When dynamite is used, the conditions are similar to those prevailing in the case of gun-cotton. Since nitro-glycerine is unaffected by water, no necessity exists for protecting it from moisture. But when a charge of dynamite is immersed in water, and not contained in a bore-hole, the nitro-glycerine rapidly exudes. The writer once made several ineffectual attempts to explode a charge of dynamite at a depth of 70 fathoms beneath the surface. The cause of failure was found to be this exudation; for subsequent experiments showed that, though the dynamite was in the form of the ordinary parchment paper cartridges, and was contained in a stout canvas bag, the kieselguhr retained hardly a trace of nitro-glycerine when the charge reached the surface from that depth, after being rapidly lowered and raised. Hence it becomes necessary to enclose dynamite within some fairly impervious substance, to prevent the exudation of the nitro-glycerine. Waxed linen, or fine canvas overlaid with the composition already described, may be used as a protective covering; for blasts in deep water, indiarubber bags and tubing are employed. When the charge is contained in a bore-hole in rock, exudation can hardly occur, and therefore in such cases waterproofing is unnecessary.
For firing subaqueous blasts with safety fuse, only the guttapercha covered kinds are suitable. Great care must be taken to render the junction of the fuse and the detonator water-tight. A stronger detonator is required under water than in dry ground. Electric fuses offer not only a cheaper, but a far more certain and suitable means of firing in water. This means is now very generally employed. When tension currents are used, the insulation must be very good. In all cases, ample power should be possessed by the firing machine or battery.
The shattering class of explosives are very suitable for subaqueous-rock blasting. In many cases, their employment renders the boring of shot-holes unnecessary, an advantage of obviously great importance. When detached or projecting masses of rock have to be broken up, it is sufficient to place the charges upon them. Of course, when so applied, larger quantities of the explosive are required; but though the method is wasteful of explosive, it is very economical of labour and time. Even when large undetached masses of rock have to be removed, the same method may often be successfully followed. Suppose a level surface of rock, for example. A few heavy charges judiciously distributed over this surface will blow out craters of a considerable radius, and more or less fracture the rock in their immediate neighbourhood. A few other blasts then fired between these shattered points will break up the intervening solid portions. Sometimes the rock will be disintegrated to a considerable depth, and so broken up generally that it may be removed by dredging. By proceeding in this way, the whole of the rock may often be removed without any labour of boring.
But when the rock is too tough to be removed in this way, recourse must be had to boring, though even when boring is necessary, an occasional “loose” shot may be found to be very efficacious.
Boring under Water.
—The percussive drills, one of which, the Darlington, was described in a [former chapter], may be used effectively under water. Compressed air is used as the motor fluid. The tripod stand, having its legs weighted to give it stability, is generally the most suitable support. These drills need the immediate attention of a diver. Sometimes the boring is carried on by hand from the deck of a vessel or from a raft provided for the purpose. The following description will give a general notion of the operations involved in subaqueous boring:—
The working vessel having been moored over the rock by means of mooring-lines attached to buoys placed about 50 yards from each quarter of the vessel, the diver descends and selects the most suitable position for the blast; he then signals, by a certain number of pulls upon his signal line, to have the drill and stand lowered to him. This being quickly done by means of a steam derrick, he guides the drill-stand to its place, and finally fixes it in position by means of its adjustable legs. This being done, he signals for air to commence drilling.
It has been found that the drill can be worked in a rapid current as well as in slack water. This allows the operations of drilling and blasting, by a proper division of time and labour, to be conducted in an extremely rapid tidal current, so that the principal work of the diver, in inserting charges for blasting and slinging stone, may be done near the periods of slack water, while the drilling may be advantageously continued during the period of rapid flow. In a rapid current, the stoppage of the drill for the purpose of “spooning out” the hole becomes unnecessary, as the motion of the drill works up the débris to the mouth of the hole, whence it is sucked out and carried off by the current in a dark stream, like the smoke from the funnel of a locomotive. In a sluggish current, or during slack water, the hose of the air-pump is sometimes introduced, and air forced into the bore-hole to create a current of water, by which means the hole is cleared more thoroughly than by the most careful “spooning out.”
As soon as the hole is drilled to the required depth, the drill is stopped; the diver then fastens the derrick chain, which is lowered to him for the purpose, to the drill-stand, and signals to hoist away, whereupon the machine is quickly hoisted on deck.
After having examined the hole and cleared away any débris remaining at the bottom, the diver comes to the surface, and taking in his hand the charge contained in a water-tight cartridge, and provided with its electric fuse to which a sufficient length of insulated wire is attached, returns with it, and inserts it into the drill hole, carefully pressing it to the bottom with a rod. The tamping, if any is used, is then inserted above the cartridge, and the diver comes up.