Mesoscopic Shear Behavior of Unsaturated Rough Joints under Constant Normal Stiffness Boundary Conditions

A Voronoi grain–based discrete-element model was generated in the three-dimensional distinct element code based on a parametric modeling battery pack established in Grasshopper (version 2024.10). The rough joint model was further developed by incorporating grains of progressively variable grain equivalent diameter and converted to a plane-strain condition to enhance computational efficiency. The generalized effective stress theory was incorporated into a rough joint model to investigate direct shear behavior in unsaturated sandstone under constant normal stiffness (CNS) boundary conditions. A calibration procedure was implemented to optimize mesoscopic parameters, including the properties of contact between grains and the capillary pressure parameter, ensuring the model’s fidelity to experimentally observed mechanical behavior in sandstone. The numerical results reveal that under CNS boundary conditions, the shear stress–shear displacement curves show significant hardening characteristics accompanied by suppressed shear dilation. The peak parameters (normal displacement, shear strength, and shear displacement) are all greater than those under constant normal load boundary conditions. Tensile microcracks were predominantly developed during shear deformation of rough joints under varying saturation conditions, with shear microcracks being a minority. Normal displacement of the joints exhibited a negative correlation with initial normal stress. In contrast, the peak normal stress increased with higher joint roughness coefficient values due to the pronounced asperity climbing effect.