Analysis on failure law of overlying strata and development height of “two zones” in repeated mining of close distance coal seams

In order to study the failure law of overlying strata and the development height of “two zones” in repeated mining of close distance coal seams, various safety accidents such as roof leakage and spontaneous combustion in goaf caused by water and air leakage channels in the mining process of working face are prevented. Taking a coal mine in Ordos City as the engineering background, the underground peeping method and numerical simulation are used to analyze the development height of “two zones” and the failure law of overlying strata after close distance coal seam mining, and the reasons for the formation of the law are revealed. The results show that under the influence of repeated mining in close distance coal seams, the development height of “two zones” is much higher than that of upper coal seam mining. When the lower coal seam advances 80 m, the maximum development height of the fracture zone is 88 m, which is connected with the surface to form a water and air leakage channel. The ratio of initial mining and repeated mining is 19.33 and 29.33 respectively. The development of the fracture zone rises step by step, and the change between stages is in the form of jump. The failure of overburden rock is “arch shape→cap shape→saddle shape→outer O inner butterfly shape”; the error rate between the development height of the “two zones” predicted by the numerical simulation and the fitting formula and the measured value is about 1%, which verifies the accuracy of the prediction and can provide reference for the mining of similar working faces and similar projects.