Effect of Confining Pressure on the Damage Evolution Process of Coal with Boreholes Under Graded Cyclic Loading–Unloading

During underground mining, the stability of in-seam gas drainage boreholes is jointly affected by multiple factors, including the in situ stress state and borehole structure. Borehole instability can reduce gas drainage efficiency and increase underground safety risks. Among these factors, confining pressure plays a decisive role in the damage evolution of the coal surrounding the borehole. To clarify the damage evolution characteristics of the coal surrounding the borehole under different confining pressure conditions, conventional triaxial graded cyclic loading–unloading numerical simulations were conducted on borehole-containing specimens using PFC2D software (version 6.0). The effects of confining pressure on acoustic emission (AE) ringing counts, microcrack propagation, crack angle distribution, damage evolution, and failure characteristics were systematically analyzed. The results show that, under graded cyclic loading–unloading, the peak AE ringing count of the borehole-containing specimens first increases and then decreases with increasing confining pressure, whereas the cumulative ringing count continues to increase. The spatial distribution of microcracks gradually evolves from dispersed development to concentration around the borehole, and the crack propagation path changes from single-path dominance to coordinated multi-path propagation. The angular distribution of tensile cracks exhibits a non-monotonic evolution pattern, namely, dispersion, concentration, and weakening, with increasing confining pressure, whereas the distributions of shear cracks and total cracks show a gradually broadened unimodal pattern with enhanced connectivity between angular intervals. At the final failure stage, both the tensile damage ratio and the shear damage ratio increase with increasing confining pressure, and their difference increases from 0.24% to 0.90%, indicating that increasing confining pressure further strengthens the dominant role of shear damage. The failure mode gradually evolves from tensile–shear mixed failure toward relatively shear-dominated failure. The results provide a theoretical basis for analyzing borehole instability and failure characteristics under different confining pressure conditions, as well as for optimizing grouting-based borehole protection parameters.