Shear Mechanical Characteristics of Slurry–Rock Composite Rock Mass with Different Fracture Undulation Angles

The undulation angle of a fracture is an important factor affecting the effectiveness of grouted reinforcement of fractured rock masses because it determines the propagation of stresses and the formation of cracks within the rock. Direct shear tests and corresponding particle flow simulations (PFC2D 5.00.35) were conducted on sandstone specimens with sawtooth fracture surfaces containing different undulation angles after grouting reinforcement to investigate the influence of joint surface undulation angles on the mechanical properties and damage mechanisms of the slurry–rock composite rock mass. The results revealed that larger undulation angles change the propagation mode of shear forces, which significantly enhances the shear resistance of the slurry–rock composite rock masses and alters the damage behavior. The occlusal behavior between the slurry and the rock mass enhances with the increase of undulation angle, leading to a change in the damage mechanism from slip tensile damage to occlusal shear damage, and a transition in the crack propagation mechanism from tensile damage dominance to tensile–shear composite damage. This reduces the sliding of the structural surfaces under shear conditions, and improves the integrity and stability of the slurry–rock composite rock mass. The strong occlusion mechanism reveals the mechanical behavior and failure mechanisms of slurry–rock composite systems, which is an important reference for the design and grouting reinforcement of fractured rock masses in underground projects.