Study on the law of top coal caving considering the shape characteristics of different coal rock blocks

In the longwall top coal caving (LTCC) process, the top coal gradually breaks into irregularly shaped granular under mining pressure. Mastering the flow characteristics and drawing mechanisms of top coal blocks with different shapes is of great significance for optimizing coal drawing process parameters, improving the top coal recovery ratio, and reducing the rock mixed rate. This paper proposes a morphological gene site-directed mutagenesis technology for coal rock blocks, through which typical coal rock blocks are expanded into models with different gene mutation sites and mutation intensities. A discrete element numerical model for top coal drawing is established to systematically study the effects of different coal rock block shapes on the top coal drawbody, top coal boundary, top coal recovery ratio, and force chains. The research results indicate that the width and volume of the top coal drawbody for ideal geometric particles are greater than those for typical coal rock blocks. For all blocks, as the mutation site shifts from macro to meso to micro, the volume of the initial drawbody continuously increases, while under high mutation intensity (2.5), the volume of the initial drawbody is minimized and the top coal boundary becomes rougher. Elongated ellipsoidal, tetrahedral, rod-like, and blade-like blocks develop thicker force chains and are more prone to arch formation during coal drawing, hindering the smooth extraction of top coal. The research results provide a theoretical basis for optimizing coal drawing process parameters and reducing the probability of top-coal arching.