Study of top-coal flow mechanism under interval and multi-grouping top-coal caving technology in extra thick coal seam

Fully-mechanized top-coal caving mining method has become the main high-yielding and high-efficiency mining method in China’s extra-thick coal seam,but it still faces the problems of low coal production rate and high mixed gangue ratio. The longwall working face No. 8222 of Tashan Coal Mine with a coal seam average thickness of 12 m,adopts a novel "large-middle-small-micro" interval top-coal caving method. In order to study the mechanism of the caving method and improve the caving efficiency,and to provide the basis for the development of intelligent caving control program,this study performed CDEM software and theoretical analysis method to investigate the grouping approach,the corresponding top-coal movement law,the characteristics of coal rock interface and the top-coal caving rate,and finally revealed the effect of the number of caving openings on the caving body shape. The numerical simulation results show that under the condition of top-coal thickness being 12 m and the principle of "gangue appears,caving opening closes",the number of caving openings in the group is 9. The field test also confirms that the optimal caving opening number is 9 in a group,of which the top-coal recovery rate,mixed gangue rate and coal recovery rate are 80.53%,4.00% and 85.31%,respectively. In addition,the number of hydraulic supports in the group has a great influence on the shape of top-coal caving body. When the number of hydraulic supports in a group is small,the influence between adjacent caving hoppers in the first stage of top-coal caving is greater,and the possibility of forming abnormal top-coal in their middle is greater. In the next stage of top-coal caving,mixed gangue is easy to occur,resulting in the increase of residual coal between the adjacent caving openings. The optimal group spacing is proportional to the coal seamthickness. For the extra-thick coal seam,reasonable increase of the group spacing can effectively reduce the amount of top coal left between hydraulic supports and improve the top-coal recovery rate.Research results is of significance to provide a reference for the reasonable determination of the technology and parameters of the fully-mechanized top-coal caving in the extra thick coal seam.