Hydration of Deconstructed Geosynthetic Clay Liners Under Daily Thermal Cycles

J.D. Garcia, R.W.I. Brachman, R.K. Rowe

In the proceedings of: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Geosynthetics II (Walls and Barrier Systems)

ABSTRACT: Geosynthetic clay liners (GCLs) have been shown to achieve lower hydraulic conductivity and, hence, better performance as a barrier material than non-prehydrated specimens provided adequate hydration has occurred. However, there is evidence that exposure to daily thermal cycles could inhibit hydration of the bentonite which is significant in field applications with delayed installation. In this paper, two deconstructed GCLs (needle-punched fibres cut) with powdered and fine granular bentonite overlying a 450mm-thick silty sand foundation at 16% moisture content are subjected to laboratory simulated daily thermal cycles of 24-60 °C. Different configurations of the deconstructed GCLs allow the roles of the needle-punched fibres, bentonite granularity, and thermal treatment on hindering or facilitating the effects of thermal cycles to be investigated.

RÉSUMÉ: Les revêtements en argile géosynthétique (GCL) ont montré une conductivité hydraulique inférieure et, par conséquent, de meilleures performances en tant que matériau barrière que les spécimens non préhydratés à condition qu'une hydratation adéquate se soit produite. Cependant, il existe des preuves que l'exposition aux cycles thermiques quotidiens pourrait inhiber l'hydratation de la bentonite, ce qui est important dans les applications sur le terrain avec une faible couverture du sol ou une installation retardée. Dans cet article, deux GCL déconstruits (fibres aiguilletées coupées) avec de la bentonite granulaire en poudre et fine recouvrant une fondation de sable limoneux de 450 mm d'épaisseur à 16% d'humidité sont soumis à des cycles thermiques quotidiens simulés en laboratoire à 24-60 ° C. Différentes configurations des GCL déconstruits permettent d'étudier les rôles des fibres aiguilletées, la granularité de la bentonite et le traitement thermique pour entraver ou faciliter les effets des cycles thermiques. 1 BACKGROUND Geosynthetic clay liners (GCLs) have been shown to achieve lower hydraulic conductivity and, hence, better performance as a barrier material, after adequate hydration has occurred compared to non-prehydrated specimens (Petrov and Rowe, 1997; Petrov et al., 1997; Jo et al., 2004). In landfill liner applications where a GCL would typically be covered by a geomembrane, the source of moisture for the GCL is the underlying foundation soil or subgrade. The factors affecting moisture uptake and the degree of hydration under isothermal conditions for have been investigated by several researchers (Daniel et al., 1997; Rayhani et al., 2011; Anderson et al., 2012). To better understand the performance of GCLs in the field when left exposed to the sun, the effects of thermal cycles on the hydration of GCLs have also been investigated (Rowe et al. 2011; Hosney et al., 2016). This is applicable to cases such as when the GCLs are not covered in a timely manner during installation and could potentially explain issues such as shrinkage (Brachman et al., 2007) and downslope bentonite erosion (Rowe et al., 2016) which have been observed in the field. 1.1 Granular GCL Under Thermal Cycles Rowe et al. (2011) performed column hydration tests to study the moisture uptake of three GCLs with granular bentonite from a silty sand subgrade exhumed from the (QUELTS) under laboratory-simulated daily thermal cycles. It was found that, at 16% subgrade water content (WFDN), the GCL equilibrium water content (WGCL) was suppressed to 30% or less than one-third of what was achieved under isothermal conditions (without thermal cycles). Particularly, GCL 2 a needle-punched and thermally treated GCL with granular bentonite, scrim-reinforced nonwoven carrier geotextile and woven cover geotextile had an isothermal to cyclic water content ratio (WISO / WCYC) of 0.31 at WFDN=16%. 1.2 Downslope Bentonite Erosion Take et al. (2015) and Rowe et al. (2016) observed significant downslope bentonite erosion in four GCLs with granular bentonite but no significant erosion in two GCLs with powdered bentonite that were covered with a high-density polyethylene geomembrane and exposed to field conditions for over two years at QUELTS. Low ionic strength water that evaporated from the GCL during daytime heating and condensed on the underside of the geomembrane during nighttime cooling can flow downslope on the surface of the GCL and lead to bentonite loss. One possible reason that the powdered GCLs tested on site such as GCL 6 needle-punched and thermally treated with woven carrier and nonwoven cover geotextiles did not develop significant erosion features may be from greater moisture retention when subjected to thermal cycles. Field observations (Figure 1) suggest that GCL 6 had a greater degree of hydration than GCL 2 the same GCL tested by Rowe et al. (2011) with granular bentonite.

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