Influence of initial stress difference on the hard rock fracture mechanism under stepwise unloading during large high-stress underground powerhouse layered excavation

With the continuous advancement of deep resource exploitation and underground engineering construction, the mechanical behavior and failure mechanisms of hard rock under stepwise unloading during Layered excavation of high-stress underground powerhouses have become critical scientific issues constraining engineering safety. Taking Shuangjiangkou granite as the research object, true triaxial stepwise unloading tests were conducted to systematically investigate the effects of different initial stress differences on rock strength, deformation, brittleness, and macro–micro failure mechanisms. In addition, AE monitoring was employed to reveal the evolution of cumulative crack damage and sudden failure. The results show that, with increasing initial stress difference, the peak strength of rock exhibits a nonlinear increase. The macroscopic failure mode evolves from diffuse shear propagation to directional splitting failure, while the microscopic fracture morphology transitions from intergranular shear cracks to transgranular tensile cracks. During the unloading process, AE energy exhibits a two-stage characteristic of steady accumulation followed by a sudden surge. Under higher initial stress differences, the duration of the rapid AE energy growth stage is significantly shortened, indicating that energy is more likely to be concentrated and released abruptly, resulting in macroscopic instability. Moreover, the degree of sudden rock failure increases logarithmically with the initial stress difference, consistent with the evolution of the brittleness index. These findings provide theoretical support for elucidating the failure mechanisms of hard rock during staged excavation of high-stress underground powerhouses. • Reveals the regulatory effects of initial stress difference on strength and deformation, and macro-micro failure mechanisms of high-stress hard rock. • Finds that increased initial stress difference induces macroscopic failure mode transition from diffuse shear to directional splitting. • Elucidates the mechanism of rock transition from progressive damage to sudden instability under stepwise unloading.