Top-coal initial caving law of continuous group caving method in extra-thick coal seams

In order to clarify the law of top coal caving with continuous group caving method in extra-thick coal seams, this study investigated the influences of the caving openings width on the morphological characteristics of caving body, the evolution law of the boundary of top coal and their spatial relationship based on the discrete element numerical simulation test and theoretical analysis.The results show that when the top coal thickness is 12 m, the increase of the ratio of coal caving height to caving width (δ) would cause the increase in the eccentricity of caving body.The eccentricity of caving body tends to have a steady value when the δ value increases to a certain value.In group coal caving, the increase of caving openings width would lead to higher single caving weight and caving efficiency with increasing influence range of the boundary of top coal increases and decreasing boundary slope.The length and height of tangent section between the initial boundary of top coal and the adjacent caving body increases continuously, which benefit top-coal recovery amount.The tangency between the top-coal caving body and the boundary of top coal of the previous adjacent caving group can be used as an effective method to evaluate continuous group caving.