Unsaturated shear strength characteristics of coarse-fine mixed soils in a wide range of degree of saturation: experimental phenomena

The coarse-fine mixed soils are widely distributed in natural and geotechnical engineering practices. Their strength is vital to the stability of geotechnical engineering (e.g., slopes, foundation pits and retaining walls). However, the unsaturated shear strength characteristics of the coarse-fine mixed soils are not evident in a wide range of degree of saturation (S_r), especially the lack of experimental data in the low range of S_r. In order to clarify the general rules that the unsaturated shear behaviors of the coarse-fine mixed soils in the full range of S_r, a series of direct shear tests are carried out on seven unsaturated coarse-fine mixed soils to study the unsaturated shear strength and deformation characteristics. The test results show that: (1) Based on the contribution of capillary water and adsorbed water to the unsaturated shear strength, the unsaturated shear strength exhibits a "peak behavior", where the shear strength reaches its peak at the residual zone, the shear strength characteristics with S_r or suction can be divided into three stages: slow growth zone, rapid rise zone and post-peak decline zone. (2) Brittle failure occurs when S_r is lower than the residual saturation. (3) The fines content (FC) significantly affects the strength and deformation characteristics of the coarse-fine mixed soils. As FC increases, the soil microstructure changes from a fines-controlled to a coarse-controlled structure. The FC corresponding to the maximum unsaturated strength is between 50%~80%.