CHAPTER VI.
SUBAQUEOUS BLASTING.
Preparation of the Charge.
—It is essential to the success of subaqueous blasting operations, that the explosive substance used should be suitable to the conditions under which it is to be applied. This is true of all blasting, but the requirement is frequently overlooked in some of the operations that have to be performed under water. In clearing a wreck for salvage purposes, gunpowder will in most cases act more effectually than either gun-cotton or dynamite. Also, in many cases, this compound will prove more suitable than the stronger substances in removing obstructions in water-courses. Examples of this will be given hereafter. But when a wreck has to be broken up, when piles, or objects of a similar character, have to be removed, or when rocks have to be blasted, the more violent compounds will be found to accomplish the purpose much more effectively. Generally, it may be stated that when it is required merely to remove objects, gunpowder is the most suitable explosive agent to employ; and that when it is required to break objects, the nitro-cotton and the nitro-glycerine compounds are the agents whose application is likely to be attended with the greatest degree of success.
When gunpowder is used, means must be adopted to protect it from the water, since a small proportion of moisture is sufficient to lessen, in a very important degree, the force developed, while a large proportion of moisture will destroy altogether its explosive properties. It is no easy matter, under the most favourable circumstances, to keep the water from the charge; but when the depth of water is considerable, it becomes very difficult to attain that object. The pressure of a considerable “head” will force the water through substances that, without a pressure, are sufficiently impervious. At ordinary depths, metal canisters are usually employed to contain gunpowder. Old oil-cans are as good as anything for this purpose. The fuse, whether safety or electric, is passed through the cork, and the latter is luted with some waterproofing composition. The best consists of:
| Tallow | 1 | part. |
| Rosin | 3 | parts. |
| Guttapercha | 4 | parts. |
| Swedish pitch | 12 | parts. |
Instead of metal canisters, indiarubber bags are sometimes used. These are, however, more expensive than the oil-cans, and, in many cases, they are scarcely more efficient or suitable. Small charges of gunpowder may be put into short lengths of indiarubber tubing, so as to form a kind of cartridge. But care must be taken to close the ends securely. The best way is to insert a cork, or if that cannot be obtained, a cylindrical piece of wood, and to tie the tubing to this very tightly with twine. The ends should then be dipped into the luting composition described above. Tubing suitable for this purpose is sold under the designation of “blasting tubes.” For large blasts, wooden casks are the most suitable receptacle for the charge. The casks should be well tarred, or, if the depth of water be great, laid over with pitch applied very hot. Great care must be taken to protect the aperture through which the safety fuse, or the wire of the electric fuse, passes.
In blasting under water with gunpowder, only the best and strongest qualities of that compound should be used. The extra strong mining powder of the Messrs. Curtis’s and Harvey’s, commercially known as the E.S.M. powder, is, of all, the most suitable. It is also highly conducive to success to detonate the charge. If the charge be not detonated, the enclosing vessel is ruptured when only a small proportion of the number of grains have been ignited, and, consequently, a large proportion of the charge is blown away into the water unburned. Were gunpowder in blasting charges always fired by a detonation, it would compare in its effects far more favourably with the nitro-cotton and the nitro-glycerine compounds than it does under the circumstances attending the common method of firing it.
When gun-cotton is used, the difficulty of waterproofing is much lessened, but not wholly removed. Inasmuch as this compound may be detonated in the wet state, it is not required to take those precautions which are necessary in the case of gunpowder. But, as we have pointed out in a former chapter, the detonation of wet gun-cotton is effected by means of that of a small quantity of the dry substance. This quantity, which is generally employed in the form of a cylinder, and is called the “priming,” must be thoroughly protected from the water. For this purpose, indiarubber tubing may be used, or, if the primer be large, indiarubber bags. When the pressure of the water is not great, a very efficient protective covering is obtained by dipping the primer into melted paraffine. Care should be taken to avoid raising the temperature of the paraffine above the degree required to melt it completely. The primer should be placed in contact with the charge, and it is desirable that the latter, when it can be conveniently made to do so, should surround the former.
Charges of gun-cotton for subaqueous blasts are usually made up of discs of a large diameter, or of slabs of a rectangular form. When, however, the charge has to be put into a bore-hole in rock, the common cartridge is employed.
Tonite, or cotton powder, is largely used in subaqueous blasting operations. This substance is always applied in a dry state, and requires, therefore, to be protected from the water. This protection it is however, not difficult to give. Being prepared for use in a very highly compressed state, it does not readily absorb moisture. In this state, it is enclosed in cartridges, which are subsequently dipped into melted paraffine. This is the form and preparation adopted for ordinary use. For application under water, especially when the depth is considerable, additional protection is given. For wreckage purposes, tonite may be obtained in convenient charges, made up in suitable forms, and sufficiently protected.