Characteristics and identification of granite rockburst under true triaxial loading-unloading with different initial stresses based on a DEM two-stage structural model

This study employed a two-stage structural model based on the discrete element method (DEM) to simulate granite rockbursts under true triaxial loading-unloading. By refining the determination method of rockburst stage, the strength properties, crack and energy evolution, and failure characteristics of rockbursts were analyzed. The stage identification, fracture patterns, and energy transformation of rockbursts were discussed. The results show that the effects of initial vertical stress ( σ zi ) and unloading direction initial stress ( σ xi ) on strength is controlled by the non-unloading direction initial stress ( σ yi ). At the yield stage, shear cracks dominate without distinct spatial zoning of shear and tensile cracks, and the rate of U e accumulation slows progressively. At the rockburst stage, tensile cracks predominate, exhibiting clear spatial zoning of shear and tensile cracks, with only approximately 0.6%–1% of ultimate elastic energy ( U emax ) converted into kinetic energy ( U k ). As the initial stresses increase, the range of the rockburst failure zone expands continuously, evolving in morphology from a symmetric V-shape to an asymmetric V-shape and finally to an inclined wedge shape. The transition of the shear-tension crack ratio from rapid increase to slow increase and from gradual increase to sharp decrease can be used as identification indicators for rockburst precursors and occurrence, respectively. • Improved the method for determining the starting point of the rockburst stage in the DEM two-stage structural model. • The influence of σ yi on strength is significantly greater than that of the σ zi and σ xi , and the effects of σ zi and σ xi on strength are governed by σ yi . • The crack spatiotemporal evolution and energy evolution of rockburst was obtained. • A method for identifying precursors and occurrence of rockburst based on the shear-tensile crack ratio was proposed.