Investigation of crack initiation and crack damage stresses in intact rock shear failure

Due to the significant advancement in the mining and tunneling industry, the depth of underground excavation has been considerably increased. As a consequence of increase in depth, the intact rock strength plays a significant role in rock mass failure. Therefore, the study of intact rock failure has become more crucial. Crack damage stress and crack initiation stress are the two important characteristics of intact rock failure, which have been studied rarely in the shear failure process. This study aims to investigate the crack damage and crack initiation stresses in the shear failure by numerical modeling of direct shear test (DST) using the discrete element method. First, the micro-mechanical properties of the numerical model were calibrated against physical experiments in uniaxial tension, uniaxial compression and triaxial compression tests. Then, the numerical models were validated by comparing against physical direct shear tests under different normal stresses. By undertaking the DST under eight different constant normal stresses, these two stress levels have been studied in brittle, brittle-ductile transition and ductile failure modes. It was revealed that the crack initiation and crack damage stresses occur in 70-91 and 85-95 percent of the peak shear strength, respectively. Also, it was observed that the ratio crack initiation and crack damage stresses to the peak shear strength decrease with an increase in the normal stress. The outcomes would be useful for studying the structures under shear stress like landslides and rock bolts.