Effect of moisture content and shearing speed on shear zone structure in fine-grained soils at large displacement

Fine-grained soils are the main component of slip zones of landslides in the Three Gorges area, China. Studying the shear zone structure in fine-grained soils is helpful to evaluate landslides. In this study, the effect of moisture content and shearing speed on the shear zone structure in fine-grained soils subjected to large displacement was investigated through a series of drained ring shear tests, grain size analyses, and microscopic observations. The concentration of clay fraction around the shear planes and corresponding variations in the shear zone structure were highlighted. The results indicate that with increasing moisture content and shearing speed, the residual strength of the soil samples gradually decreased. The soil grains within the samples were redistributed during extended shearing. The clays tended to concentrate around the shear planes and decrease in number away from the shear planes, which could be caused by grain crushing and the diffusion of silty water. The most obvious concentration of clay fraction was observed at high moisture content and high shearing speed. Moreover, the effect of moisture content and shearing speed on the shear zone structure depended on the grain size. Increasing moisture content and shearing speed resulted in a smaller median grain size and ratio of the amount of sand fraction after shearing to before shearing, while a larger ratio of clay fraction. The corresponding change in the shear zone structure caused significant deterioration in the shear strength and porosity of shear zone soils, to the detriment of landslide stability.