Abstract
Three different chilled-mirror hygrometer test procedures were developed to investigate the time-dependent unsaturated 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 CCL structure and bentonite forms governed the effect of measurement 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.