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

by George G. André

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.

Supposing such a system in use, it now remains to consider the operations of boring, and the subsequent operations of charging, firing, and removing the rock dislodged by the blast. Of the method of executing the boring, little remains to be said. It may, however, be well to direct attention to the necessity of keeping the holes clear of the débris. To ensure this, the bits should be chosen of a form suitable to the nature and the structure of the rock, and the hole kept well supplied with water. When the hole becomes deep, it should be cleared out with a scraper during the time of changing the bit, and in very argillaceous rock it may become necessary sometimes to withdraw the tool, and to remove the accumulation with the scraper. When the débris does not work out freely, its escape may be facilitated by giving a slow motion to the tool, and then suddenly changing to a rapid motion. When several machines are employed, the maximum number that can be applied with advantage is one to the square yard of working face. The absolute number of holes required in any case, will, of course, depend upon the tenacity of the rock and the development of the joint planes, and also, in some degree, by the lines of fracture due to the preceding blast. The same circumstance will determine the distribution of the holes. Leaving minor variations out of account, however, the same distribution will be adhered to throughout the driving.

The manner of distributing the holes over the face of the heading may be varied according to the judgment of the engineer in charge; that is, the general features of the distribution to be adopted may be chosen to suit the requirements of the machines and their supports. Also, it should be noted that one method of distributing the shot-holes will require a less number of them than another. Some examples will be found on [Plate IX.], where there are represented the Göschenen end of the St. Gothard tunnel; the Airolo end of the same tunnel; the face of a stone drift driven at Marihaye; that of a similar drift at Anzin; and that of a drift of the same character at Ronchamp; the latter three examples being typical of the distribution adopted in the French collieries.

The same mode of unkeying the face is adopted with machine as with hand boring. Generally, two parallel rows of holes, from two to five in a row, are bored in the middle of the face or fore-breast, the rows being from 18 inches to 30 inches, according to the strength of the rock, apart on the surface, and angled so as to be from 9 inches to 15 inches apart at the bottom. These shots unkey the fore-breast; and it is greatly conducive to a successful accomplishment of the operation, to fire these shots simultaneously. Sometimes, when dynamite is used, another method is adopted. A hole is bored horizontally in the centre; at about three inches distant, are bored three other holes at an equal distance apart. These latter are heavily charged with dynamite, the centre hole being left empty. When these charges are fired, the rock between them is crushed, and a large hole made. The lines of fracture of the subsequent shots run into this hole. In this case, it is even more desirable than in the preceding to fire the central shots simultaneously.

In shaft sinking, if the strata are horizontal or nearly so, it is usual to unkey from the centre, as in the heading. But if they be highly inclined, it will be better to unkey from one side of the excavation. The water which flows into the workings must be collected into one place, both for convenience in raising it, and for the purpose of keeping the surface of the rock clear for the sinkers. The depression caused by the removal of the key serves to collect the water, and, on that account, it is called “the sump.” Into this sump, the tub dips, or, when pumps are used, the suction pipe drops. When the strata are highly inclined, the water gravitates towards the dip side of the excavation, and it becomes, therefore, necessary to place the sump in that situation. The unkeying of the rock from this direction is, moreover, favourable to the effect of the shots. In putting in the shot-holes, it is well to avoid, as far as possible, terminating them in, or nearly in, a bedding plane, because when so terminated, the force of the charge expends itself along this plane. The position and the direction of the holes will, however, be determined in some degree by the character of the support used for the drills, and by other conditions of convenience.

Charging and Firing.

—The operations of charging the holes and firing the shots demand particular attention when machine labour is employed. It has been pointed out in the foregoing paragraphs that holes bored by machine drills cannot be placed or directed strictly in accordance with the requirements of the lines of least resistance; but that, on the contrary, these requirements can only be approximately complied with, and in some cases must be wholly neglected. To compensate in some degree this defect of machine labour, the strength of the charges should be varied according to the resistance which they will be required to overcome. That is, the principles of blasting described in a former chapter, which cannot be complied with by the borer, should be strictly followed by the blaster in apportioning his charges. By this means, a great saving of the explosive compound may be effected, and that without difficulty or loss of time, if the blaster be intelligent and understand his work. A glance will be sufficient to show what charge a given hole of a known depth will require, and as cartridges of different sizes are ready at hand, no delay is occasioned in making up the charge. The holes in the centre, which are intended to unkey the face, require, of course, the heaviest charge, since the conditions are there most unfavourable to the effects of the explosion. And the more complete is the unkeying resulting from this first explosion, and the more fractured and jointed is the rock surrounding the cavity thus formed, the more may the charges placed behind these unsupported faces be reduced.

As economy of time is, in machine boring, the chief end to be attained, the tamping should be done with dried clay pellets previously prepared. This material gives the greatest resistance, and thereby ensures the maximum of useful effect; and if prepared beforehand, in the manner described in the preceding chapter, the time consumed in tamping will be reduced to a minimum. An abundant supply of such pellets should always be ready at hand. In downward holes, such as are used in shaft sinking, the plastic clay pellet and sand may be employed. This tamping may be put in very rapidly, and, in all but very shallow holes, it is very effective. When it is desired to use sand tamping in horizontal holes, and holes bored in an ascending direction, the sand should be made up in paper cartridges. The tamping employed in the St. Gothard tunnel consisted of sand prepared in this manner. At the Mont Cenis tunnel, an argillaceous earth was similarly prepared in paper cartridges for tamping.

Firing the charges also affords an occasion for the exercise of knowledge and judgment. A skilful determination of the order in which the charges are to be fired will in a great measure compensate the ill effects of badly-placed holes. The firing of a shot leaves the surrounding rock more or less unsupported on certain sides; and it is evident that to profit fully by the existence of these unsupported faces, the succession of explosions must be regulated so that each shall have the advantage of those formed by the preceding shots. This condition can be wholly fulfilled only by simultaneous firing; but when the firing is to take place successively, the condition may be approximated to by regulating the succession according to the indications observed on a careful inspection of the rock. Before firing the charges, the blaster should consider the relative positions of the holes, the stratification and jointing of the rock, the fissures caused by the preceding blast, and any other circumstances that may influence the results. The charges intended to unkey the face will be fired first, and those in the concentric series will be then fired, in the order determined upon, by means of different lengths of fuse. The series will follow each other from the centre outwards. When a large number of shots regularly placed in series have to be fired, a convenient practical means of ensuring the successive explosion of the series, in the case of the whole being lighted simultaneously, consists in bringing the fuses from all the shot-holes together to one point at the centre. This method of regulating the length of the fuses was adopted at the St. Gothard tunnel.

It is obvious that the acceleration of the labour of excavation, which has been effected in so remarkable a degree by the introduction of machine drills and strong explosives, may be still further promoted by the adoption of electricity as the firing agent. The advantages of firing a number of shots simultaneously, some of which have already been pointed out, are great and manifest. In the case of a driving, for example, when all the holes have been bored and charged, and the machines withdrawn, it is clearly desirable to blast down the face as quickly and as effectively as possible. If the whole of the shots can be fired at once, the time is reduced to a minimum, and, consequently, the maximum of progress in a given time is ensured. Electricity affords, indeed, the most convenient, the most effective, and the most safe means of firing blasts. Hofrath Ritter von Pischof, the Austrian Chief Inspector of Railways, in one of his reports, says:—“A greatly increased amount of work and a notable saving of cost are effected when the shots can be so disposed and fired as to mutually aid one another. These results are obtained by employing electricity as the firing agent. The experience which has been gained at the Büchenberg cutting, where electrical firing has been extensively adopted, has shown that, when properly employed, this means allows, in comparison with the ordinary methods, twice the amount of work to be performed in a given time. It is therefore highly desirable to adopt electrical blasting whenever it is a question of economy of time and money.”

Removing the dislodged Rock.

—As the removal of the rock brought down by the blast consumes a large proportion of the time saved by machine boring, it becomes necessary to provide means for reducing this loss to a minimum. The most important of these means is a suitable provision for the rapid removal of the machine to a place of safety, and a conveniently designed and well-laid tramway, upon which the rock may be quickly run out without confusion and its consequent delay. The number of wagons required to remove a given cube of rock may be readily ascertained, and sufficient provision should be made for the transport of these to “day” in the most rapid succession. The wagons should be of such dimensions as to be capable of being handled without great difficulty; the importance of this condition will be understood when the frequency of derailments is borne in mind. The shovelling up of the rubbish is greatly facilitated by laying iron plates in front of the face to be brought down previously to the firing of the blast. This expedient is often adopted in important drivings. It has also been remarked that the dislodged rock can be more rapidly removed when it exists in small blocks. Thus there will be an advantage in placing the charges and in regulating their strength so as to completely break up the rock. Another matter of importance in the arrangements for the rapid removal of the rock brought down by the blast, is the proportioning of the number of hands employed to the requirements of the case. This number will increase with the size of the blocks to be lifted, the distance to be run over, and the want of suitability in the matériel employed.

Division of Labour.

—A proper division of labour is greatly conducive to rapid and economical progress. The operations may be divided into three series, namely: boring the shot-holes, charging and firing, and removing the rock dislodged. Each of these series of operations may be performed by different sets of men, and in several instances this division of labour has been adopted. But it does not appear that such a division leads to the most satisfactory results. The work of boring occupies a much longer time than either of the other two series of operations, and hence the distribution of the time is unequal. It has been found that, generally, where all the arrangements have been well considered, the labour of charging the shot-holes, firing the blast, and removing the rock brought down, can be performed in about the same time as that of boring. Thus it would seem to be more conducive to economy of time to divide the men employed into only two sets: one set to bore the holes, the other to perform all the subsequent operations. This division has been adopted in numerous instances with favourable results. Sometimes the whole of the operations have been performed by the same set; but such an arrangement is not to be recommended. The labour of directing the machines is of too distinct and skilled a character to be confounded with that of removing the débris, without a strong reason for such a proceeding, which does not appear to exist. Besides reserving a set of men specially for this portion of the work, it is desirable to keep the same men to the same machine, for in such a case each man gets accustomed to the peculiarities of the machine entrusted to him, and besides conceives a kind of affection for it that leads to careful handling and watchful attention. In addition to the men required for the operations referred to above, smiths will be needed to re-sharpen the bits and to repair the machines. The amount of this labour will obviously depend upon the number of machines employed, and the hardness of the rock to be passed through.