Impact of multiple drying–wetting cycles on shear behaviour of an unsaturated compacted clay

Experimental investigations on the impact of multiple drying–wetting cycles on mechanical behaviour of unsaturated soils, particularly on shear behaviour, are limited. Suction-controlled direct shear tests were carried out to investigate the impact of multiple drying–wetting cycles on shear behaviour of an unsaturated compacted clay. One drying–wetting cycle was applied on a soil specimen by increasing its suction to 400 kPa followed by returning its suction to 1 or 200 kPa at constant vertical stress. The experimental results showed that more significant volume contraction occurred during the first drying–wetting cycle as compared with the subsequent drying–wetting cycles. At higher net normal stress (i.e. 200 kPa), a transition from shear-induced contraction to dilation was found. Nevertheless, such transition from contraction to dilation was not observed for the specimens at lower net normal stress (i.e. 50 kPa). The results also showed that amplitude of drying–wetting cycles and vertical stress influences the shear strength. The shear strength increases slightly after the first drying–wetting cycle at lower net normal stress; at higher net normal stress the effect of the first cycle on the shear strength is contrast.