Wetting-induced softening behavior of an unsaturated expansive clay

Studying the wetting-induced softening behavior of unsaturated soils is important for understanding rain-induced slope failures in unsaturated soils. However, experimental studies on expansive soils are limited. This study investigates the wetting-induced softening characteristics of an unsaturated expansive clay via suction-controlled triaxial tests on recompacted and natural specimens. It was found that the wetting-induced swelling of the soil included a significant component of plastic strain and its magnitude decreased with the net confining stress. When a soil specimen was subjected to wetting at a constant deviator stress, a threshold value of soil suction was identified after which the axial strain increased greatly. This value tended to increase with the applied net stress ratio. After the threshold suction, the wetting process for recompacted specimens could be divided into a yielding process associated with volumetric strain hardening/softening and a failure process in which the volumetric behavior depended on the applied net stress ratio. The natural specimens exhibited a brittle failure like a heavily overconsolidated soil during wetting at a constant deviator stress and continuous volumetric dilation occurred after the threshold suction. The wetting-induced softening soil behavior revealed from the present study could interpret the rain-induced progressive slope failure in unsaturated expansive soils.