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To explore the energy evolution characteristics of rockbursts and the mechanism of excess energy in rockbursts, a self-developed rockburst experimental system was used. We performed true triaxial rapid unloading rockburst simulations and triaxial experiments based on the complete stress-displacement surface (CSDS) model prediction of the post-peak curve of rock uniaxial compression. By analyzing the characteristics of stress–strain curve and energy evolution of the rockburst experiment and comparing the energy storage characteristics of the two experiments, we arrived at the following conclusions Unlike the static failure of rock due to uniaxial compression, the occurrence of rockburst is mainly due to the generation of excess energy, which depends on the elastic strain energy stored before rock failure, outside energy input after rock failure and residual elastic strain energy. The intermediate principal stress of the rockburst experiment stored considerable residual elastic strain energy after the rockburst, resulting in an insufficient release of the elastic strain energy stored before the rockburst. The intensity of the rockburst is closely related to its excess energy release rate (EERR). With an increase in the EERR value, the cumulative energy of acoustic emission (AE) during rockbursts, the fractal dimension of rockburst fragments, and the rockburst intensity gradually increase. Highlights: The energy evolution and energy storage characteristics of true triaxial single-sided rapid-unloading rockburst experiment were studied using the graphic area method. The excess energy mechanism of the rockburst was discussed by comparing the energy storage characteristics of the true triaxial rockburst experiment and the static failure of the uniaxial compression rock. The correlation between rockburst intensity and EERR value was explored. [ABSTRACT FROM AUTHOR]

This study aimed to explore the effect of four-sided rapid unloading on delayed pillar rockbursts under high pressures. To this end, true triaxial four-sided rapid unloading rockbursts and uniaxial compression experiments were conducted on two sandstones with different strengths. The differences in damage under two different loading paths and characteristics of the rapid unloading point of the pillar rockburst were analyzed. The results indicate that rapid unloading on the four sides under true triaxial high-pressure loading caused unloading damage. The stress and strain at the rapid unloading point would produce an unloading platform, and the acoustic emission (AE) signal and dissipated energy would increase rapidly. The rapid unloading damage in the elastic stage is mainly caused by the tension cracks generated around the specimen because of the rapid expansion of the specimen. As the center part of the specimen was not affected, the stress–strain curve after unloading was still in the elastic stage. Although the initial stress of the high-strength sandstone pillar rockburst during rapid unloading was higher than that of low-strength sandstone, the unloading damage caused was less than that of low-strength sandstone, indicating that rapid unloading damage is closely related to the rock properties. [ABSTRACT FROM AUTHOR]

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]