Experimental study on the failure characteristic and mechanism of granite time-delayed rockburst under true triaxial condition

A series of tests for time-delayed rockburst of granite under true triaxial condition was designed and carried out. By using the true triaxial rockburst test system, an acoustic emission (AE) system, a high-speed camera system, and a digital image motion analysis software, the time-delayed rockburst development process was monitored and studied. Four stages were found in the failure of granite time-delayed rockburst, i.e. grains ejection, slab breaks and ejects, first fragments ejection and second fragments ejection. There is a “V” shear crack generated in the time-delayed rockburst sample, and several tensile cracks in the lower part of the rock sample cross through the “V” shear crack. The longer the duration (the time elapsed between the moments the AE hits rises rapidly and the rockburst occurs), the smaller the depth of the rockburst pit. The time-delayed rockburst debris are mainly composed of blocks and fragments. The longer the duration, the smaller the total debris mass, the percentage of ejected debris and the ejection kinetic energy. The fractal dimension of the debris is positively correlated with the duration. The longer the duration, the higher the degree of fragmentation. The cracks generated are tensile-shear composite cracks. In the loading stage, it is dominated by shear cracks. However, in the time-lag stage, it is dominated by tensile cracks. With the increase of the duration, the proportion of tensile cracks increased and the proportion of shear cracks decreased. The research results will have a certain reference value for the warning and risk mitigation of time-delayed rockbursts.