Experimental study and acoustic emission frequency domain analysis of spalling-rockburst in high-stress tunnel surrounding rock

To elucidate the spalling-rockburst mechanisms in tunnel surrounding rock under deep, high-stress environments, true triaxial loading tests were conducted on limestone specimens with circular through-holes. These experiments simulated the conditions of deep-buried tunnels subjected to ''high geostress + stress adjustment''. A micro-camera and an acoustic emission(AE) monitoring system were used to record the macroscopic failure phenomena around the boreholes and the transient elastic waves released by micro-damage. The AE waveforms were transformed from the time domain to the frequency domain using Fast Fourier Transform (FFT), and the frequency energy migration was characterized by establishing the FR parameter. The study focused on the spalling-rockburst failure process and the evolution of frequency-domain acoustic emission signals. Results indicate that the spalling-rockburst process of the surrounding rock can be divided into four stages: the quiet stage, particle ejection stage, spalling detachment stage, and rockburst ejection stage. Tensile damage induced by vertical stress and shear damage caused by horizontal stress jointly contribute to the formation of a "V"-shaped rockburst crater. The peak frequency and frequency centroid of AE signals classify them into four types: low-frequency, sub-low-frequency, sub-high-frequency, and high-frequency signals, corresponding to different crack scales and damage modes in the rock. The co-occurrence of these frequency signals and the stepwise changes in FR parameters reflect significant internal structural changes within the rock. Rockburst occurrence is primarily determined by the stress state of the surrounding rock and the energy storage capacity of the rock mass. While spalling is a sufficient but not necessary condition for rockburst, it accelerates its evolution.