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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]

Rockburst warnings are a significant problem in the field of rock engineering. In this study, a rockburst experimental system was developed to conduct a true triaxial single-side rapid unloading rockburst experiment. The rockburst process and time were recorded using a high-speed camera. Based on the critical slowing-down theory, the acoustic emission (AE) RA/AF, duration, and rise time in the rockburst process were studied to obtain rockburst precursor information. The main conclusions of the experiment are as follows. (1) In the early stage of rockburst, the AE signal appears to be a critical-slowing phenomenon at the critical point, which is manifested as an increase in the variance and autocorrelation coefficient, and it also appears in the increase in the variance and autocorrelation coefficient during rapid unloading. A recording made using a high-speed camera shows that the rockburst occurrence time does not necessarily correspond to the peak stress point of the stress–strain curve. (2) The variances in the AE RA/AF value, duration, and rise time are the same. The sudden change time of the precursor signal is the same, the waveform fluctuation is similar, and they all appear before the rockburst. At this time, the AE localization event shows that many high-energy events occur in the sandstone, and cracks appear on the sandstone surface. (3) The autocorrelation coefficient fluctuates considerably compared with the variance; therefore, the variance can be considered the main criterion for rockburst precursors. This study provides a theoretical basis for predicting rockbursts. [ABSTRACT FROM AUTHOR]