Mechanism and application of cross-interface fracturing for permeability enhancement through the roof blasting of tectonic coal seams

Deep coal mining is challenging due to deteriorating geological conditions. After undergoing multiphase tectonic movements, primary coals have evolved into soft tectonic coal seams with low permeability and high gas content, complicating drilling in coal seams. In this case, the blasting-induced fractures are not well-developed and are prone to be re-compacted. So far, no fundamental breakthrough has been achieved in the key technology of fracturing for permeability enhancement technology through the roof blasting of tectonic coal seams. Given this, this study established test models for similar simulation tests and numerical analysis on the roof blasting of coal seams. By monitoring the cross-interface stress wave propagation and the macroscopic three-dimensional fracture evolutionary morphologies, this study reproduced the blasting stress wave propagation, as well as the internal damage and failure processes of coals and rocks. Key findings are as follows: (1) The blasting-induced damage to tectonic coal seams is primarily distributed in blasting boreholes and coals near the upper and lower coal-rock interfaces. The roof blasting of tectonic coals produces cross-interface fractures for pressure relief, and the blasting-induced damage propagates along blasting boreholes to the surrounding rock masses, ultimately spreading to the footwall of coal seams. Serious damage is found in blasting boreholes, near the upper coal-rock interfaces, and inside coal seams. (2) The blasting stress waves undergo transmission and reflection when propagating to coal-rock interfaces from the roof of the tectonic coal seams. The transmitted compressive stress waves damage coals, while the reflected tensile stress waves react on the rock masses at coal-rock interfaces. (3) The pressure relief induced by cross-interface fractures, generated by blasting-induced cumulative damage, connect the rock fractures on the roof with the fractures within tectonic coal seams, facilitating the vertical migration of gas in tectonic coal seams and gas drainage with stress relief. The on-site application shows that gas drainage following the roof blasting of coal seams manifested rapidly increased pure gas flow and concentration, which increased from 0.07 m³/min to 1.73 m³/min and from 10.46% to 68.50% (volume fraction), respectively and maintained at high levels for a prolonged period. The results of this study can provide a theoretical basis and technical support for efficient gas drainage from deep tectonic coal seams.