Study on Stability and Control of Surrounding Rock in Large-Section Mining Space with Thick-Hard Roof Under Fully-Mechanized Top Coal Caving Mining in Extra-Thick Coal Seams

Disaster prevention and control of thick and hard roof breakage in longwall mining is a major technical problem faced by coal production enterprises. Taking 15311-south workings face of a mine as the engineering background, a mechanical model of thick and hard roof breakage in longwall workings was established to analyze the main influencing factors of the energy released by the initial and cyclical breakage of thick and hard roofs and the disaster-causing mechanism. The results show that the total energy released from hard roof breakage is positively correlated with the mining thickness, hard roof thickness, hard roof tensile strength and negatively correlated with the direct roof thickness, and the total energy released from the initial breakage of the main roof, is more than twice as much as that released from the cyclic breakage. Based on the production geological conditions, the parameters of directional long-drilling fracturing of thick-hard roof (fine sandstone) in 15311-south working face were determined as follows: single-point water injection volume of 20m3, spacing of fracturing points of 15m. Based on the control idea of “energy mitigation - structural weakening - stress regulation”, the “fixed-length drilling - hydraulic fracturing of thick-hard roof” was proposed. Based on the idea of “energy mitigation, structural weakening and stress control”, the three-dimensional fracturing technology of “fixed-length drilling, segmental fracturing and layered control” was applied in the field practice. On-site monitoring shows that: the maximum amount of coal pillar gangs moving in 15311-south tailgate is 236mm, the maximum amount of solid coal moving is 135mm, and the maximum value of roof and floor convergence is 287mm; the initial pressure step of the working face is reduced from 45m to 18m; compared with the non-fractured working face, the average reduction of the cycle pressure step of the quarry is 35%, and the scheme effectively ensures the safe and efficient mining of the comprehensive workface under the thick-hard roof.