Borehole disturbance of ultra deep-buried pulverized coal seams with low-permeability under high ground stress

The M1-7 to M1-9 coal seams traversed by the Zhaotong Tunnel feature high ground stress, low permeability, and gas outburst risks. Frequent borehole collapse and drilling jamming during construction severely constrain gas drainage efficiency due to inadequate borehole completion rates. This study therefore attempts to investigate the instability mechanisms of boreholes in high-stress pulverized coal seams and optimize drilling parameters (diameter and depth) for enhanced gas extraction via a multidisciplinary approach. Specifically, this study (1) developed a mechanical model for borehole disturbance under non-isobaric stress fields and, analytically characterized the "dumbbell-shaped" plastic zone under lateral pressure coefficients (λ>1); (2) employed COMSOL Multiphysics simulations to analyze stress distributions for boreholes with diameters of 350/500 mm and depths of 5~20 m; (3) conducted dual-aperture field tests and borehole imaging quantify fracture development and gas drainage dynamics. We found that under high ground stress, lateral pressure coefficient λ governs plastic zone morphology; at λ = 1.6, a dumbbell-shaped stress concentration zone forms, with disturbance ranges extending to 1.29 times the borehole depth. The plastic zone of a 500 mm aperture borehole expands to 1.9 times that of a 350 mm borehole, with gas drainage volume in the tunnel construction section increased by 60 %, though requiring enhanced support for transverse fractures.