Response law and limit of triple bore pressure relief in coal seams under mining induced load.

This study investigates the response characteristics and ultimate failure mechanisms of large-diameter triple-bore pressure-relief drill holes in coal seams under mining-induced incremental static loading. Combining theoretical analysis, laboratory experiments, and numerical simulations, we systematically examine borehole mechanical behavior, energy evolution, and structural stability. Theoretical results show incremental loading initially expands fracture and plastic zones, promoting stress relief. Continued loading induces borehole collapse where fractured coal expands, filling the cavity and restricting deformation space, ultimately attenuating pressure relief capacity. Laboratory tests reveal a 30.9% increase in coal sample peak strength when loading rates rise from 0.1 to 1.5 mm/min. Input elastic energy grows while dissipated energy declines, indicating reduced energy dissipation and increasingly brittle failure at higher loading rates. PFC simulations demonstrate tensile-dominated crack development under high loading increments. Force chain instability and abrupt contact force increases cause progressive deterioration of pressure relief effectiveness.FLAC3D simulations further confirm reduced plastic zone extent and impaired stress transfer capacity under high stress concentrations, forming localized high-energy zones that significantly weaken pressure relief. This work provides theoretical and engineering foundations for designing pressure relief boreholes in rockburst-prone coal seams.