Effect of Wetting-Drying Cycles on Shear Strength of the Clayey Soils in the Three Gorges Area

Slope along the water level fluctuation zone in the Three Gorges reservoir area of China is suffering from failure frequently. Shear strength reduction of the slope materials due to wetting-drying cycles along the water level fluctuation zone may be one of the major causes leading to slope failures there. In this study, shear strength of clayey soils derived from three kinds of highly weathered argillaceous rocks, which are the most susceptible to landsliding in the area, was experimentally investigated following different wetting-drying (W-D) cycles and with different initial water content. Test results reveal that shear strength and their parameters of the three clayey soils reduced significantly after experiencing W-D cycles regardless of their initial water content. Reduction in both shear strength and their parameters of the soils derived from highly weathered mudstone occurred significantly when W-D cycles were less than 9, while such reduction of the soil derived from highly weather marl linearly lowered with W-D cycles even when the cycles were up to 13 times. All the three kinds of clayey soils with lower initial water content had greater reduction in both shear strength and their parameters, and vice versa. It is further discussed that two effects: swelling and desiccation of clay size particles (mainly clay minerals) and slaking of rock fragments coarser than silt, which occur during W-D cycles, lead to reduction of shear strength and their parameters of the clayey soils. Difference in reduction degree of shear strength and their parameters among the soils should be related to their difference in abundance of clay fraction, percentage of active clay minerals and rock fragments prone to slaking.