Design of direct shear test conditions.

In Loess Plateau, low shear strength of the loess–mudstone contact interface leads to the loess–mudstone landslides. However, the influencing factors of shear strength of loess-mudstone interface and the disaster-causing effect of interface landslide are not clear. Therefore, this study focus on the loess-mudstone interface, and conducts direct shear tests under different water content, density and morphology conditions. The results show that the shear strength of loess-mudstone interface is always lower than that of homogeneous loess and mudstone. An increase in moisture content leads to a continuous reduction in shear strength, whereas an increase in dry density enhances shear strength. A decrease in interface roughness also results in a reduction in shear strength parameters. Specifically, when interface roughness decreases from 1.72 to 0, cohesion decreases by 42%, and friction angle decreases by 13%. The failure modes in different interface types can be classified into three: interface shear, locked-segment, and partial locked-segment failure model. This study analyzes the influencing factors and internal mechanism of shear strength of loess-mudstone interface, reveals its landslide disaster effect, and puts forward suggestions for risk assessment of loess-mudstone landslide. The results are of great value to the potential risk assessment of loess-mudstone landslide.