Influence of loose gangue thickness on top coal recovery ratio in extrathick coal seam in longwall top coal caving

This paper uses PFC 2D software to carry out DEM numerical simulations of coal caving, calculates the top coal recovery under the same coal seam thickness and different rock layer thicknesses, analyzes the influence of the rock layer thickness on the top coal caving law in two different stages, and explains the necessity of a thick rock layer and the optimal thickness of the rock layer. The results show that as the thickness of the loose gangue layer increases from 1 to 8 m, the recovery ratio of top coal increases from 84 to 96%, showing zigzag growth. The lateral diameter of the drawing body gradually increases. Due to the particularity of the coal caving method, in the subsequent coal caving stage, the uneven thickness of the top rock layer causes the direction of the drawing body to change, and the existence of a coal ridge causes the displacement field on the right side of the rock to be larger than that on the left side above the drawing opening. This leads to different sizes of the left and right secondary coal ridges. Through simple geometric relationship analysis, this paper explains the principle that gangue particles intrude into coal seams due to the weakening of rock constraints and an increase in the free movement of coal particles. This further demonstrates the necessity of a loose and thick rock layer. The optimum thickness of the rock layer and the relationship between the thickness of coal and rock and parameters of coal caving are calculated. Finally, the necessity of a thick rock layer is extended to three typical top coal caving methods to verify the universality of this necessity.