Impact of wetting–drying cycles on hydro-mechanical behavior of an unsaturated compacted clay

To further our understanding on the impact of wetting–drying cycles on the hydro-mechanical behavior of unsaturated soils, this paper presents experimental results from suction-controlled isotropic compression tests on an unsaturated compacted clay subjected to different wetting–drying histories. This clay exhibited complicated volumetric response to wetting–drying cycles such as irreversible swelling upon wetting, irreversible shrinkage upon subsequent drying and accumulated swelling after a wetting–drying–wetting cycle. The wetting-induced irreversible swelling contributes to a significant reduction in pre-consolidation stress. It was observed that a wetting–drying cycle leads to a smaller pre-consolidation stress and downward shifting of the post-yield compression curve at a given suction, whereas a wetting–drying–wetting cycle shows an opposite effect. These observations are attributed to both irreversible swelling and irreversible change in the degree of saturation resulting from wetting–drying cycles. It was found that irreversible swelling or an irreversible increase in degree of saturation makes the soil more susceptible to yield, exhibiting a softening effect. Regarding water phase, its response to isotropic compression is mostly related to the recent wetting–drying history rather than the overall wetting–drying history.