Optimisation of recovery from coal seams locked under surface/sub-surface features by Wide Stall Mining
During optimisation of recovery from a coal seam beneath a surface/sub-surface feature, thickness and quality of inter-burden between working horizon and surface/sub-surface feature are observed to be the most important parameters from ground movement point of view. Both parameters affect percentage of recovery from the coal seam. The application of a concept, termed as non-effective width, to control the dimensions of underground excavations for optimal recovery of coal, trapped in pillars below surface/subsurface features, is an established practice in the Indian coal industry. A higher value of non-effective width at a deeper seam provides enough width of the excavation to accommodate row/rows of the developed pillars for final extraction. At shallow cover, when the value of the safe excavation width becomes smaller than the width of a single pillar then even alternate row of pillars cannot be extracted without endangering stability of the surface/sub-surface features. In this case, the situation becomes more difficult as the chance of occurrence of discontinuous subsidence increases.
Here, conventional method, splitting and stooking of a pillar, to optimise a recovery of coal under built-up surface structures has limited scope for a multisection developed thick seam under massive immediate roof strata and shallow depth cover. Taking advantage of rapid increase of pillar strength with the increase of its size, the conventional method to optimise recovery under built up surface structure is replaced by widening of the existing galleries in a particular configuration and the method is called wide stall mining. First field trial of the method was conducted under the geomining conditions of East Bhuggatdih colliery. Successful application of this method at East Bhuggatdih colliery provided nearly 42% coal from a multisection developed 17 m thick coal seam without endangering the surface and subsurface structures. Wide stall mining with and without hydraulic sand stowing/filling has successfully been practiced under different geo-mining conditions for optimization of coal recovery.