The cycle step length (CSL) is a significant parameter for longwall top-coal drawing technology that remarkably affects the top-coal recovery rate and the rock-mixing rate, especially for extra-thick coal seams. In this study, a particle-block element coupling approach is performed to investigate a reasonable CSL for extra-thick coal seams. By comparing this approach to the Bergmark-Roos analytical result, the proposed numerical model is verified, showing good performance in modeling top-coal caving. A 2-D numerical model of hydraulic support considering the mechanical behavior of the legs is established, which can be used for modeling the interaction between the hydraulic support and the top coals during the top-coal drawing process. The top-coal recovery rate, the top-coal drawing body shape, and the evolution characteristics of the coal-rock interface under different CSL conditions are compared. In addition, the mechanism of the lost top coal affected by the CSL is revealed. The results show that the CSL of top-coal drawing has a significant effect on the morphology of the coal-rock interface and the mutual invasion of coal and rock, which is the primary reason for coal loss and further affects the top-coal recovery rate and the rock-mixing rate. It is suggested that the CSL should be 0.8 m when the top-coal thickness is 12 m.

The cycle step length (CSL) is a significant parameter for longwall top-coal drawing technology that remarkably affects the top-coal recovery rate and the rock-mixing rate, especially for extra-thick coal seams. In this study, a particle-block element coupling approach is performed to investigate a reasonable CSL for extrathick coal seams. By comparing this approach to the Bergmark-Roos analytical result, the proposed numerical model is verified, showing good performance in modeling top-coal caving. A 2-D numerical model of hydraulic support considering the mechanical behavior of the legs is established, which can be used for modeling the interaction between the hydraulic support and the top coals during the top-coal drawing process. The top-coal recovery rate, the top-coal drawing body shape, and the evolution characteristics of the coal-rock interface under different CSL conditions are compared. In addition, the mechanism of the lost top coal affected by the CSL is revealed. The results show that the CSL of top-coal drawing has a significant effect on the morphology of the coal-rock interface and the mutual invasion of coal and rock, which is the primary reason for coal loss and further affects the top-coal recovery rate and the rock-mixing rate. It is suggested that the CSL should be 0.8 m when the top-coal thickness is 12 m.