Abstract: In the southeast Tibetan region, the stratigraphic lithology is complex, the clastic accumulation is widely distributed, and the base cover interface is mostly the potential slip surface of the slope of such accumulation, whose shear mechanical properties are the key factor to control the stability of the slope. To investigate the mechanical properties of the base cover interface of the accumulation, a 3-level indoor orthogonal direct shear experiment with 4 factors (rock content, joint roughness coefficient (JRC), water content, and bedrock softness) was conducted. The results show that the shear stress-shear displacement curve pattern of the soil-stone mixture-bedrock interface is dominated by plastic strain, but there is strain softening in the test curve with a JRC value of 18.7 at high vertical pressure, and the length of the strain softening phase is positively correlated with the size of the rock content. In addition, the shear stress-shear displacement curve is prone to the phenomenon of“stress jump”, which is mainly related to the block and bedrock embedded occlusion, block and bedrock fragmentation, block in the bedrock surface position adjustment; for the shear strength of soil-stone mixture-bedrock interface, the degree of bedrock softness and water content have the most significant degree of influence. The key factors affecting cohesion and internal friction angle are the degree of bedrock softness and water content, while the key factors affecting shear expansion are JRC and bedrock softness and hardness, and the key factors affecting shear contraction are rock content and water content.

Key words: southeast Tibet, clastic accumulation, bedrock-cover interface, JRC, shear mechanical properties, orthogonal experimental method