Numerical investigation on the caving mechanism with different standard deviations of top coal block size in LTCC

The size distribution of the broken top coal blocks is an important factor, affecting the recovery ratio and the efficiency of drawing top coal in longwall top coal caving (LTCC) mining panel. The standard deviation of top coal block size (δ_t) is one of the main parameters to reflect the size distribution of top coal. To find the effect of δ_t on the caving mechanism, this study simulates experiments with 9 different δ_t by using discrete element software PFC. The δ_t is divided into two stages: uniform distribution stage (UDS) whose δ_t is less than 0.1 (Schemes 1–5), and nonuniform distribution stage (NDS) whose δ_t is more than 0.1 (Schemes 6–9). This research mainly investigates the variation of recovery ratio, drawing body shape, boundary of top coal, and contact force between particles in the two stages, respectively. The results showed that with the increasing δ_t, the recovery ratio of the panel increases first and then decreases in UDS. It is the largest in Scheme 3, which mainly increases the drawing volume at the side of starting drawing end. However, the recovery ratio decreases first and then increases quickly in NDS, and it is the largest in Scheme 9, where the drawing volume at the side of finishing drawing end are relatively higher. In UDS, the major size of top coal is basically medium, while in NDS, the size varies from medium to small, and then to large, with a distinct difference in shape and volume of the drawing body. When the major size of top coal is medium and small, the cross-section width of the initial boundary of top coal at each height is relatively small. Conversely, when the top coal size is large, the initial boundary of top coal has a larger opening range, the rotating angle of lower boundary is relatively small in the normal drawing stage, which is conducive to the development of drawing body and reduces the residual top coal, and the maximum particle velocity and the particles movement angle are both larger. This study lays a foundation for the prediction of recovery ratio, and suggests that the uniform top coal is more manageable and has a larger recovery ratio.