Shear characteristics and mechanisms of sandstone-mudstone interface under coupled stress and dry-wet cycling

Under the combined influence of periodic water level fluctuations and the overlying rock mass, the hydro-fluctuation belt of reservoir bank slopes exhibits significant degradation in physical and mechanical properties, particularly in interbedded sandstone and mudstone formations. To investigate the shear mechanical characteristics of sandstone-mudstone interfaces, typical interfaces from the Three Gorges Reservoir area were examined through stress and dry-wet cycle coupling tests. The results indicate that this coupling effect leads to a nonlinear degradation trend in interfacial shear strength, cohesion, and internal friction angle, characterized by an initial rapid decline followed by a gradual reduction. The degradation intensifies with increasing overburden stress. A shear strength degradation model based on the Mohr-Coulomb criterion showed strong agreement with the experimental data. Microscopic analysis revealed a transition in the interface structure from dense to porous, with mudstone degradation playing a dominant role in the weakening of mechanical properties. These findings provide a theoretical basis for the degradation analysis and stability evaluation of clastic rock bank slopes in the Three Gorges Reservoir area.