Elsevier

Geotextiles and Geomembranes

Volume 49, Issue 5, October 2021, Pages 1165-1175
Geotextiles and Geomembranes

Effect of wet-dry cycles on standard & polymer-amended GCLs in covers subjected to flow over the GCL

https://doi.org/10.1016/j.geotexmem.2021.03.010Get rights and content

Highlights

  • First study of the effects water flow over the GCL on inferred washout of polymers.

  • First study of the effects of different lengths of drying cycles.

  • First assessment of the relative performance of standard and polymer amended GCLs when subjected to wet-dry cycles.

Abstract

The performance of five different GCLs (two GCLs with standard sodium bentonite and three GCLs with polymer enhanced bentonite) subjected to three different climatic modes of wet-dry cycles simulating conditions to which a GCL might expose in cover systems over a prolonged time is reported. The wetting cycles lasted for 8 h, while the drying cycles varied between 16 h, seven days, and 14 days. It is shown that after around a year of accelerated aging, the hydraulic conductivity of the aged GCLs increased notably when permeated with tap water at an applied effective stress of 15 kPa for a range of heads (0.07, 0.14, 0.21, 0.49, and 1.2 m). The combined effects of the number and the duration of the wet-dry cycles, the GCL's mass per unit area, the carrier geotextile, the size and the number of the needle punch bundles, and the thermal treatment to bond the needle-punch bundles to the carrier geotextile are discussed. The poor hydraulic performance of the polymer-amended/modified bentonite GCLs is discussed.

Introduction

Geosynthetic clay liners (GCLs) are primarily being used as hydraulic barriers for containment and sealing applications in modern landfills (Petrov et al., 1997b, Petrov et al., 1997a; Petrov and Rowe 1997; Bouazza 2002; Meer and Benson 2007; Rowe 1998, 2005, 2012, 2014, 2016, 2018, 2020a, 2020b; Yu et al., 2018; Scalia et al., 2018; Rowe et al., 2019; Yu and El-Zein 2019; McWatters et al., 2019; Mukunoki et al., 2019; Li and Rowe 2020; Yu et al., 2020) because of their ability to restrict moisture infiltration due to rainfall and exfiltration of landfill gas through a cover/cap and to restrict the release of leachate from the waste.

It is now well known that the wet-dry cycles can damage the GCL in a GMB/GCL composite liner if left exposed for weeks to years. This damage may be in the form of desiccation (Lin and Benson 2000), panel shrinkage (Thiel and Richardson 2005; Koerner and Koerner 2005; Thiel et al., 2006; Rowe et al. 2010, 2011a, 2011b, 2012a, 2012b; Bostwick et al., 2010; Brachman et al., 2018), and downslope bentonite erosion (Ashe et al., 2015, Take et al., 2015b, Take et al., 2015a; Brachman et al., 2015).

Lin and Benson (2000) investigated how wet-dry cycling affects bentonite plasticity, swelling, and the hydraulic conductivity of deionized (DI) water, tap water, and 0.0125-M CaCl2 solution hydrated geosynthetic clay liners (GCLs). The wetting cycles lasted for one month in the Flexible Wall Permeameter (FWP) cell under 17.5 kPa effective stress and hydraulic gradient of 80, after which the specimen was air-dried on the bench until the weight of the specimen was constant, and the specimen was again placed inside the FWP cell for another permeation cycle. They found that the DI water and the tap water wet-dry cycles did not affect the bentonite's swelling capacity, and the hydraulic conductivity remained in a low range. However, the k of all specimens permeated with the 0.0125-M CaCl2 solution increased substantially within five to eight cycles to as high as 7.6 × 10−8 m/s. As cracks formed during desiccation did not fully heal when the bentonite was rehydrated, the hydraulic conductivity increased.

Rowe et al. (2017) exhumed four GCLs after 5 and 7 years used as single liners and covered by about 0.7 m silty sand on a 3H:1 V slope at QUELTS in Godfry (north of Kingston), Ontario, Canada. The 300 mm GCL panels overlap showed a good physical overlap intact. However, the bound sodium of the bentonite was replaced completely by divalent cations. The GCLs that had a small needle punch bundles size (average of 0.7 mm) and a percentage area covered by needle punch bundles of 4% had a low k to a 10 mmol/L CaCl2 solution when tested using FWP at 15 kPa effective stress and a range of heads (0.07–1.2 m). The GCLs that have bigger needle punch bundles (1.1–1.6 mm) covering a percentage area of a (9–14%) had a low k under a low head of 0.07 m. However, k increased by 1–4 orders of magnitude when the applied head increased up to 1.2 m. Rowe et al. (2017) explained that the GCL can perform well as a single cover liner if good attention was paid to the hydraulic gradient, the number and the size of the needle punch bundles, and the dry mass per unit area, Ma, when the cover system is designed.

Tian et al., (2019) examined the k of two needle-punched GCLs (BP GCL and BPS GCL) with nonwoven cover geotextiles and the same dry blend of bentonite and polymer. The GCLs were similar except that the BPS GCL had a woven carrier geotextile with a slit-film geotextile intact to illuminate the polymer elution. DI water and a variety of CaCl2 solutions (20–500 mM CaCl2) were used to permeate the two GCLs. Over the range of CaCl2 solutions tested, the k of both GCLs was one to four times lower than the k of standard unmodified bentonite. The retention of the polymer and the clogging of the hydraulically active pores resulted in lower k of the used GCLs, according to scanning electron microscopy (SEM). When permeated with 50 and 100 mM/L CaCl2, however, the polymer elution and the k of the BPS GCL were about three times lower than those of the BP GCL. During the permeation test, the woven slit-film reinforced carrier geotextile is thought to have been able to limit the polymer elution. When the BPS GCL was permeated with 200 mM/L CaCl2, however, the conformation of the polymer on the clay surface and the polymer charge were greatly changed, and the BPS GCL's carrier geotextile was unsuccessful in avoiding polymer washout.

The primary outstanding question concerning polymer (chemically) modified bentonites in GCLs is the GCL's long-term performance and durability. During the first swell cycles, the polymer additive can provide an advantage (Bohnhoff and Shackelford, 2014; Malusis and McKeehan, 2013; De Camillis et al. 2014, 2016; Mazzieri et al., 2017). De Camillis et al. (2017) assessed the impact of wet-dry cycles on untreated sodium bentonite and HYPER clay with seawater. HYPER clay is polymer-amended bentonite with improved efficiency in the presence of electrolyte solutions. Bentonite and bentonite treated with 2% and 8% polymer by dry clay weight were evaluated for their swelling capacity, self-healing capacity, crack formation, and hydraulic conductivity. The results showed that HYPER clay 8% had swollen the most and that its thickness after the 6th wet-dry cycle was comparable to the original thickness of the untreated bentonite during its maximum swelling in deionized water. Unlike the untreated clay, HYPER clays maintained a low permeability to seawater throughout the wet-dry cycles.

Despite the extensive prior research, the implications of wet-dry cycles combined with the flow over the GCL from a rainfall event followed by a dry warm period, either when the GCL is used alone or in a composite liner with a preserved wrinkle with a hole in the overlying geomembrane are still largely unknown; especially, for polymer-amended bentonite GCLs. Thus, the objectives of this study are to (i) examine the changes in the hydraulic conductivity, k, of five different GCLs, including three with polymer modification, when subject to about one year of wet-dry cycles, (ii) investigate the role of the length of the dry period between wet periods in affecting k for approximately the same total period of exposure to wet-dry cycles, (iii) explore the effect of the bentonite's mass per unit area on k, (iv) examine the role of the number and the size of the needle-punch bundles in the changes of k, (v) investigate the influence of the thermal treatment of the needle punch bundles on k.

Section snippets

GCLs

Five different GCLs were examined. All had a needle-punched nonwoven cover geotextile, but different types of carrier geotextile, bentonite granularity, polymer enhancement, and thermal lock for the needle-punch bundles on the carrier geotextile. The properties of the GCLs are given in (Table 1).

Flow-over the GCL test

Ideally, a GCL used alone or in the composite liner is covered with at least 30 cm of cover soil immediately after installation to protect the GCL from desiccation, vehicles, and people/animals. The

Visual observations

Observations were made to visually identify the changes in the GCLs both during and at the end of about one year of the wet-dry cycles. For GCL2 (with fine granular bentonite, an NWSR carrier, and thermally treated but with no polymer added to the bentonite), no notable physical changes were observed for any of the different wet-dry cycles, and that can be compared to the findings of Rowe et al. (2017) where the exhumed GCL1 and GCL2 (with fine granular bentonite) samples appeared to be

General discussion

Subjected to approximately one year wet-dry cycles, GCL2 with a scrim-reinforced thermally treated carrier geotextile and standard sodium bentonite performed best with less than an order of magnitude increase in k (to 3 × 10−10 m/s) consistent with cation exchange. Second-best was GCL6 with a woven thermally treated carrier and traditional sodium bentonite GCL, which also generally experienced about an order of magnitude increase in k (to 5 × 10−10 m/s) consistent with cation exchange, except

Conclusion

Five different GCLs were aged in a long-term process of wetting for 8 h by tap water flowing over the GCL on a 3H:1 V slope, and three drying cycles (16 h, 7 days, and 14 days), intended to simulate field conditions where there was water flow in the sand above the GCL when used alone in a cover or below a holed wrinkle in the GMB in a composite liner.

After prolonged wet-dry cycles, hydraulic conductivity tests were conducted using tap water as a permeant and under low effective stress (15 kPa),

Acknowledgements

The research was funded by Discovery Grant A1007 to the senior author from the Natural Sciences and Engineering Research Council of Canada (NSERC) with additional support from Naue.

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