Report: APP CMHS Project 1

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3.4.6. Steep Seam Mining

In the north eastern part of India there exists tertiary coal deposits in excess of 945 million tonnes of proven, indicated and inferred reserves of soft, friable, sulphur 1.5-4 %, low ash 4 % - 25 % and good caking properties. There are three underground and two opencast mines operated in this area by the state owned North Eastern Coal fields (NEC). The NEC operates from Margherita area in the coal filed is named as Makum coal field having two seams named the 20 ft seam and the 60 ft seam with gradients ranging from 25° to 80° to the horizontal and having mineable reserves of 30 million tonnes. All the underground coal mines are degree-III gassy mines (methane gas emission of > 10 m3 per tonne of coal produced) and are highly prone to spon com and having very soft roof and floor.

The permanent drivages in the mine are driven in the hard strata either in the foot wall or the hanging wall and the access to the coal seam is made by cross measure drifts with pick mining heavily supported by wooden supports. The present method of extraction of the highly steep seams is the Tipong method in two of the mines with gradients 40°-50°, and the scraper assisted Chamber method in one mine with gradients of 30°-35°.

The Tipong method was derived from the earlier Bhaska method of working and is considered to be safer and yields more extraction. In the Bhaska method the miners had to work under unsupported roof. Other methods like flexible roofing method and descending shield method are also practiced but with limited success. In the Tipong method, levels were driven along the middle section of the seam 18 m apart and with rises to the dip connections made every 9 m. Two more sub levels were driven between the two levels. The development and the caving were done simultaneously. The rise gallery between the levels is approached from the bottom sublevel and heightened and widened in the shape of a funnel with a loading chute at the bottom. The gallery at the upper sub level is widened to the full thickness of the seam. Drilling and blasting is done in the sublevels to break the coal in the hanging wall and the foot wall. The schematic diagram of the method of working is explained in Figure 53.

Figure 53 Schematic of Tipong method

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