Blasting

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 sticking fast; in double-hand work, one man strikes, while the other holds and rotates the drill. For large and deep holes, two hammermen are sometimes employed.

Fig. 2.—Sledge-hammer.

A miner’s drill is a steel bar, occasionally round but generally of octagonal cross-section, one end of which is forged out to a cutting edge (fig. 1). 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 ¾ to 1 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 10 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 weight from 3½ to 4½ ℔ for single-hand drilling, up to 8 or 10 ℔ 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 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 drilling; 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 upward. Other tools used in connexion with rock-drilling are the pick and gad.

Fig. 3.—Ingersoll-Sergeant Mining Drill.

Holes drilled by hand usually vary in depth from say 18 to 36 in., 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 essential, to secure a reasonable speed of work.

Machine Drills.—The introduction of machine drills in the latter part of the 19th century exerted an important influence on the work of rock excavation in general, and specially 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. Nearly all machine drills are of the reciprocating or percussive type, in which the drill bit is firmly clamped to the piston rod and delivers a rapid succession of strong blows on the bottom of the hole. The ordinary compressed air drill (which may, for surface work, be operated also by steam) may be taken as an illustration. The piston works in a cylinder, provided with a valve motion somewhat similar to that of a steam-engine, together with an automatic device for producing the necessary rotation of the piston and drill bit. While at work the machine is mounted on a heavy tripod (fig. 3); or, if underground, 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. As the hole is deepened, the entire drill head is gradually fed forward on its support by a screw feed, a succession of longer and longer drill bits being used as required.

Among the numerous types and makes of percussion drill may be named the following:—Adelaide, Climax, Darlington, Dubois-François, Ferroux, Froelich, Hirnant, Ingersoll, Jeffrey, Leyner, McKiernan, Rand, Schram, Sergeant, Sullivan and Wood.

Figs. 4 and 5.—Darlington’s Rock Drill.

One of the simplest of the machine drills is the Darlington (figs. 4 and 5): a is the cylinder; b, piston rod; c, bit; d, d, air inlets, either being used according to the position of the drill while at work; h, piston; j, rifle-bar for rotating piston and bit; k, ratchet attached to j; l, brass nut, screwed into h, and in which j works; f, chuck for holding drill-bit; n, air port communicating between ends of cylinder, front and back of piston; o, exhaust port. This machine has no valve. From its construction, the compressed air (or steam) is always acting on the annular shoulder round the forward end of the piston. The piston is thereby forced back on the in-stroke until the port n is uncovered. This admits the compressed air to the rear end of the cylinder, and as the area of this end of the piston is much greater than that of the shoulder on the other end, the piston is driven forward and strikes its blow. When it has advanced far enough to cover the exhaust port o, the air behind the piston is exhausted, and, under the constant inward pressure noted above, the stroke is reversed. The rotation of piston and bit is caused by the rifle-bar j. On the outward stroke, j, with its ratchet k, is free to turn under a couple of pawls and springs, and consequently the piston delivers its blow without rotation. On the inward stroke the ratchet is held fast by the pawls, and the piston and bit are forced to rotate through a small part of a revolution. The cylinder is fed forward with respect to the shell r, by rotating the handle p, which works a long screw-bar engaging with a nut on the under side of the cylinder. The shell r is bolted to the clamp s, which in turn is mounted on the hollow column or bar g, or on a tripod, according to the character of the work. By means of the adjustable clamp s, the machine can be set for drilling a hole in any desired direction. The drill makes from 400 to 800 strokes per minute.

The “New Ingersoll” drill, which may be taken as an example of the numerous machines in which valves are used, is shown in section in fig. 6. The steam or compressed air is distributed through the ports alternately to the ends of the cylinder, by the reciprocations of a spool-valve working in a chest mounted on the cylinder. The movements of this valve are caused by the strokes of the main piston, which, by means of the wide annular groove around the middle of the piston, alternately open and close the spool-valve exhaust ports. Fig. 3 shows the Ingersoll “Light Mining drill,” as mounted on a tripod, and in position for drilling a hole vertically downward. In the Leyner drill the drill-bit is not connected to the piston, but is struck a quick succession of blows by the latter. An important feature of this machine is the provision for directing a stream of water into the hole for clearing out the cuttings. For this purpose the shank of the drill-bit is perforated longitudinally, the water being supplied under pressure from a small tank, to which compressed air is led.

A rock drill of entirely different design, the Brandt, has been successfully used in Europe for driving railway tunnels. It is operated by hydraulic power, the pressure water being supplied by a pump. The hollow drill-bit, which has a serrated cutting edge, is forced under heavy pressure against the bottom of the hole, and is rotated slowly—at six to eight revolutions per minute—by a pair of small hydraulic cylinders, thus grinding and crushing the rock instead of chipping it. The bottom of the hole is kept clean and the drill-bit cooled by a stream of water passing down through its hollow shank. On account of its size and weight, this machine is not suitable for mine work.

Fig. 6.—New Ingersoll Drill.

Most of the machine drills are made in a number of sizes, from 2 in. up to 5 in. diameter of cylinder, the larger sizes being capable of drilling holes 5 in. diameter and 30 ft. deep. They range in weight from say 95 to 690 ℔ for the drill head (unmounted), the tripods weighing from 40 to 260 ℔, exclusive of the weights placed for stability on the tripod legs (fig. 3). The sizes in most common use for mining are from 2½ in. to 31⁄8 in. diameter of cylinder. In rock of average hardness the best drills make from 4 to 7.5 linear ft. of hole per hour. For use in narrow veins, or other confined workings underground, several extremely small and light compressed air drills have been introduced, as, for example, the Franke and Wonder, the first of which weighs complete only 16 ℔, and the second 18 ℔ These drills are held in the hands of the miner in the required position, and strike a rapid succession of light blows. A large number of mechanical drills operated by hand power have been invented. Some imitate hand-drilling in the mode of delivering the blow; in others the drill-bit is caused to reciprocate by means of combinations of crank and spring. None of these machines is entirely satisfactory, and but few are in use.

Among percussion rock-drills operated by electricity are the Bladray, Box, Durkee, Marvin and Siemens-Halske. The Marvin drill works with a solenoid; most of the others have crank and spring movements for producing the reciprocations of the piston. Power is furnished by a small electric motor, either mounted on the machine itself, as with the Box drill, or more often standing on the ground and transmitting its power through a flexible shaft. Although rather frequently used, electric percussion drills cannot yet be considered entirely successful, at least for mine service, in competition with compressed air machines. Another type of electric drill, however, has been successfully used in collieries, viz. rotary auger drills, mounted on light columns and driven through gearing by diminutive motors. These are intended for boring in coal, slate or other similar soft material. Hand augers resembling a carpenter’s brace and bit are also often used in collieries.

Whatever may be the method of drilling, after the hole has been completed to the depth required, it is finally cleaned out by a scraper or swab; or, when compressed air drills are used, by a jet of air directed into the hole by a short piece of pipe connected through a flexible hose with the compressed air supply pipe. The hole is then ready for the charge.

Fig. 7.	Fig. 8.

Location and Arrangement of Holes.—For hand drilling in mining the position of the holes is determined largely by the character and shape of the face of rock to be blasted. The miner observes the joints and cracks of the rock, placing the holes to take advantage of them and so obtain the best result from the blast. In driving a tunnel or drift, as in figs. 7 and 8, the rock joints can be made of material assistance by beginning with hole No. 1 and following in succession by Nos. 2, 3 and 4. Frequently the ore, or vein matter, is separated from the wall-rock by a thin, soft layer of clay (D, D, fig. 8). This would act almost as a free face, and the first holes of the round would be directed at an angle towards it, for blasting out a wedge; after which the positions of the other holes would be chosen.

Fig. 9.	Fig. 10.

When machine drills are employed, less attention is given to natural cracks or joints, chiefly because when the drill is once set up several holes at different angles can be drilled in succession by merely swinging the cylinder of the machine into a new position with respect to its mounting. According to one method, the holes are placed with some degree of symmetry, in roughly concentric rings, as shown by figs. 9 and 10. The centre holes are blasted first, and are followed by the others in one or more volleys as indicated by the dotted lines. Another method is the “centre cut,” in which the holes are drilled in parallel rows on each side of the centre line of the tunnel, drift or shaft. Those in the two rows nearest the middle are directed towards each other, and enclose a prism of rock, which is first blasted put by heavy charges, after which the rows of side holes will break with relatively light charges.

Explosives.—A great variety of explosives are in use for blasting purposes. Up to 1864, gunpowder was the only available explosive, but in that year Alfred Nobel first applied nitroglycerin for blasting, and in 1867 invented dynamite. This name was originally applied to his mixture of nitroglycerin with kieselguhr, but now includes also other mechanical mixtures or chemical compounds which develop a high explosive force as compared with gunpowder. Besides these there are the so-called flameless or safety explosives, used in collieries where inflammable gases are given off from the coal.

Gunpowder, or black powder, is seldom used for rock-blasting, except in quarrying building-stone, where slow explosives of relatively low power are desirable to avoid shattering the stone, and in such collieries as do not require the use of safety explosives. Gunpowder is exploded by deflagration, by means of a fuze, and exerts a comparatively slow and rending force. The high explosives, on the other hand, are exploded by detonation, through the agency of a fuze and fulminating cap, exerting a quick, shattering, rather than a rending force. Dynamites and flameless explosives are made in a variety of strengths, and are packed in waterproofed cartridges of different sizes. The grades of dynamite most commonly employed contain from 35 to 60% of nitroglycerin; the stronger are used for tough rock or deep holes, or for holes unfavourably placed in narrow mine workings, as sometimes in shaft-sinking or tunnelling. When of good quality high explosives are safer to handle than gunpowder, as they cannot be ignited by sparks and are not so easily exploded. The ordinary dynamites used in mining are about four times as powerful as gunpowder.

Nitroglycerin in its liquid form is now rarely used for blasting, partly because its full strength is not often necessary but chiefly because of the difficulty and danger of transporting, handling and charging it. If employed at all, it is charged in thin tinned plate cases or rubber-cloth cartridges.

Blasting with Black Powder.—The powder is coarse-grained, usually from 1⁄8 to 3⁄16 in. in size, and is charged in paper cartridges, 8 to 10 in. long and of a proper diameter to fit loosely in the drill hole. A piece of fuze, long enough to reach a little beyond the mouth of the hole, is inserted in the cartridge and tied fast. For wet holes paraffined paper is used, the miner waterproofing the joints with grease. When more than one cartridge is required for the blast, that which has the fuze 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 fuze 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 fuze covering. As the fuze burns slowly, at the rate of 2 or 3 ft. per minute, the miner uses a sufficient length to allow him to reach a place of safety.

For blasting in coal, “squibs” instead of fuzes are often used. A squib is simply a tiny paper rocket, about 1⁄8 in. diameter by 3 in. 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 ¼ in. 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 connexion it may be noted that for breaking ground in gassy 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 fuze and detonating cap or by electric fuze. Detonating caps of ordinary strength contain 10 to 15 grains of fulminating mixture. The cap is crimped tight on the end of the fuze, embedded in the cartridge, and on being exploded by fire from the fuze 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 are not exploded by sparks, they should nevertheless always be handled carefully. It is not so essential to fill the hole completely and 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 say 15 grains of fulminating powder, produce the best results. Weaker caps are not economical, as they do not produce complete detonation of the dynamite. 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 80° 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 ventilation is imperfect.

Like nitroglycerin, the common dynamites freeze at a temperature 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 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 dangerously 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 360° F.) before all is consumed, an explosion will result. 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 nitroglycerin from the cartridges, which would be a source of danger.

For the storage of explosives at mines, &c., 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.

Fig 11. Electrical Fuze.

Firing blasts by electricity has a wide application for both surface and underground work. An electrical fuze (fig. 11) consists of a pair of fine, insulated copper wires, several feet long and about 1⁄40 of an inch in diameter, with their bare ends inserted in a detonating cap. For firing, the fuze wires are joined to long leading wires, connected with some source of electric current. By joining the fuze wires in series or in groups, any number of holes may be fired simultaneously, 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 explosive 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 advantage is that there is no danger of a hole “hanging fire,” which sometimes causes accidents in using ordinary fuzes.

Hanging fire may be due to a cut, broken or damaged powder fuze, which may smoulder for some time before communicating fire to the charge. “Miss-fires,” which also are of not infrequent occurrence with both ordinary and electric fuzes, are cases where explosion from any cause fails to take place. After waiting 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.

High tension electricity, generated by a frictional machine, provided with a condenser, was formerly much used for blasting. The bare ends of the fuze wires in the detonating cap are placed say 1⁄8 in. apart, leaving a gap across which a spark is discharged, passing through a priming charge of some sensitive composition. The priming is not only combustible but also a conductor of electricity, such as an intimate mixture of potassium chlorate with copper sulphide and phosphide. By the combustion of the priming the fulminate mixture in the cap is detonated. As these fuzes are more apt to deteriorate when exposed to dampness than fuzes for low-tension current, and the generating machine is rather clumsy and fragile, low-tension current is more generally employed. It may be generated by a small, portable dynamo, operated by hand, or may be derived from a battery or from any convenient electric circuit. The ends of the fuze wires in the detonating cap are connected by a fine platinum filament (fig. 11), embedded in a guncotton priming on top of the fulminating mixture, and explosion results from the heat generated by the resistance opposed to the passage of the current through the filament. Blasting machines are made in several sizes, the smaller ones being capable of firing simultaneously from ten to twenty holes. The fuzes must obviously be of uniform electrical resistance, to ensure that all the connected charges will explode simultaneously. The premature explosion of any one of the fuzes would break the circuit.

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 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 occasionally 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 gunpowder or dynamite are at times exploded simultaneously, breaking or dislodging thousands, or even hundreds of thousands, of tons of rock. This method is adopted for getting stone cheaply, as for building macadamized roads, dams and breakwaters, obtaining limestone for blast furnace flux, and occasionally in excavating large railway cuttings. It is also applied in submarine blasting for the removal of reefs obstructing navigation, and sometimes for loosening extensive banks of partly cemented gold-bearing gravel, preparatory to washing by hydraulic mining.

Authorities.—For further information on drilling and blasting see:—Callon, Lectures on Mining (1876), vol. i. chs. v. and vi.; Foster, Text-book of Ore and Stone Mining, (1900), ch. iv.; Hughes, Text-book of Coal Mining (1901), ch. iii.; H.S. Drinker, Tunnelling, Explosive Compounds and Rock Drills (1878); M.C. Ihlseng, Manual of Mining (1905), pp. 596-696; Köhler, Der Bergbaukunde (1897), pp. 104-208; Daw, The Blasting of Rock (1898); Prelini, Earth and Rock Excavation (1905), chs. v., vi. and vii.; Gillette, The Excavation of Rock (1904); Guttmann, Blasting (1892); Spon’s Dictionary of Engineering, art. “Boring and Blasting”; Eissler, Modern High Explosives (1893), pts. ii. and iii.; Walke, Lectures on Explosives (1897), chs. xix.-xxii. Also: Proc. Inst. Civ. Eng. (London), vol. lxxxv. p. 264; Trans. Inst. Min. Eng. (England), vols. xiv., xv. and xvi. (arts, by W. Maurice), vol. xxvi. pp. 322, 348, vol. xxiv. p. 526 and vol. xxv. p. 108; Trans. Amer. Soc. Civ. Eng., vol. xxvii. p. 530; Trans. Amer. Inst. Min. Eng., vol. xviii. p. 370, vol. xxix p. 405 and vol. xxxiv. p. 871; South Wales Inst. Eng. (1888); Jour. Ass. Eng. Socs., vol. vii. p. 58; Jour. Chem. Met. and Mining Soc. of South Africa, August 1905; School of Mines Quarterly, N.Y., vol. ix. p. 308; Colliery Guardian, April 15, 1898, and February 6, 1903; Mines and Minerals, February 1905, p. 348, January 1906, p. 259, and April 1906, p. 393; Eng. and Mining Jour., April 19, 1902, p. 552; The Engineer, February 24, 1905; Elec. Rev., June 9, 1899; Eng. News, vol. xxxii. p. 249, and August 3, 1905; Gluckauf, September 28, 1901, and July 5, 1902; Österr. Zeitschr. f. Berg- u. Hüttenwesen, May 18, 25, 1901, April 18, 1903 and November, 18, 1905; Annales des mines, vol. xviii. pp. 217-248.