ACOUSTIC EMISSION CHARACTERISTICS OF COAL UNDER DIFFERENT TRIAXIAL UNLOADING CONDITIONS

The mechanism of coal and rock damage caused by unloading during deep mining differs from that caused by continuous loading, and failure often results from a combination of loading and unloading. Triaxial unloading tests are useful for studying the damage characteristics of coal and rock under different conditions from a unloading point of view to better understand dynamic disasters of deep coal and rock. In this study, we performed triaxial tests involving loading axial stress and unloading confining pressure (4, 7, 10 MPa) using different unloading rates (0.02, 0.05, 0.08, 0.11, 0.14 MPa/s), and acoustic emission (AE) events were recorded simultaneously. The results show that the maximum AE ring and energy counting rates do not appear at peak stress but rather at the stress drop stage following peak stress. Similarly, the maximum AE impact counting rate occurs after the peak stress, which indicates that the internal cracking of the coal samples reaches a maximum at the fracture stage following peak stress. The AE information indicates a relatively quiet period prior to the occurrence of large-scale AE events. Higher initial unloading confining pressure is associated with earlier and more severe failure after peak stress. Faster unloading rates are also associated with earlier sample destruction after peak stress because the coal rapidly changes from a triaxial stress state to a uniaxial stress state with higher unloading rate, crack propagation is insufficient, and more elastic energy is released. Compared with conventional triaxial tests, the coal damage variable increases faster under triaxial unloading tests, which indicates that unloading failure is more severe.