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 overcom ing a variety of geotechnical problems encountered during underground extra ction of thick coal seams in the different conditions of some of India's co alfields and so increasing production, productivity and safety. Strata-cont rol observations were recorded during extraction by mechanized longwall sub -level caving, wide-stall mining, mechanized depillaring based on cable-bol ting, sub-level caving with artificial roofing and hydraulic mining. The st rengths 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 a ttributable to top-section working had to be considered in the design of su pport for such workings. By taking advantage of competent roof strata, an adequate depth of cover an d the rapid increase of pillar strength with increase in its size the split ting and stooking method that has been used to optimize the recovery of coa l from a seam developed under surface structures in a built-up area is bein g replaced by widening of existing galleries. This method of partial extrac tion, called 'wide-stall mining', provides better safety and recovery than the conventional method. In another investigation the massiveness of the coal seam played an importa nt role during the application of cable-bolting to provide effective suppor t to the high roof as well as the overlying coal bed. Encouraging results w ere obtained in a field trial of depillaring the complete thickness of a de veloped thick seam in one lift with the aid of full-column grouted steel ca ble. During underground extraction of a steeply dipping thick seam under weak an d thinly laminated roof strata no strata control problems were experienced with sublevel caving with artificial roofing. A separate attempt to use hyd raulic mining failed, however, as a result of mismatch between the pressure of the jet and the strength of the coal.