Investigation of Overburden Fracture Evolution and Feasibility of Upward Mining in Shallow-Buried Coal Seams

Taking Yujialiang Coal Mine as the engineering background, aiming at the actual demand of 5-2 coal seam mining and 4-4 coal seam upward mining, the temporal evolution and spatial distribution characteristics of overburden failure height after 5-2 coal seam mining are systematically investigated by using multi-source field detection technology such as ground drilling, logging, and borehole peeping, combined with a numerical simulation method. The field detection results show that after the 5-2 coal seam is mined, the development height of the water-conducting fracture zone (WCFZ) is 116.25–129.92 m, and the height of the caving zone is 9.32–21.56 m. The 4-4 coal seam is located within the fracture zone, 15.99–22.88 m above the caving zone. The strength of the 4-4 coal seam and its surrounding rock affected by mining is reduced, with a more significant decrease in the middle of the goaf. The numerical simulation further reveals the law of overburden movement and deformation. After the 5-2 coal seam mining, the maximum subsidence of the 4-4 coal seam floor reaches 4.57 m, and there is stress concentration above the remaining coal pillars. The maximum vertical stress after mining all three working faces (52,204, 52,205, 52,206) is 4.10 MPa, and the stress environment above the goaf is better. The results show that the average distance between the 4-4 coal seam and 5-2 coal seam is about 39.45 m, and the upward mining is feasible, but the stability of the rock strata in the fracture zone should be paid more attention to. Based on the movement law of overlying strata and the characteristics of stress distribution, it is suggested that the mining gateway of the 4-4 coal seam should be arranged in the middle of the remaining coal pillar of the 5-2 coal seam or the corresponding area in the middle of the goaf so as to ensure the stability of the roadway surrounding the rock during mining. The research results provide a reliable theoretical basis and technical support for the upward mining design of the 4-4 coal seam in Yujialiang Coal Mine and have important reference value for the upward mining projects of coal mines under similar conditions.