The strength of the sliding zone soil determines the stability of reservoir landslides. Fluctuations in water levels cause a change in the seepage field, which serves as both the external hydrogeological environment and the internal component of a landslide. Therefore, considering the strength changes of the sliding zone with seepage effects, they correspond with the actual hydrogeological circumstances. To investigate the shear behavior of sliding zone soil under various seepage pressures, 24 samples were conducted by a self-developed apparatus to observe the shear strength and measure the permeability coefficients at different deformation stages. After seepage-shear tests, the composition of clay minerals and microscopic structure on the shear surface were analyzed through X-ray and scanning electron microscope (SEM) to understand the coupling effects of seepage on strength. The results revealed that the sliding zone soil exhibited strain-hardening without seepage pressure. However, the introduction of seepage caused a significant reduction in shear strength, resulting in strain-softening characterized by a three-stage process. Long-term seepage action softened clay particles and transported broken particles into effective seepage channels, causing continuous damage to the interior structure and reducing the permeability coefficient. Increased seepage pressure decreased the peak strength by disrupting occlusal and frictional forces between sliding zone soil particles, which carried away more clay particles, contributing to an overhead structure in the soil that raised the permeability coefficient and decreased residual strength. The internal friction angle was less sensitive to variations in seepage pressure than cohesion.

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Keywords: Sliding zone soil, Permeability coefficient, Shear strength, Seepage pressure, Reservoir landslides