Experimental Study on the Fracture Evolution Process of Pillar Burst Based on Acoustic Emission Under Horizontal Bidirectional Unloading

The pillar burst disaster in deep engineering significantly constrains project construction. To reveal the fracture mechanism of rock pillars under instantaneous unloading, a self-designed rigid true triaxial experimental apparatus, equipped with two high-speed cameras and an acoustic emission (AE) monitoring system, was utilized to conduct instantaneous horizontal bidirectional unloading tests on granite with different stress states. The results indicate that granite undergoes severe and irreversible deformation during instantaneous unloading, with vertical strain rapidly increasing at higher axial stress. Analysis of AE time- and frequency-domain indicates that different stress states can alter crack type and crack mechanism. The wavelet transform was performed on all AE signal waveforms, and it revealed that the energy of AE signals caused by unloading was concentrated in low-frequency bands (15.625-62.5 kHz). Additionally, nonlinear characteristics of the b-value and the Hurst index explained the fracture evolution process, and the mutation point of the b-value and the Hurst index can be considered warning precursors of rock pillar burst. Finally, the damage evolution process based on the accumulated AE counts was segmented into two stages and fitted by a linear function and the Weibull distribution. The parameter m of the Weibull distribution was used to reflect the brittleness, and the effect of parameter m on the growth speed of damage was discussed.