Study on the laws of “activation-rebalance” of large goaf roof

During the mining of the working face, the small coal pillar cannot bear the roof pressure of the instability and breaking, which leads to the “activation” instability of the roof structure of the adjacent large goaf roof, and the change of the stress field near the large goaf is easy to induce rock burst and other disasters. Based on the background of 3104, 3106 and 3107 working faces in Menkeqing Coal Mine, this paper comprehensively uses field monitoring and theoretical analysis to explore the fracture evolution law of roof overburden in large goaf and clarify its “activation-rebalance” mechanism. The research shows that: ① during the second square of the 3107 working face, the microseismic events above 10⁴ J on the side of the large goaf increased by 7 times compared with the first time, and the energy surged by about 28 times, indicating that the roof is “activated” and unstable. By the third time, the large energy event is reduced to 1 time, the energy plummeted by 93.9%, and the roof returned to the “rebalance” state. ② During 3107 working face stoping, the microseismic frequency of 78-198 m strata is about 4 times that of 3104 and 3106 working faces, but its total energy is one order of magnitude lower, which confirms that the instability of roof “activation” in large goaf does not extend to high strata. ③ The mechanical model and criterion of roof activation and instability in goaf under small coal pillar mining are established. It is revealed that the rotary activation and instability of the lower basic roof occurs, while the middle key stratum does not reach the instability condition and remains stable.