To remove these, large surface charges were again tried, but with the same ineffective result. Their only effect was to pile up the sand and small fragments of stone into irregular windrows on the surface of the rock. Small holes had, therefore, to be drilled at each of these points to blast them off. This occupied much more time than could reasonably have been expected; so that it was not until two months’ labour had been expended, that all the points were finally reduced to the required level.
Obstructions in Water-courses.
—The removal of obstructions from water-courses often leads to much subaqueous blasting. Trees that have fallen into the stream are most effectively broken up by charges of gunpowder fired by a detonation. The success of the operation will, however, be greatly dependent upon the judicious placing of the charges. Brickwork may also be very effectively dealt with by charges of gunpowder. But stone masonry and blocks of rock may be more effectively broken up by gun-cotton, tonite, or dynamite. For work of this character, electrical firing offers great advantages, for, besides its convenience, it allows of several charges being exploded simultaneously, a condition that is always favourable, and in many cases essential, to success.
The following highly interesting and instructive account of the removal by blasting of some obstructions in certain rivers in India is given by Lieut. A. O. Green, R.E.
Fig. 51.
Fig. 52.
[Large Fig. 52]
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He, in company with some assistants, left Calcutta for Maldah on the 8th of April, 1874, where they commenced work on the following day upon the wreck of a large county boat, which lay on the top of a tree in mid-stream, as shown in [Fig. 51]. Soundings were taken over and around this tree, which was found to be about 3 feet 6 inches in diameter at its base. The gunpowder intended to be used in these operations not having arrived, three 5-lb. charges of gun-cotton were made up; it was thought that under the 15-feet head of water, these would have been sufficient to break the tree in half. The gun-cotton was in the form of compressed discs, 21⁄2 inches in diameter, and 2 inches thick, each disc weighing about 5 ounces. These discs were filled into a tin cylinder to within about 4 inches of the top. An electric fuse, with wires attached, having been securely pushed into the hole in the centre of the top disc, the empty space above was filled up, with first a layer of sawdust, and then a layer of plastic clay, well rammed. The whole was then painted over, and the upper end tied up in a covering of waxed cloth, the holes through which the fuse wires passed being carefully luted. The charge, thus made up, is shown in section in [Fig. 52]. It was fired by means of a dynamo-electric machine.