Study on the evolution and transport pattern of overburden fracture induced by mining in cave mines

Coal mines located in southwestern China are generally faced with the complex situation of repeated mining of coal seams in areas with dense karst caves. In view of the problems brought about by overburden fracture transport-induced karst cave instability during repeated mining of karst mines, this paper takes the repeated mining under the karst mines of Qinglong Coal Mine in Guizhou, China, as a background for the study.Using a combination of theoretical analyses, physical experiments and numerical simulations, the influence of karst caves on the transport of the overlying rock layers and the height of the development of water-conducting fissure zones was analysed from the conditions of repeated mining in karst mines.The results show that the existence of repeated mining karst caves accelerates the overburden fracture fissure penetration into the upper coal seam air-mining zone, and at the same time, the analysis of the stress distribution of the caves obtains the judgement basis for the instability of the caves.The DIC digital scatter monitoring system found that the displacement of the overlying rock layer of the cave model at the monitoring point was significantly larger than the displacement of the overlying rock layer of the no-cave model as the working face progressed.The displacement rate of the cavern model is obviously larger under repeated mining, but the cavern plays a key role in controlling the fracture of the overburden rock when the working face advances to the bottom of the cavern, and the cavern is re-stabilised after instability, and it still has a certain supporting effect on the overburden rock above.In order to verify the above analysis using 3DEC numerical simulation to verify it again, it was found that the results are still shifted more at the karst cave, and showed asymmetric subsidence after the cave was perturbed to return to the steady state, which is highly similar to the results of the physical experiments.Therefore, before the coal seam is mined to the vicinity of the karst hole, the roof of the working face can be grouted and reinforced in advance, and the working face can be pushed through the vicinity of the karst hole quickly, and the results of the above research provide certain reference for the safe mining in karst mines.