Rock Blasting / A Practical Treatise on the Means Employed in Blasting Rocks for Industrial Purposes

by George G. André

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.