—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.
Examples of Drivings.
The St. Gothard Tunnel.
—The St. Gothard tunnel is driven in five sections. First, the “heading” is driven at the roof level 6 feet 6 inches wide, and 7 feet high. The position of the holes is shown in the drawings on [Plate IX.] The number of holes at the Göschenen end is 28, and the depth about 40 inches. The shots are fired by means of safety fuse, the ends of the fuse being brought together at the centre. This arrangement causes the shots to explode in the proper order of succession. At a certain distance back from the face, is the “right enlargement;” this is a widening of the heading to the limits of the tunnel in that direction. Farther back is the “left enlargement,” by which the heading is widened to the full width of the tunnel. Still farther back is the first “bench cut,” in which one half of the floor is blasted out to the full depth of the tunnel, and behind this again is the second bench cut, in which the remaining half is removed. The boring machines employed are the Dubois-François, the McKean, and the Ferroux. The explosive agent used is dynamite. The rock is a tough granite.
The Hoosac Tunnel.
—At the west end of the Hoosac tunnel, the system adopted was the following. First, a centre cut was made by drilling two rows of five or six holes each, about 9 feet apart on the face, and converging to about 3 feet at their lower ends. The depth of these holes was from 9 to 12 feet, according to the hardness of the rock. These holes are numbered from 1 to 11 on [Plate X.] They were charged with nitro-glycerine, and fired by electricity, Mowbray’s frictional machine being used. As soon as the rock had been removed, the next series of fourteen holes, numbered from 12 to 25, were drilled. These holes were then charged and fired simultaneously like those of the first series. When the rock dislodged had been removed, the third series of holes, numbered from 26 to 41, were bored. This series, like the other two, were charged, and fired by electricity. The effect of these three blasts, which were fired within twenty-four hours, was to advance the heading, 9 feet in height by the full width of 24 feet, to the extent of 7 feet 6 inches. The drawings on [Plate XI.] are: an elevation of the fore-breast, which shows the positions of the shot-holes; a sectional plan, which shows the directions of the first series of holes; a similar plan, showing the directions of the second series of holes, and the centre cut removed; and a sectional plan of the heading after the second series have been fired, showing the direction of the third series of holes.
The operations of taking out the “bench” were carried on at a distance of about 170 yards back from the fore-breast. This was effected by first drilling six holes 7 feet deep; two of these were each about 4 feet from the face of the bench and close to the side of the tunnel, whilst two others were each 4 feet behind these first holes, and the remaining two holes were 8 feet from the face, 8 feet from the sides of the tunnel, and 8 feet from each other. These were fired simultaneously, the result being to lower the bench about 7 feet throughout the full width of the tunnel. At a safe distance beyond this first bench cut, the same operations were carried on by another gang of men, whereby the bench was lowered to the floor of the tunnel, the full area of 24 feet in width by 22 in height being thus completed. The rock was a moderately tough granite.