Acoustic Emission Characteristics of Coal Failure Under Triaxial Loading and Unloading Disturbance

The utilization of deep mineral resources is severely restricted by dynamic disasters, such as rock burst, caused by mining disturbances. This study is aimed at investigating the instability failure mechanism and precursory response of coal specimen, with the aid of acoustic emission (AE) monitoring, under triaxial loading and unloading disturbance. The AE time-varying characteristics were analyzed, the crack propagation mode was explored, and the statistical b value and beta(t) value (damage parameters) of AE events were introduced to evaluate the damage state of the loaded coal under disturbance. The results show that: the dynamic disturbance loads will accelerate the deformation process of coal specimen under high static confining stress, resulting in a positive relation between the AE characteristic parameters (AE counts and energy) and the increasing disturbance amplitude. The proportion of low-frequency and high-amplitude AE signals increases with the disturbance amplitude, indicating the disturbance with large amplitude will promote the formation of large-magnitude cracking events. Those stress-induced cracks, judged from the RA-AF value of AE signals, are mainly tensile mode (i.e., mode-I) under the applied disturbance load, but shear cracks are the critical factor affecting the instability and failure of the loaded coal specimen. Both the b value and beta(t) value can reflect the time-varying evolution process of internal damage for the loaded coal specimen. The sudden decreasing of the b value together with the exponential increasing of the beta(t) value can be regarded as a short-impending precursor indicator for the coal instability and failure. The results of this study will provide some experiment basic and theoretical guidance for dynamic disaster monitoring and early-warning during the deep coal mining.