Extraction of contiguous coal seams

Abstract

Underground mining methods are still used to excavate large proportions of coal in India. This accounts for up to 70 % of the total coal produced. The opencast is only profitable up to a certain depth; below that underground mining is the only profitable alternative. So more effective underground mining methods are required to be searched for and presented. When two seams are above one another, the workings in one tend to affect the working in another. In India coal seams that are near to one another such that the parting thickness between them is below 9 m are called contiguous. Coal mine regulation number 104 of Coal Mine Regulations, 1957 apply to such seams. The objective of this project was to evaluate the effect of parting thickness and depth cover of coal seams on stress distribution over pillars, stooks and ribs at different stages of depillaring, through numerical modelling using FLAC 2D software. Contagious seams can be extracted in three possible ways, viz, First extraction in the upper seam then followed by the lower seam, first extraction in the lower seam then followed by extraction in upper seam or simultaneous extraction of both the seams. In India such seams are extracted preferably by caving or stowing. For such seams when extracted the pillars of one seam shall be vertically above or bellow the pillars of other seam. In the example taken under consideration in this project, the two seams contiguous to one another were generated. Their depth cover (from 150 m to 750 m) and parting thickness (from 3 m to 9 m) were varied and the stress distribution over pillars, stooks, ribs and parting were evaluated. The evaluation was carried at two stages of mining, i.e. at development stage and after extraction of two and a half pillars with a single rib left. The maximum stress in the pillar increased with increase in parting thickness and depth cover. For stooks change in parting thickness was found to have no effects, while it increased with increase in depth cover. Maximum stress in ribs were found to be increasing with increase in parting thickness for top seam but remained more or less constant for bottom seam. While with increase in depth cover the maximum stress was found to be increasing first then decreasing. It can be inferred that the ribs may have yielded for larger depths.