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.