Timing of concurrent mining and reclamation in coal-grain overlapping areas with mining-induced subsidence,Eastern China

Mining-induced subsidence in the coal-grain overlapping areas of eastern China has caused a large surface ponding, exacerbating the contradiction between coal supply and grain production. In order to coordinate the relation- ship between farmland protection and underground coal mining, we use the concurrent mining and reclamation (CMR) technology, which can realize the coupling of mine reclamation and underground mining, and can significant- ly improve the reclamation rate in mining areas with high groundwater levels.The timing of reclamation is the key to the CMR and directly determines the success or failure of the reclamation project. This study analyzes and summaries the natural and human factors that influence the choice of reclamation timing and proposes two types of rec- lamation timing: 1 the theoretical reclamation timing under the critical ponding and 2 the actual reclamation timing under the shallow ponding on the surface caused by coal mining. Taking a single coal seam mining as a case, the spa- tio-temporal relationship between surface ponding and underground mining was studied, and the starting distance of surface ponding caused by underground mining and the boundary angle of surface ponding were proposed to reveal the dynamic relationship between workface advancement and theoretical reclamation timing. A model of the crit- ical width of skip-panels and back-panels based on the subsidence value of the critical ponding on the surface and a model of the actual reclamation timing under shallow ponding were constructed. The case study shows that the criti- cal width of working face proposed with the critical subsidence value (1.00 m) of surface ponding in the first min- ing district of the Guqiao Coal Mine of China can delay the reclamation timing and farmland life by 12.5 years. The change of reclaimed farmland area in the Longgu Coal Mine of China under different depths of surface ponding was simulated. A quantitative model between reclamation timing and reclamation rate was constructed to predict the optimal reclamation timing under different reclamation rate indexes and the optimal reclamation rate under shallow ponding conditions. The optimal reclamation timing under the mining geological condition is when the relative depth of ponded water is 0.6. When the difference of reclamation rate is controlled at 5%, the relative ponding depth of 0.706 7 is the optimal time for reclamation. Finally, the input-output benefits of the CMR were discussed. The CMR can sig- nificantly improve the land reclamation benefits and internalize the negative social and ecological benefits caused by coal mining at the sources. The technology has the advantages of being technically feasible, economically reasona- ble and eco-friendly. The study can provide a theoretical support and case reference for the optimal timing and the coupling of the CMR.