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BLASTING

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Originally appearing in Volume V04, Page 45 of the 1911 Encyclopedia Britannica.
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BLASTING , the See also:

process of rending or breaking apart a solid See also:body, such as See also:rock, by exploding within it or in contact with it some explosive substance. The See also:explosion is accompanied by the sudden development of See also:gas at a high temperature and under a tension sufficiently See also:great to overcome the resistance of the enclosing body, which is thus shattered and disintegrated. Before the introduction of See also:explosives, rock was laboriously excavated by See also:hammer and See also:chisel, or by the See also:ancient process of " See also:fire-setting," i.e. See also:building a fire against the rock, which, on cooling, splits and flakes off. To hasten disintegration, See also:water was often applied to the heated rock, the loosened portion being afterwards removed by pick or hammer and See also:wedge. In See also:modern times blasting has become a See also:necessity for the excavation of rock and other hard material, as in open See also:surface cuts, See also:quarrying, tunnelling, See also:shaft-sinking and See also:mining operations in See also:general. For blasting, a hole is generally drilled to receive the See also:charge of explosive. The See also:depth and See also:diameter of the hole and the quantity of explosive used are all variable, depending on the See also:character of the rock and of the explosive, the shape of the See also:mass to be blasted, the presence or See also:absence of cracks or fissures, and the position of the hole with respect to the See also:free surface of the rock. The See also:shock of a blast produces impulsive waves acting radially in all directions, the force being greatest at the centre of explosion and varying inversely as the square of the distance from the charge. This is evidenced by the observed facts. Immediately surrounding the explosive, the rock is often finely, splintered and crushed. Beyond this is a See also:zone in which it is completely broken and displaced or projected, leaving an enveloping mass of more or less ragged fractured rock only partially loosened. Lastly, the diminishing waves produce vibrations which are transmitted to considerable distances.

Theoretically, if a charge of explosive be fired in a solid material of perfectly homogeneous texture and at a proper distance from the free surface, a conical mass will be blown out to the full depth of the See also:

drill hole, leaving a See also:funnel-shaped cavity. No rock, however, is of See also:uniform mineralogical and See also:physical character, so that in practice there is only a rough approximation to the conical See also:crater, even under the most favour-able conditions. Generally, the shape of the mass blasted out is extremely irregular, because of the variable texture of the rock and the presence of cracks, fissures and cleavage planes. The ultimate or resultant useful effect of the explosion of a confined charge is in the direction where the least resistance is presented. In the actual See also:work of rock excavation it is only by trial, or by deductions based on experience, that the behaviour of a given rock can be determined and the quantity of explosive required properly proportioned. Blasting, as usually carried on, comprises several operations: (1) drilling holes in the rock to be blasted; (2) placing in the hole the charge of explosive, with its See also:fuze; (3) tamping the charge, i.e. compacting it and filling the See also:remainder of the hole with some suitable material for preventing the charge from blowing out without breaking the ground; (4) igniting or detonating the charge; (5) clearing away the broken material. The holes for blasting are made either by See also:hand, with hammer and drill or jumper, or by See also:machine drill, the latter being driven by See also:steam, compressed See also:air, or See also:electricity, or,in rare cases,by See also:hydraulic See also: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. or more. The deeper holes are made only in surface excavation of rock, the shallower, to a maximum depth of say 12 ft., being suitable for tunnelling and mining operations. Hand Drilling.—The work is either " single-hand " or " See also:double-hand." In single-hand drilling, the miner wields the hammer with one hand, and with the other holds the drill or " See also:bit," rotating it slightly after every See also:blow in See also:order to keep the hole See also:round and prevent the drill from sticking fast; in double-hand work, one See also:man strikes, while the other holds and rotates the drill. For large and deep holes, two hammermen are sometimes employed. ' A miner's drill is a See also:steel See also:bar, occasionally round but generally of octagonal See also:cross-See also:section, one end of which is forged out to a cutting edge (fig.

I). The edge of the drill is made either straight, like that of a chisel, or with a See also:

convex See also:curve, the latter shape being best for very hard rock. For hard rock the cutting edge should be rather thicker and blunter, and therefore stronger, than for soft rock. Drills are made of high-grade steel, as they must drill steel for hand work is usually from to 1 in., and the length of cutting edge, or See also:gauge, of the drill is always greater than the diameter of the shank, in the proportion of from 7.4 to 4.3. Holes over to or 12 in. deep generally require the use of a set of drills of different lengths and depending in number on the depth required. The shortest drill, for starting the hole, has the widest cutting edge, the edges of the others being successively narrower and graduated to follow each other properly, as drill after drill is dulled in deepening the hole. Thus the hole decreases in diameter as it is made deeper. The miner's hammer (fig. 2) ranges in See also:weight from 32 to 42 lb for single-hand drilling, up to 8 or io lb for double-hand. If the hole is directed downward, a little water is poured into it at intervals, to keep the cutting edge of the drill cool and make a thin mud of the cuttings. From See also:time to time the hole is cleaned out by the " scraper " or " See also:spoon," a See also:long slender See also:iron bar, forged in the shape of a hollow semi-See also:cylinder, with one end flattened and turned over at right angles. If the hole is directed steeply upward and the rock is dry, the cuttings will run out continuously during the drilling; other- See also:wise the scraper is necessary, or a small See also:pipe with a plunger like a See also:syringe is used for washing out the cuttings.

The " jumper " is a long steel bar, with cutting edges on one or both ends, which is alternately raised and dropped in the hole by one or two men.

End of Article: BLASTING

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