Numerical study on micro-fracture mechanism of rock dynamic failure induced by abrupt unloading under high in-situ stresses

• The combined effect of the stress state transition and the unloading effect result in rock unloading failure. • The vertical and horizontal in-situ stresses determine the unloading stress state and the unloading effect, respectively. • The longitudinal stress waves induced by unloading mainly contribute to the unloading effect. Rock burst is a kind of severe engineering disaster resulted from dynamic fracture process of rocks. The macro failure behaviors of rocks are primarily formed after experiencing the initiation, propagation, and coalescence of micro-cracks. In this paper, the grain-based discretized virtual internal bond model is employed to investigate the fracturing process of unloaded rock under high in-situ stresses from the micro-fracture perspective. The simulated micro-fracturing process reveals that the longitudinal stress waves induced by unloading lead to the visible unloading effect. The influences of in-situ stresses, mineral grain sizes, and grain heterogeneity on rock macro and micro fracture are investigated. Micro-crack areas of tensile and shear cracks and micro-crack angles are statistically analyzed to reveal the rock micro-fracture characteristics. The simulated results indicate that the combined effect of the stress state transition and the unloading effect dominates the rock unloading failure. The vertical and horizontal in-situ stresses determine the stress state of surrounding rock after unloading and the unloading effect, respectively. As the vertical stress increases, the stress level after unloading is higher, and the shear failure characteristics become more obvious. As the horizontal stress increases, the unloading effect increases, leading to the intensification of tensile failure. The mineral grain size and grain heterogeneity also have nonnegligible influences on rock unloading failure. The micro-fracture perspective provides further insight into the unloading failure mechanism of deep rock excavation.