“And what part of the Alps?”
“Through Mont Cenis. The tunnel will be about 7½ miles long, and the mountain over it will rise 5400 feet at one point.”
“And when do they expect to finish it?”
“I cannot say. They will begin on the southern—that is, the Italian—side first, and later on the French side. Through the tunnel will pass one of the principal routes from the West to the East.”
This conversation, we may suppose, took place in 1857, the year when the tunnel was commenced. For four years hand work was used, though blasting was in operation from the first; but in 1861 drilling by machinery was brought into play, and the rate of progress became much greater.
The machine was the first practical boring apparatus for rock, and was used first in making the Mont Cenis Tunnel. With explosives, as gun-cotton, dynamite, etc., the time occupied in cutting tunnels has been much reduced. Thus the Mont Cenis Tunnel occupied about thirteen years, and cost three millions of pounds. The St. Gotthard—another Alpine subway—occupied eight years, though it is 9¼ miles in length; and the Arlberg—yet another Alpine tunnel—a little over 6 miles long, occupied something more than three years.
Further, the railway of which the St. Gotthard Tunnel forms part, has been commercially very successful. This tunnel was commenced in 1872 and completed in 1880, the same year that saw the beginning of the Arlberg.
Tunnels through hard rock do not always need a lining of brickwork; but if the soil be clay, or loose earth of any kind, the lining of brick or stone must be brought up close to the scene of actual excavation. The Mont Cenis is lined with stone or brick almost entirely, about 900 feet, however, being without such lining.
And now, how was the actual work of tunnelling carried on? It will be seen at once that the problem was quite different from that of boring fifteen feet under the Thames, and sometimes through watery mud. In boring through mountains the quickest way of cutting and carting away rock is one of the chief points to be considered. At the Mont Cenis Tunnel the blasting took place by driving a series of shot holes into the soil, all over the surface to be cut, filling them with explosives, and firing them simultaneously in rings. Such explosives may be fired by a time-fuse or by electricity, giving the workmen ample time to escape out of reach. The shaken and shattered soil can then be cleared away.
The blast holes in this small-shot system are about 1 to 1½ inch in diameter, and from 1½ to 7 or 9 feet in the rock. The explosive is forced to the end of each, and the hole is then tamped—that is, closed with clay or sand—and fired in due time.