Reasonable drilling angle and technology application for pre-cracking thick-hard roofs before driving gob-side roadways in ultra-thick seams.

To address the challenges of controlling the surrounding rock deformation in gob-side roadways caused by thick and hard roof suspension in ultra-thick coal seams, a pre-cracking roof pressure relief technique before roadway excavation is proposed. Through on-site surveys, theoretical analysis, numerical simulations, and industrial tests, the effect of roof cracking angle on the pressure relief effect in gob-side roadway surrounding rock was studied based on the lateral overburden stress transfer mechanism. The optimal parameters for pre-cracking roof cracking in gob-side roadways were determined. The results indicate that the larger the roof cracking angle, the smaller the additional load exerted on the coal body by the overlying rock, leading to a more significant pressure relief effect. Numerical simulations showed that as the cracking angle increases, the second invariant of the deviatoric stress tensor (J2) in the surrounding rock gradually decreases. When the cracking angle is less than 90°, the range of plastic failure decreases with an increase in angle. However, when the cracking angle exceeds 90°, the range of plastic failure in the surrounding rock increases. At a cracking angle of 90°, the accumulated distortion energy in the deep surrounding rock is minimal, and the shallow surrounding rock has sufficient bearing capacity, resulting in the best pressure relief effect. On-site monitoring results show that the use of CO2 phase change-induced fracture pre-cracking for thick and hard roof layers can reduce surrounding rock deformation by more than 75%, demonstrating excellent control performance.