Effects of intermediate principal stress on strainburst in granite: Insights from true-triaxial unloading experiments and PFC3D-GBM simulations

To improve the accuracy of rockburst risk evaluation in mining and tunnelling engineering, the influence of intermediate principal stress σ 2 deserves further consideration, which has been neglected in general prediction frameworks. This study employs an integrated approach that combines true-triaxial unloading experiments with three-dimensional grain-based discrete element modeling (PFC3D-GBM) to examine the effects of σ 2 on strain systematically burst and elucidate the underlying mechanisms. Through this dual experimental–numerical methodology, the strainburst characteristics under varying σ 2 are analyzed in detail regarding mechanical responses, failure evolution and patterns, microscope fracture mechanisms, and energy partitioning. The results indicate that elevated σ 2 can enhance the bearing capacity of rock, thereby necessitating a higher stress condition required for strainburst. However, it also enlarges the potential strainburst intensity, manifesting as deeper rockburst pits and more violent ejection of rock fragments. An increasing σ 2 facilitates the microscope transgranular fractures, inhibits intergranular tensile fractures, and raises the kinetic energy conversion ratio slightly. It affects the intensity of strainburst through the following mechanisms, including the increase of energy storage limit, the intensification of Poisson effect for lateral expansion, and the enhancement of the transgranular fracturing mechanism. In practical engineering, the depth and range of support needs to be ensured under high σ 2 conditions, and it is recommended to use prestressing techniques to control the development of significant slabbing.