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

by George G. André

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.