Abstract: Expansive soil is highly sensitive to climate change. Understanding the response of engineering properties of expansive soil under wettingdrying cycles is therefore significant for revealing the disaster mechanism as natural expansive soil is subjected to periodic evaporation and infiltration. In this investigation, a series of shear tests were conducted on unsaturated remolded expansive soil which was subjected to three wettingdrying cycles. The influence of drying process, normal stress and wettingdrying cycles on shear strength behavior were analyzed and discussed. The results show that (1) during drying, specimen stiffness, shear strength, shear strength parameters (cohesion and internal friction angle), failure brittleness and peak strength loss increase with decreasing water content, while the shear displacement decreases accordingly; (2) higher normal stress gives rise to higher shear strength as well as residual shear strength, but lower peak strength loses, and fracture toughness of the specimen increases generally with increasing normal stress applied during shearing; (3) the shear strength behavior of unsaturated expansive soil is significantly influenced by wettingdrying cycles, during which both shear strength and cohesion increase first and then decrease, and reach their peak values at the second drying path; however, the internal friction angle is slightly influenced by wettingdrying cycles; (4) after the specimens were subjected to multiple wettingdrying cycles, the shear characteristics are similar to that of over consolidated soil, and the specimen presents more brittleness at failure; (5) the residual shear strength seems to be independent of drying process and wettingdrying cycles as the obtained values are generally kept around 100 kPa; and (6) shear strength of unsaturated expansive oil during wettingdrying cycles is significantly conditioned by suction, microstructure and cracking development; it is necessary to combine unsaturated soil mechanics and soil science to make an accurate evaluation.

Key words: expansive soil, wettingdrying cycle, shear strength, unsaturated soil, suction, microstructure