Local seismicity induced unstable crack propagation in a coal and its effect on coal burst characteristics

In this study, the acoustic emission evolution and stress drop processes of brittle coal specimens under quasi-static compression were investigated by experiments and numerical simulations, to reveal the pattern of mesoscopic crack evolution and its effect on the macroscopic coal burst characteristics. The results indicate that the crack evolution law during an instantaneous stress drop is dramatically different from that under quasi-static compression, in terms of both the inducement mechanism and the pattern and scope of crack propagation. Upon an instantaneous stress drop, mesoscopic tensile cracks around the free face rapidly propagate and coalesce. Subsequently, massive debris are formed and ejected, due to slippage-triggered high-energy seismicity events. During the process, tensile cracks are dominant while shear cracks are mainly generated during crack coalescence. In addition, high-energy seismicity events are localized and then extend peripherally, where the dynamic disturbance leads to rapid evolution of local mesoscopic cracks and structural instability. As a result, stress drop and coal burst take place at the macroscopic scale. Generally, temporality among seismicity, stress drop and coal burst was found. Moreover, coal burst intensity is correlated with the stiffness ratio and released energy of a stress drop event. The critical stiffness ratio is 18.2 for coal specimens in this study, below which no coal burst occurs. With the increase of stiffness ratio and energy release, four types of coal burst behavior, i.e. slight debris ejection, massive debris ejection, block ejection, and coal bump, successively take place. The mean ejection velocity and kinetic energy of debris, as well as the maximum debris ejection velocity, generally increase with the rise of stiffness ratio. Conclusions and findings in this paper are beneficial to the understanding of coal burst mechanism under quasi-static compression.