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 remains a long-standing challenge. 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 reducing 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 Under the geological and mining conditions of this mine, upward mining achieves smaller surface subsidence than downward mining, not only after the mining of the first coal seam (2.64 m vs. 5.93 m), but also after the complete mining of both coal seams (8.21 m vs. 9.04 m). A reasonable explanation for this phenomenon obtained in this study 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 to the mined coal seam. Under multi-coal seam mining conditions, the final total weakening degree is lower when the key strata's strength first experiences a small weakening stage. Upward mining first mines the coal seam farther from the key strata, and the key strata's 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 is recommended to be carried out in turn during bed separation grouting. For upward mining, the bed separation space during the mining of the second coal seam is significantly larger than that during the mining of the first coal seam. The maximum bed separation space of the three key strata is 208 m², 265 m², and 237 m² for the first mining stage, and 856 m², 793 m², and 1287 m² for the second mining stage, respectively. Accordingly, the grouting volume is suggested to be mainly concentrated in the mining process of the second coal seam. For downward mining, the difference in bed separation space is relatively small. The maximum bed separation space of the three key strata is 396 m², 586 m², and 800 m² for the first mining stage, and 336 m², 286 m², and 802 m² for the second mining stage, respectively. Thus, the grouting volume should be applied evenly. The bed separation space and grouting timing under the two mining sequences are accordingly obtained.