Manual opencast working in coal
The quarriable area is divided into sections along the strike so that overburden extraction takes place in some sections arid coal extraction in others which have the coal already exposed after 'overburden removal. Small pillars of the rocks excavated are left for measurement. These are called witnesses or 'Sakhi' in Hindi. They are removed after measurement' of excavated area, usually once a week, is over. The height of such 'sakhi' should not exceed 2.5 m and where the height of such pillar exceeds 1.25 m its base should not be less than 1.5 m in diameter.
Removal of overburden:
The soft material like earth and weathered rock is cut by earth cutting picks. A team of workers consists of 3 or 4 members, one cutting and two loading. As female workers are allowed in quarries, a team of 3 workers usually includes one or two female members who are normally permitted to work only between 5 a.m. and 7 p.m. under the Mines Act. Each team is allowed a small plot usually 4.5 m x 4.5 m. The average output per worker in soft rock or earth is about 2.8 m3 (in situ) per day.
In the hard rocks which need blasting holes are drilled
(a) manually with the help of hexagonal steel rod with chisel end or
(b) by compressed air hammer drill (jack hammer).
Method (a) is used in a small quarry where the output is only 50 to 100 te of coal per day, electricity or compressed air is not available and labour is cheap. A hole is 1.2 m to 1.5 m deep and one man can dri-11 5 to 8 holes, each 1.2 m deep in a shift (8 hrs.) Method (b) is now-a-days commonly employed. Compressed air is supplied by steel pipes 50 to 100 mm dia, up to central places and branch pipes supply air to drills through hose pipes. Two workers (one driller-and one helper) drill 40 to 50 holes, each 1.5 m deep, in one shift. The holes one placed 1.2 to 1.5 m apart and are blasted with gun powder or other suitable explosives. Blasting is done during the rest interval of the workers to prevent frequent interruption of work.
The overburden is loaded into tipping tubs (0.73 m3 capacity) which are hauled by direct or endless haulages. In seams of mild gradients locomotives may be used. The haulage track is taken to each bench or alternate benches. Blasted overburden of higher bench is sometimes dropped on the lower bench for loading into tubs.
Overburden is dumped at the rise of the outcrop or beyond the quarriable limit but as the coal extraction proceeds, the overburden may be dumped in the area from which coal is extracted— an operation known as back filling. The overburden should be so dumped that it does not roll down at the coal benches when it assumes its angle of repose, nor should it choke water courses of damage paddy fields, other agricultural area or water reservoirs.
Extraction of Coal:
The coal which is exposed after removal of overburden is blasted after drilling holes. The same drills which are used for stone may be utilised for coal also but if the compressor has a limited capacity electrically operated drills are used for coal due to their relative lightness and better performance. The spacing of holes in coal is 1.5 m to 2.2 m, the depth varying from 1,2 m to 2 m. The coal available per kg of high explosive like special gelatine (60 to 80%) is generally 10-12 tonnes. With blasted coal, the average loading performance per loader is nearly 4 tubs (1.1 m3 capacity).
Fig. 5.1 shows the layout of a large manual quarry, after it has advanced some distance along the dip. The entrance to the working places is by steps and inclined roads. The permanent installations like haulages, compressors, power transmission lines, etc. are installed in such places that their frequent shifting is not necessary when the stone benches or coal benches advance.
On a level track hand pushing of empty tubs having pedestal bearings is not uncommon for a distance of 100 m. If the same track is used for loads as well as empties, the gradient of the track should be nearly 1 in 80 in favour of loads. These factors limit the length of benches.
The property is divided into blocks, each 120 to 180 m long along the strike. Each block has a direct or endless haulage in the middle and the benches are formed along the strike on either side of the haulage. Block A which is shown more advanced than Block B, raises coal from the coal benches CBl, CB2, CB3, CB4. Each bench is 3 m high and has a haulage track on its floor. From the clipping point PI to the junction P2 the haulage track is either level or slightly rising inbye at 1 in 80 or so. The stone benches SB1, SB2, are ahead of the coal benches so that the exposed coal lasts for 2—4 weeks. Stone from the higher bench SB2, is dropped on the lower bench SB1 which has haulage tracks on its floor. The stone is taken in tipping tubs along a level track TA by hand pushing for dumping in the de-coaled area. The track TA is along the barrier. The tubs cross the direct haulage track over a bridge K, to fully utilise the decoaled area for dumping. As the stone benches advance the position of the bridge Kt has to be shifted to the dip side. Another level haulage track is taken from the stone bench SB, to the decoaled area over a ledge LL. This ledge is 3m wide at the top and is a solid barrier of coal and stone left between blocks A and B. The coal of the ledge is not recoverable.
In block B, there is emphasis on removal of overburden. The direct haulage track is along the floor of the coal seam and more stone benches than in block A are provided for employment of a large number of workers on overburden removal. The stone benches SB5 SB6 SB7 SB8 are served by the central direct haulage. Stone benches SB3 and SB4 are worked by a level track TB and another level track passing over the ledge LL1, Stone is taken over these tracks in tipping tubs for dumping in the decoaled area in the same manner as described for block A. The bridge K2 serves the same purpose as bridge K, in block A, and is advanced towards the dip at intervals.
During rainy season coal raising may be suspended in block A which is on the dip side, and only overburden removal may take place from the higher stone benches which are free from water. From block B, only coal raising may take place during monsoon and the block can be kept free from water by making a through connection 1.8 m high in the ledge LL1 for the water to gravitate to block A.
Drains for water, as shown in the figure, called garland drains are cut to minimise inflow of surface water into the quarry. Pumps are installed at possible places of heavy accumulation of water. The installation of compressed air pipes and the overhead power lines is as shown in the figure.
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