Mechanism of coal mass fracture expansion under drilling and pressure relief

To clarify the nonlinear transformation of coal from elastic energy storage to macroscopic fracture under borehole unloading, this study develops a multistage loading–unloading experimental system, incorporates acoustic emission monitoring, and utilises techniques such as moment tensor inversion, time–frequency analysis, and stress field inversion to examine the fracture propagation mechanisms and stress response characteristics of coal. The experimental findings indicate that borehole unloading causes local stress disturbances, which significantly influenced the fracture propagation paths and failure modes, thereby governing the macro-damage evolution process. The three-stage source mechanism analysis reveals that shear failure predominates throughout, while tensile failure notably increases during the secondary loading stage, indicating stage-dependent restructuring of stress distributions within the coal. Stress field inversion results reveal that in the initial loading stage, the maximum principal stress aligns southwest–northeast. During the secondary loading stage, the stress field shifts to a southeast–northwest inclined shear-dominated configuration, promoting shear-tension coupled fracturing. Borehole diameter affects fracturing behaviour: a 10 mm-diameter borehole initiates local high-energy events that inhibit crack propagation, whereas a 12-mm-diameter borehole enables energy cascade release and chain-like destruction. Time–frequency analysis shows that shear fractures mainly concentrate at 80–90 kHz with short-term energy aggregation; tensile fractures occur at 40–50 kHz, displaying mid-frequency expansion characteristics; and compression fractures are characterised by continuous low-frequency outputs at 10–20 kHz. Through theoretical analysis, a borehole pressure relief coal elastic–plastic damage model was developed to elucidate the control mechanisms of lateral pressure coefficient, borehole diameter, and stress level on the evolution of the plastic zone. This study enhances the understanding of crack propagation mechanisms in coal subjected to drilling-induced pressure relief in mines.