Unsaturated behaviour and gas transport through geosynthetic clay liners (GCLs) and geomembrane (GMB)/GCL composite liners

Abstract

Geosynthetic clay liners (GCLs) are widely used as hydraulic barriers because of their low saturated hydraulic conductivity and sealing capacity. However, with increasing interest in their use as a component of cover systems for landfills as well as in mining applications, the capability of GCLs to minimise gas emissions, when under an unsaturated condition, are of some concern. This research work has been undertaken to enhance general understanding of the unsaturated behaviour of GCLs and specifically, the effectiveness of unsaturated GCLs to minimise gas migration. In this investigation, the hydration (adsorption and desorption) state of bentonite in GCLs was controlled by vapour phase contact with solutions held at various relative humidity (RH) states under stress and no stress conditions. Gas permeability tests were conducted on three GCLs while monitoring gravimetric water content and total suction under different hydration conditions. A gas flow unified measurement system (UMS-G) for sequential measurement of gas diffusion and gas permeability of geosynthetic clay liners (GCLs) was developed and validated both experimentally and numerically. A series of gas diffusion and gas permeability tests were performed sequentially on two partially hydrated GCLs using UMS-G under a range of gravimetric water contents and hydration conditions. An experimental column study was conducted to understand the hydration (due to infiltration of rainfall water), dehydration (caused by solar radiation or heat) and gas flow through a defected geomembrane (GMB)/GCL composite liner where the GMB contains a defect. Results indicated that RH, applied stress, GCL bentonite form (powdered or granular) and the mineralogy of the bentonite component of the GCL, all play key roles in the hydration/dehydration behaviour with time when GCL specimens were in equilibrium with water vapour. The gas permeability was strongly influenced by gravimetric water content, total suction and applied stress. The UMS-G measured GCL gas diffusivity and gas permeability decreased with the increase of water content and applied stress. The differences in the form of bentonite and GCL structures (scrim reinforced vs non-scrim reinforced) had significant effects on the variation of gas diffusivity and permeability. The hydration behaviour of GCL underlying a defective GMB in a model composite liner was largely influenced by the bentonite form of the GCL, applied rainfall amount and rainfall events. Gas diffusion and gas permeability of GCL part of the composite liner and gas leakage rate of defective GMB/GCL composite liner were influenced by average GCL gravimetric water content and average GCL wet surface area. Uniformly hydrated GCL tended to give lower gas diffusion and permeability than non-uniformly hydrated GCL. Gas diffusion coefficient and gas permeability of GCLs were also influenced by wetting and drying cycle due to the formation of cracks in dehydrated bentonite. The results highlighted that the adjacent layers of GCL in cover system should have sufficiently high gravimetric water content to hydrate the total surface area and thickness of GCL uniformly before coming in contact with gas. It also should be ensured that there is enough cover above the GCL to prevent it from subsequent drying. The results presented here can be useful for design engineers and regulatory agencies to improve the design of landfill and mine tailing cover system.