Directional development mechanism of hard rock fracture under true triaxial stress: Insights from macroscopic and microscopic fracture perspectives

Abstract

The excavation of hard rocks in deep engineering frequently leads to disasters such as rockbursts, spallings, and collapses. Traditional triaxial tests, which neglect the effect of the intermediate principal stress, fail to reveal the failure mechanism of deep hard rocks. Under true triaxial stress, the intermediate principal stress inhibits volumetric deformation and expansion of rocks, resulting in directional failure. However, the fracture characteristics of hard rocks remain qualitative, lacking a mechanistic explanation. Therefore, based on true triaxial compression tests, complemented by acoustic emission and scanning electron microscopy, the directional development mechanism of hard rock fracture under true triaxial conditions is interpreted from both macroscopic and microscopic fracture perspectives. The macroscopic results show that as the intermediate principal stress increases and the minimum principal stress decreases, the macroscopic failure angle increases, the roughness of the rock failure surface decreases, and the fracture plane becomes increasingly perpendicular to the minimum principal stress, leading to the directional development of fractures. Microscopically, the development of microcracks is suppressed by the intermediate principal stress. As the intermediate principal stress increases and the minimum principal stress decreases, microcracks extend more severely in directions parallel to the intermediate principal stress, reducing intergranular fracture and increasing transcrystalline fracture and the proportion of tensile cracks. The differences in macroscopic deformation and microscopic tensile and shear fractures induced by true triaxial stress reveal the mechanism of directional rock fracture.