Blasting

“ Blasting, the method of loosening or breaking in pieces masses of rock by means of the explosion of gunpowder or dynamite. It is used in sinking shafts, cutting tunnels, building roads and railroads and clearing channels. ”

“ 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. ”

“ But to return—to cut into so hard a rock the contractors have to adopt the process of blasting, which is carried on upon a magnitude which has never been equalled. To penetrate the rocks sets of workmen, in twos, are employed in different parts of the face of the mountain, and these men drive a heading or gallery into the solid rock, about five feet high and three wide, for a distance of 30 to 40 feet, which is accomplished chiefly by blasting. ”

“ There was no one at the quarry who really knew how to blast efficiently. But then one day a man came out and showed Earl how to use electric blasting caps instead of the fuses he had been using. By drilling shallow holes, placing less explosives in each hole, and setting them off all at once, electrically, the blasting was much more efficient and a lot safer. Before that time, the custom was to set off a small blast in the bottom of a deep hole for the purpose of opening up a "pocket" large enough to hold as many as eight cans of powder and 80 sticks of dynamite. ”

“ The Roman method of blasting consisted in building a fire against the rock and pouring vinegar, or even water, upon the red-hot stone which then disintegrated. It would have been an easy matter to kindle great fires at every corner of the pedestal which, by the time this kind of destruction became popular, had already lost much of their original height through the gradual rise of the ground level. This method of blasting by fire would account for the all but universal gnawing away, or rough rounding off of the lower corners of the shaft, in which the bronze dowels were so firmly embedded. ”

“ The relative strength, bulk for bulk, is, however, of greater importance in rock blasting. This is easily computed from the foregoing table and the specific gravity of the substances, which is 1·00 for gunpowder and compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite. Compared in this way, bulk for bulk, these explosives range as follows:—
Hence, for a given height of charge in a bore-hole, gun-cotton exerts about 21⁄2 times the force of gunpowder, and dynamite about 41⁄4 times that force. ”

“ In blasting, the powder is pressed tightly about the cap and fuse, and tamping, of sand or other proper material, added, and pressed but not pounded in. A tamping firmly pressed is as good as if rammed in the most solid manner.
The fuse explodes the cap, and this explosion explodes the powder.
I will add here that by carefully packing the end of a good fuse amidst the powder of a charge enclosed, like a blasting charge, in a tight place, the fuse alone will explode the powder, especially if the powder is strongly charged with nitro-glycerine. ”

“ 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 [170] economical of labour and time. ”

“ In the shaft we were obliged to blast, as I said before. This was done by drilling holes in the rock, which were then loaded with the blasting powder, the fuse inserted, and then the hole tamped down hard and fired. The firing was done by lighting a bit of candle over the flame of which we bent the fuse. While the easing of the fuse was burning through whoever fired the shot would have plenty of time to put his foot in the hook, shake the rope and be hoisted up out of danger. ”

“ It will be observed, from a consideration of the foregoing facts, that a very large proportion of the force developed by an explosion is due to the heat liberated by the chemical reactions which take place. And hence it will plainly appear that, in the practical application of explosive agents to rock blasting, care should be taken to avoid a loss of the heat upon which the effects of the explosion manifestly so largely depend. ”

“ In most cases this necessitates the cutting down of the adjacent lots to get a frontage on the street level. The amount of explosives being used for this purpose must be very great. Blasting is constantly going on and the visitor can easily fancy himself in a beseiged town. ”

“ In the actual operations of blasting, definite rules for the proportioning of the charges are rarely observed, and although the blasts made by a skilful miner seldom fail to do their work, it is a common fault that too much, rather than too little, explosive is used. ”

“ The holes for blasting are made either by hand, with hammer and drill or jumper, or by machine drill, the latter being driven by steam, compressed air, or electricity, or, in rare cases, by hydraulic power. Drill holes ordinarily vary in diameter from 1 to 3 in., and in depth from a few inches up to 15 or 20 ft. ”

“ Better to face the difficulty, construct efficient electric apparatus, convey the electricity along wires of perfect insulation to a safe priming, and by complete and violent explosion of the Nitro-Glycerin, or powder, make such effective blasting as not to throw away the labor of drilling, candles, power, and blasting materials. I believe this the true economy. These details may seem wearisome, but the casualties of blasting can best be diminished by avoiding missed holes, a result only attainable by using materials absolutely reliable ”

“ Blasting.—In some hydraulic claims, the dirt, in dry seasons, is blasted, so as to loosen it. A drift or hole is cut into the bottom of a hill, one or two hundred feet high, and a number of kegs of powder (from twenty to two hundred) are introduced, and they are fired with a slow match. The explosion makes an earthquake in the vicinity; and the ground is loosened to such an extent that there is a great saving of labor. The breaking up of the dirt and the exposure to the air are supposed to facilitate the washing greatly. ”

“ As the pressure increases, the hot gases are forced more and more deeply into the grains, and combustion, consequently, proceeds more and more rapidly.
Detonation. —By detonation is meant the simultaneous breaking up of all the molecules of which the explosive substance is composed. Properly the term is applicable to the chemical compounds only. But it is applied to gunpowder to denote the simultaneous ignition of all the grains. The mode of firing by detonation is obviously very favourable to the rending effect required of blasting powder, since it reduces to a minimum the time of explosion. ”

“ Also in blasting operations, the explosive has to be adapted to the nature of the work, and it has been found that a substance which has worked well in driving a heading for a tunnel through one kind of rock may prove both slow in progress, and more costly in expenditure, when some different kind of rock is reached. ”

“ In thinly bedded sandstones, where vertical joints are frequent, it is often possible to separate the desired slabs and Hagstones with crowbars and wedges, without drilling or the use of explosives. When blasting is necessary, some form of gunpowder is generally used, rather than a violent explosive like dynamite, in order to avoid shattering the rock. ”

“ To deliver the blow upon a rock drill, hammers and sledges are used. The distinction between a hammer and a sledge is founded on dimensions only: the hammer being intended for use in one hand, is made comparatively light and is furnished with a short handle, while the sledge, being intended for use in both hands, is furnished with a much longer handle and is made heavier. The striking face of the blasting sledge should be flat, to enable the striker to deliver a direct blow with certainty upon the head of the drill ”

“ For it is obvious that holes cannot be angled with the same ease when a machine drill is used, as they can when the boring is executed by hand.
Boring the Shot-holes. —It has already been remarked that the exigencies of machine labour render it impracticable to follow closely the principles which lead to economy of labour and material in blasting. In hand boring, economy is gained by reducing to a minimum the number of holes and the quantity of explosive substance required. ”

“ It will be seen at once that the problem was quite different from that of boring fifteen feet under the Thames, and sometimes through watery mud. In boring through mountains the quickest way of cutting and carting away rock is one of the chief points to be considered. At the Mont Cenis Tunnel the blasting took place by driving a series of shot holes into the soil, all over the surface to be cut, filling them with explosives, and firing them simultaneously in rings. Such explosives may be fired by a time-fuse or by electricity, giving the workmen ample time to escape out of reach. ”

“ This operation is of course very dangerous, and is generally impossible unless the fire of the defence along the ditch is previously silenced. To expedite the work of the miner a gun is sometimes brought down through the gallery, and the face of the wall is shattered by its fire before the miner is “attached.” 178
CHAPTER IV.
BLASTING AND DEMOLITIONS. BLASTING. 94. Blasts are small mines used generally for breaking up rocks, or masonry in demolitions. ”

“ Moreover, as the depth of the holes is increased, so must the diameter be increased in proportion to the depth when powder is the blasting agent, but when the drilled hole is to be blasted out with Nitro-Glycerin, a diameter of 13⁄4 inches is sufficient for a hole having a depth of ten feet, and a line of least resistance of eight feet, a depth wholly inadmissible for powder, because the rock at that depth would act like the breech of a cannon, and the explosion would issue direct from the hole, only fracturing the edge, i. ”

“ And not until every such block has been removed, do they resume the boring for the second blast. Sometimes, to facilitate the removal of the rock dislodged by the shots, iron plates are laid in front of the face in a heading. The rock falling upon these plates is removed as quickly as possible, to allow the boring for the succeeding blast to commence. It is important, in the organization of work of this character, that one gang of men be not kept waiting for the completion of the labour of another. ”

“ To cause the sledge to fly off from the head of the drill in the case of a false blow being struck, and thereby to prevent it from striking the hand of the man who holds the drill, the edges of the striking face should be chamfered or bevelled down till the diameter is reduced by nearly one-half. ”

“ Shells follow us, high explosive and shrapnel. Three times I find myself within the deadly cone of bursting shrapnel: the bullets hiss into the earth about me, smashing heads and shattering feet.
We march through an inferno of smoke, from time to time obtaining a glimpse, through momentary clearings, of the village and the river running beneath the trees. But there is no truce to the shells which follow us in hundreds. ”

“ The fire is made up again into the shape before described, a stratum of fresh ore spread upon the part I, and the operation of stirring, breaking the lumps upon the workstone, and picking out the hard slags repeated, after the expiration of a few minutes, exactly in the same manner. At every stirring a fresh peat is put above the nozzle of the bellows, which divides the blast, and causes it to be distributed all over the hearth; and as it burns away into light ashes, an opening is left for the blast to issue freely into the body of the brouse. ”

“ Now, one can better make out the form of our "guests." At each shot a tuft of sulphurous white underlined in black forms sixty yards up in the air, unfolds and mottles itself, and we catch in the explosion the whistling of the charge of bullets that the yellow cloud hurls angrily to the ground. It bursts in sixfold squalls, one after another—bang, bang, bang, bang, bang, bang. It is the 77 mm. gun.
We disdain the 77 mm. shrapnel, in spite of the fact that Blesbois was killed by one of them three days ago. They nearly always burst too high. ”

“ That [Pg 113] foot bellows were employed before the system of working bellows, and producing a continuous blast by means of a water-wheel, is most probable. The foot bellows are known to most primitive people, but in Agricola's illustration of the smelting of tin none are shown. On the contrary, Æolus is represented in the corner as blowing a natural blast.
SLAG-POUNDING HOLLOWS, GOBBETTS.
The book of Agricola, published in 1556, shows that this primitive method was still in practice so late as the middle of the sixteenth century. ”

“ But the material point is the extension of the invention claimed, by the addition of claim 1 of the reissue, the words in which, 'substantially as set forth,' refer to the new matter in the reissue, that the in the original specification which indicates cooled with water, in such a manner that the tymp, or forehearth, and the wall stone can be dispensed with.' The intention manifestly was to construe the first claim so as to cover any kind of blast furnace with a closed breast, having a slagdischarge opening, cooled in any manner or to any extent by water. ”