Granite strainbursts induced by true triaxial transient unloading at different stress levels: Insights from excess energy ΔE

In this study, based on the rockburst disaster mechanism of excess energy ΔE > 0, true triaxial transient unloading strainburst (including instantaneous strainburst and delayed strainburst) experiments were performed on granite specimens at different maximum principal stress levels. The experimental results were then analyzed, with the strainburst characteristics and acoustic emission (AE) responses of the granite specimens being examined. The excess energy ΔE was derived through a comparison with the results of conventional biaxial compression tests. The following beneficial conclusions were drawn. The mechanical strength of delayed strainburst specimens initially increases and then decreases with the rise of the unloading stress level. In contrast, the mechanical strength of instantaneous strainburst specimens is higher than that of delayed ones, increasing with the unloading stress level. In terms of fragment ejection velocity and scale, the rockburst intensity of a specimen is positively correlated with its mechanical strength. A pronounced linear relationship exists between the excess energy ΔE and the fragment ejection velocity (as well as weight), indicating that ΔE is intimately linked to the kinetic energy of rockbursts. Rockbursts lead to the formation of burst pits and typical V-shaped damage zones near the free face of the specimens, within which tensile cracks dominate. Additionally, the distribution of AE AF-RA values indicates that the proportion of tensile cracks increases with the rise of unloading stress level, suggesting that transient unloading under high stress levels significantly promotes tensile fracture. It is anticipated that this study will provide further elucidation on the mechanism of rockburst kinetic energy generation, thereby establishing a foundation for the design of rockburst support measures in engineering applications.