Underground extraction of thick and difficult coal seams
Multi-slice working of a thick coal seam is a normal choice to reduce operational constraints and optimize recovery and safety. Orientation of a slice is controlled by seam gradient and massiveness of the coal seam while thickness of the slice is mainly dependent upon the quality and dimension of the available roof support system. The order of slices to win a complete thickness is decided according to the local geo-mining conditions of the site. Slices in ascending order (from bottom to top) require stowing/filling of the void created due to mining which decreases productivity and becomes expensive. Slices in descending order (from top to bottom) experience strata control problems because the working in the lower slices are to be carried out directly below the broken overlying strata due to top slice working. Parting plane controlled caving of the massive overlying roof strata also creates problems during slicing of a thick coal seam in descending order. Even under weak and laminated roof strata, slices in descending order require a technology/system (generally artificial roofing) to differentiate the caved rock piles from the excavated coal.
Single lift working of a thick coal seam has always an edge over the multi-slice working due to favourable economics and high production and productivity. The suitable geo-mining conditions of Europe and many other countries have led to the wide range application of single lift working by sublevel caving of thick coal seams. Unfortunately, the thick coal seams of India posses a relatively higher strength, which restricts the scope of application of single lift working by mechanised longwall sublevel caving. Large capital investment, presence of massive roof strata (difficult for caving) and shallow depth cover are other major problems associated with introduction of mechanised longwall sublevel caving faces for underground thick seam working in India. In fact, a number of prestigious projects including longwall (powered support) face of Charcha colliery (technically supported by UK), longwall (powered support) face of Kottadih colliery (technically supported by Cdf, France) etc. failed due to the single technical reason: the presence of massive overlying roof strata. The coal measure formations of India and Australia are similar as both belong to Lower Gondwana. Australian coal mining industry have achieved great success in mechanised coal production while, in India, the strata control and rock mechanics problems created havoc for the mechanisation in the beginning itself. A review of the support capacity applied to the longwall faces of the two countries shows that the average support capacity of an Australian longwall face is almost double to that of an Indian longwall face. The existing difficulties in designing support capacity for Indian longwall faces will further increase for mechanised longwall sublevel caving of a thick coal seam due to increase in extraction height.
As discussed above, the inherited geotechnical problems for underground exploitation of a thick coal seam becomes even worse under the Indian geo-mining conditions mainly due to the presence of roof strata caveable with difficulty. On the other hand, the production target is being increased every year in the country under the thrust of increasing coal demand. Under the condition, it is practically difficult to ignore the potential of underground mining of thick coal seams. Therefore different successful indigenous attempts are made to overcome the technical problems of underground mining of thick coal seams in the interest of production, productivity and safety.
Thick coal seam deposits of India may broadly be divided into five categories; (a) virgin thick coal seams, (b) developed thick coal seams under protected surface features, (c) developed thick coal seams under caveable roof/surface, (d) highly inclined thick coal seams under weak roof strata and (e) thick coal seams of friable nature. The department conducted extensive field and laboratory investigations for development of suitable mining method for all these geo-mining conditions which indicated that the impact of indirect increase of extraction height, stiffness of roof strata, stiffness of natural support, in situ stress condition and depth cover are important parameters to be considered for the design. Both massiveness and hardness of thick coal seams of the country circumscribe the scope of underwinning of roof coal band through natural gravitational flow during working of a thick seam along the floor horizon. Under these conditions, induced caving of the roof coal band is done during depillaring of the thick seams.