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The fast unloading delayed rockburst experiments on single-side, double-side, three-side, and four-side were carried out using the self-developed true triaxial rockburst experimental apparatus, which can realize the fast unloading of multiple loading surfaces in the horizontal direction under high stress. The rockburst experiment's stress–strain curve, energy evolution, failure characteristics, and acoustic emission (AE) characteristics are investigated. The results show that the rockburst gradually reduces the axial peak stress and strain when increasing the number of unloading surfaces (NUS). The rise in the axial strain at the fast unloading point will progressively grow when the NUS increases, indicating that the sample's expansion in the unloading direction will gradually increase. Fast unloading causes unloading damage, which causes a sudden rise in dissipated energy at the unloading point. The total energy and elastic strain energy input by axial stress at the bursting point gradually declines when the NUS increases, but the dissipated energy increases. The release rate of elastic strain energy at the bursting point increases when the NUS increases, and the rockburst becomes more intense. The abrupt value of the cumulative AE hits at the fast unloading point and the cumulative AE hits at the burst point increase with the NUS increase. The changing trend of the multifractal parameter, which initially decreases and then rises abruptly, can be regarded as the precursor of rockburst. The hysteresis of the precursor of rockburst grows when the NUS increases. Highlights: The fast unloading delayed rockburst tests on single-side, double-side, three-side, and four-side were carried out using the self-developed true triaxial rockburst experimental apparatus. Energy analysis and high-speed recording of the rockburst process found that the severity of the rockburst does not depend on the amount of elastic strain energy stored in the rock sample but rather on the release rate of energy at the rockburst point. The changing trend of the multifractal parameter, which initially decreases and then rises abruptly, can be regarded as the precursor of rockburst. The hysteresis of the precursor of rockburst grows when the NUS increases. [ABSTRACT FROM AUTHOR]

To explore the effect of rapid excavation unloading under high pressure on delayed rockburst, a true triaxial double-sided rapid unloading rockburst experiment and a biaxial compression failure experiment without unloading were carried out. The results show that the rapid unloading of excavation before the occurrence of rockburst will cause unloading damage, and the axial strain, acoustic emission (AE) count, and AE energy will suddenly increase at the unloading point. The AE source shows that the rapid unloading produces many tensile cracks, mainly distributed near the free surface. The axial peak stress of the rockburst and the stored strain energy before failure are higher than those of biaxial compression, so the ejection velocity of free surface fragments is also higher at failure. [ABSTRACT FROM AUTHOR]

To study the effect of excavation unloading on hard rock failure, a series of true-triaxial compression tests, biaxial compression tests, and true-triaxial unloading compression tests (two different unloading rates) at different confining pressures was conducted on red sandstone cube samples. The strength and failure characteristics and their relationship for red sandstone unloading at different unloading rates and confining pressures were analyzed. Based on the test results, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were explored, and a reasonable explanation for unloading-induced spalling in hard rock tunnels was presented. The results show the stress-strain curve of highly stressed red sandstone exhibits a stress step during unloading, and the higher the unloading rate, the lower the stress level required for a stress step. The rock strength-weakening effect induced by unloading was confirmed. The mechanical properties of red sandstone become more unstable and complicated after unloading. After the red sandstone is unloaded to a two-dimensional stress state, with increasing confining pressure, the strength increases first and then decreases; the failure mode changes from a low-confining pressure tensile-shear failure to a high-confining pressure tensile failure; and the geometries of the slabs change from large thick plates and wedges to medium- and small-sized thin plates. At equal confining pressures, the higher the unloading rate, the lower the strength (i.e., the strength-weakening effect is more pronounced), the thinner the slab, and the lower the confining pressure required for the failure mode to change from tensile-shear failure to tensile failure. The unloading rate and confining pressure affect the strength and failure characteristics by affecting the crack initiation type and propagation direction in hard rock. For deep hard rock tunnels with high unloading rate and axial stress, neglecting the effects of unloading rate and axial stress will lead to a dangerous support design. For deep hard rock ore, if the maximal horizontal principal stress exceeds the critical confining pressure, the mining surface should be perpendicular to the direction of the minimal horizontal principal stress. The results of this study are of great engineering significance for guiding deep hard rock tunnel construction and mining. [ABSTRACT FROM AUTHOR]