Landslides developing in bedding-plane sediments are predominantly controlled by basal shear zones, where clay-rich materials localize deformation along bedrock surfaces. The mechanical behavior of shear-zone soil is further influenced by the characteristics of soil-rock interface. This paper investigates the residual strength of the soil-rock interface samples through ring shear and large-scale direct shear tests under varying stress and rate conditions. Shear zone materials from two landslides sites are paired with manufactured base and natural rock to compose the interface samples. Experimental results find that the residual strengths of shear zone materials are altered by different interfaces. At a low normal stress level, the mechanical behaviors of soils show strong dependence on surface asperities. As driven by increasing shear stress, the smooth interface sample exhibits accelerated failure progression with significant loss of resistance. The surface morphology and rheological behavior explain that the basal shearing easily occur along a relatively smooth interface, resulting weakening at high velocity and stress states.
