Frequency domain analysis of AE waveforms in rockburst specimens under different stress gradients

To investigate the influence of stress gradients in the surrounding rock of tunnels on the frequency features of acoustic emission (AE) waveforms, a true-triaxial gradient, and hydraulic-pneumatic loading apparatus were used to conduct rockburst experiments under four different stress gradients. The AE waveforms were recorded in real time throughout the entire loading process using an AE instrument. A fast Fourier transform was applied to extract feature points from the frequency spectrum of the AE waveforms. The main contribution of this study is the qualitative analysis of the feature points using the proposed variance/mean ratio (VMR) under different stress gradients. It was found that: (1) the cumulative amplitude of the dominant frequency of the feature points decreases with increasing stress gradients, indicating a reduction in crack size in the specimens. (2) The feature points exhibit varying evolutionary patterns with loading under different stress gradients, with low-frequency, high-amplitude feature points tending to concentrate at the moment of rockburst as stress gradients increase. (3) Fluctuations in the VMR curve can be utilized to identify stages of specimen deformation, and regular changes in these stages were observed with increasing stress gradients. (4) As the stress gradient increases, the fluctuation of the VMR curve gradually decreases, indicating that the increase in stress gradient accelerates the alternating development of macroscopic scale cracks and small-scale cracks. The results of the experiments are expected to provide insights into the mechanism of rockburst in underground constructions.