Study on the Evolution Law of Overlying Strata Movement Inducing Surface Subsidence Under Different Mining Sequences of Multiple Coal Seams: A Case Study

Reducing surface subsidence caused by coal mining is important for the coordinated development of efficient coal resource development and ecological environmental protection and it is a long-term difficult problem. Taking the on-site conditions of a mine in Fengfeng Mining Area as the engineering background, this study uses numerical simulation to research the evolution law of overlying strata movement closely related to surface subsidence under different mining sequences of multiple coal seams due to the deficiencies of existing research. The research results are as follows: ① The evolution law of overlying strata fractures is obtained. The key strata play a major role in inhibiting the vertical fracture propagation of overlying strata and thus controlling surface subsidence. The overlying strata will form a trapezoidal propagation structure with the horizontal fractures of the key strata as the upper base and the vertical fractures as the sides. When multiple key strata all inhibit vertical fracture propagation, the relatively higher key stratum plays the main inhibitory role, and the overlying strata fracture propagation process of the three key strata is determined. ② Compared with downward mining, upward mining not only results in smaller surface subsidence after the first coal seam mining but also smaller total subsidence after the two coal seams mining. The explanation for this phenomenon is that the key strata weaken in strength while inhibiting fracture propagation. The degree of fracture propagation inhibition (degree of surface subsidence control) by the key strata is proportional to the degree of strength weakening, and both are inversely proportional to the distance from the mined coal seam. Under multi-coal seam mining conditions, the final total weakening degree is lower when the key strata strength first experiences a small weakening stage. Upward mining first mines the coal seam farther from the key strata, and the key strata strength is less weakened during the mining of this coal seam, leading to a lower final total weakening degree and thus smaller final surface subsidence. ③ The evolution law of overlying strata bed separation is determined. The maximum values of bed separation space under the three key strata appear in the order of increasing stratum position, and grouting should be carried out in turn during bed separation grouting. The bed separation space during the second coal seam mining in upward mining is significantly larger than that during the first coal seam mining, so the grouting volume should be mainly concentrated in the second coal seam mining process; while the difference in downward mining is small, and the grouting volume should be implemented evenly. The bed separation space and grouting timing under the two mining sequences are obtained.