The speed with which holes may be sunk varies of course with the hardness of the rock and the diameter of the hole. At Holyhead the average work done by three men in hard quartz rock with 11⁄2-inch drills was 14 inches an hour; one man holding the drill, and two striking. In granite of good quality, it has been ascertained by experience that three men are able to sink with a 3-inch jumper 4 feet in a day; with a 21⁄2-inch jumper, 5 feet; with a 21⁄4-inch, 6 feet; with a 2-inch, 8 feet; and with a 13⁄4-inch, 12 feet. A strong man with a 1-inch jumper will bore 8 feet in a day. The weight of the hammers used with drills is a matter deserving attention; for if too heavy they fatigue the men, and consequently fewer blows are given and the effect produced lessened; while, on the other hand, if too light, the strength of the workman is not fully employed. The usual weight is from 5 to 7 lb.
As the labour of boring a shot-hole in a given kind of rock is dependent on the diameter, it is obviously desirable to make the hole as small as possible, due regard being had to the size of the charge; for it must be borne in mind in determining the diameter of the boring that the charge should not occupy a great length in it. Various expedients have been resorted to for the purpose of enlarging the hole at the bottom so as to form a chamber for the powder. If this could be easily effected, such a mode of placing the charge would be highly advantageous, as a very small bore-hole would be sufficient, and the difficulties of tamping much lessened. One of these expedients is to place a small charge at the bottom of the bore and to fire it after being properly tamped. The charge being insufficient to cause fracture, the parts in immediate contact with it are compressed and crushed to dust, and the cavity is thereby enlarged. The proper charge may then be inserted in the chamber thus formed by boring through the tamping. Another method, applicable chiefly to calcareous rock, has been tried with satisfactory results at Marseilles. When the bore-hole has been sunk to the required depth, a copper pipe, [Fig. 22], of a diameter to fit the bore loosely, is introduced, the end A reaching to the bottom of the hole, which is closed up tight at B with clay so that no air may escape. The pipe is provided with a bent neck C. A small leaden pipe e, about half an inch in diameter, with a funnel f at the top, is introduced into the copper pipe at D and passed down to within about an inch of the bottom. The annular space between the leaden and copper pipes at g is filled with a packing of hemp. Dilute nitric acid is then poured through the funnel and leaden pipe. The acid dissolves the calcareous rock at the bottom, causing an effervescence, and a substance containing the dissolved lime is forced out of the orifice C. This process is continued until from the quantity of acid consumed it is judged that the chamber is sufficiently enlarged. Other acids, such as muriatic or sulphuric, will produce the same effects, but the result of the chemical solution will of course depend upon the nature of the stone.
After the shot-hole has been bored, it is cleaned out and dried with a wisp of hay, and the powder poured down; or, when the hole is not vertical, pushed in with a wooden rammer. The quantity of powder should always be determined by weight. One pound, when loosely poured out, will occupy about 30 cubic inches, and 1 cubic foot weighs 57 pounds. A hole 1 inch in diameter will therefore contain ·414 ounce for every inch of depth. Hence to find the weight of powder to an inch of depth in any given hole, we have only to multiply ·414 ounce by the square of the diameter of the hole in inches, and we are enabled to determine either the length of hole for a given charge, or the charge in a given space. It is important to use strong powder in blasting operations, because, as a smaller quantity will be sufficient, it will occupy less space, and thereby save labour in boring.
Figs. 23-25.
When the hole is in wet stone, means must be provided for keeping the powder dry. For this purpose, tin cartridges are sometimes used. These are tin cylinders of suitable dimensions, fitted with a small tin stem through which the powder is ignited. The effect of the powder is, however, much lessened by the use of these tin cases. Generally a paper cartridge, well greased to prevent the water from penetrating, will give far more satisfactory results. When the paper shot is used, the hole should, previous to the insertion of the charge, be partially filled with stiff clay, and a round iron bar, called a clay-iron or bull, [Figs. 24, 25], driven down to force the clay into the interstices of the rock through which the water enters. By this means the hole will be kept comparatively dry. The bull is withdrawn by placing a bar through the eye near the top of the former, provided for that purpose, and lifting it straight out. The cartridge is placed upon the point of a pricker and pushed down the hole. The pricker, shown in [Fig. 23], is a taper piece of metal, usually of copper to prevent accidents, pointed at one end and having a ring at the other. When the cartridge has been placed in its position by this means, a little oakum is laid over it, and a Bickford fuse inserted. This fuse is inexpensive, very certain in its effects, not easily injured by tamping, and is unaffected by moisture. The No. 8 fuse is preferred for wet ground; and when it is required to fire the charge from the bottom in deep holes, No. 18 is the most suitable.
When the line of least resistance has been decided upon, care must be taken that it remains the line of least resistance; for if the space in bore-hole is not properly filled, the elastic gases may find an easier vent in that direction than in any other. The materials employed to fill this space are, when so applied, called tamping, and they consist of the chips and dust from the sinking, sand, well-dried clay, or broken brick or stones. Various opinions are held concerning the relative value of these materials as tamping. Sand offers very great resistance from the friction of the particles amongst themselves and against the sides of the bore-hole; it may be easily applied by pouring it in, and is always readily obtainable. Clay, if thoroughly baked, offers a somewhat greater resistance than sand, and, where readily procurable, may be advantageously employed. Broken stone is much inferior to either of these substances in resisting power. The favour in which it is held by sinkers and quarrymen, and the frequent use they make of it as tamping, must be attributed to the fact of its being always ready to hand, rather than to any excellent results obtained from its use. The tamping is forced down with a stemmer or tamping bar similar to [Figs. 26, 27], too frequently made of iron, but which should be either of copper or bronze. The tamping end of the bar is grooved on one side, to admit of its clearing the pricker, or the fuse, lying along the side of the hole. The other end is left plain for the hand or for being struck with a hammer.