Section VI.—Some Varieties of the Nitro-cellulose and the Nitro-glycerine Compounds.
Nitrated Gun-cotton.
—It has been shown that gun-cotton contains an insufficient quantity of oxygen for its complete combustion. To furnish that which is wanting, gun-cotton has sometimes incorporated with it a certain proportion of nitrate of potash, or of nitrate of baryta. This compound, which, it will be observed, is at once a chemical compound and a mechanical mixture, is known as “nitrated gun-cotton.”
Cotton Powder, or Tonite.
—The explosive which is now well known as “tonite” or “cotton powder,” is essentially nitrated gun-cotton. It is produced in a granulated form, and is compressed into cartridges of various dimensions to suit the requirements of practice. The convenient form in which tonite is made up, ready to the miner’s hand, has greatly contributed towards bringing it into favour. But irrespective of this, the fact of its being so highly compressed as to give it a density equal, or nearly equal, to dynamite gives it a decided advantage over the other nitro-cotton compounds as they are at present used.
Schultze’s Powder.
—In Schultze’s powder, the cellulose is obtained from wood. The wood is first sawn into sheets, about 1⁄16 inch thick, and then passed through a machine, which punches it up into grains of a uniform size. These are deprived of their resinous matters by a process of boiling in carbonate of soda, and are further cleansed by washing in water, steaming, and bleaching by chloride of lime. The grains, which are then pure cellulose, are converted into nitro-cellulose in the same way as cotton, namely, by being treated with a mixture of nitric and sulphuric acids. The nitro-cellulose thus produced is subsequently steeped in a solution of nitrate of potash. Thus the finished compound is similar in character to nitrated gun-cotton.
Lithofracteur.
—Lithofracteur is a nitro-glycerine compound in which a portion of the base is made explosive. In dynamite, the base, or absorbent material, is, as we have said, a silicious earth, called “kieselguhr.” In lithofracteur, the same substance is used; but in addition, a mixture of nitrate of baryta and charcoal, a kind of gunpowder, is introduced. The object of employing this explosive mixture is to increase the force of the explosion, the kieselguhr being an inert substance. Obviously this object would be attained if the explosive mixture possessed the same absorbent power as the kieselguhr. But unfortunately it does not, and, as a consequence, less nitro-glycerine is used. Thus what is gained in the absorbent is lost in the substance absorbed. The composition of lithofracteur varies somewhat; but its average proportion of ingredients are the following:—
| Nitro-glycerine | 52·50 |
| Nitrate of baryta | 16·40 |
| Charcoal | 2·85 |
| Sulphur | 25·75 |
| Kieselguhr | 22·50 |
| 100·00 |
Brain’s Powder.
—Brain’s powder is a nitro-glycerine compound, similar in character to lithofracteur. The exact composition of the base has never been published, so far as relates to the proportions of the ingredients. But it is composed of chlorate of potash, charcoal, and nitrated sawdust. The proportion of nitro-glycerine never exceeds 40 per cent. Horseley’s powder contains about the same proportion of nitro-glycerine in a base of chlorate of potash and nut-galls.
Cellulose Dynamite.
—In Germany, gun-cotton is used as an absorbent for nitro-glycerine, the compound being known as “Cellulose dynamite.” It is chiefly used for primers to explode frozen dynamite. It is more sensitive to blows than the kieselguhr dynamite.
CHAPTER III.
The Principles of Rock Blasting.
Line of Least Resistance.
—The pressure of a fluid is exerted equally in all directions; consequently the surrounding mass subjected to the force will yield, if it yield at all, in its weakest part, that is, the part which offers least resistance. The line along which the mass yields, or line of rupture, is called the “line of least resistance.” If the surrounding mass were perfectly homogeneous, it would always be a straight line, and it would be the shortest distance from the centre of the charge to the surface. Such, however, is never the case, and the line of rupture is, therefore, always a more or less irregular line, and often much longer than that from the centre direct to the surface. It will be obvious, on reflection, that the line of least resistance will be greatly dependent upon (1) the texture of the rock, which may vary from one point to another; (2) its structure, which renders it more easily cleavable in one direction than in another; (3) the position, direction, and number of the joints, which separate the rock into more or less detached portions; and (4) the number and relative position of the unsupported faces of the rock. All these circumstances must be ascertained, and the position and the direction of the bore-hole determined in accordance with them, in order to obtain the maximum effect from a given quantity of explosive. It must not be supposed, however, that this is a labour involving minute examination and long consideration. On the contrary, a glance is generally sufficient to enable the trained eye to estimate the value of those circumstances, and to determine accordingly the most effective position for the shot. In practice, the line of least resistance is taken as the shortest distance from the centre of the charge to the surface of the rock, unless the existence of joint planes, a difference of texture, or some other circumstance, shows it to lie in some other direction.
Force required to cause Disruption.
—When the line of least resistance is known, it remains to determine the quantity of the explosive compound required to overcome the resistance along that line. This matter is one of great importance, for not only is all excess waste, but this waste will be expended in doing mischief. In mining operations, the dislodged rock is violently projected, and the air is vitiated in an unnecessary degree; and in quarrying, stones are shattered which it is desirable to extract in a sound state. The evil effects of overcharging, in occasioning the formation of noxious gases, was pointed out in the last chapter. Of course it is not possible so to proportion a charge to the resistance that the rock shall be just lifted out, and no more; because neither the force developed by the charge, nor the value of the resistance can be known with precision. But a sufficient approximation may be easily arrived at to enable us to avoid the loud report that is indicative of wasted force.
Charges of an explosive compound of uniform strength produce effects that vary as the weight of those charges, that is, a double charge will move a double mass. And, as homogeneous masses vary as the cube of any similar line within them, the general rule is established that charges of powder capable of producing the same effects are to each other as the cubes of the lines of least resistance. Generally, the quantity of black blasting powder requisite to overcome the resistance will vary from 1⁄20 to 1⁄30 of the cube of the line of least resistance, the latter being measured in feet and the former in pounds. Thus, if the rock to be blasted be moderately strong limestone, for example, and the shortest distance from the centre of the charge to the surface of the rock be 3 feet, we shall have 3 × 3 × 3 = 27, the cube of the line, and 27⁄25 lb. = 12⁄25 lb., or about 1 lb. 1 oz., as the weight of the powder required. If dynamite be used, and we assume it to be four times as strong as common black powder, of course, only one-fourth of this quantity will be required. Also if gun-cotton, or cotton-powder, be used, and we assume its strength to be three times that of black powder, one-third only will be needed. Again, if Curtis’s and Harvey’s new extra-strong mining powder fired by a detonator be employed, we may assume it to be twice as strong as common black powder fired by the ordinary means, and consequently we shall need only one-half the quantity indicated by the formula.
It is neither practicable nor desirable that such calculations and measurements as these should be made for every blast; their practical value lies in this, namely, that if the principles involved in them be clearly understood, the blaster is enabled to proportion his charges by sight to the resistance to be overcome, with a sufficient degree of precision. A few experiments in various kinds of rock, followed by some practice, will enable a man to acquire this power.
As it is a common and a convenient practice to make use of the bore-hole as a measure of the quantity of explosive to be employed, we have calculated the following table:—
| Diameter of the Hole. | Black Powder in 1 inch. | Gun- cotton in 1 inch. | Dynamite (or Tonite) in 1 inch. | |
|---|---|---|---|---|
| ins. | ozs. | ozs. | ozs. | |
| 1 | 0·419 | 0·419 | 0·670 | |
| 1 | 1⁄4 | 0·654 | 0·654 | 1·046 |
| 1 | 1⁄2 | 0·942 | 0·942 | 1·507 |
| 1 | 3⁄4 | 1·283 | 1·283 | 2·053 |
| 2 | 1·675 | 1·675 | 2·680 | |
| 2 | 1⁄4 | 2·120 | 2·120 | 3·392 |
| 2 | 1⁄2 | 2·618 | 2·618 | 4·189 |
| 2 | 3⁄4 | 3·166 | 3·166 | 5·066 |
| 3 | 3·769 | 3·769 | 6·030 | |
Fig. 40.
Fig. 41.
Fig. 42.
Fig. 43.
Conditions of Disruption.
—Having explained the law according to which the elastic gases evolved by an explosion act upon the surrounding rock, and shown how the force required to cause disruption may be calculated, it now remains to consider the conditions under which disruption may take place. Suppose a block of unfissured rock detached on all sides, as shown in plan, in [Fig. 40], and a bore-hole placed in the centre of this block. If a charge be fired in this position, the lines of rupture will radiate from the centre towards any two, or towards all four of the unsupported faces of the block, because the forces developed will act equally in all directions, and the lines of rupture will be those of least resistance. Evidently this is the most favourable condition possible for the charge, since the rock offers an unsupported face on every side; and it is evident that the line of rupture must reach an unsupported face to allow of dislodgement taking place. Suppose, again, as shown in [Fig. 41], the block to be unsupported on three sides only, and the charge placed at h. In this case, the lines of rupture may run to any two, or to all three, of the unsupported faces; and hence this will be the next most favourable condition for the action of the charge. The greatest useful effect, however, will be obtained in this case by placing the charge farther back at h′, when the lines of rupture must necessarily run to the opposite faces b c, and, consequently, the whole of the block will be dislodged. Assume another case, in which the rock is unsupported upon only two sides, as shown in [Fig. 42], and the charge placed at h. In this case, the lines of rupture must run to each of the unsupported faces a b. Thus, it is evident that this condition, though still a favourable one for the good effect of the charge, is inferior to the preceding. As rock is never homogeneous in composition nor uniform in texture, the lines of rupture, which, as before remarked, will be those of least resistance, may reach the faces at any point, as at m n, m′ n′, or any point intermediate between these. But it will be seen that the useful effect will be greatest when these lines, radiating from the charge, make an angle of 180°, or, in other words, run in directly contrary directions, and that the useful effect diminishes with the angle made by these lines of rupture. Suppose, again, the rock to be unsupported upon one side only, as shown in [Fig. 43], and the charge placed at h. In this case, the lines of rupture must run to the face a, and the condition must therefore be considered as less favourable than the preceding. As in those cases, the useful effect will depend upon the angle made by the lines of rupture h m and h n, which angle may be very small, and which must necessarily be much less than 180°. A greater effect may be obtained, under this condition, by firing several charges simultaneously. If, for example, we have two charges placed, one at h, and the other at h′, and fired successively, the lines of rupture will run in or near the directions h m, h n, h′ m′, h′ n′, and the portion of rock dislodged will be m h n h′ n′. But if these two charges be fired simultaneously, the lines of rupture will be h m, h o, h′ o, h′ n′, and the mass of rock dislodged will be m h h′ n′. Simultaneous firing is in this way productive of a greatly increased useful effect in numerous cases, and the mining engineer, and the quarryman especially, will do well to direct their attention to this source of economy. There is yet another case to be considered, in which the conditions are still less favourable. Suppose two unsupported faces at right angles to each other, and the charge placed at h, as shown in [Fig. 44]. In this case, the lines of rupture will run to each of the two unsupported faces; but as these lines must necessarily make a very small angle with each other—for the length of the lines increases rapidly with the angle—the useful effect will be less than in the last case. It follows, therefore, that this is the most unfavourable condition possible, and as such it should be avoided in practice.
Fig. 44.
Fig. 45.
Fig. 46.
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.
Fig. 47.
Fig. 48.
Fig. 49.
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.
Fig. 50.
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.
| Firing Table for Frictional Machine. | |||
|---|---|---|---|
| When the Machine sparks with 10 Turns. | When the Machine sparks with 12 Turns. | When the Machine sparks with 14 Turns. | |
| Fuses in Circuit. | Number of Turns. | Number of Turns. | Number of Turns. |
| 4 | 12 | 15 | 17 |
| 5 | 12 | 15 | 17 |
| 6 | 14 | 17 | 20 |
| 7 | 16 | 19 | 22 |
| 8 | 18 | 22 | 25 |
| 9 | 20 | 24 | 28 |
| 10 | 22 | 26 | 31 |
| 11 | 24 | 28 | 34 |
| 12 | 25 | 30 | 35 |
| 13 | 26 | 31 | 36 |
| 14 | 27 | 33 | 38 |
| 15 | 28 | 34 | 39 |
| Note.—If the machine does notspark with 14 turns, the rubber should be taken out and brushed. | |||
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.