Shear Failure Behavior and a Shear Strength Prediction Model of Unbonded Rock–Cement Interface

Grouting has been widely used in the reinforcement of jointed rock masses, and the interface between the rock and cement serves as a crucial binary interface in controlling the strength effect after grouting. In order to understand the shear failure behavior of the rock–cement interface, a series of unbonded sawtooth-shaped rock–cement interface specimens were fabricated and tested. Direct shear tests were conducted under a constant normal load to investigate the shear mechanical characteristics, considering the effect of the rock materials, interface morphology, and normal stress level. In addition, to investigate the mesoscopic failure behavior in detail, the particle flow code framework (PFC2D) was employed to simulate the shear behavior of unbonded rock–cement interfaces under combined compression and shear load action. It was found that the shear failure behavior was mainly concentrated at the cement block asperities, and the failure mode changed from wearing of the asperities on the cement block to cutting failure with increasing asperity inclination. The mesoscopic failure of the unbonded rock–cement interface was dominated by tensile failure of the cement block asperities. The progressive shear failure of the unbonded rock–cement interface showed as an extension of the damage from the surface to the interior of the asperities. Based on the discovered shear mechanism, a theoretical model for predicting the shear strength of unbonded rock–cement interfaces was derived by adopting the deduction method of the peak dilation angle. The model has clear physical significance because it considered both the interface morphology and shear properties, and was verified as having high prediction precision, providing theoretical guidance for studying the shear behavior of rock–cement interfaces.