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.
