Experimental investigation on mechanical behaviors and shear fracture mechanisms of rough jointed sandstone under conventional triaxial compression

Conventional triaxial compression tests were conducted on rough jointed sandstone to investigate the mechanical behaviors and failure mechanisms of fractured rocks. The influences of confining pressure, axial loading rate, joint dip angle, and joint surface roughness on the failure mechanisms of jointed sandstone were investigated. The stress-strain curve, shear stress, shear displacement, friction coefficient, and energy release process during the failure of jointed sandstone were analyzed. Failure modes and the evolution of rough surface wear in jointed sandstone were observed through both macroscopic and microscopic analysis. Results indicate that as the dip angle increases from 30° to 60°, the triaxial peak strength, crack damage threshold, and peak axial strain of jointed sandstone gradually decrease. With the increase in confining pressure, axial loading rate, and joint surface roughness, peak strength, deformation, crack damage threshold, and residual strength of jointed sandstone with a 60° dip angle gradually increase. For the jointed sandstone with 30° and 40° dip angles, shear fracture failure occurs along the direction perpendicular to the joint surface, resulting in the joint surface fractures into two segments. For the jointed sandstone with 50° and 60° dip angles, shear slip instability occurs along the joint surface, accompanied by deformation and fracture at the joint surface ends. Shear slip along the joint surface reduces the roughness and fractal dimension of the jointed sandstone and transgranular crack failure is the dominant failure mode of the joint surface. The debris and powder formed after the shear slip wear of the joint surface form fault gouge and attach to the micro-pores and micro-cracks on the joint surface. As the dip angle increases, the damage to the joint surface gradually intensifies. These findings can provide a theoretical basis for understanding the failure mechanisms of fractured rock masses induced by underground engineering construction.