Effect of hydraulic hysteresis on unsaturated shear strength of expansive soil under low stress level

The hydraulic hysteresis effect on the unsaturated shear strength of expansive soil under low stress and its influence on slope stability were investigated. Suction-controlled direct shear tests were conducted on Jingmen expansive soil over several years using an unsaturated soil direct shear apparatus along both drying and wetting paths to characterize its water retention, shear strength, and deformation behavior. The results show that, under the same matric suction and net normal stress, the shear strength on the drying path is significantly higher than that on the wetting path. This difference cannot be explained by variations in water content or dry density, but is mainly attributed to water-migration-induced changes in soil fabric. During drying, residual capillary bridges between fine particles enhance interparticle bonding and increase the apparent cohesion, thereby strengthening the soil fabric. During wetting, however, water preferentially enters large pores, and effective liquid bridges are difficult to form within the micropore domain, resulting in a weaker and looser fabric. Based on the test results, the shear strength and hydraulic parameters for both hydraulic paths were calibrated, and seepage-stability analyses of an expansive soil slope under rainfall infiltration were performed using GeoStudio. The results indicate that neglecting hydraulic hysteresis leads to an overestimation of slope stability and may thus result in unsafe engineering evaluation.