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The various explosives and accessories employed for well sinking are manufactured by Indian Explosives Ltd., Gomia.
Explosives used for well sinking must have the following properties:
(i) High Density:
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Comparatively smaller holes are drilled in well sinking and it is important that the charges should be concentrated at the bottom of the holes.
(ii) High Water Resistance:
Since the wells are sunk in water bearing strata, it is essential that the explosive has good water resistance. Special Gelatine 80% will be usually effective, but if the strata is particularly hard, special Gelatine 90% strength may be used. Special Gelatines are supplied in 25 mm dia × 200 mm length cartridges packed in cases containing 25 kg net explosives.
Either electric or safety fuse shot firing can be employed for well sinking. Plain detonators, comprising a metal tube closed at one end, filled with a powerful explosive consolidated under pressure, are used in conjunction with safety fuse, the ‘spit’ from the fuse causing the detonator to explode. The safety fuse consists of a thin core of black powder wrapped in layers of jute textiles and waterproof coatings. On lighting, it burns at controlled rate from one end to the other. The types of safety fuse recommended are given in Table 11.1.
Other accessories for preparation of charges and stemming the short holes, are crimper, pricker, scraper and stemming rod.
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In electric shot firing, an electric detonator is used. On passage of electric current through the fuse head coupled to a pair of leading wires, a flash is produced which ignites the detonator charge. Instantaneous electric detonators fire instantaneously on application of current. Short delay detonators with a nominal delay interval of 25 minuntes and long delay detonators with a nominal delay interval of 300 min are available in delay numbers from 0 to 10.
Other accessories in electric shot firing are exploder (Beethoven Dynamo Condenser exploder capable of firing up to 100 shots is commonly used). Ohm-meter (manufactured by Gyro Laboratories Ltd. Bombay), shot firing cable (a well insulated 2-core cable about 300 m long, each core consisting of a conductor of at least four copper wires of not less than 0.46 mm diameter), stemming rod, scraper and pricker may be used.
1. Delay Blasting:
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The usual technique in well sinking is to drill concentric rings of holes. The inner ring known as the ‘sumper’ or the ‘cut’ is drilled so that the bottom of the holes almost meet at the centre, thus forming a cone which is blasted first and provides a free face for the subsequent rings. The cavity is enlarged further by the subsequent rings of holes around it spaced at 0.6 to 0.9 m.
The last ring holes or ‘trimmers’ at the side of the well should be drilled with slightly smaller burden and should be more closely spaced than in the preceding rings. By this means the over-break can be reduced and there will be less likelihood of the wall being damaged. The placement of holes for a well of 6 m diameter is shown in Fig. 11.14. Short delay detonators of 1, 2, 3 and 4 delay numbers are used respectively in the round, with the approximate charges of explosives as shown in Fig. 11.14.
The charges are initiated inversely, i.e., the primer cartridge is placed at the bottom of the hole with detonator pointing towards the main charge. This reduces the chances of a misfire due to cut-off. The depth of pull depends upon the diameter of the well, and is restricted to a maximum of half the diameter. The minimum angle of the cut holes cone should be 45°.
The blasting ratio generally varies between 0.05 and 0.2 m3 of solid rock broken per N of explosives used, depending on the nature of the rock and the size of the wells. The smaller the well, the higher is the charge of explosive required per cubic meter of rock blasted. If instantaneous electric detonators are used to initiate the charges, the various rings of holes will have to be blasted separately.
2. Instantaneous Blasting:
In narrow wells, say 3 m or less in diameter, short vertical holes (1 m depth) are drilled at a regular spacing of 0.5-0.6 m only and charged to within 0.15 m of the collar. All the holes in a round are fired simultaneously. The spacing between the peripheral holes may be reduced to 0.3-0.4 m for proper breakage.
When well sinking is carried out in built up areas or in the vicinity of buildings, vibrations can be minimised by reducing the amount of charge fired at a time, and also by using delay detonators. Flying debris can be controlled by covering the mouth of the well by strong wire nets weighted down by sand bags.
Thus with a gadget of RB units which comprises air compressors, pneumatic hammer or rock drills, portable magazines and detonators, and a few debris removers, it is possible to deepen the wells quickly and revitalise them for agricultural or drinking water purposes. The diameter of agricultural wells generally varies from 3 to 7.5 m, whereas that of drinking water wells ranges between 1.8 and 6 m only. The wells are deepened to attain a depth of about 12-15 m.
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