Law Regarding Damage Caused by Repeated Mining in Close Coal Seams

Coal mining in China is increasingly moving towards deeper resources. In certain Carboniferous–Permian coal fields in North China, there is a typical problem of repeated mining over short distances and multiple coal seams, and the distance between the main coal seam and the mined shallow coal seam is relatively small. This leads to repetitive disturbance damage to the surrounding rock during closed-coal seam mining. This was followed by more serious threats from water disasters. Therefore, it is particularly important to systematically study the disturbance and failure characteristics of rock masses under repeated mining conditions in multiple coal seams and to investigate repeated mining damage law and characteristics of the overlying rock of a close coal seam. This study focuses on six mining areas in the Jiyang coal mine. This study also predicts the damage height of the top and bottom slabs of mined No. 7 coal and main mined No. 10 coal based on an empirical formula. An on-site investigation was conducted using downhole drilling and segmental water injection to determine the damage depths of the two seams. A numerical simulation was then conducted to study the height, displacement, and stress of the overlying rock damage caused by mining close to the coal seams. Changes in height, displacement, and stress of the overburden rock mining damage were also studied through numerical simulations. Results indicated the following: (1) The height of the plastic zone of the overburden rock increased by 6.25%, (2) maximum settlement displacement increased by 5.05%, and maximum horizontal displacement increased by 9%. It is important to note that these findings are objective and based solely on presented data. Repetitive disturbances with larger amplitudes caused a 27% change in horizontal displacement. (3) Maximum principal stress in the stress field decreased with the overall value of the vertical stress. The influence range increased slightly and the two sides of the hollow area at the open cutting eye and stopping line were also affected. These findings were obtained through numerical simulations. Stress concentration was more pronounced in the open-cut eye and near-the-stop line.