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.”