In India, Blasting Gallery (BG) and Cable Bolting (CB)-based pillar extraction methods are adopted for the single-lift underground extraction of total thickness of a thick coal seam, developed on pillars along the floor. Here thick coal seams are, first, developed with normal height pillars along the floor and then the overlying coal band is taken during retreat of the face by longhole drilling and blasting. This pattern of roof coal winning created a higher goaf, resulting in an increase in the height of pillars. In particular, barrier pillars, between exhausted panels and panels being mined, encountered considerable increases in their height. The adverse effects of the increase in the height of barrier pillars affected production, productivity and safety. Two case studies, the Cable Bolt (CB) method at the North Chirimiri Ponri Hill (NCPH) colliery in the Chirimiri area, and the Blasting Gallery (BG) method at the GDK10 Incline in the Rmamagundam area, are briefly presented in this paper to demonstrate the influence of the indirect increase in the height of pillars and the caving nature of overlying strata on the stability of the natural support.
In India, coal mining by opencast methods attracted mechanization and automation due to favourable geotechnical conditions; the method provided better production, productivity and safety compared to underground mining. The importance of underground coal mining is increasing, not only due to growing environmental concerns, but also because the mineable opencast coal reserve is becoming exhausted. However, the performance of underground coal mines of the country is relatively poor, mainly due to a lack of mechanization. Underground mining of a thick coal seam, by total extraction of the whole seam in a single pass, has always had the edge over multi-pass working, due to favourable economics and safety.