Dynamic cumulative damage characteristics of deep-buried granite from Shuangjiangkou hydropower station under true triaxial constraint

Deformation and failure of rocks under loading is an energy-driven progressive process of damage evolution, and the abrupt energy release from rocks storing strain energy under high-geostress conditions is a determinant inducing catastrophic effects. Therefore, studying the progressive fracture mechanism of rocks from the perspectives of energy accumulation and dissipation is conducive to prediction and evaluation of catastrophic effects of surrounding rocks. In this study, granite was taken from an underground powerhouse of Shuangjiangkou Hydropower Station, Sichuan Province, China to study cumulative damage characteristics of the rock under dynamic triaxial loading by considering factors, such as in-situ stress, number of impacts and strain rate through use of a true-triaxial split Hopkinson pressure bar (SHPB) as a loading device and a three-dimensional (3-d) Xray computed tomography (CT) system. Mechanisms inducing catastrophic effects in deep-buried granite were explored from the perspectives of energy evolution, progressive damage, and failure mode. The research results demonstrate that under a single impact, the dynamic compressive strength of granite increases exponentially with the impact load, while the rate of increase gradually decreases. Moreover, failure effects of the rock are strengthened with increasing impact load. Under multiple impacts, the energy absorbed by the rock shows cumulative effects and the amount of energy dissipated per unit volume increases. Furthermore, progressive damage is significant, and the damage evolution exhibits obvious anisotropy. Under dynamic triaxial loading, shear failure predominates in granite. The confining pressure inhibits damage to the rock during static loading and the rock is severely damaged in the direction of its free face, where it is more likely to induce catastrophic effects. The 3-d visualization of fracture networks reveals the progressive damage process of granite under dynamic triaxial loading and the cumulative absorbed energy and volume of microcracks in rock increase linearly with the number of impacts. Therefore, taking cumulative damage effects caused by frequent blasting excavation into consideration can better evaluate catastrophic effects, such as rock burst or rib spalling of surrounding rock of side wall of deep-buried caverns in Shuangjiangkou Hydropower Station.