Failure mechanism of gently inclined shallow landslides along the soil-bedrock interface on ring shear tests

There were many gently inclined shallow landslides along the soil-bedrock interface at a depth of 3–5 m in Nanjiang County, Sichuan Province, China. To explain why slope failure occurs at the soil-bedrock interface at a depth of 3–5 m, we conducted ring shear tests on samples of the slip zone soils, slip zone soil-to-the smooth bedrock, slip zone soil-to-the natural bedrock, and slip zone soil-to-the rough bedrock interfaces under various normal stresses. Twelve specimens of the slip zone soils and the slip zone soil-to-the bedrock of the gently inclined shallow landslides were tested by means of a ring shear apparatus. The results show that the peak shear strength and residual strength of the slip zone soils were larger than those of the slip zone soil-to-bedrock interface under a normal stress of 60–100 kPa. Thus, the slope slides along the slip zone soil-bedrock interface at a depth of 3–5 m. This can explain why the slope fails along the slip zone soil-bedrock interface at a depth of 3–5 m. The peak shear strength and residual strength of the slip zone soil-to-bedrock were larger than those of the slip zone soils under a normal stress of more than 100 kPa. Thus, the slope slides along the slip zone at a depth of more than 5 m. This can explain why the slope does not fail along the interface between the slip zone soils and the bedrock at a depth of more than 5 m.