In the foregoing considerations, the holes have been assumed to be vertical, and for this reason the unsupported face which is perpendicular to the hole, that is, the face into which the hole is bored, has been neglected. For it is evident that, under the conditions assumed, the lines of rupture cannot reach this face, which, therefore, has practically no existence. Suppose, for example, a bore-hole placed at h, in Fig. 45, and the rock to be supported upon every side except that at right angles to the hole. The forces acting perpendicularly to the direction of the bore-hole are opposed on all sides by an infinite resistance. Hence, in this case, either the tamping will be blown out, or, if the forces developed are unequal to the work, no effect will be produced beyond a slight enlargement of the hole at the base. This, however, is a case of frequent occurrence in practice, and it becomes necessary to adopt measures for making this unsupported face available. Evidently this object can be attained only by so directing the bore-hole that a line perpendicular to it may reach the face; that is, the line of the bore-hole must make with the unsupported face an angle less than 90°. This direction of the bore-hole is shown in Fig. 46, which may be regarded as a sectional elevation of Fig. 45. In this case, the lines of rupture, which will run similarly to those produced in the case shown in Fig. 43, will reach the unsupported face at b, and the length of these lines, and consequently the depth of the excavation, for a given length of bore-hole, will depend upon the angle which the latter makes with the face. This mode of rendering a single exposed surface available is called “angling the holes,” and it is generally resorted to in shaft sinking and in driving headings. The conditions involved in “angling” are favourable to the action of strong explosives.

Example of a Heading.

—To show how these principles are applied in practice, we will take a typical case of a heading, 7 feet by 9 feet, as shown in Fig. 47. In this case, we have at starting only one exposed face, which is perpendicular to the direction of the driving. Hence it is evident that we shall have to proceed by angling the holes. We might begin in any part of the exposed face; but, as it will hereafter appear, the most favourable position is the centre. We therefore begin at this point by boring a series of holes, numbered 1 on the drawing. These holes are angled towards each other; that is, the two sets of three holes vertically above each other converge in the direction of their lower ends, as shown in the sectional plan, Fig. 48. In this instance, we have assumed six holes as necessary and sufficient. But it is obvious that the number of holes, as well as their distance apart horizontally, will be determined by their depth, the tenacity of the rock, and the strength of the explosive used. When these holes are fired, a wedge-shaped portion of the rock will be forced out, and this result will be more effectually and certainly obtained if the charges be fired simultaneously. The removal of this portion of the rock is called “taking out the key.” The effect of removing this key is to leave the surrounding rock unsupported on the side towards the centre; that is, another face is formed perpendicular to the first.

Having thus unkeyed the rock by the removal of this portion from the centre, it will evidently be unnecessary, except for convenience or increased effect, to angle any more of the shot-holes. The second series therefore, numbered 2 in the drawing, may be bored perpendicularly to the face of the heading. When this series is fired, the lines of rupture will all run to the unsupported face in the centre—and from hole to hole, if the shots be fired simultaneously—and the annular portion of rock included between the dotted lines 1 and 2 will be removed. If the shots be fired successively, the first will act under the condition of one unsupported face, as illustrated in Fig. 43; but as another unsupported face will be formed by the removal of the rock in front of this charge, the succeeding shots will be subject to the more favourable condition represented in Fig. 42. The firing of this second series of shots still leaves the surrounding rock unsupported towards the centre, and consequently the same conditions will exist for the third series, numbered 3 on the drawing, the firing of which series will complete the excavation. Fig. 49 shows the appearance of Fig. 48 after the firing of the central holes.

It may be remarked here that, owing to the want of homogeneity in the rock, and to the existence of joints and fissures, the outer line of rupture will not, in practice, run so regularly as indicated, in this assumed case, by the dotted lines. This circumstance will influence the position of the holes, or the quantity of explosive, in the next series, and furnish an opportunity for the exercise of judgment on the part of the blaster.

There exist also other circumstances which will influence the position and the number of the holes in a very important degree, and which therefore must be taken fully into account at every advance. One of these is the irregularity of the face of the excavation. Instead of forming an unbroken plane at right angles to the direction of the heading, or of the shaft, this face is broken up by projecting bosses and more or less deep depressions. Obviously these protuberances and cavities will influence, in no inconsiderable degree, the lines of least resistance; the latter being lengthened or shortened, or changed in direction, by the presence of the former, which give existence to unsupported faces to which the lines may radiate. These conditions must, in every case, be taken into account when determining the best position for the bore-hole. Of yet greater importance, is the existence of joint planes and bedding planes. A bed of rock may be, and frequently is, cut up by these planes into detached blocks of greater or less dimensions, according to the more or less perfect development of the different sets. Hence it becomes necessary, in determining a suitable position for blasting the charge, to consider such planes as unsupported faces, and to ascertain the direction and length of the lines of resistance under such conditions. If a charge be placed in close proximity to one of these planes, not only may the lines of rupture run in unforeseen directions, but the greater part of the force of the explosion will be lost by the escape of the gases along the plane. The same loss of force may be occasioned by the presence of a cavity, such as are of frequent occurrence in cellular or vughy rock. When the joint planes are fully developed, their existence can be ascertained by inspection; but when their development is imperfect, there may be considerable difficulty in discovering them. In such cases, the rock should be carefully inspected, and sounded with a hammer or pick. When a cavity is bored into, it may be rammed full of clay, and the boring continued through the clay; or if sufficient depth has been obtained, the charge may be placed upon the clay, which will prevent the wasteful dissipation of the gases. As none of the aforementioned circumstances occur under precisely similar conditions, no general rule of much service can be laid down; they are matters upon which the blaster must be left to use his own judgment, and to do this effectively, it is necessary that he possess some knowledge of the materials with which he deals.

Economical Considerations.

—Besides the important economical considerations involved in the foregoing, there are others which claim attention. Foremost among these is the question whether, for a given effect, it be better to augment or to diminish the individual importance of the shots; that is, whether it be better to diminish the number of the holes and to increase their diameter, or to diminish their diameter and increase their number; or, again, to diminish their diameter and to increase their depth, or to increase their diameter and to diminish their number and their depth. It may be readily shown mathematically, and the results are confirmed by experience, that there is an important gain in reducing the diameter of the shot-holes to the lowest limit allowed by the strength and the gravimetric density of the explosive, and increasing their depth. The gain is mainly in the direction of a saving of labour, and it is especially remarkable in the case of machine boring. Here again we perceive the advantage of strength in the explosive agent employed.

The simultaneous firing of the shots offers several important advantages. It has already been shown how one charge aids another, under such a condition, and in what way the line of rupture is affected by it. When the shots are fired successively, each one has to tear out the portion of rock allotted to it; but when they are fired simultaneously, their collective force is brought to bear upon the whole mass to be dislodged. This is seen in the diagram, Fig. 43. When deep holes are used, the greater useful effect caused by simultaneous firing becomes very marked. Hence electricity associates itself naturally with machine drills and strong explosives.

Tamping.

—To “tamp” a shot-hole is to fill it up above the charge of explosive with some material, which, when so applied, is called the “tamping.” The object of tamping is to oppose a resistance to the escape of the gases in the direction of the bore-hole. Hence a primary condition is that the materials used shall be of a strongly resisting character. A second determining condition is that these materials shall be of easy application. This condition precludes the use of all such devices as plugs, wedges, and forms of a similar character, which have been from time to time proposed.

The only material that, in practice, has been found to satisfactorily fulfil the requirements, is rock in a broken, pulverulent, or plastic state. As, however, all rock is not equally suitable, either from the point of view of its resisting character, or from that of convenience of handling, it becomes necessary to consider which satisfies the two conditions in the most complete manner.

Though it is not easy to assign a perfectly satisfactory reason why one kind of rock substance opposes a greater resistance to motion in a bore-hole than another, yet it is certain that this resistance is mainly due to the friction among the particles of that substance. If a column of solid, hard rock, of the same diameter as the bore-hole, be driven down upon the charge, the resistance opposed by the column to the imprisoned gases will be, neglecting the weight of the former, that of the friction between the sides of the column and those of the hole. But if disintegrated rock be used, not only is an absolute motion imparted to the particles, but, on account of the varying resistances, a relative motion also. Consequently, friction occurs amongst the particles, and as the number of these is immense, the sum of the slight friction of one particle against another, and of the great friction of the outside particles against the sides of the hole, amounts to a much greater value than that of the outside particles of the solid column against the sides of the bore-hole. If this view of the facts alone be taken, it follows that dry sand is the most resistant material, and that the finer the grains, the greater will be the resistance which it offers. In practice, however, it has been found that though the resistance offered by sand tamping is very great, and though also the foregoing inference is true when the tamping is lifted by the pressure of a solid against it from below, this substance is notably inferior to some others when acted upon by an explosion of gases. The explanation of this apparent anomaly is that the gases, under the enormous tension to which they are subjected in the bore-hole, insinuate themselves between the particles, and so prevent the friction which would otherwise take place. When the readiness with which water, through the influence of gravity alone, permeates even closely compacted sand, is borne in mind, there will be no difficulty in conceiving a similar action on the part of more subtile gases in a state of extreme tension. Under such conditions as these, there is no resistance whatever due to friction, and the only resistance opposed to the escape of the gases is that proceeding from the inertia of the mass. How this resistance may be very great, we have shown in the case of air tamping. Hence, it becomes necessary to have recourse to some other material of a composition less liable to be thus acted upon, or to seek means of remedying the defect which renders such action possible.

Clay, dried either in the sun, or, preferably, by a fire, appears to fulfil the requirements of a tamping material in the fullest degree. This substance is composed of exceedingly minute grains of silicious matters, bound together by an aluminous and calcareous or ferruginous cement. Thus constituted, there are no voids between the particles, as in porous substances, and, consequently, there is no passage for the gases, the substance being impervious alike to water and gas. Hence, when this material is employed as tamping, the forces act only upon the lower surface, friction takes place among the particles, and the requisite degree of resistance is produced. By reason of its possession of this property, clay is generally used as the tamping material.

In rock blasting, it is usual to prepare the clay beforehand, and this practice is conducive both to effective results and to rapidity of tamping. The latter consideration is an important one, inasmuch as the operation, as commonly performed, requires a good deal of time. To prepare the pellets of clay, a lump is taken and rolled between the palms of the hands until it has assumed the form of a sausage, from three to four inches in length, and of the diameter of the bore-hole. These pellets are then baked until they are thoroughly dry, when they are ready for use. In making them up to the requisite diameter, a little excess should be allowed for shrinkage, since it is essential that they fit tightly into the hole. When the charge has been put in, and covered with a wad of hay, or a handful of sand or rubbish, one of these pellets is inserted and pushed home with a wooden rammer. Considerable pressure should be applied to make the clay fill the hole completely, but blows should be avoided. A second pellet is then pushed down in the same way, and the operations are repeated until the whole of the hole is tamped. To consolidate the whole, light blows may be applied to the outer pellet. It will be found advantageous to place an undried pellet immediately above the charge, because the plasticity of such a pellet enables it to fill all the irregularities of the sides of the hole, and to securely seal the passage between the sides and the tamping, along which the gases might otherwise force their way. In coal blasting, soft shale is always used for tamping, because it is ready at hand, and heavy shots are not required.

Broken brick constitutes a fairly good tamping material, especially when tempered with a little moisture; but as it is not readily procurable, its application is necessarily limited. The dust and chippings of the excavated rock are largely employed as tamping in quarries. This material, however, has but little to recommend it for the purpose beyond its readiness to hand.

It now remains to consider what means are available for remedying the defect inherent in sand as a tamping material. This constitutes a very important practical question, because if the defect can be removed, sand will constitute by far the most suitable material whenever the bore-hole has a downward direction. It can be everywhere obtained at a low cost; it may be poured into the hole as readily as water; and its application gives rise to no danger. Obviously the difficulty will be overcome if we can find suitable means for preventing the gases from penetrating the sand.

The end proposed may be successfully attained by means of the plastic clay pellet applied in the following manner. Immediately above the charge, place a handful of perfectly dry and very fine sand. This may be obtained by sifting, if not otherwise procurable. Upon this sand, force firmly down with a wooden rammer, so as to fill every irregularity, a plastic clay pellet, about four inches in length, and of the same diameter as the bore-hole, prepared by rolling between the hands in the manner already described. Above this pellet, fill the hole with dry sand. The impervious nature of the clay prevents the gases from reaching the sand, except along the line of junction of the clay with the sides of the hole. Tamped in this way, a resistance is obtained scarcely, if at all, inferior to that opposed by the most carefully placed dried clay.

By the employment of a detonator, the defect due to the porous character of sand is not removed, but its influence is greatly diminished. When detonation is produced in an explosive compound, the full force of the elastic gases is developed instantaneously; and it has already been shown that, under such conditions, the resistance occasioned by the presence of any substance in the bore-hole, even the air alone, in the case of nitro-glycerine, is sufficient to throw the chief portion of the force upon the sides of the hole. Loose sand, therefore, may be successfully employed as tamping under these conditions, since its inertia will oppose a sufficient resistance to the escape of the gases. But though the rock may be dislodged when light tampings are used with detonation, there can be no doubt that a considerable proportion of the force of the explosion is lost; and hence it will always be advantageous to tamp securely by means of the clay pellet, as already described. The highest degree of economy is to be obtained by detonating the charge, and tamping in this manner.

CHAPTER IV.
THE OPERATIONS OF ROCK BLASTING.

Hand Boring.

—When the positions and the directions of the shot-holes have been determined, the operations of blasting are begun by striking a few blows with the hammer upon the spot from which the hole is to start, for the purpose of preparing the surface to receive the drill. In some cases, this preliminary operation will not be needed; but generally some preparation is desirable, especially if the surface be smooth, and the hole be to be bored at an angle with it. For the purpose of illustration, we will take the case of a hole bored vertically downwards, and will suppose the boring to be carried on by double-hand.

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.

When gun-cotton is the explosive agent employed, the wet material which constitutes the charge is put into the shot-hole in cartridges, one after another, until a sufficient quantity has been introduced. Each cartridge must be rammed down tightly with a wooden rammer to rupture the case and to make the cotton fill the hole completely. A length of safety fuse is then cut off, and one end of it is inserted into a detonator cap. This cap is fixed to the fuse by pressing the open end into firm contact with the latter by means of a pair of nippers constructed for the purpose. The cap, with the fuse attached, is then placed into the central hole of a dry “primer,” which should be well protected from moisture. When an electric fuse is used, the cap of the fuse is inserted in the same way into the primer. The primer is put into the shot-hole and pushed gently down upon the charge. As both the dry gun-cotton and the detonator may be exploded by a blow, this operation must be performed with caution.

Cotton-powder or tonite requires a somewhat different mode of handling. It is made up in a highly compressed state into cartridges, having a small central hole for the reception of the detonator cap. This cap, with the safety fuse attached in the way described, or the cap of the electric fuse, is inserted into the hole, and fixed there by tying up the neck of the cartridge with a piece of copper wire placed round the neck for that purpose. The cartridge is then pushed gently down the shot-hole, or, if a heavier charge is required, a cartridge without a detonator is first pushed down, and the “primed” cartridge put in upon it. No ramming may be resorted to, as the substance is in the dry state.

When dynamite is the explosive agent used, a sufficient number of cartridges is inserted into the shot-hole to make up the charge required. Each cartridge should be rammed home with a moderate degree of force to make it fill the hole completely. Provided a wooden rammer be employed, there is no danger to be feared from explosion. A detonator cap is fixed to the end of a piece of safety fuse, and, if water tamping is to be used, grease, or white-lead, is applied to the junction of the cap with the fuse. A “primer,” that is, a small cartridge designed to explode the charge, is then opened at one end, and the detonator cap, or the cap of the electric fuse, is pushed into the dynamite to a depth equal to about two-thirds of its length, and the paper covering of the primer is firmly tied to the cap with a string. If the cap be pushed too far into the dynamite, the latter may be fired by the safety fuse, in which case the substance is only burned, not detonated. With an electric fuse this cannot occur. The same result ensues if the cap be not in contact with the dynamite. The object of tying in the cap is to prevent its being pulled out. The primer thus attached to the fuse is then pushed gently down upon the charge in the shot-hole. It should be constantly borne in mind that no ramming may take place after the detonator is inserted.

Gun-cotton and tonite require a light tamping. This should consist of plastic clay; or sand may be used in downward holes. The tamping should be merely pushed in, blows being dangerous. A better effect is obtained from dynamite when tamped in this way than when no tamping is used. In downward holes, water is commonly employed as tamping for a dynamite charge, especially in shaft sinking, when the holes usually tamp themselves. But in other cases, it is a common practice to omit the tamping altogether to save time.

Firing the Charges.

—When all the holes bored have been charged, or as many of them as it is desirable to fire at one time, preparation is made for firing them. The charge-men retire, taking with them the tools they have used, and leaving only him of their number who is to fire the shots, in the case of squibs or safety fuse being employed. When this man has clearly ascertained that all are under shelter, he assures himself that his own way of retreat is open. If, for example, he is at the bottom of a shaft, he calls to those above, in order to learn whether they be ready to raise him, and waits till he receives a reply. When this reply has been given, he lights the matches of the squibs or the ends of the safety fuse, and shouts to be hauled up; or if in any other situation than a shaft, he retires to a place of safety. Here he awaits the explosion, and carefully counts the reports as they occur. After all the shots have exploded, a short time is allowed for the fumes and the smoke to clear away, and then the workmen return to remove the dislodged rock. If one of the shots has failed to explode, fifteen or twenty minutes must be allowed to elapse before returning to the place. Nine out of ten of the accidents that occur are due to these delayed shots. Some defect in the fuse, or some injury done to it, may cause it to smoulder for a long time, and the blaster, thinking the shot has missed, approaches the fuse to see the effects produced by the shots that have fired. The defective portion of the fuse having burned through, the train again starts, and the explosion takes place, probably with fatal consequences. Thus missed shots are not only a cause of long delays, but are sources of great danger. Accidents may occur also from premature explosion. In this case, the fuse is said to “run,” that is, burn so rapidly that there is not sufficient time for retreat.

When the firing is to take place by means of electricity, the man to whom the duty is entrusted connects the wires of the fuses in the manner described in a former chapter, and as shown in Fig. 50. He then connects the two outer wires to the cables, and retires from the place. Premature explosion is, in this case, impossible. When he has ascertained that all are under shelter, he goes to the firing machine, and, having attached the cables to the terminals, excites and sends off the electric current. The shots explode simultaneously, so that only one report is heard. But there is no danger to be feared from a misfire, since there can be no smouldering in an electric fuse. The face may, therefore, be approached immediately, so that no delay occurs, and there is no risk of accident. Moreover, as all the holes can be fired at the moment when all is in readiness, a considerable saving of time is effected. It is essential to the success of a blast fired by this means that a sufficient charge of electricity be generated to allow for a considerable loss by leakage. If Siemens’ large dynamo-machine be used, the handle should be turned slowly till a click is heard inside, and then, not before, the cable wires should be attached to the terminals. To fire, the handle must be turned as rapidly as possible, a jerky motion being avoided. As considerable force is required, the machine must be firmly fixed. If a frictional machine be used, care must be had to give a sufficient number of turns. As this kind of machine varies greatly, according to the state of the rubbing surfaces and the degree of moisture in the atmosphere, it should always be tested for a spark before firing a blast. In this way only, can the number of turns required be ascertained. It is important that the discharging knob should be pushed in, or, as the case may be, the handle turned backward, suddenly. A slow motion may be fatal to the success of a blast. In testing Bornhardt’s machine, the handle should always be turned forwards; but in firing, half the number of turns should be given in one direction and half in the other. The following table shows the number of turns required for a given number of André’s fuses with Bornhardt’s machine. The first column, containing the least number of turns, may be taken also for Julian Smith’s machine as manufactured by the Silvertown Company with the modifications suggested by W. B. Brain.

Places of refuge, called man-holes, are often provided in headings for the blaster to retire into; these man-holes are small excavations made in the sides of the heading. Sometimes it is necessary to erect a shield of timbers in the heading for the protection of the men; such a shield is frequently needed to protect machine drills from the effects of a blast. In Belgium, it is a common practice to provide man-holes in the sides of a shaft as places of retreat for the men; these holes are called caponnières. Instead of caponnières, a hollow iron cylinder is sometimes used as a protection to the men. This cylinder is suspended in the shaft at a height of a few yards from the bottom, and is lowered as the sinking progresses. The men climb into this cylinder to await the explosion of the shots beneath them.

The workmen, on returning to the working face, remove the dislodged rock, and break down every block that has been sufficiently loosened. For this purpose, they use wedges and sledges, picks, and crowbars. And not until every such block has been removed, do they resume the boring for the second blast. Sometimes, to facilitate the removal of the rock dislodged by the shots, iron plates are laid in front of the face in a heading. The rock falling upon these plates is removed as quickly as possible, to allow the boring for the succeeding blast to commence. It is important, in the organization of work of this character, that one gang of men be not kept waiting for the completion of the labour of another.

Machine Boring.

—In machine drilling, the operations necessarily differ somewhat in their details from those of hand boring, and, in some cases, other methods of procedure will be adopted more suitable to the requirements of machine labour. It may even be, and in most cases indeed is, inexpedient to follow closely the principles which lead to economy of the explosive substance employed, since the more restricted conditions under which machine power may be applied may point to more important gains in other directions. Thus it may be found more conducive to rapidity of execution to determine the position and the direction of the shot-holes rather to satisfy the requirements of the machine than those of the lines of least resistance; or, at least, these requirements must be allowed to have a modifying influence in determining those positions and directions. For it is obvious that holes cannot be angled with the same ease when a machine drill is used, as they can when the boring is executed by hand.

Boring the Shot-holes.

—It has already been remarked that the exigencies of machine labour render it impracticable to follow closely the principles which lead to economy of labour and material in blasting. In hand boring, economy is gained by reducing to a minimum the number of holes and the quantity of explosive substance required. But in machine boring, economy is to be sought mainly in the reduction of the time needed to accomplish the driving.

Attempts have been made to assimilate the methods of machine boring to those adopted for hand labour, but the results have not been satisfactory. On the contrary, the conditions determining the position and the direction of the holes relatively to the production of the greatest useful effect have been wholly ignored in favour of those which determine the most rapid boring. This system has been attended with more satisfactory results. Another system, partaking of both the preceding, is widely adopted, and hitherto the best results have been obtained from this, which may be regarded as a compromise between conflicting conditions. Thus we have three systems of executing machine boring: one in which a single machine is used upon a support capable of holding it in any position, so as to be able to bore at any angle, and in which the holes are placed according to the lines of least resistance, as in hand boring. A second, in which several machines are fixed upon a heavy support, allowing but little lateral or angular motion, and in which the holes are placed at regular intervals apart, and bored parallel, or nearly parallel, with the axis of the excavation, irrespective of the varying nature of the rock, and the lines of least resistance. And a third, in which it is sought, by the employment of one, two, or at the most three machines, upon a simple and light support allowing the position and direction of the machine to be readily changed, to satisfy in some degree the two sets of conditions determining the two former systems, by placing the shot-holes as far in accordance with the lines of resistance as the exigencies of a fairly rapid handling of the machine will allow.

In the first of these systems, the necessity for extreme lightness in the machine is unfavourable to its efficient action, and the great length of time consumed in changing the position of the machine, so as to comply with the conditions of resistance in the rock, render it impossible to attain a much higher rate of progress than is reached by a well-regulated system of hand boring. With such a result, there is nothing to compensate the first cost of the machinery, or in any way to justify its adoption. In the second system, the time consumed in removing and fixing the machines is reduced to a minimum, and the chief portion of the time during which the machines are at the working face is, consequently, occupied in actual boring, a circumstance that is highly favourable to machine labour. Hence the rate of progress attained by this system is greatly in excess of that accomplished by hand labour; and this superiority has led to the adoption of the system in several important cases, and has also led many to regard it as the most favourable to the exigencies of machine drilling. But as the holes are bored to suit the requirements of the machine, quite irrespectively of the resistance of the rock, their positions and directions are very unfavourable to the action of the explosive. This circumstance necessitates a much greater number of holes to ensure the fracture of the rock around each charge, and hence the time saved in shifting the machines is in part lost in extra boring; besides which, the consumption of powder is enormously increased. It would, therefore, appear that the full advantages of machine boring are to be obtained from the intermediate system, if carried out in accordance with all the conditions of the case.

Assuming that we have a machine and a support of such dimensions, weight, and construction as to be capable of being readily placed in position, it is evident that we shall still require a much larger number of holes than would be needed if the boring were performed by hand, because they are not placed wholly in accordance with the lines of least resistance. In some parts of the heading, indeed, these lines will have to be wholly neglected, in order to avoid the loss of time involved in shifting the supports; for the principle upon which an intermediate system is based is to fulfil the requirements of the lines of least resistance, when that can be conveniently done, and to neglect them, when such fulfilment would involve a considerable expenditure of labour and time.

In this way, the time both for fixing and removing the machines and of boring is reduced to a minimum, and thus two conditions favourable to rapid and economical progress is ensured. It is evident that when this system is followed, the face will not require the same number of holes at each blast. Another circumstance operating to increase the number of shot-holes is the desirability of bringing down the face in fragments small enough to be lifted without great difficulty. When the rock is completely broken up, the labour, and, consequently, the time of removing it after each blast, are lessened in an important degree. Hence there will be an advantage in placing the shot-holes sufficiently close together to ensure the fracture of the mass between each. These circumstances render it necessary to bore a large number of holes when the work is done by mechanical means. The boring of the additional holes reduces the superiority of machine over hand labour, and the additional quantity of the explosive required augments the cost of the work. To counterbalance these disadvantages, the shot-holes should be bored deep. It has already been pointed out that when a hole is once started with a machine, the rate of progress is immensely superior to that attained in hand boring, and to profit by this advantage, the hole should be continued to as great a depth as practicable. This is sufficiently obvious, since it amounts to increasing the proportion of the whole time consumed that is occupied in actual boring; for as it is in the rapidity of the operation of boring alone that the superiority of machine labour exists, it is plain that the longer the proportion of the time so occupied, the more marked that superiority will be. Thus, by increasing the depth of the holes to the farthest practicable limit, we approximate as much as possible to the condition most favourable to machine boring. The intermediate system, therefore, which takes full advantage of this means, will lead to the best results. To recapitulate the main points of such a system; it should follow the lines of least resistance when that can be conveniently done, and neglect them when the fulfilment of their requirements would occasion a considerable expenditure of time; and to counterbalance the disadvantages of machine boring, it should employ shot-holes of as great a depth as is practicable.

Supposing such a system in use, it now remains to consider the operations of boring, and the subsequent operations of charging, firing, and removing the rock dislodged by the blast. Of the method of executing the boring, little remains to be said. It may, however, be well to direct attention to the necessity of keeping the holes clear of the débris. To ensure this, the bits should be chosen of a form suitable to the nature and the structure of the rock, and the hole kept well supplied with water. When the hole becomes deep, it should be cleared out with a scraper during the time of changing the bit, and in very argillaceous rock it may become necessary sometimes to withdraw the tool, and to remove the accumulation with the scraper. When the débris does not work out freely, its escape may be facilitated by giving a slow motion to the tool, and then suddenly changing to a rapid motion. When several machines are employed, the maximum number that can be applied with advantage is one to the square yard of working face. The absolute number of holes required in any case, will, of course, depend upon the tenacity of the rock and the development of the joint planes, and also, in some degree, by the lines of fracture due to the preceding blast. The same circumstance will determine the distribution of the holes. Leaving minor variations out of account, however, the same distribution will be adhered to throughout the driving.

The manner of distributing the holes over the face of the heading may be varied according to the judgment of the engineer in charge; that is, the general features of the distribution to be adopted may be chosen to suit the requirements of the machines and their supports. Also, it should be noted that one method of distributing the shot-holes will require a less number of them than another. Some examples will be found on Plate IX., where there are represented the Göschenen end of the St. Gothard tunnel; the Airolo end of the same tunnel; the face of a stone drift driven at Marihaye; that of a similar drift at Anzin; and that of a drift of the same character at Ronchamp; the latter three examples being typical of the distribution adopted in the French collieries.

The same mode of unkeying the face is adopted with machine as with hand boring. Generally, two parallel rows of holes, from two to five in a row, are bored in the middle of the face or fore-breast, the rows being from 18 inches to 30 inches, according to the strength of the rock, apart on the surface, and angled so as to be from 9 inches to 15 inches apart at the bottom. These shots unkey the fore-breast; and it is greatly conducive to a successful accomplishment of the operation, to fire these shots simultaneously. Sometimes, when dynamite is used, another method is adopted. A hole is bored horizontally in the centre; at about three inches distant, are bored three other holes at an equal distance apart. These latter are heavily charged with dynamite, the centre hole being left empty. When these charges are fired, the rock between them is crushed, and a large hole made. The lines of fracture of the subsequent shots run into this hole. In this case, it is even more desirable than in the preceding to fire the central shots simultaneously.

In shaft sinking, if the strata are horizontal or nearly so, it is usual to unkey from the centre, as in the heading. But if they be highly inclined, it will be better to unkey from one side of the excavation. The water which flows into the workings must be collected into one place, both for convenience in raising it, and for the purpose of keeping the surface of the rock clear for the sinkers. The depression caused by the removal of the key serves to collect the water, and, on that account, it is called “the sump.” Into this sump, the tub dips, or, when pumps are used, the suction pipe drops. When the strata are highly inclined, the water gravitates towards the dip side of the excavation, and it becomes, therefore, necessary to place the sump in that situation. The unkeying of the rock from this direction is, moreover, favourable to the effect of the shots. In putting in the shot-holes, it is well to avoid, as far as possible, terminating them in, or nearly in, a bedding plane, because when so terminated, the force of the charge expends itself along this plane. The position and the direction of the holes will, however, be determined in some degree by the character of the support used for the drills, and by other conditions of convenience.

Charging and Firing.

—The operations of charging the holes and firing the shots demand particular attention when machine labour is employed. It has been pointed out in the foregoing paragraphs that holes bored by machine drills cannot be placed or directed strictly in accordance with the requirements of the lines of least resistance; but that, on the contrary, these requirements can only be approximately complied with, and in some cases must be wholly neglected. To compensate in some degree this defect of machine labour, the strength of the charges should be varied according to the resistance which they will be required to overcome. That is, the principles of blasting described in a former chapter, which cannot be complied with by the borer, should be strictly followed by the blaster in apportioning his charges. By this means, a great saving of the explosive compound may be effected, and that without difficulty or loss of time, if the blaster be intelligent and understand his work. A glance will be sufficient to show what charge a given hole of a known depth will require, and as cartridges of different sizes are ready at hand, no delay is occasioned in making up the charge. The holes in the centre, which are intended to unkey the face, require, of course, the heaviest charge, since the conditions are there most unfavourable to the effects of the explosion. And the more complete is the unkeying resulting from this first explosion, and the more fractured and jointed is the rock surrounding the cavity thus formed, the more may the charges placed behind these unsupported faces be reduced.

As economy of time is, in machine boring, the chief end to be attained, the tamping should be done with dried clay pellets previously prepared. This material gives the greatest resistance, and thereby ensures the maximum of useful effect; and if prepared beforehand, in the manner described in the preceding chapter, the time consumed in tamping will be reduced to a minimum. An abundant supply of such pellets should always be ready at hand. In downward holes, such as are used in shaft sinking, the plastic clay pellet and sand may be employed. This tamping may be put in very rapidly, and, in all but very shallow holes, it is very effective. When it is desired to use sand tamping in horizontal holes, and holes bored in an ascending direction, the sand should be made up in paper cartridges. The tamping employed in the St. Gothard tunnel consisted of sand prepared in this manner. At the Mont Cenis tunnel, an argillaceous earth was similarly prepared in paper cartridges for tamping.

Firing the charges also affords an occasion for the exercise of knowledge and judgment. A skilful determination of the order in which the charges are to be fired will in a great measure compensate the ill effects of badly-placed holes. The firing of a shot leaves the surrounding rock more or less unsupported on certain sides; and it is evident that to profit fully by the existence of these unsupported faces, the succession of explosions must be regulated so that each shall have the advantage of those formed by the preceding shots. This condition can be wholly fulfilled only by simultaneous firing; but when the firing is to take place successively, the condition may be approximated to by regulating the succession according to the indications observed on a careful inspection of the rock. Before firing the charges, the blaster should consider the relative positions of the holes, the stratification and jointing of the rock, the fissures caused by the preceding blast, and any other circumstances that may influence the results. The charges intended to unkey the face will be fired first, and those in the concentric series will be then fired, in the order determined upon, by means of different lengths of fuse. The series will follow each other from the centre outwards. When a large number of shots regularly placed in series have to be fired, a convenient practical means of ensuring the successive explosion of the series, in the case of the whole being lighted simultaneously, consists in bringing the fuses from all the shot-holes together to one point at the centre. This method of regulating the length of the fuses was adopted at the St. Gothard tunnel.

It is obvious that the acceleration of the labour of excavation, which has been effected in so remarkable a degree by the introduction of machine drills and strong explosives, may be still further promoted by the adoption of electricity as the firing agent. The advantages of firing a number of shots simultaneously, some of which have already been pointed out, are great and manifest. In the case of a driving, for example, when all the holes have been bored and charged, and the machines withdrawn, it is clearly desirable to blast down the face as quickly and as effectively as possible. If the whole of the shots can be fired at once, the time is reduced to a minimum, and, consequently, the maximum of progress in a given time is ensured. Electricity affords, indeed, the most convenient, the most effective, and the most safe means of firing blasts. Hofrath Ritter von Pischof, the Austrian Chief Inspector of Railways, in one of his reports, says:—“A greatly increased amount of work and a notable saving of cost are effected when the shots can be so disposed and fired as to mutually aid one another. These results are obtained by employing electricity as the firing agent. The experience which has been gained at the Büchenberg cutting, where electrical firing has been extensively adopted, has shown that, when properly employed, this means allows, in comparison with the ordinary methods, twice the amount of work to be performed in a given time. It is therefore highly desirable to adopt electrical blasting whenever it is a question of economy of time and money.”

Removing the dislodged Rock.

—As the removal of the rock brought down by the blast consumes a large proportion of the time saved by machine boring, it becomes necessary to provide means for reducing this loss to a minimum. The most important of these means is a suitable provision for the rapid removal of the machine to a place of safety, and a conveniently designed and well-laid tramway, upon which the rock may be quickly run out without confusion and its consequent delay. The number of wagons required to remove a given cube of rock may be readily ascertained, and sufficient provision should be made for the transport of these to “day” in the most rapid succession. The wagons should be of such dimensions as to be capable of being handled without great difficulty; the importance of this condition will be understood when the frequency of derailments is borne in mind. The shovelling up of the rubbish is greatly facilitated by laying iron plates in front of the face to be brought down previously to the firing of the blast. This expedient is often adopted in important drivings. It has also been remarked that the dislodged rock can be more rapidly removed when it exists in small blocks. Thus there will be an advantage in placing the charges and in regulating their strength so as to completely break up the rock. Another matter of importance in the arrangements for the rapid removal of the rock brought down by the blast, is the proportioning of the number of hands employed to the requirements of the case. This number will increase with the size of the blocks to be lifted, the distance to be run over, and the want of suitability in the matériel employed.

Division of Labour.

—A proper division of labour is greatly conducive to rapid and economical progress. The operations may be divided into three series, namely: boring the shot-holes, charging and firing, and removing the rock dislodged. Each of these series of operations may be performed by different sets of men, and in several instances this division of labour has been adopted. But it does not appear that such a division leads to the most satisfactory results. The work of boring occupies a much longer time than either of the other two series of operations, and hence the distribution of the time is unequal. It has been found that, generally, where all the arrangements have been well considered, the labour of charging the shot-holes, firing the blast, and removing the rock brought down, can be performed in about the same time as that of boring. Thus it would seem to be more conducive to economy of time to divide the men employed into only two sets: one set to bore the holes, the other to perform all the subsequent operations. This division has been adopted in numerous instances with favourable results. Sometimes the whole of the operations have been performed by the same set; but such an arrangement is not to be recommended. The labour of directing the machines is of too distinct and skilled a character to be confounded with that of removing the débris, without a strong reason for such a proceeding, which does not appear to exist. Besides reserving a set of men specially for this portion of the work, it is desirable to keep the same men to the same machine, for in such a case each man gets accustomed to the peculiarities of the machine entrusted to him, and besides conceives a kind of affection for it that leads to careful handling and watchful attention. In addition to the men required for the operations referred to above, smiths will be needed to re-sharpen the bits and to repair the machines. The amount of this labour will obviously depend upon the number of machines employed, and the hardness of the rock to be passed through.

Examples of Drivings.

The St. Gothard Tunnel.

—The St. Gothard tunnel is driven in five sections. First, the “heading” is driven at the roof level 6 feet 6 inches wide, and 7 feet high. The position of the holes is shown in the drawings on Plate IX. The number of holes at the Göschenen end is 28, and the depth about 40 inches. The shots are fired by means of safety fuse, the ends of the fuse being brought together at the centre. This arrangement causes the shots to explode in the proper order of succession. At a certain distance back from the face, is the “right enlargement;” this is a widening of the heading to the limits of the tunnel in that direction. Farther back is the “left enlargement,” by which the heading is widened to the full width of the tunnel. Still farther back is the first “bench cut,” in which one half of the floor is blasted out to the full depth of the tunnel, and behind this again is the second bench cut, in which the remaining half is removed. The boring machines employed are the Dubois-François, the McKean, and the Ferroux. The explosive agent used is dynamite. The rock is a tough granite.

The Hoosac Tunnel.

—At the west end of the Hoosac tunnel, the system adopted was the following. First, a centre cut was made by drilling two rows of five or six holes each, about 9 feet apart on the face, and converging to about 3 feet at their lower ends. The depth of these holes was from 9 to 12 feet, according to the hardness of the rock. These holes are numbered from 1 to 11 on Plate X. They were charged with nitro-glycerine, and fired by electricity, Mowbray’s frictional machine being used. As soon as the rock had been removed, the next series of fourteen holes, numbered from 12 to 25, were drilled. These holes were then charged and fired simultaneously like those of the first series. When the rock dislodged had been removed, the third series of holes, numbered from 26 to 41, were bored. This series, like the other two, were charged, and fired by electricity. The effect of these three blasts, which were fired within twenty-four hours, was to advance the heading, 9 feet in height by the full width of 24 feet, to the extent of 7 feet 6 inches. The drawings on Plate XI. are: an elevation of the fore-breast, which shows the positions of the shot-holes; a sectional plan, which shows the directions of the first series of holes; a similar plan, showing the directions of the second series of holes, and the centre cut removed; and a sectional plan of the heading after the second series have been fired, showing the direction of the third series of holes.

The operations of taking out the “bench” were carried on at a distance of about 170 yards back from the fore-breast. This was effected by first drilling six holes 7 feet deep; two of these were each about 4 feet from the face of the bench and close to the side of the tunnel, whilst two others were each 4 feet behind these first holes, and the remaining two holes were 8 feet from the face, 8 feet from the sides of the tunnel, and 8 feet from each other. These were fired simultaneously, the result being to lower the bench about 7 feet throughout the full width of the tunnel. At a safe distance beyond this first bench cut, the same operations were carried on by another gang of men, whereby the bench was lowered to the floor of the tunnel, the full area of 24 feet in width by 22 in height being thus completed. The rock was a moderately tough granite.

The Musconetcong Tunnel.

—The heading of the tunnel, shown on Plate XII., like that of the Hoosac, was driven to the full width of the tunnel. It is clear from theoretical considerations, and experience has confirmed the conclusions, that the method of taking, with machine drills, the whole width of the excavation at once conduces to rapidity of advance, and to economy of explosive. In the example under consideration, three tram lines were laid up to the face. The carriages carrying the drills were run upon the two outside lines. These carriages were simply stout frameworks of oak, each having in front three horizontal iron bars, on which the drills were clamped in a way that ensured easy lateral and vertical motion. After the firing of a blast, all hands were set to shovel the dislodged rock into the middle between the machine lines for the purpose of clearing the latter as soon as possible to make way for the machines to be brought up for the next boring. The lines being thus cleared, drilling was recommenced, and the broken rock removed in wagons upon the centre line of rails. The heading being 26 feet wide, there was ample room, and, a convenient system of switching having been adopted, no delay was occasioned by a want of wagons.

The system followed was that of centre cuts, and subsequent squaring up. It consists in first blasting out an entering wedge or “key,” about 10 feet deep in this case, in the centre, and afterwards squaring up the sides by several blasts. In the Musconetcong heading, twelve holes were first drilled, as shown in the drawing, and marked C, A being the floor of the heading. These holes were drilled with from 1¹⁄₂-inch to 2³⁄₄-inch “bits,” in two rows of six, 9 feet apart on the face, and angled to meet at the bottom. They were charged with 25 lb. of No. 1 and 50 lb. of No. 2 dynamite, and fired simultaneously by electricity. The No. 1 dynamite was used in the bottom of these centre holes; in all the subsequent blasts in squaring up, No. 2 only was used.

As soon as the cut was out, a second round of holes was started for the first squaring up, as shown in the drawings, where they are numbered 1, 1, 1, 1, &c. In these and in the subsequent rounds, numbered 2, 2, 2, 2, &c., and 3, 3, 3, 3, &c., the resistance to be overcome is, of course, not so great as in the cut. In the first and the second squaring-up rounds, from 50 lb. to 60 lb. of dynamite was used, and, in the third round, this quantity was increased to 80 lb. or 90 lb., the resistance becoming greater as the roof arch falls at the sides. In this third round, there were generally one or two additional roof holes; these are not shown in the drawing, as their position varied, according to the lay of the rock. The top holes in the first round are also intended to bring down any roof not shaken by the cut, and these are therefore angled sharply towards the centre, and bored from 12 feet to 14 feet deep. In the plan, Plate XII., the number 3 indicates the cut holes, and 4, 5, and 6, the squaring-up rounds. The holes of the first squaring round were always drilled about a foot deeper than the cut holes; when blasted, these generally brought out an additional foot of shaken rock at the apex of the cut. The following table shows approximately the number and the depth of the holes required, and the quantity of dynamite used for a linear advance of 10 feet.