Numerical Simulation Study on Shear Properties of Rock Joint Under CNS Boundary Conditions

In this study, the shear mechanical property of rock joints under constant normal stiffness (CNS) boundary conditions was investigated by the PFC discrete element software. The effects of joint roughness coefficient (JRC), initial normal stress (σn0), normal stiffness (kn) and shear rate (v) on shear stress (τ), shear dilation (normal displacement, δv), and normal stress (σn) were quantitatively investigated. The stress evolution and damage process inside the samples during the CNS shearing process of rock joints were analyzed. The test results showed that the increase of JRC significantly improved the shear performance of rock joints. Meanwhile, shear stress, normal displacement, normal stress and the number of tensile cracks of samples were significantly increased. Excessive initial normal stress (greater than or equal to 4 MPa) will cause irreversible failure of the joints. The protrusion of the joints is sheared or ground and detached from the samples, which leads to shear shrinkage of the sample. The increase of normal stiffness and shear rate will restrain the shear expansion of the joints, which will also aggravate the failure of the samples. Three empirical formulas for predicting shear stress, normal stress, normal displacement of rock joint samples under CNS boundary conditions are proposed and laboratory tests were carried out to verify them. The four factors involved above and shear displacement (δh) are considered in the prediction model. The calculated results of the prediction model are in good overall agreement with the laboratory test results, and the error decreases with the increase of shear displacement.