Numerical Study on the Load-Bearing Characteristics and Damage Behavior of Boreholed Coal Specimens Under the Combined Effects of Compression–Shear Loading and Borehole Diameter

Coal in deep inclined-seam roadways is subjected to long-term high-stress and coupled compression–shear loading conditions. After pressure-relief boreholes are drilled, local high stresses are transferred while the load-bearing capacity of the coal is also weakened. Therefore, clarifying the load-bearing damage behavior of coal after drilling under coupled compression–shear loading conditions is important for the stability control of deep roadways. Based on the discrete element code PFC2D, models of intact coal specimens and coal specimens containing boreholes with diameters of 8, 12, and 16 mm were established to investigate the macroscopic mechanical response, damage behavior, and mesoscopic load-bearing characteristics of specimens at five compression–shear angles of 0°, 5°, 10°, 15°, and 20°. The results show that, as the compression–shear angle increases from 0° to 20°, the compression–shear strength of specimens with different borehole diameters decreases by 53.77%~62.45%; the damage initiation stress decreases by 50.5–61.4%, and damage initiates earlier; the post-peak response progressively exhibits plateau softening, stepwise softening, and multi-peak drop characteristics. Increasing borehole diameter reduces the effective load-bearing width on both sides of the borehole and the available load-transfer paths bypassing the borehole, whereas increasing the compression–shear angle causes the primary force chains to deflect and intensifies the stress asymmetry on the two sides of the borehole. With the propagation of cracks around the borehole, the primary load-transfer paths are interrupted successively, the load is redistributed to residual force chains, and through-going failure forms after continued failure of the residual paths. From the perspectives of internal load transfer and load-bearing-network evolution, this study reveals the mesoscopic process by which coal evolves from stable load bearing to unstable failure under the combined effects of borehole-induced structural weakening and compression–shear loading, providing supplementary insight into the evolution of the internal load-bearing structure corresponding to the stress–strain response and macroscopic failure pattern. The results can provide a theoretical reference for the selection of pressure-relief borehole parameters and the evaluation of post-drilling load-bearing damage of coal under inclined-seam conditions.