Production challenges in underground coal mining: scenario planning and extraction strategics below surface and sub-surface properties

摘要

Coal operators are mostly posed with problems of winning coal below important surface or sub-surface properties with the previous extraction nearby. It may be noted that after restricting the extraction span (volume or void, the more general term) conceptually and having applied the engineering judgment, it is essential that the pillars left in the panel i.e. between two extraction spans or the pillars as barrier must be long-term stable, taking into consideration the combined effect of extraction in neighbouring panels. The fundamental reason is that failure of these left-out pillars may increase the extracted span more than NEW, which may jeopardize the primary objective of protecting the surface or sub-surface properties. Extraction spans are to be decided, based on non-effective width (NEW), nature of subsidence continuous or discontinuous, localised or extensive type, and study of other geo-mining parameters like depth of cover, width-to-height ratios, height and percentage of extraction, method of goaf treatment and overllying rocks of the proposed panels. In case, a large number of such proposed panels are to be worked out, the load on left-out pillars need to be estimated using numerical modeling technique only, where the proposed, neighbouring panels are to be considered, to take into the combined effects on load distributions. The present developed/to-be-developed workings are to assumed to be extracted while doing numerical simulation. The factor of safety calculation and assessment of long-term stability of left-out pillars will then be made. The pattern of extraction should therefore be finalized on case-by-case basis. Many panels in Eastern Coalfields Ltd., South Eastern Coalfields Ltd. and Bharat Coking Coal Ltd. (subsidiaries of CIL) and some panels in SCCL are being worked, after Bord & Pillar Mining and Numerical Modelling Department of CMRI has undertaken such scientific and numerical modelling exercises with rational engineering judgments. These studies have opened up whole new vistas for subsidence engineering, an detailed account of which is given in this paper.