Abstract: The reduction in shear strength of muddy interlayer structural planes under hydro-mechanical coupling is a key factor contributing to the instability of rock bedding slopes. In this study,natural specimens of muddy interlayer structural planes from bedding slopes were used to conduct shear failure and shear strength tests under both saturated and unsaturated conditions. The contributions of shear resistance from surface protrusions and frictional resistance from the structural plane were quantitatively separated within the overall shear strength. This approach clarified the failure mechanism of muddy interlayer structural planes under coupled moisture-loading effects and led to the development of a corresponding shear mechanical model. The results indicate that: (1)For natural muddy interlayer structural planes,the shear stress-displacement curves primarily exhibit three stages: elastic deformation,plastic deformation,and a stable residual stage. (2)Under low normal stress,fracture surfaces remain relatively rough,with some fine protrusions remaining intact; under high normal stress,surfaces become smoother as failure progresses through crack initiation at the base of protrusions,followed by crack propagation and complete shearing. (3)In natural structural planes,shear failure specimens showed a 2.56% higher friction coefficient and 11.46% higher cohesion compared to specimens subjected to direct shear. Under saturated conditions,shear failure specimens exhibited a 3.38% higher friction coefficient and 82.29% higher cohesion. (4)The proposed shear failure and shear resistance models enable estimation of the overall shear performance of concave-convex structural planes with muddy interlayers based on the strength parameters of flat structural planes.