Abstract: To investigate unsaturated shear strength of compacted expansive soil within the entire range of moisture and density state, the unsaturated direct shear tests are conducted on Jingmen expansive soil associated with 38 types of moisture and density state combinations. The distribution of shear strength are obtained with respect to moisture state varying from air-dry to saturation and density represented by void ratio ranging from 0.539 to 1.089. Conclusions are as follows: (1) Strain-softening phenomenon is observed in shear for most samples. In contrast, strain-hardening phenomenon is observed in shear only for the lower-density and higher-moisture state combination. Stress plummeting phenomenon is observed in shear for the higher-density and lower-moisture state combination. Plastic deformation capacity of the soil samples is improved by increasing moisture under the same density or improved by decreasing density under the same moisture. (2) With the moisture increasing from air-dry to saturation, the unsaturated shear strength and total cohesion increase and then decrease, which can be called the peak effect. (3) Unsaturated shear strength, total cohesion, and total friction angle all decrease with decreasing density, which can be called the density effect.

Key words: unsaturated soils, compacted soils, shear strength, stress plummeting, peak effect, density effect