Water retention curves of a geosynthetic clay liner under non-uniform temperature-stress paths

Highlights

• A novel suction-controlled chamber (SSC) that permits the WRC determination over the full range of suctions under non-uniform temperature-stress paths is presented.

• The unified SSC allows measurements of suctions sequentially on a single GCL specimen.

• WRCs are presented for typical field conditions with particular emphasis on heap leach pads and brine ponds.

• Paper provides a new and unique insight on the water retention properties of GCLs.

Abstract

This paper presents a novel suction-controlled chamber that permits the determination of the full water retention curves of geosynthetic clay liners (GCLs) under non-uniform temperature-stress paths. It investigates field conditions encountered in brine ponds (low confining stress settings) and heap leach pads (high confining stress settings) during construction and operation stages. Consequently, the analysis of the moisture dynamics in a GCL was defined under the wetting path (construction) and drying path (operation). High vertical stresses were found to facilitate a more rapid water uptake as capillarity is established faster than at low, confined stresses. In general, the drying curves increase the water desorption over the suction range investigated due to the low water viscosity caused by high temperatures. The wetting of the GCL at 20 °C and drying at 70 °C under either low, confined stress (2 kPa) or high confining stress (130 kPa) shows a reduction in the volumetric water contents. Furthermore, on the drying path, the coupled effect of elevated temperature and high confining stress accelerates water desorption leading possibly to potential desiccation.

Introduction

Geosynthetic clay liners (GCLs) are hydraulic barriers made typically of a thin layer of sodium bentonite (5–10 mm) encased between two geotextiles through needle punching or stitch bonding process and can also be bonded to a geomembrane (Bouazza, 2002; Bouazza and Bowders, 2009, Gates et al., 2018). They also include multicomponent GCLs, which are conventional GCLs with a coating or attached film (Bannour and Touze-Foltz, 2015). The GCLs primary function is to prevent or slow the flow of fluids from a pollution source, being well suited for this owing to the low hydraulic conductivity of hydrated bentonite, a condition achieved when placed on-site where it takes up water from the subsoil. They are now a ubiquitous part of landfills, mining waste and oil and gas facilities containment barrier systems (Gates et al., 2009; Hornsey et al., 2010; Fourie et al., 2010; Bouazza and Gates, 2014; Liu et al., 2015, 2019; Bouazza et al., 2013, 2014; Touze-Foltz et al., 2016, 2021; McWatters et al., 2016; Chen et al., 2019; Ören et al., 2019; Naka et al., 2019; Bouazza, 2020; Rowe, 2014, 2020a,b; Li et al., 2021; Rowe and AbdelRazek, 2021).

In applications such as brine ponds, heap leach pads and tailing dams, GCLs can be exposed to coupled effects of elevated temperatures and low or high confining stresses during the operation of these facilities. Consequently, their hydration and dehydration processes could be affected, which, in turn, could affect their hydraulic performance (Bouazza et al., 2017a; Tincopa, 2020; Ghavam-Nasiri et al., 2020). Although a GCL is commonly expected to absorb water from the subgrade soil to hydrate fully, it is not always that straightforward, as evidenced recently by Bouazza et al. (2017b), Acikel et al. (2018a) and Rowe (2020b). An estimation of the GCL hydration can be obtained using its water retention curve (WRC), which is defined as the relationship between the moisture or volumetric water content and suction (Lu and Likos, 2004). This relationship varies according to the conditions that might be encountered on site. For example, the hydration of a GCL placed on a subsoil under ambient temperatures and low confining stresses will differ from the hydration process under elevated temperatures and high confining stresses. The limitation of the bentonite swelling due to the confining stress (Bannour et al., 2014) and the acceleration of moisture transfer due to temperature changes (Romero et al., 2001; Laloui et al., 2013; Ghavam-Nasiri et al., 2019a) may affect moisture movements in GCLs.

Many studies have been conducted over recent decades to establish the WRCs of GCLs, as shown in Table 1. However, most of these studies have not considered the operation processes experienced in applications where GCLs will be exposed to non-uniform temperature-stress paths during hydration and dehydration processes. Typical of these applications are brine ponds or heap leach pads/tailings dams, among other applications. Furthermore, separate devices are traditionally used to establish the GCLs WRCs due to the wide suction range of the bentonite component. This process of measurement makes it cumbersome to evaluate the water retention properties of the GCLs accurately as such procedure is fraught with uncertainties since specimens used in separate devices may differ substantially (i.e., different mass per unit area, thickness, etc.) even if they are from the same GCL roll.

This paper presents a suction measurement system that allows the measurement of suctions to be performed over the whole suction range of GCLs using the axis translation technique, osmotic technique, vapour equilibrium technique sequentially on one single specimen. Furthermore, it examines the GCL water content-suction relationship under representative field conditions; in particular, it addresses the effect of non-uniform temperature-stress paths.