Dynamic caving interval technology and its mechanisms for improving top coal recovery at the upward–downward transition of longwall top coal caving panel

The longwall top coal caving mining (LTCC) is one of the effective mining methods for thick coal seams in China. With the advancement of LTCC panel, upward mining, downward mining and alternate upward–downward mining always occur in the whole mining process, which has great effect on the top coal drawing law. By summarizing the motion characteristics of top coal and gangue in upward mining or downward mining, the dynamic caving interval technology for the upward–downward transition is proposed in this paper, and the variation of top coal boundary, caving body and top coal recovery at different caving stages in this technology is analyzed by discrete element numerical simulation. The results show that when the LTCC panel adopts the dynamic caving interval technology, the top coal boundary is relatively smooth and the caving body develops preferentially in vertical direction at the upward mining stage with the interval of one-cutting and one-caving. When the interval of three-cutting and one-caving is used at the downward mining stage, the intrusion of gangue can be effectively reduced toward the goaf, and the horizontal development of caving body is obvious. Compared with the conventional fixed caving interval technology, the top coal recovery of the whole panel under the dynamic caving interval technology is improved by about 3.64%. In the excessive caving process, the terminated gangue mixed ratio at downward mining stage and horizontal mining stage is low under the same high top coal recovery, at 10–15%. The research results provide a new approach for improving the top coal recovery at the upward–downward transition stage and obtain a theoretical basis for determining the closing time of caving opening in intelligent LTCC.