Experimental investigation on pillar rockburst of true triaxial four-face rapid unloading under high stress

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.