Study on the influence of fracture geometric characteristics on the microscopic crack propagation and energy release mechanism in granite

Rockburst is a major safety risk in deep tunnel engineering, closely related to the propagation of fractures and energy release in rock masses. Therefore, a thorough understanding of the laws governing fracture propagation, energy release mechanisms, and their intrinsic relationship with rock failure modes is crucial for the prevention and control of high-energy events. This study, based on the geological characteristics of granite, utilizes PFC2D to establish a numerical model and calibrates the mesoscopic parameters using the trial-and-error method. The mechanical response and energy damage evolution characteristics of fractured granite under different fracture inclinations and lengths are systematically investigated. The results indicate that the inclination and length of fractures significantly affect the failure mode, energy release process, and the extent of crack development in the rock. The failure mechanism is mainly characterized by a shear-dominated, tension-assisted oblique shear-tensile failure mode. When the fracture inclination is 0°, granite is most prone to fracture. As the fracture length increases, the number of macroscopic cracks and the degree of failure reach their maximum when the fracture inclination gradually decreases (60°-30°-15°). A smaller YU value corresponds to greater energy release, an increased number of macroscopic cracks, and a higher degree of failure, which is more conducive to crack formation. A damage constitutive model for granite based on dissipated energy density is established, and the analysis shows good agreement between the theoretical and experimental curves. The results provide a theoretical basis for understanding rock mass failure and safety issues during deep tunnel excavation.