Experimental and Numerical Investigations on the Macro-Meso Shear Mechanical Behaviors of Artificial Rock Discontinuities with Multiscale Asperities

In the Three Gorges Reservoir Area (TGRA) has frequently occurred a large number of landslides in the stratified rock slopes with typical saw-tooth rock discontinuities since its impoundment in 2003, in which the shear mechanical behaviors of the saw-tooth rock discontinuities have been explored by few scholars. In this study, the macro-meso shear mechanical behaviors of the rock discontinuities with multiscale triangular-shaped asperities under two different lithologic combination types, when subjected to a static constant normal load (CNL), were investigated using experimental and numerical approaches. The hard layer and soft layer are, respectively, limestone and clayey soil taken from the TGRA, and the normal loading rate for the combination of hard layers and the combination of soft–hard layers is 0.5 kN/min and 0.05 kN/min, respectively, while the shear loading rate of the two different lithologic combination types is 0.5 kN/min. The laboratory direct shear tests on the rock discontinuities with different first-order asperity angles, i.e., 30°, 45° and 60°, and the same second-order asperity angle of 45°, were first conducted under different normal stresses and moisture contents. Four various types of shear-tensile failure modes, i.e., climbing failure, climbing-gnawing failure, gnawing failure and sliding failure, were observed for the combination of hard layers; while three diverse types of shear-tensile failure modes, i.e., disengaging failure, dislocating failure and gnawing failure, were observed for the combination of soft–hard layers. Subsequently, the Two Dimensional Particle Flow Code discrete element method (PFC 2D DEM) was used for the numerical simulations, in which the meso-damage evolution process of the rock discontinuities was simulated, and the variation laws of the meso-damage crack number and energy were obtained. Based on the good agreement between the experimental observations and numerical results, a shear strength equation for the rock discontinuities was proposed, and its rationality was validated by comparing the factor of safety (FOS) of a slope calculated by the limit equilibrium method (LEM) and the strength reduction method (SRM).