Determining a Reasonable Time Interval for Upward Mining in Longwall Mining Goaf Conditions

Aiming to a reasonable optimal time interval for mining the upper coal seam in an upstream direction under longwall goaf conditions, the support pressure distribution law, bottom displacement change law, compression law, and surface subsidence velocity of the upward working face were analyzed through simulation test, gangue compression test, and field measurement. The results indicate that the essence of determining a reasonable time interval is primarily to ensure that the balanced structure formed by the broken rock blocks and the overlying strata (the floor of the upper coal seam working face) remains intact and does not experience significant dislocation under the influence of leading support pressure and other factors. The primary factor influencing the continuity and integrity of the upper coal seam is the compression of the broken gangue in the floor of the upward working face under support pressure. Over time, as the goaf stabilizes under the weight of the overburden and gangue compression reaches stability, it becomes possible to mine the upper coal seam without causing significant step displacement or further compromising its continuity and integrity. Therefore, the key to determining an appropriate time interval is identifying when the gangue compression displacement in the goaf floor stabilizes. The subsidence curve of the gangue in the goaf is similar to that of surface points, indicating that as surface movement stabilizes, so does the strata movement and the gangue compression in the goaf. A subsidence velocity of 50 mm/day serves as a critical threshold, which, when combined with the subsidence velocity of surface points, leads to a formula for calculating the optimal time interval for upward mining. These findings offer valuable guidance for mining operations in the upper coal seam.