Mining methods to overcome geotechnical problems during underground working of thick coal seams - case studies

Several mining methods have recently been tried as possible ways of overcoming a variety of geotechnical problems encountered during underground extraction of thick coal seams in the different conditions of some of India's coalfields and so increasing production, productivity and safety. Strata-control observations were recorded during extraction by mechanized longwall sub-level caving, wide-stall mining, mechanized depillaring based on cable-bolting, sub-level caving with artificial roofing and hydraulic mining. The strengths and weaknesses of these methods are discussed in the context of the geotechnical problems. Field monitoring revealed excessive load on chock shield support during the mechanized longwall sublevel caving of a thick seam. Laboratory simulation of the mining conditions in the coalfield indicated that the impact of an indirect increase of extraction height and the load of broken roof strata attributable to top-section working had to be considered in the design of support for such workings. By taking advantage of competent roof strata, an adequate depth of cover and the rapid increase of pillar strength with increase in its size the splitting and stooking method that has been used to optimize the recovery of coal from a seam developed under surface structures in a built-up area is being replaced by widening of existing galleries. This method of partial extraction, called 'wide-stall mining', provides better safety and recovery than the conventional method. In another investigation the massiveness of the coal seam played an important role during the application of cable-bolting to provide effective support to the high roof as well as the overlying coal bed. Encouraging results were obtained in a field trial of depillaring the complete thickness of a developed thick seam in one lift with the aid of full-column grouted steel cable. During underground extraction of a steeply dipping thick seam under weak and thinly laminated roof strata no strata control problems were experienced with sublevel caving with artificial roofing. A separate attempt to use hydraulic mining failed, however, as a result of mismatch between the pressure of the jet and the strength of the coal.