Abstract: In this paper, the experimental studies on the water retention and strength properties of unsaturated weakly expansive soil in a wide suction range were carried out, and a strength model of unsaturated soil over a wide suction range was proposed. The results show that in the wide suction range, the stress‒strain curve increases with the increase of suction. The strain hardening occurs in the low suction range, while the strain softening appears in the high suction range. In the low suction range, the sample presents shear contraction, while in the high suction range, the sample begins to show a shear dilation trend at the axial strain of 2%. In addition, a soil-water retention curve model was proposed to distinguish adsorbed water from capillary water, assuming that the strength increment caused by suction is mainly determined by capillary water. The degree of saturation of capillary water was used to replace the effective stress coefficient and then it was substituted into the Bishop’s unsaturated soil strength formula. The proposed model was verified by test data of various soil types and compared with the models in the literature. The results show that the proposed model can well describe the strength of unsaturated soil in a wide suction range.

Key words: unsaturated soil, shear strength, soil-water retention curve, effective stress coefficient, wide suction range