Safe feasibility of retaining sand-proof coal rock pillars in full-mechanized caving mining of extra-thick coal seam

To solve the problem of increasing the mining upper limit by reasonably setting sand-proof coal rock pillars in an extra-thick coal seam under a thick loose stratum and thin bedrock of Yangcun Coal Mine in Yanzhou Mining Area, the geological and hydrogeological conditions of working face were systematically analyzed by taking 325 Working Face in No. 3 coal mining area of the mine as the background and combining with the mining experience of its adjacent working faces. The mining grade under the water body was determined to be grade II, consistent with the conditions for retaining sand-proof coal rock pillars on the roof of coal seam. On this basis, to avoid the limitation of a single method, this paper adopts the key stratum theory, empirical formula analogy, and numerical simulation; the heights of the maximum caved zone (CZ) and the water-conducting fractured zone (WCFZ) were determined as 32.8 m and 82.0 m, respectively. The thickness of protective stratum is 16.4 m; the height of sand-proof coal rock pillars in 325 Working Face is 49.2 m; the minimum thickness of bedrock pillar in 325 Working Face is 53.1 m. Thus, full-mechanized caving mining can be carried out according to the reservation of sand-proof coal rock pillars. The comprehensive application of various methods can effectively improve the reliability and accuracy of the calculation results. The research results provide the main reference for this working face and similar working faces in Yanzhou Mining Area in the future to increase the mining upper limit and set up safe coal rock pillars according to roof sand control. © 2021, Saudi Society for Geosciences.