Experiment and particle flow simulation of unloading induced shearing properties for jointed sandstone materials

Shear tests on sandstone containing non-connected joints under the unloading normal stress but fixed shear stress conditions were conducted with various initial normal stress and unloading rate. During the unloading process, continuous shear dilation could be observed. For initial normal stress = 5–20 MPa, the sliding initiation normal displacement decreased by 17.14–66.32 % but the sliding initiation shear displacement increased by 21.91–98.19 %. The sliding initiation normal stress declined by 7.18–15.57 % for unloading rate = 0.002–0.01 MPa/s but increased by a factor of 3.18–3.53 with the initial normal stress. Shear sliding of the jointed samples was more prone to occur at a smaller initial normal stress with a larger unloading rate. From the numerical simulation results via PFC2D, during the unloading process, cracking was first initiated from central part of the rock bridge and propagated gradually to coalesce with cracks developed from the joint tips. The penetrating fracture surfaces led to shear sliding of the jointed samples, and shear wear occurred on the fractures as shear displacement continued increasing. For unloading rate = 0.002–20 MPa/s, quantity of cracks corresponding to shear displacement of 1.2 mm decreased by 12.18–36.34 %. Cracking degree was more serious for the samples at a low unloading rate due to progressively developed cracks near the main failure planes before shear sliding and more accumulated cracks in the shearing wear stage for fractures under a larger normal stress.