Soil-water retention and shear strength characteristics of intact unsaturated expansive soil

Using the axis translation method and the isopiestic humidity control technique, the soil-water retention behavior of intact brown expansive soil from Jingmen is systematically investigated over the full suction range. The drying boundary curve, wetting boundary curve, and the complete scanning curve originating from the natural water content state are determined. The results indicate a pronounced hysteresis region between the drying and wetting boundary curves, demonstrating a significant hydraulic hysteresis effect. Consolidated drained direct shear tests are further conducted under saturated conditions and under unsaturated conditions at natural water contents, yielding the shear strength characteristics and associated parameters of the intact expansive soil. On this basis, the Fredlund-Xing equation is employed to fit the soil-water retention curves along different hydraulic paths, and the relationship between unsaturated shear strength and water content is predicted by combining the saturated and unsaturated shear strength parameters. The results show that hydraulic hysteresis exerts a significant influence on the prediction of unsaturated shear strength: at the same water content, differences in suction associated with different hydraulic paths lead to pronounced divergence in the predicted shear strength, with values based on the drying boundary curve consistently higher than those derived from the complete scanning curve and the wetting boundary curve. Therefore, the hydraulic hysteresis effect cannot be neglected in the prediction of unsaturated shear strength of intact expansive soil; direct use of the drying path in engineering practice may result in systematic overestimation of shear strength and an unsafe bias in stability evaluation.