A Comparative Study of Fracture Evolution and Failure Characteristics of Coal Specimens Under Different Mining-Induced Stress Paths Using True Triaxial Tests and Acoustic Emission Monitoring

To investigate the fracture evolution and failure mechanisms of coal specimens under different mining-induced stress paths, true triaxial tests were conducted along six paths grouped into three categories: bidirectional loading–unloading, unidirectional double-face unloading, and unidirectional single-face unloading. Acoustic emission (AE) monitoring was used to analyze specimen failure and the corresponding AE responses. The results show that brittle shear failure dominated under the σ1–σ2 loading–unloading path, whereas ductile failure was more pronounced under the σ1–σ3 loading–unloading path. Under unidirectional double-face unloading of σ2 or σ3, tensile–shear cracking became more pronounced, and the failure mode shifted from shear-dominated failure to tensile–shear composite failure. Under unidirectional single-face unloading, the specimens unloaded along σ2 exhibited better stability than those unloaded along σ3. Fractures were mainly concentrated in the half region adjacent to the unloading face and were dominated by tensile cracks and their associated secondary fractures. AE activity under all testing conditions showed clear stage-dependent characteristics. In particular, high-intensity AE activity appeared earlier and was more pronounced when σ2 was unloaded. The RA–AF distributions indicate the coexistence of tensile- and shear-type cracking. This interpretation is consistent with the macroscopic failure patterns associated with the intermediate principal stress effect. These findings provide a reference for early warning and stability control of surrounding rock in underground coal mining engineering.