Excess energy characteristics of true triaxial multi-faceted rapid unloading rockburst

Delayed rockburst experiments with different numbers of unloading surfaces (DNUS) were performed using an independently developed true triaxial multisurface unloading rockburst experimental system. Based on the rockburst excess energy theory, the energy storage characteristics, excess energy, excess energy release rate (EERR), and crack evolution characteristics of rockbursts with DNUS were studied, and the following main conclusions were obtained. The occurrence of rockbursts is mainly due to the generation of an excess energy ΔE. ΔE depends on the elastic strain energy stored in the rock before the rockburst, the energy input by the equipment after the peak, and the residual elastic strain energy. As the DNUS increases, ΔE gradually decreases, but the EERR value increases, and the rockburst becomes increasingly severe; Rapid unloading of the specimen under true triaxial high-pressure loading will produce an unloading platform in the stress–strain curve, causing unloading damage. The damage is mainly concentrated near the free surface in the form of tension failure, and the unloading damage gradually increases with increasing DNUS; Tensile cracks play a dominant role in the damage and destruction of sandstone. In the final rockburst stage, the slope of the shear crack curve was greater than that of the tensile cracks, indicating that shear cracks were a critical factor affecting the instability and failure of the specimen.