The mesoscopic fracture mechanism of rockbursts under gradient stresses

Abstract   

The tangential stress of surrounding rocks is large on tunnel walls. While from tunnel walls to the interior of surrounding rocks, the tangential stress declines and approaches the in situ stress in a gradient manner. To study the influences of stress gradient on the failure mechanism of rockbursts, a mesoscopic model was established based on the discrete element software particle flow code (PFC). The model was used to simulate rockburst disasters in the loading process of gradient stresses and analyze the failure modes and energy evolution process under different gradient stresses. Using the PFC platform, an acoustic emission (AE)–based simulation method at the mesoscopic scale was proposed according to the moment tensor theory to explore features of AE events, including the spatio-temporal distribution and fracture strength of the model during rockbursts. By analyzing the failure process, the increase in the applied stress gradient is found to accelerate the deterioration process of materials and promotes samples to fracture rapidly along dominant main cracks. The number of derivative cracks and the total number of cracks are significantly reduced, and the model shows a change from tensile failure to shear failure. As the applied stress gradient grows, the proportion of elastic energy storage in the model increases before a rockburst, and the rate of release of energy rises accordingly during the rockburst. The AE count at fracture points on the unloading face of the model is normally distributed with changes in the strength M, and the overall AE intensity is enhanced as the gradient increases.