Weakening and fracturing mechanism of roof strength through borehole pressure relief under true triaxial stress

Focusing on the engineering characteristics of high stress, strong disturbance, and large deformation in deep coal mining. Borehole pressure relief technology, however, can control the stability of the surrounding rocks in mining faces. The work employed laboratory true triaxial tests on borehole pressure relief rocks, numerical simulations, and a secondary development approach incorporating representative volume elements (REV). The approach was to investigate the stability of surrounding rock structures under mining-induced conditions and the multiscale fracture evolution law of overlying strata in mining faces. The key findings are as follows. (1) True triaxial tests on rocks with holes were conducted to analyze the mechanical response and failure characteristics under different hole parameters. Stress concentration around holes significantly altered rock fracture modes, forming directional fracture zones aligned with the hole arrangement. (2) A multiscale fracture evolution model for rocks at macro- and micro-levels was established, employing REV and the Weibull function to characterize rock heterogeneity. The Mogi-Coulomb criterion was used to describe the damage evolution process of micro-level elements. A macro-micro numerical computation program for coal-rock masses was developed, with the progressive fracture evolution law of holed rocks under true triaxial conditions analyzed. (3) Stress concentration near holes and the X-shaped fracture propagation pattern were revealed to investigate the influences of hole quantity, size, and distribution on regulating rock fracture paths. The stability mechanism of surrounding rocks controlled by borehole pressure relief technology was elucidated.