was taken during the examination to prevent disturbance of the "bres and prevent a loss of bentonite from the GCL.
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Fig. 10. Comparison of thermal versus non-thermally treated GCLs (constant stress swell tests).
After preliminary observations, the GCLs were transferred from the optical microscope to an oedometer pot and a constant stress swell test was performed on each sample with distilled water. Once equilibrium occurred, the GCLs were transferred from the oedometer pot to the optical microscope. The photographic procedure was repeated on the GCLs to provide a visual before and after comparison of the "bre `bundlesa on the carrier geotextile. A non-thermal "bre `bundlea was considered to be remaining on the bottom of the GCL after the swell test if the density of the "bre `bundlea was approximately 50% to 100% of that before the swell test (even if the length had decreased). Since the resistance to pullout for these types of GCLs relies on entanglement of the "bres, the density of the individual `bundlesa is one measure of remaining e!ectiveness of the "bres. For the thermally treated "bres, the remaining "bre `bundlesa that had been fused together were either present or absent on the carrier geotextile after the swell test. Fig. 10 shows the swell test results for the WNWT and WNW GCLs. For the 6 kPa stress level, after 35 days the thermally treated GCL had essentially reached equilibrium, while the non-thermally treated GCL appeared to be still gradually swelling. During this time, the WNWT GCL swelled to a height of 11.54 mm, while the WNW GCL swelled to a height of 17.09 mm. This is almost a 50% increase in height for the non-thermally treated, needlepunched GCL and is signi"cant considering that there was essentially no di!erence between the GCLs except thermal treatment of the "bres. Table 2 shows a comparison of the number of "bre `bundlesa visible by the optical microscope before and after the 6 kPa swell tests. There are more "bre `bundlesa visible on the thermally treated GCL (69%) compared to the non-thermally treated
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Table 2 Number of visible "bre `bundlesa for thermal and non-thermally treated GCLs, before and after constant stress swell tests (6 kPa)
Grid number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Totals
Non-thermally treated GCL
Thermally treated GCL
Before
Before
After
% remaining
After
% remaining
6 8 7 10 7 6 5 7 7 9 6 4 9 9 10 6
0 1 1 0 1 2 2 0 0 2 2 0 1 1 1 0
0 13 14 0 14 33 40 0 0 22 33 0 11 11 10 0
10 8 8 8 12 11 11 10 7 7 6 5 9 10 9 10
10 5 5 6 5 9 8 7 4 4 4 3 8 6 5 8
100 63 63 75 42 82 73 70 57 57 67 60 89 60 56 80
116
14
12
141
97
69
GCL (12%) after completion of the swell tests. A statistical t-test was performed on the data shown in Table 2 and it was shown that there was a signi"cant di!erence when comparing the means of the non-thermally treated and thermally treated GCLs (two tailed p-value of 3]10~12). It would appear that the "bre `bundlesa that are not visible have been pulled out from the carrier woven geotextile and are no longer participating in the restriction of swelling of the GCL. When "rst observed under the microscope, the non-thermally treated "bres were punched through the carrier geotextile and entangled. After the 6 kPa swell tests, some individual "bres were still visible on the bottom of the geotextile where bundles had one been, however the density of the "bre `bundlesa through the holes had decreased greatly (Fig. 11). Fig. 12 provides visual evidence of how some of the thermally treated "bres have remained attached to the carrier geotextile, providing restriction to the swelling of the GCL. This was emphasized when the carrier geotextiles were compared after the tests. The WNWT GCL exhibited dimples on the bottom of the GCL, showing how some of the thermally treated needlepunching had remained in place during the swelling process. This was in contrast to the #at surface observed for the carrier geotextile of the WNW GCL. The results of the 20 kPa and 100 kPa swell test results are compared with the 6 kPa swell test results on Fig. 10. At the 20 kPa and 100 kPa stress levels, the non-thermally treated needlepunched GCLs swelled to a greater GCL thickness than
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Fig. 11. Comparison of non-thermally treated GCL before and after swell tests (6 kPa).
the respective thermally treated GCL. Also, the relative di!erence in "nal GCL height between the non-thermally and thermally treated GCLs at 6 kPa is greater than that at 100 kPa implying that as the stress level increases, the in#uence of the thermally treated "bres decreases. This is con"rmed by comparing Figs. 13 and 14 as well as
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Fig. 12. Comparison of thermally treated GCL before and after swell tests (6 kPa).
Figs. 15 and 16. At 100 kPa, the "bres of both the non-thermal and thermally treated GCLs are still visible in the carrier geotextile while at 20 kPa it appears that the non-thermally treated "bre `bundlesa have decreased substantially compared to the thermally treated "bres for the two grids examined. Since only two grids were
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Fig. 13. Comparison of non-thermally treated GCL before and after swell tests (20 kPa).
examined for the 20 kPa and 100 kPa stresses (Tables 3 and 4), no statistical conclusions can be drawn as the di!erences between the means for these two samples. Regardless of the stress level tested, the thermally treated "bres appear to be at least as
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95
Fig. 14. Comparison of thermally treated GCL before and after swell tests (20 kPa).
or more e!ective at restricting the GCL swell height, probably due to the resistance of the pullout as shown in Figs. 11 and 12. These results may only be applicable for the type of GCL tested (WNW and WNWT) and should not be extended for other types of GCLs without further testing.
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Table 3 Number of visible "bre `bundlesa for thermal and non-thermally treated GCLs, before and after constant stress swell tests (20 kPa)
Grid number 1 2 Totals
Non-thermally treated GCL
Thermally treated GCL
Before
Before
After
% remaining
After
% remaining
8 5
1 0
13 0
7 9
4 8
57 89
13
1
8
16
12
75
Table 4 Number of visible "bres for thermal and non-thermally treated GCLs, before and after constant stress swell tests (100 kPa)
Grid number 1 2 Totals
Non-thermally treated GCL
Thermally treated GCL
Before
Before
After
% remaining
After
% remaining
6 7
5 7
83 100
9 8
9 8
100 100
13
12
92
17
17
100
Factors such as the density of the needlepunching, the mass per unit area of the carrier geotextile, the mass of bentonite in the GCL, and the type of thermal treatment manufacturing process may e!ect the number of "bres pulled out from the carrier geotextile and the relative di!erence in GCL swell height between a non-thermal and thermally treated GCL. However, these results do show that these simple swell tests perhaps in combination with peel test results can be an e!ective index test if "bre pullout must be evaluated. These observations are applicable to situations where a GCL may be hydrated under low to medium stress conditions. The question remains as to whether there will be any signi"cant di!erence in GCL di!usion coe$cients at low stresses when comparing the WNW and WNWT GCLs.
5. E4ect of thermal treatment on Na` and Cl~ di4usion coe7cients To examine the e!ect of GCL thermal treatment on Na` and Cl~ di!usion coe$cients under free swell conditions (3 kPa), constant stress di!usion (CSD) tests were performed on the WNW and WNWT GCLs (Table 1). Details regarding the test procedures and methods of data analysis for the CSD test are given in Rowe et al.
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Fig. 15. Comparison of non-thermally treated GCL before and after swell tests (100 kPa).
(2000). Both GCL samples were hydrated with deionized, deaired water (DDW) from the bottom of the sample. The water uptake was recorded using a burette attached to the bottom of the GCL di!usion testing apparatus. When water uptake ceased, 1 cm of DDW was added to the source compartment and the sample was left until chemical
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Fig. 16. Comparison of thermally treated GCL before and after swell tests (100 kPa).
equilibrium was reached in the source and receptor compartments. A di!usion test was then initiated by replacing the source #uid in the upper chamber by a 4.6 g/L (0.08 M) NaCl solution. Changes in GCL heights during hydration were recorded and 3 mL samples were taken daily from the source and receptor reservoirs during
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99
di!usion testing. Experimental concentrations obtained from each di!usion test were modelled with a contaminant transport program, POLLUTE (Rowe and Booker, 1999) as described by Rowe et al. (2000). The results for the two tests showed that under free swell conditions, the thermally treated GCL swelled to an equilibrium height of 13.57 mm (bulk GCL void ratio of 4.7) compared to the non-thermally treated, needlepunched GCL equilibrium height of 22.07 mm (bulk GCL void ratio of 8.2). This is over a 60% increase in swell height and the trend is consistent with that for the swell tests performed at 6 kPa described earlier. Post-di!usion test inspection of each GCL carrier geotextile revealed a `dimpleda bottom for the thermally treated GCL and a very smooth bottom for the non-thermally treated GCL indicating the "bres of the thermally treated GCL were restricting the swelling of bentonite GCL core. A chloride di!usion coe$cient of 3.5]10~10 m2/s and sodium di!usion coe$cient 5.1]10~10 m2/s for the thermally treated GCL compared to a chloride di!usion coe$cient of 4.1]10~10 m2/s and sodium di!usion coe$cient of 5.9]10~10 m2/s for the non-thermally treated needlepunched GCL suggest that thermal treatment may be bene"cial for minimizing di!usion at low con"ning stresses. These results suggest that if it is desired to minimize di!usion through GCLs at low con"ning stresses, thermal treatment may be bene"cial compared to non-thermally treated "bres because the thermally treated needlepunched "bres appear to be more e!ective at restricting swelling at lower con"ning stresses during hydration. These results are also consistent with those reported in the companion paper (Lake and Rowe, 2000) that have shown that when comparing similar testing conditions, lower bulk GCL void ratios result in lower di!usion coe$cients.
6. Conclusions Swell tests were performed on several di!erent types of needlepunched GCLs to examine any di!erences in swelling behavior for a range of stress and hydration conditions. Generally there was not much di!erence in swelling behavior for the GCLs examined at higher stresses but as the stress level decreased, the method of manufacturing appeared to exhibit more control over the swelling behavior. The following conclusions can be drawn from the results presented herein: (1) At stresses below 100 kPa, the GCL with bentonite impregnated in the cover geotextile (denoted here as WNWBT) swelled to equilibrium heights and bulk GCL void ratios up to 2.25 times higher than that of two other needlepunched, thermally treated GCLs (denoted NWNWT, WNWT herein). Given that the other needlepunched GCLs swelled to similar but smaller void ratios and that one of these GCLs (WNWT) had a very similar construction to the GCL with the powdered bentonite in the cover (WNWBT) except for the surface bentonite, implies that the di!erence in swelling behavior is due to the uncon"ned nature of the bentonite in the cover geotextile. Since the bentonite core in all surface bentonite GCLs can be expected to be similar, this implies that at low con"ning
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(2) (3)
(4)
(5)
(6)
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stresses the GCL with powdered bentonite in the cover geotextile will have a high contrast between the void ratio of the surface bentonite and that in the core (i.e. that needlepunched between the cover and carrier geotextile). A comparison of the results for the three GCLs also con"rms that needlepunching of the GCL signi"cantly reduces the swelling of the bentonite core at con"ning stresses below 100 kPa. At con"ning stresses above 100 kPa, each of the needlepunched GCLs swelled to similar bulk GCL void ratios for a particular stress. When the three thermally treated needlepunched GCLs (NWNWT, WNWT and WNWBT) were hydrated under a 6 kPa con"ning stress and then subsequently consolidated to a higher stress level, all three GCLs converged to similar void ratios at stresses greater than about 400 kPa. This suggests that the type of GCL manufacturing process has a smaller e!ect on the bulk GCL void ratio when the stress is higher if a GCL has been initially hydrated under low stresses. A comparison of constant stress swell test data suggests that thermal treatment of needlepunched GCL "bres restricts the bulk void ratio compared to a similar non-thermally treated GCL at low stresses. A more detailed examination of two similar GCL samples subjected to constant stress swell tests at 6 kPa showed that a non-thermally treated GCL swelled to almost a 50% greater thickness than a similar thermally treated GCL. Microscopic examination of each GCL before and after the swell tests revealed that 69% of the thermally treated, needlepunched GCL "bre `bundlesa were still visible by microscope compared to 12% of the non-thermally treated "bre `bundlesa. Comparison of thermal and non-thermally treated GCLs at higher stresses of 20 kPa and 100 kPa showed that the e!ect of the type of needlepunching begins to decrease as the stress level increases above 100 kPa for the GCLs examined herein. These results may depend on the type of carrier geotextile, the density of needlepunching and the mass per unit area of bentonite in the GCL and therefore should not be generalized. Regardless of the stress level, the thermally treated "bres appear to be as or more e!ective at restricting the GCL swell height. Di!usion tests on a thermally treated, needlepunched GCL and a non-thermally treated GCL showed that under free swell conditions, the thermally treated GCL swelled to a much lower bulk CGL void and exhibited lower di!usion coe$cients. By being more e!ective at controlling the swelling of the GCL at low con"ning stresses, the thermally treated needlepunched "bres appears to be more e!ective limiting the di!usive #ux through the GCLs at these low stresses.
Acknowledgements The research reported in this paper was funded by Terra"x Geosynthetics, the National Research Council of Canada (IRAP), and the Natural Sciences and Engineering Research Council of Canada. The authors very gratefully acknowledge the discussions with Messrs. Don Stewart and Cal Reaume of Terra"x and Mr. Kent von
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Maubeuge of Naue Fasertechnik during the preparation of this paper. The information in this paper should not be used without independent examination and veri"cation of its suitability for any particular project.
References ASTM, 1996. One dimensional swell or settlement potential of cohesive soils. Annual book of ASTM Standards, ASTM D4546-96, Vol. 4.08, Soil and Rock; Dimension Stone; Geosynthetics. Daniel, D.E., Shan, H.Y., Anderson, J.D., 1993. E!ects of partial wetting on the performance of the bentonite component of a geosynthetic clay liner. In: Proceedings Geosynthetics '93, Vancouver, B.C., Canada,. IFAI, St. Paul, MN, USA, pp. 1483}1496. Dixon, D.A., Wan, A.W.L., Graham, J., Kjartarson, B.H., 1991. Assessing pressure-volume equilibrium in bentonite based materials. In Proceedings of the 44th Canadian Geotechnical Conference, Calgary, Alberta, 2: Paper 61. Fox, P.J., 1998. Research on geosynthetic clay liners at Purdue University. Geotechnical News 16 (1), 35}40. Koerner, R.M., 1998. Designing with Geosynthetics, 4th edition. Prentice-Hall, Englewood Cli!s, N.J. Kraus, J.F., Benson, C.H., Erickson, A.E., Chamberlain, E.J., 1997. Freeze thaw cycling and hydraulic conductivity of bentonite barriers. ASCE Journal of Geotechnical and Geoenvironmental Engineering 123 (3), 229}238. Lake, C.B., Rowe, R.K., 2000. Di!usion of sodium and chloride through geosynthetic clay liners. Geotextiles and Geomembranes 18 (2}4), 103}131. Madsen, F.T., Muller-Vonmoos, 1989. The swelling behavior of clays. Applied Clay Science 4, 143}156. Petrov, R.J., Rowe, R.K., 1997. GCL } chemical compatibility by hydraulic conductivity testing and factors impacting its performance. Canadian Geotechnical Journal 34 (6), 863}885. Petrov, R.J., Rowe, R.K., Quigley, R.M., 1997. Selected factors in#uencing GCL hydraulic conductivity. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 123, 683}695. Richardson, G.N., 1997. GCL internal shear strength requirements. Geotechnical Fabrics Report 15 (2), 20}25. Rowe, R.K., Lake, C.B., Petrov, R.J., 2000. Apparatus and procedures for assessing inorganic di!usion coe$cients through geosynthetic clay liners. ASTM Geotechnical Testing J. (in press). Rowe, R.K., Booker, J.R., 1999. POLLUTE v.6.4 } 1-D pollutant migration through a non-homogeneous soil(. Distributed by GAEA Environmental Engineering Ltd., 44 Canadian Oakes Dr., Whitby, Ontario, L1N 6W8. Scranton, H.B., 1996. Field performance of sloping test plots containing geosynthetic clay liners. Master of Science in Engineering Thesis. University of Texas at Austin. Shan, H.Y., Daniel, D.E., 1991. Results of laboratory tests on a geotextile/bentonite material. In: Geosynthetics '91, Atlanta. IFAI, St. Paul, MN, USA, pp. 517}535. USEPA, 1993. Report of workshop on geosynthetic clay liners. EPA/600/R-93/171. Risk Reduction Engineering Laboratory, Cincinnati, Ohio, 106p. von Maubeuge, K.P., Eberle, M.A., 1998. Can geosynthetic clay liners be used on slopes to achieve long term stability? Proceedings of the Third International Congress on Environmental Geotechnics, Lisboa, Portugal, 1998, pp. 375}380.