Coal pillar extraction at deep cover: With special reference to Indian coalfields

Majority of the developed Indian coal seams are lying under strong and massive overlying strata. Depth of cover of these coal seams, standing on pillars, varies from 50 to 500 m. Present pillar extraction (depillaring) practises of the country have, predominantly, adopted intermediate mechanisation along with few fully mechanised depillaring faces. Most of these faces are operating at shallow cover. But the existing techno-economical conditions of the industry are attracting a fully mechanised depillaring system for deep seated coal pillars, which is likely to grow in near future. Here the circumstances of bord and pillar mining are to face difficult surroundings, mainly, due to rock-mass characteristics and stress conditions of the deep horizon. Although these two parameters are difficult to be pre-assessed accurately, a couple of recent experiences of depillaring at deep cover have shown their influence over efficiency and safety of the depillaring. A review of pillar extraction practises of other coal producing countries, under varying geo-mining conditions, showed importance of these parameters for the depillaring. Providing a brief review of underground coal pillar extraction scenario, this paper presents geo-technical challenges encountered during caving of massive roof strata at two deep Indian depillaring faces. These two trials experienced problems of (a) roof instability (b) barrier pillar instability and (c) pillar-spalling/floor-heaving. In absence of an effective hard roof management approach these problems could simply be dealt, up to some extent, by improvement in competency of the natural support.

Research Highlights► Indian geo-mining scenario of coal pillar extraction is provided. ► Status of bord and pillar mining for deep coal seams is reviewed. ► Inherited instability issues of natural support during depillaring of a deep seam under competent roof (high stress conditions) are discussed. ► Influence of caving of competent overlying strata during extraction of deep seated pillars is shown by case studies.