Rock Blasting / A Practical Treatise on the Means Employed in Blasting Rocks for Industrial Purposes

by George G. André

Boring the Shot-holes.

—The surface of the rock having been prepared to receive the drill, one man sits down, and placing the shortest drill between his knees, holds it vertically, with both hands. The other man, who stands opposite, if possible, then strikes the drill upon the head with the sledge, lightly at first, but more heavily when the tool has fairly entered the rock. The man who holds the drill raises it a little after each blow, and turns it partly round, the degree of turn usually given being about one-eighth of a revolution. By this means, the hole is kept circular, and the cutting edge of the drill is prevented from falling twice in the same place. To keep the tool cool, and to convert the dust and chippings into sludge, the hole is kept partially filled with water, whenever it is inclined downwards. For this reason, downward holes are sometimes described as “wet” holes, and upward holes as “dry” holes. The presence of water greatly facilitates the work of boring. It has been found by experience that the rate of boring in a dry and in a wet hole varies as 1 : 1·5; that is, it takes one and a half times as long to bore a dry hole as to bore a wet hole. Thus, by using water, the time may be reduced by one-third. To prevent the water from spurting out at each stroke and splashing the man who holds the drill, a kind of leathern washer is placed upon the drill immediately above the hole, or a band of straw is tied round it. When the hole has become too deep for the short drill, the next length is substituted for it, which is in its turn replaced by the third or longest drill as the depth becomes greater. Each drill, on the completion of the length of hole for which it is intended, is sent away to the smithy to be re-sharpened. In very hard rock, the drills may have to be frequently changed, a circumstance that renders it necessary to have several of the same length at hand. The depth of shot-holes varies from 1 foot to 10 feet, according to the nature of the rock, the character of the excavation, and the strength of the explosive to be used. In shafts and in headings, the depth varies generally between 2 feet 6 inches and 4 feet, a common depth being 3 feet.

The débris which accumulates at the bottom of the hole must be removed from time to time to keep the rock exposed to the edge of the drill. The removal of this sludge is effected by means of the tool called a “scraper.” If the sludge is in too liquid a state to allow of its ready removal by this means, a few handfuls of dust are thrown in to render the mass more viscous. The importance of keeping the bore-hole clear of sludge, and of shortening the time expended in using the scraper, has led, in some localities, to the adoption of means for rendering the sludge sufficiently viscous to adhere to the drill. When in this state, the sludge accumulates around the tool rather than beneath it, the fresh portion formed pushing the mass upward till it forms a thick coating upon the drill throughout a length of several inches. When the tool is withdrawn from the hole, this mass of débris is withdrawn with it; in this way, the employment of a scraper is rendered unnecessary. This mode of clearing the bore-hole is commonly adopted by the Hartz miners, who use slaked lime for the purpose. This lime they reduce to the consistency of thick paste by the addition of water, and they store it, covered with water, in a small tin box, which they carry with them to their work. To use this paste, they take a piece about the size of a walnut, dilute it with water, and pour it into the bore-hole. This lime paste is, for the purpose intended, very effective in friable rock, especially if it be of a granular structure, as sandstone. As the grains of sand resulting from the trituration of such rocks have no more tendency to adhere to each other than to the drill, each of them becomes covered with a coating of lime, which causes them to agglutinate into a viscous mass possessing sufficient adhesiveness to enable it to cling to the tool in the manner described.

When the hole has been bored to the required depth, it is prepared for the reception of the charge. The sludge is all carefully scraped out to clear the hole, and to render it as dry as possible. This is necessary in all cases; but the subsequent operations will be determined by the nature of the explosive, and the manner in which it is to be used. If black powder be employed in a loose state, the hole must be dried. This is done by passing a piece of rag, tow, or a wisp of hay, through the eye of the scraper and forcing it slowly up and down the hole, to absorb the moisture. If water is likely to flow into the hole from the top, a little dam of clay is made round the hole to keep it back. When water finds its way into the hole through crevices, claying by means of the “bull” must be resorted to. In such cases, however, it is far more economical of time and powder to employ the latter in waterproof cartridges. Indeed, excepting a few cases that occur in quarrying, gunpowder should always be applied in this way. For not only is a notable saving of time effected by avoiding the operations of drying the hole, but the weakening of the charge occasioned by a large proportion of the grains being in contact with moist rock is prevented. But besides these advantages, the cartridge offers security from accident, prevents waste, and affords a convenient means of handling the explosive. It may be inserted as easily into upward as into downward holes, and it allows none of the powder to be lost against the sides of the hole, or by spilling outside. These numerous and great advantages are leading to the general adoption of the cartridge.

Charging the Shot-holes.

—When the hole is ready to receive the explosive, the operations of charging are commenced. If the powder be used loose, the required quantity is poured down the hole, care being taken to prevent the grains from touching and sticking to the sides of the hole. This precaution is important, since not only is the force of the grains so lodged lost, but they might be the cause of a premature explosion. As it is difficult to prevent contact with the sides when the hole is vertical, and impossible when it is inclined, recourse is had to a tin or a copper tube. This tube is rested upon the bottom of the hole, and the powder is poured in at the upper end; when the tube is raised, the powder is left at the bottom of the hole. In horizontal holes, the powder is put in by means of a kind of spoon. In holes that are inclined upwards, loose powder cannot be used. When the powder is used in cartridges, the cartridge is inserted into the hole and pushed to the bottom with a wooden rammer.

If the charge is to be fired by means of a squib, a pointed metal rod, preferably of bronze, of small diameter, called a “pricker,” is placed against the side of the bore-hole, with its lower pointed end in the charge. The tamping is then put in, in small portions at a time, and firmly pressed down with the tamping iron, the latter being so held that the pricker lies in the groove. The nature of tamping has been already fully described. When the tamping is completed, the pricker is withdrawn, leaving a small circular passage through the tamping down to the charge. Care must be taken in withdrawing the pricker not to loosen the tamping, so as to close up this passage. A squib is then placed in the hole thus left, and the charge is ready for firing.

If the charge is to be fired by means of safety fuse, a piece sufficiently long to project a few inches from the hole is cut off and placed in the hole in the same position as the pricker. When the powder is in cartridges, the end of the fuse is inserted into the cartridge before the latter is pushed into the bore-hole. The fuse is held in its position during the operation of tamping by a lump of clay placed upon the end which projects from the hole, this end being turned over upon the rock. The tamping is effected in precisely the same manner as when the pricker is used.

If the charge is to be fired by electricity, the fuse is inserted into the charge, and the wires are treated in the same way as the safety fuse. When the tamping is completed, the wires are connected for firing in the manner described in a former chapter.

In all cases, before tamping a gunpowder charge placed loose in the hole, a wad of tow, hay, turf, or paper is placed over the powder previously to putting in the tamping. If the powder is in cartridges, a pellet of plastic clay is gently forced down upon the charge. Heavy blows of the tamping iron are to be avoided until five or six inches of tamping have been put in.