Staggered development of a thick coal seam for full height working in a single lift by the blasting gallery method

Abstract

In line with the Coal Mines Regulations (CMR) of India, superimposed development was made along the floor for full height extraction of a thick coal seam (which had already been developed leaving pillars along the roof) in a single lift by the blasting gallery (BG) method. Depillaring of the seam induced roof failure at roadway junctions in the bottom section near the goaf edge during a major roof fall. Dilution by paring between the two close developments caused a considerable drop in width/height (w/h) ratio of the pillars, resulting in poor strength and stiffness of the natural support at the goaf edge. The presence of strong and massive overlying roof strata caused high values of the mining induced stress over the pillars facing the goaf line during the depillaring. Ultimately, the pillar extraction caused collapse ahead of the face. Based on simple ideas and the results of laboratory investigations on simulated models, conventional superimposed development of the thick seam was replaced by a staggered one for the depillaring and underwinning of the roof coal by the BG method. The field performance of this was monitored during a field trial. Extraction of a 10.5 m thick seam in a single lift by the BG method with staggered bottom section development did not cause any strata control problem, even during the major roof fall. Thus, this paper describes the philosophy of staggered development of a thick coal seam, which has already been developed along the roof horizon, for single lift working, together with the results of the laboratory and field investigations.