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Time-Dependent Unsaturated Behaviour of Geosynthetic Clay Liners A.S

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Time-Dependent Unsaturated Behaviour of Geosynthetic Clay Liners A.S 1824 ARTICLE Time-dependent unsaturated behaviour of geosynthetic clay liners A.S. Acikel, W.P. Gates, R.M. Singh, A. Bouazza, D.G. Fredlund, and R.K. Rowe Abstract: Three different chilled-mirror hygrometer test procedures were developed to investigate the time-dependent unsat- urated behaviour of powdered and granular bentonite based needle-punched geosynthetic clay liners (GCLs) on both the wetting and drying paths of the water retention curve (WRC). The GCL structure and bentonite forms governed the effect of measure- ment time and duration as well as the time-dependent suction changes of the bentonite component at a constant gravimetric water content. A conceptual model is proposed to explain the observed time-dependent unsaturated behaviour of the GCLs. The model suggests that the cross-over points on WRCs correspond to the point where bentonite crystallite separation is maximized within the crystalline swelling regime of smectite, forming a four-layer hydrate state where smectite interlayer spaces are filled with water. At gravimetric water contents below this point, the interlayer space dominated the suction, while at higher water contents, mesopores and macropores played increasingly important roles in determining the suction. The results reported herein provide further proof that the unsaturated behaviour of GCLs is largely controlled by the bentonite component. Key words: geosynthetics, geosynthetic clay liner (GCL), unsaturated, bentonite, water retention curve, time dependency. Résumé : Trois procédures d’essai d’hygromètres sur un miroir refroidi ont été développées pour étudier le comportement insaturé dépendant du temps des doublures d’argile géosynthétique (GCL) perforées et granulaires à base de bentonite sur les deux chemins de mouillage et de séchage de la courbe de rétention d’eau (WRC). La structure de GCL et les formes de bentonite ont régi l’effet du temps et de la durée de mesure ainsi que les changements d’aspiration dépendant du temps du composant de bentonite à une teneur en eau gravimétrique constante. Un modèle conceptuel est proposé pour expliquer le comportement insaturé dépendant du temps observé des GCL. Le modèle suggère que les points de croisement sur les WRC correspondent au point où la séparation des cristallites de bentonite est maximisée dans le régime de gonflement cristallin de la smectite, formant un état hydrate à quatre couches où les espaces interstratifiés de smectite sont remplis d’eau. À des teneurs en eau gravimétrique inférieures à ce point, l’espace intercouche dominait l’aspiration tandis qu’à des teneurs en eau plus élevées, les mésopores et macropores jouaient des rôles de plus en plus importants dans la détermination de l’aspiration. Les résultats rapportés ici fournissent une preuve supplémentaire que le comportement insaturé des GCL est largement contrôlé par le composant For personal use only. bentonite. [Traduit par la Rédaction] Mots-clés : géosynthétique, doublures d’argile géosynthétique (GCL), insaturé, bentonite, courbe de rétention d’eau, dépendant du temps. 1. Introduction hydraulic barrier (Rayhani et al. 2011; Liu et al. 2015; Bouazza et al. Geosynthetic clay liners (GCLs), typically 5–10 mm thick, are 2017). Key to the hydration process is the GCL water retention composed of a thin layer of sodium bentonite contained between curve (WRC) relationship, which is fundamental for estimating two layers of geotextile, commonly held together by needle the ultimate degree of saturation attained by GCLs and the equi- punching (Bouazza 2002). They are widely used in lining systems librium times required for hydration or dehydration. of waste containment facilities to minimize the escape of pollut- While complex, quantifying the water retention behaviour of ants into soil and groundwater (Hornsey et al. 2010; McWatters GCLs is crucial for the evaluation of their unsaturated hydraulic et al. 2016; Rowe 2014; Touze-Foltz et al. 2016). Geosynthetic clay performance (Abuel-Naga and Bouazza 2010; Beddoe et al. 2010; liners are well suited for this application owing to the low perme- Bouazza et al. 2013; Bannour et al. 2014; Acikel et al. 2015; Rouf ability of bentonite (the active component of GCL) when hydrated, et al. 2016). In this respect, several factors need to be taken into usually by uptake of water from the underlying soil (Daniel et al. consideration including (i) the time-dependent unsaturated be- 1993; Rowe 2007; Rayhani et al. 2011; Rowe et al. 2011; Anderson haviour of the bentonite component, (ii) the time dependency Can. Geotech. J. Downloaded from www.nrcresearchpress.com by MONASH UNIVERSITY on 01/21/19 et al. 2012; Chevrier et al. 2012). The final equilibrium gravimetric of the suction measurement and (or) control techniques when water content reached by the GCL component via hydration from considering a partially confined highly plastic material such as the subsoil is therefore critically important to its success as a bentonite, and (iii) remembering that the GCL is a composite man- Received 8 November 2017. Accepted 4 April 2018. A.S. Acikel and A. Bouazza. Monash University, Melbourne, Australia. W.P. Gates. Deakin University, Burwood, Australia. R.M. Singh. University of Surrey, Surrey, UK. D.G. Fredlund. Golder Associates Ltd., Saskatoon, SK, Canada. R.K. Rowe.* Queen’s University, Kingston, ON, Canada. Corresponding author: A.S. Acikel (email: [email protected]). *R.K. Rowe currently serves as an Associate Editor; peer review and editorial decisions regarding this manuscript were handled by C. Lake. Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. Can. Geotech. J. 55: 1824–1836 (2018) dx.doi.org/10.1139/cgj-2017-0646 Published at www.nrcresearchpress.com/cgj on 25 September 2018. Acikel et al. 1825 Fig. 1. Representative microstructural and macrostructural units and pores of bentonite and GCL trimodal structure with macropores, mesopores, and micropores (modified from Acikel et al 2018). GCL aggregates aggregate geotextile particle pores inter-aggregate pores intra-aggregate or inter-particle pores intra-particle quasi-crystal pores inter-layer pores crystallites GCL meso-pores GCL macro-pores or bentonite micro-pores bentonite macro-pores ufactured material with a trimodal pore structure. Currently, pores, whereas in this usage, interparticle pores include all the there are insufficient published studies that specifically investi- pores that are not interlayer pores (e.g., Gates et al. 2012). Such gate the impact of the time dependency of the many factors influ- differentiation has been useful to many researchers (Gens and encing the unsaturated behaviour of GCLs as a composite Alonso 1992; Alonso et al. 1999; Sanchez et al. 2006, 2016; Delage material. The previous studies mostly aimed at obtaining WRCs of 2007; Villar and Lloret 2008; Romero et al. 2011; Seiphoori 2014; GCLs at a homogenized condition and did not point out the im- Seiphoori et al. 2014; Navarro et al. 2015; Cui 2016) who described For personal use only. portance of the impact of measurement on the reported suction bentonite as having a bimodal pore structure. The pores between values. The suction values of unsaturated GCLs are not always in bentonite aggregates (interaggregate pores) are generally within a homogenized condition in practice, but they are in a time- the macropore size range, and the pores within aggregates (intra- dependent suction range at given gravimetric water content due aggregate pores) are of micropore sizes. Other researchers have to the aging behaviour of the bentonite component (Schanz et al. used a trimodal pore structure to define bentonite pores (Yong 2010; Delage et al. 2006). Considering the conditioning (aging) and 1999; Pusch and Yong 2003; Salles et al. 2009; Nowamooz and testing time dependencies of WRCs can lead to more analysis Masrouri 2010). Herein, the pores within bentonite — interlayer, — accuracy when the WRC is implemented in numerical analysis. interparticle–intraparticle, and interaggregate–intra-aggregate Thus, the time dependency of unsaturated GCLs needs to be fur- are considered to provide dual (macro and micro) porosity within the unsaturated regime. ther investigated to improve numerical modelling accuracy and Geosynthetic clay liners have a more complex porosity pattern the understanding of the unsaturated performance of GCL in field over the range of sizes, with average pore sizes that are different applications. for the bentonite and the geotextile components of GCLs. In this The objective of this paper is to explore the time dependency of context, the whole GCL can be considered as having a trimodal the GCL unsaturated behaviour as affected by conditioning time, pore structure that, in addition to the interlayer, intra-aggregate, testing time, and duration. Three different chilled-mirror hy- and interaggregate pores of bentonite described earlier, includes grometer test procedures were used on both the wetting and dry- the geotextile pores. In the current study, the geotextile pores are ing paths of the WRC of three GCL types to investigate the effect of Can. Geotech. J. Downloaded from www.nrcresearchpress.com by MONASH UNIVERSITY on 01/21/19 referred to as “macro”, the interaggregate pores as “meso”, the conditioning and testing times and duration on total suction mea- intra-aggregate pores and all pore sizes smaller than intra- surements with different levels of partial confinements provided aggregate (such as interparticle, intraparticle,
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