Blasting

BLASTING, the process of rending or breaking apart a solid body, such as rock, by exploding within it or in contact with it some explosive substance. The explosion is accompanied by the sudden development of gas at a high temperature and under a tension sufficiently great to overcome the resistance of the en closing body, which is thus shattered and disintegrated. Before the introduction of explosives, rock was laboriously excavated by hammer and chisel, or by the ancient process of "fire-setting," i.e., building a fire against the rock, which, on cooling, splits and flakes off. To hasten disintegration, water was often applied to the heated rock, the loosened portion being afterwards removed by pick or hammer and wedge. In modern times blasting has become a necessity for the excavation of rock and other hard material, as in open surface cuts, quarrying, tunnelling, shaft sinking and mining operations in general.

For blasting, a hole is generally drilled to receive the charge of explosive. The depth and diameter of the hole and the quan tity of explosive used are all variable, depending on the character of the rock and of the explosive, the shape of the mass to be blasted, the presence or absence of cracks or fissures, and the position of the hole with respect to the free surface of the rock. The 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 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 trans mitted 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 drill hole, leaving a funnel-shaped cavity. No rock, however, is of uniform miner alogical and physical character, so that in practice there is only a rough approximation to the conical crater, even under the most favourable 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 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: (I) drilling holes in the rock to be blasted; (2) placing in each hole the charge of explosive, with its fuse; (3) tamping the charge, i.e., compacting it and filling the remainder of the hole with clay or fine rock cuttings, to prevent the charge from blow ing out without breaking the rock; (4) igniting or detonating the charge; (5) clearing away the broken material. Drill holes ordinarily vary in diameter from to 3in., and in depth from a few inches up to 15 or 2oft. or more. The deeper holes are made only in surface excavation of rock, the shallower, to a maximum depth of say 2f t., being suitable for tunnelling and underground mining.

Hand Drilling.

The work is either "single-hand" or "double hand." In single-hand drilling, the miner wields the hammer with one hand, and with the other holds the drill or "bit," rotating it slightly after every blow in order to keep the hole round and prevent the drill from wedging fast; in hand work, one man strikes, while the other holds and rotates the drill. For large and deep holes, two hammer men are sometimes employed.

A miner's drill is a steel bar, occasion ally round but generally of octagonal cross-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 convex 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 be tempered accurately and uniformly. The diameter of drill steel for hand work is usually from to 'in., and the length of cutting edge, or 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 io or i2in. deep generally require the use of a set of drills of different lengths and depending in number on the depth required. The shortest drill, f or 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 af ter 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 weight from 34 to 441b.

for single-hand drilling, up to 8 or olb. for double-hand. If the hole is directed downward, a little water is poured into it at inter vals, to keep the cutting edge of the drill cool and make a thin mud of the cuttings. From time to time the hole is cleaned out by the "scraper" or "spoon," a long slender iron bar, forged in the shape of a hollow semi-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 drill ing; otherwise the scraper is necessary, or a small pipe with a plunger like a 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. In rock work the jumper is rarely used except for holes directed steeply downward, though for coal or soft shale or slate it may be employed for drilling holes horizontally or even upward. Other tools used f or rock-drilling are the pick and gad.

Holes drilled by hand usually vary in depth from say 18 to 36in., according to the nature of the rock and purpose of the work; though deeper holes are often made. For soft rock, single hand drilling is from 20 tO 30% cheaper than double-hand, but this difference does not hold good for the harder rocks. For these double-hand drilling is preferable, and may even be essen tial, to secure a reasonable speed of work.

Machine Drills.

The introduction of machine drills in the latter part of the igth century exerted an important influence on the work of rock excavation in general, and especially on the art of mining. By their use many great tunnels and other works involving rock excavation under adverse conditions have been rapidly and successfully carried out. Before the invention of machine drills such work progressed slowly and with difficulty.

General Classification.

(I) Compressed-air recipro cating or piston drills; (2) hammer drills; (3) electric auger drills; (4) electric reciprocating drills; (5) hydraulic drills. Type (I) has become nearly obsolete having been almost entirely supplanted by type (2) . Type (3) is largely used in coal mining. The others are of minor importance.

Although PISTON DRILLS (fig. 3) are now rarely listed by makers, some are still in service, and a brief description of them will lead to a better understanding of the hammer drill. They are operated by compressed air (or, for surface excavation, often by steam). The drill bit is firmly clamped to the piston rod and delivers a rapid succession of strong blows (3oo-40o per min.) on the bottom of the hole. The force of the blow is the resultant of the weight of the reciprocating parts, the length of stroke (both depending on the size of the drill cylinder), and the air pressure. These factors, together with the speed of stroke, determine the rate of advance, or drilling speed. Air pressures are commonly from 7o to ioolb. per sq. in. Practice has tended toward the higher pressures; but, though faster drilling may be done in some rocks by pressures above golb., the life of the drill is shortened and the repair cost increased. The softer rocks do not require pressure above, say, 8olb. Piston-drill cylinders are usually from 21 to Sin. diameter, though they have been made in smaller and larger sizes; normal length of stroke, 4 to 6in. The admission of compressed air, and therefore the speed and force of the blow, are controlled by a hand valve in the air pipe, close to the valve-chest of the drill. The piston works in a cylinder, provided with a valve motion somewhat similar to that of a steam-engine, together with a rifle-bar, working in the rear end of the piston, for producing the necessary rotation of the drill bit, to keep the hole round and prevent the bit from wedg ing. The machine is mounted on a tripod (fig. 3) ; or, if under ground, sometimes on an iron column or bar, firmly wedged in position between the roof and floor, or side walls, of the tunnel or mine working.

While at work, the entire drill head is fed forward by a long, hand-operated screw, as the hole is deepened. When it has ad vanced as far as the length of the screw and supporting parts per mit, the drill is stopped and a longer bit inserted. If the feed is faster than the hole is being deepened the stroke shortens, because the bit strikes the bot tom before the full length of stroke is reached; conversely, with too slow a feed, the piston will strike the front cylinder head. Thus, by regulating the feed, the force of the blow is varied to suit the conditions. When starting a hole, the stroke should be short, until the bit has adjusted itself to the shape of the bottom of the hole. For hard rock, a short, rapid stroke, is best; a longer stroke for soft or tough rock. The cutting edge of the bit is usually cross or X-shaped.

Hammer Drills.

These first appeared about ago, but were little used until the beginning of the loth century. Since about 1 g' o they have almost entirely replaced piston or reciprocating drills for both surface and underground rock excavation, and are made in a great variety of forms and sizes. They weigh less, are more mobile and drill faster than reciprocating drills. Their principle was first applied in pneumatic riveters, and tools for chipping and other machine-shop work. They are best adapted to drilling holes directed steeply upward or downward, but can be used also for tunnelling and similar horizontal work.

Classification of Hammer Drills.

(') Drills designed for tripod or column mounting (fig. 4), usually of the larger sizes, and comparable in their field of work to the old reciprocating machines; (2) drills with either a cross or D shaped handle (fig. 5), chiefly for holes directed steeply downward, the weight of the drill resting on the bit itself ; (3) drills supported by an automatic air-feed standard, primarily for holes directed upwards. They may be used for holes on the horizontal (fig. 6).

Details.

The bit of the hammer drill does not reciprocate; its shank projects into the forward end of the cylinder, where it is held loosely, and is struck a rapid succession of blows by the piston-like hammer. The cutting edge of the bit is in constant contact with the bottom of the hole, except during the slight rebound caused by' each blow of the hammer. In machines of class (I) rotation of the bit is automatic, as in the old reciprocat ing drills. In classes (2) and (3) the bit is rotated by turning the drill on its axis by hand ; though, in many of the later designs, a rifle-bar produces automatic rotation, as in class (i). To aid in keeping the hole round and true, the bit is commonly of the star or rosette shape, with six or eight radial cutting edges. As the bit does not reciprocate, it is evident that, for holes directed downwards, some auto matic means must be provided for removing the drill dust or sludge ; for, if the bottom of the hole is not kept clean, the useful effect of the hammer blows is decreased. To accomplish this a hollow bit is generally used, having a small-diameter hole longitudinally through its centre.

By injecting through this hole a jet of water, the drillings are displaced and the bit is kept cool.

The water, supplied from a small tank, in which water is kept under pressure by admitting compressed air from the air pipe, is delivered to the hollow bit by a small tube, passing through the rifle-bar and hammer, in the axis of the drill. When an air jet is used, it passes through a similar axial tube, the rear end of which is connected with the air supply; or, compressed air may be led directly to the bit from the valve chest. Details of the valve motion, for causing movements of the hammer, are too varied and technical to be described here (see Peele's Compressed Air Plant, 4th ed., 1 q 2 5) .

As regards the feed of hammer drills, class (I) drills (fig. 4) are fed forward as the hole is deepened by a hand-operated feed screw, as in reciprocating drills (fig. 3) . Class (2) drills (fig. 5 ) require no mechanical feed; the weight of the drill rests on the bit in down holes, and, for holes in other directions, the drill is simply pushed forward by the operator. Class (3) drills (fig. 6), chiefly for holes directed steeply upward. are supported by a lone proofing the joints with grease. When more than one cartridge is required for the blast, that which has the fuse attached is usually charged last. The cartridges are carefully rammed down by a wooden tamping bar and the remainder of the hole filled with tamping. This consists of finely broken rock, dry clay or other comminuted material, carefully compacted by the tamping bar on top of the charge. The fuse is a cord, having in the centre a core of gunpowder, enclosed in several layers of linen or hemp waterproofed covering. It is ignited by the miner's candle or lamp, or by a candle end so placed at the mouth of the hole that the flame must burn its way through the fuse covering. As the fuse burns slowly, at the rate of 2 or aft. per minute, the miner uses a sufficient length to allow him to reach a place of safety. For blasting in coal, "squibs" instead of fuses are often used. A squib is simply a tiny paper rocket, about kin. diameter by Sin. long, containing fine gunpowder and having a sulphur slow-match at one end. It is fired into the charge through a channel in the tamping. This channel may be formed by a piece of gas pipe, tamped in the hole and reaching the charge ; or a "needle," a long taper iron rod, is laid longitudinally in the hole, with its point entering the charge, and after the tamping is finished, by carefully withdrawing the needle a little channel is left, through which the squib is fired. In this connection it may be noted that for breaking ground in grassy collieries several substitutes for explosives have been used to a limited extent, e.g., plugs of dry wood driven tightly into a row of drill holes, and which, on being wetted, swell and split the coal; quicklime cartridges, which expand powerfully on the application of water; simple wedges, driven by hammer into the drill holes ; multiple wedges, inserted in the holes and operated by hydraulic pressure from a small hand force-pump.

Blasting with High Explosives.

High explosives are fired either by ordinary fuse and detonating cap or by electric fuse. Detonating caps of ordinary strength contain to to 15 grains of fulminating mixture. The cap is crimped tight on the end of the fuse, embedded in the cartridge, and on being exploded by fire from the fuse detonates the charge. The number of cartridges charged depends on the depth of hole, the length of the line of least resistance, and the toughness and other characteristics of the rock. Each cartridge should be solidly tamped, and, to avoid waste spaces in the hole, which would reduce the effect of the blast, it is customary to split the paper covering lengthwise with a knife. This allows the dynamite to spread under the pressure of the tamping bar. The cap is often placed in the cartridge preceding the last one charged, but it is better to insert it last, in a piece of cartridge called a "primer." Though the dynamites cannot be exploded by sparks, they should nevertheless always be handled carefully. It is not so essential to fill the hole com pletely nor so thoroughly to compact the tamping, as in charging black powder, because of the greater rapidity and shattering force of the explosion of dynamite; tamping, however, should never be omitted, as it increases the efficiency of the blast. In exploding dynamite, strong caps, containing not less than 15 grains of fulminating powder, produce the best results. Weaker caps are not economical, as they do not cause complete detona tion. This is specially true if the weather be cold. Dynamite then becomes less sensitive, and the cartridges should be gently warmed before charging to a temperature of not more than 8o° F. Poisonous fumes are often produced by the explosion of the nitroglycerin compounds. These are probably largely due to incomplete detonation, by which part of the nitroglycerin is vaporized or merely burned. This is most likely to occur when the dynamite is chilled, or of poor quality, or when the cap is too weak. There is generally but little inconvenience from the fumes, except in confined underground workings, where ventila tion is imperfect.

Like nitroglycerin, the common dynamites freeze at a tempera ture of from 42° to 46° F. They are then comparatively safe, and so far as possible should be transported in the frozen state. At very low temperatures dynamite again becomes somewhat sensitive to shock. When it is frozen at ordinary temperatures even the strongest detonating caps may fail to develop the full force. In thawing dynamite, care must be exercised. The fact that a small quantity will often burn quietly has led to the danger ously mistaken notion that mere heating will not cause explosion. It is chiefly a question of temperature. If the quantity ignited by flame be large enough to heat the entire mass to the detonating point (say 36o°) before all is consumed, an explosion will re sult. Furthermore, dynamite, when even moderately heated, becomes extremely sensitive to shocks. There are several accepted modes of thawing dynamite : (1) In a water bath, the cartridges being placed in a vessel surrounded on the sides and bottom by warm water contained in a larger enclosing vessel. The warm water may be renewed from time to time, or the water bath placed over a candle or small lamp, not on a stove. (2) In two vessels, similar to the above, with the space between them occupied by air, provided the heat applied can be definitely limited, as by using a candle. (3) When large quantities of dynamite are used a supply may be kept on shelves in a wooden room or chamber, warmed by a stove, or by a coil of pipe heated by exhaust steam from an engine. Live steam should not be used, as the heat might become excessive. Thawing should always take place slowly, never before an open fire or by direct contact with a stove or steam pipes and care must be taken that the heat does not rise high enough to cause sweating or exudation of liquid nitro glycerin from the cartridges, which would be a source of danger.

For the storage of explosives at mines, etc., proper magazines must be provided, situated in a safe place, not too near other buildings, and preferably of light though fireproof construction. Masonry magazines, though safer from some points of view, may be the cause of greater damage in event of an explosion, because the brick or stones act as projectiles. Isolated and abandoned mine workings, if dry, are sometimes used as magazines.

Firing Blasts by Electricity.

This has a wide application for both surface and underground work. An electrical fuse (fig.

consists of a pair of fine, insulated copper wires, several feet long and about s o0 of an inch in diameter, with their bare ends inserted in a detonating cap.

For firing, the fuse wires are joined to long leading wires, connected with some source of electric current. By joining the fuse wires in series or in groups, any number of holes may be fired simultane ously, according to the current available.

A

round of holes fired in this way, as for driving tunnels, sinking shafts, or in large surface excavations, produces better results, both in economy of ex plosive and effect of the blast, than when the holes are fired singly or in succession.

Also, the miners are enabled to prepare for the blast with more care and deliberation, and then to reach a place of safety before the current is transmitted. Another ad vantage is that there is no danger of a hole "hanging fire," which sometimes causes accidents in using ordinary powder fuses.

Hanging fire may be due to a cut, broken or damaged powder fuse, which may smoulder for some time before communicating fire to the charge. "Misfires," which also are of not infrequent occurrence with both ordinary fuse and electric fuses, are cases where explosion from any cause fails to take place. After wait ing a sufficient length of time before approaching the charged hole, the miner carefully removes the tamping down to within a few inches of the explosives and inserts and fires another cartridge, the concussion usually detonating the entire charge. Sometimes another hole is drilled near the one which has missed. No attempt to remove the old charge should ever be made.

Low-tension electricity is now always used. It is generated by a small, portable, hand-operated dynamo, or taken from any convenient electric circuit. High-tension machines are too fragile and the detonators are more apt to deteriorate. The end of the fuse wires in the low-tension cap are connected by a fine platinum filament (fig. 1 1), embedded in loose fulminate on top of a compressed fulminating mixture, and explosion results from the heat generated by the resistance opposed to the passage of the I current through the filament. Blasting machines are made in several sizes, the smaller ones being capable of firing simultane ously from o to 20 holes. Fuses must be of uniform electrical resistance, to ensure that all the connected charges will explode simultaneously. The premature explosion of any one fuse would break the circuit.

Delay electric blasting caps are for firing groups of holes in two or three volleys, with one application of the electric current. The cap shell is extra long. Between the fulminate and the prim ing ignited by the current is a short piece of slow-burning ordi nary powder-fuse. Hence there is an appreciable interval between the passage of the current and the detonation of the cap.

These caps cannot be used in series with ordinary electric caps. They require a special "no-delay" detonator, because first- and second-period delay-action caps are less sensitive to ignition. The entire round of holes is wired as for simultaneous firing. When current is trans mitted, about one second elapses before the first-delay caps detonate, and the same interval between these and the second delay. Delay caps are specially useful in tunnelling and shaft-sinking, as they enable the blaster to fire an entire round of holes without returning to the working place.

Electric fuse igniters serve the same purpose ; they light simultaneously powder fuses of different lengths. Electric squibs, for firing black powder, are useful in coal mining. For further details of electric blasting, see Peele's Mining Engineer's Handbook (2nd ed., 1927).

Definite rules for the proportioning of blasting charges are rarely observed, and although the blasts made by a skilful miner seldom fail to do their work, too much, rather than too little, explosive is often used. High explosives are specially liable to be wasted, probably through lack of appreciation of their power as compared with that of black powder. Among the indications of excessive charges are the production of much finely broken rock or of crushed and splintered rock around the bottom of the hole, and excessive displacement or projection of the rock broken by the blast. In beginning any new piece of work, such waste may be avoided or reduced by making trial shots with different charges and depths of hole, and noting the results; also by letting contracts under which the workmen pay for the explosive. In surface rock excavation the location and determination of the depth of the holes and the quantity of explosive used are occasion ally put in charge of one or more skilled men, who direct the work and are responsible for the results obtained.

Blasting in surface excavations and quarries is sometimes done on an immense scale-called "mammoth blasting." Shafts are sunk, or tunnels driven, in the mass of rock to be blasted, and, connected with them, a number of chambers are excavated to receive the charges of explosive. The preparation for such blasts may occupy months, and many tons of black powder or dynamite may be exploded simultaneously, breaking or dislodging thousands of tons of rock. This method is adopted for getting stone cheaply, as for building macadamized roads, dams and break waters, obtaining limestone for blast furnace flux, and occasion ally in excavating large railway cuttings. It is also applied in submarine blasting for the removal of reefs obstructing naviga tion, and sometimes for loosening extensive banks of partly cemented gold-bearing gravel, preparatory to washing by hydraulic mining.

BIBLIOGRAPHY.-For

further information on drilling and blasting Bibliography.-For further information on drilling and blasting see Daw, The Blasting of Rock (1898) ; Foster, Text-book of Ore and Stone Mining (i9oo) ; Hughes, Text-book of Coal Mining 0900 ; Gillette, The Excavation of Rock (19o4) ; Peele, Mining Engineer's Handbook (2nd ed., 1927), sections 4, 5, 6, 7, io and 15. Also Proc. Inst. Civ. Eng. (London), vol. lxxxv. p. 264; Trans. Inst. Min. Eng. (England), vols. xiv.-xvi. (arts. by W. Maurice), vol. xxvi. pp. 322, 348, vol. xxiv. p. 526; Trans. Amer. Soc. Civ. Eng., vol. xxvii. p. 53o; Trans. Amer. Inst. Min. Eng., vol. lxvii. p. 213, lviii. p. 3, ixvi. p. 743, lxviii. p. 216, lxiv. p. 271, lxxi. p. 1248, lxxiv. pp. 512 and 69o; Trans.

Inst. Min. and Met., vol. xxxii. p. xis, xxxv. p. 390; Eng. and Min. Jour., serial, Aug. 17, 1921, P. 34o to Oct. 1, 1921, p. 538, jan. 21, 1922, p. 104, March 18, 1922, P. 446, April 14, 1923, p. 67o, Nov. 29, 1924, p. 845, Feb. 2, 1924, p. 197, July io, 1926, P• 44, Nov. 20, 1926, p. 8o4, Dec. 4, 1926, p. 897; Iron and Coal Trades Rev., Jan. 15, 1926, p. Ioo; Coll. Eng., July 1925, p. 323; Coal Age, Dec. 24, 1925, p. 881; U.S. Bur. of Mines, Tech. Papers 210, 294 and 364; Coll. Guardian, Nov. 27, 1925, p. 1279, Dec. 4, 1925, p. 134I• (R. PE.)