Technique for water-preserved coal mining based on zonal height limits along mining face and mechanisms behind overburden failure

Objective The exploitation of shallow coal seams under water bodies faces a contradiction between water resource preservation and overburden disturbance. In this case, achieving safe mining while preventing water-conducting fracture zones from affecting key aquifers and exploring techniques that place equal emphasis on water preservation and efficient resource recovery have emerged as popular research topics.

Methods An integrated aquifer-overburden-coal seam model was constructed using 3D geological visualization technology. This model, combined with the principles for the setting up of waterproof coal pillars, enabled accurate delineation of zones with water inrush risks. Accordingly, a technical scheme for mining based on zonal height limits along a single mining face was proposed. By introducing bending moment and shear force continuity conditions at mining height transition boundaries, this study derived the deflection curve equations for both full-seam and height-limited mining zones using the theory of beams on elastic foundations. The heights of water flowing fractured zones were determined by comparing the maximum bending deflection-induced subsidence of hard rock layers with their free-space height. Furthermore, by combining the 3D geological modeling with FLAC^3D-based numerical simulation, this study systematically investigated the overburden failure characteristics and stress evolutionary patterns under the condition of mining based on zonal height limits.

Results This study proposed a zonal mining scheme for mining face 200 in the Shuanglong Coal Mine, Shaanxi Province. In the zonal mining schemes, the mining heights for the full-seam and height-limited mining zones were set at 3.2 m and 2.5 m, respectively. The results indicate that in the early mining stage, the development of the water flowing fractured zone in the height-limited mining zone exhibited significant spatiotemporal lags. Theoretical analysis reveals that the height of the water flowing fractured zone decreased from 83.6 m to 68.9 m as the mining face advanced from 45 m to 100 m. In contrast, the numerical simulation indicates that the height decreased from 86.8 m to 69.5 m as the advancing distance reached 50 m. The reduction in the mining height significantly enhanced the concentration effect of coal support pressure in front of the mining face. Specifically, the coal support pressure increased sharply from 12.0 MPa to 15.2 MPa (growth rate: 26.7%) at an advancing distance of 50 m, decreased to 8.7 MPa at an advancing distance of 250 m, and finally reverted to 10.9 MPa after mining completion. Therefore, the coal support pressure generally exhibited a dynamic evolutionary pattern from a sharp increase to a gradual decrease and then to a gradual increase.

Conclusions The mining technology based on zonal height limits can effectively inhibit the development of local water flowing fractured zones and reduce damage to aquifers caused by the overburden disturbance, thereby achieving water-preserved coal mining. Furthermore, this study reveals the stress redistribution patterns in coals following a reduction in mining height, along with the mechanisms behind the lagged response of the development of water flowing fractured zones. The results of this study will provide a theoretical basis and technical reference for the zoning parameter optimization and support design for water-preserved mining under water bodies in similar geological conditions.