Study of automated top-coal caving in extra-thick coal seams using the continuum-discontinuum element method

In order to investigate top-coal drawing regularity and automated top-coal caving technology in extra-thick coal seams, an approach coupling the particle element and the block element is presented based on the continuum-discontinuum element method (CDEM). Then, the constitutive model of hydraulic support is introduced into the CDEM to simulate the top-coal drawing process. Meanwhile, the Bergmark-Roos model for coal drawing of single support is extended into the collaborative coal drawing of multiple supports to optimize the automated top-coal caving technology. Finally, CDEM models of hydraulic support, armored face conveyer and coal-rock strata are established according to the field conditions of the "Tongxin Coal Mine". Then, the top-coal drawing mechanism is analyzed, and an automated top-coal caving technology is proposed. The results show that under the interaction between hydraulic support and surrounding rocks, the final drawing body of initial top-coal drawing appears as an irregular, deflected ellipsoid shape. During different support-moving cycles, the coal-rock structure affects the coal drawing amount and support resistance. A statistical comparison of the coal-rock drawing amount based on 100 drawing openings indicates that the standard deviation of the coal drawing amount in automated, one-round coal caving is 1.83 m(2) less than that in traditional coal caving (8.23 m(2)), and the coal drawing amount from each drawing window is more uniform. For multi-round coal caving techniques, the average recovery rate remains at 79.4% and the rock mixed rate is less than 1%. Based on a comparison of several coal caving techniques, automated four-round coal caving technology is the optimal technology, in which the change rate of the standard deviation of the coal drawing amount as well as the top-coal recovery rate are both largest.