The Underground World: A mirror of life below the surface

by Thomas Wallace Knox

Modane, or, more properly speaking, Fourneaux, was the base of operations. Fourneaux is a miserable little village in a narrow gorge in the valley of the Arc, and its inhabitants are chiefly remarkable for their deformity and idiocy. The Grand Vallon is eleven thousand feet above the sea level, and crowned with snow. Its sides are steep, and it would be quite impossible to carry a railway over it. The other mountains on the route are equally rugged in character, but their height above makes little difference with the workings carried on in their interior.

The Mont Cenis Tunnel is the largest in the world, extending from Fourneaux, on the French side, to Bardouneche, on the Italian side. When it was begun, with the ordinary system, of hand drills, it was found that at the ordinary rate of progress, it would take thirty or forty years to finish the work. With an ordinary tunnel, where the elevation of earth or rock is not very great, shafts are sunk along the line, as before stated; but in this case it was impossible to sink these vertical shafts, on account of the great distance. A necessity arose for penetrating the rock much faster than by ordinary means, and there was also a necessity for supplying the workmen with fresh air.

SOMMELIER’S AIR COMPRESSOR.

These necessities led to Sommelier’s invention of drills worked by compressed air, and of the machinery for compressing the air. The machines have already been described in connection with the Hoosac Tunnel. A great many experiments were made before the air could be successfully used; but finally, when they were completed, the work progressed rapidly. By means of the compressors that were worked by a stream of water from the mountain, the air was reduced to one sixth of its natural bulk, and thus, when liberated, it exercised an expansive force equal to six atmospheres. The compressing machines used at most tunnels to-day are simply enormous and very powerful pumps, but the machine of Sommelier used the weight of water. Twenty or more large iron tubes were placed in an upright position. The “head” of the supply was far up the mountain side, and the water was brought to the machine in an iron pipe. A piston perfectly tight was fitted to the tube, the water was turned on, and its weight, added to the head it had received, compressed the air in the tube. As it was compressed, a valve was opened, through which it could escape into a reservoir. From this reservoir the air was conveyed in an iron pipe into the tunnel, where it was used to work the perforators.

We found that the entrance to the tunnel was quite a distance up the side of the mountain, and it was evident that considerable engineering skill would be required to bring the railway track thither when the work was completed. Opposite the mouth of the tunnel, my attention was called to a large target, made of boards painted white, and securely fastened against the rock. The target was used for the proper alignment of the work. At every foot of progress into the mountain, bearings were carefully taken. At night a Drummond light was placed in the centre of the target, so that it could be visible from the middle of the mountain.

It will be seen that it was a work requiring the utmost caution to lay out the route and direction of the tunnel through the mountain. A variation of a hundredth part of an inch at any point in the surveys would have changed the course of the working on one side or the other, so that the two ends would not meet. Bear in mind that these surveys were carried from the valley of the Arc to the valley of the Dora,—the opposite points being eight miles apart,—and the route lying, not through level fields and meadows, but over three rough and high mountains, where there was no path beyond that which the surveyors and their assistants laid out. And yet, so carefully was the work performed from the two sides, that the workings were brought together exactly, without a variation of a single foot.

RULES IN REGARD TO VISITORS.

The entrance to the tunnel is about twenty-five feet wide, and the same in height. To go inside the workings, you are clad with a rubber suit, and supplied with a lamp, and accompanied by a guide. For some time after the working began, almost any one could be admitted; but it was found that the workmen were greatly hindered by frequent visits, so that the rules became very strict. No one could enter the tunnel, unless employed there, without a pass from the management, though it was not very difficult for a journalist or a person of influence or prominence to obtain admission. As fast as the work progressed, a double railway was laid down to carry in the materials used in the working, and to bring out the broken rock. There was a narrow sidewalk of flagged stone on each side. The pipes for the air were ranged along the side of the tunnel, and between the lines of the rails, in a deep trench, were the gas and water pipes.

Like all tunnels this one was damp, from the streams of water coming through the roof; and if you wondered before entering, why you should be asked to wear a rubber coat, your wonder speedily ceased. At the time of my visit the workmen were nearly three miles from the entrance,—that is to say, the tunnel was finished for that distance,—while for about a quarter of a mile the men had cut the heading, but the upper part of the tunnel had not been opened.

The heading is the most difficult part of the work, and in all tunnel operations the workmen at the heading are kept sufficiently in advance of the enlargers, so that one party will not be in the other’s way.