Popular Science Monthly/Volume 17/August 1880/The St Gothard Tunnel
|THE ST. GOTHARD TUNNEL.|
THE boring of the St. Gothard Tunnel is pronounced by one of the engineers to be the greatest work hitherto attempted by man; certainly its importance and magnitude can hardly be overrated. It deserves to be regarded with especial admiration as a work which was marked in every department and at every stage by triumphs of the highest skill in scientific engineering. The tunnel is intended to form part of the railway connecting the North Sea with the Mediterranean, and is situated on the most direct route between these regions, passing through the chain of the Alps at a central point. The railroad through this line was preceded by the road over the Brenner Pass of the Tyrol, for that was easier of execution, and by that through the Mont Cenis Tunnel, for the French Government had political reasons for constructing a road which should be under its own control. The Swiss soon saw, after the rival lines were constructed, that the traffic which belonged to them would be diverted to pass around them, and immediately began operations to open a road through their own most direct route. In this they enjoyed the coöperation of Germany and Italy. The operations at St. Gothard were begun under the advantage of the possession of the experience, knowledge, and skill that had been gained in constructing the tunnel of Mont Cenis.
The preliminary surveys made it certain that the only points at which the opening of the tunnel could be made were near Goeschenen, in the Canton Uri, on the north, 1,109 metres or 3,604 feet above the sea, and near Airolo, in the Canton Tessin, on the south, 1,145 metres or 3,721 feet above the sea. Considerable works were necessary to reach these elevations. An examination of the map showed that the tunnel might be straight and that it would be about 15,000 metres or 48,750 feet long. The engineers of the tunnel of Mont Cenis had a point at the highest part of the ground from which they could see at once objects indicating the positions of both openings. No such advantage existed at St. Gothard, and some of the summits are so steep
Fig. 1.—Profile along the Length of the Tunnel.
and high that it is impracticable to attempt any direct tracing of the line over the mountain. The relative position of the two openings and the direction of the tunnel had then to be calculated indirectly, from triangulations. The directions and levels were thus ascertained. Observatories were then placed at the tunnel-mouths to serve as direction-points for the miners. At Goeschenen it was necessary, in order to get a long enough line of sight, to make borings of considerable length through two projecting rocks. The surveys, originally made by M. Gelpke, were verified by a second series of triangulations made in 1874 by another engineer, M. Koppe, on a different system. M. Gelpke had based his surveys on summits in the neighborhood, and had used triangles of only moderate size. M. Koppe made his triangles as large as possible, so that he might connect the two openings of the tunnel by a minimum number of intermediary stations. The two triangulations gave results agreeing within two seconds of direction with each other. M. Koppe also verified his survey practically by projecting a line from the opening at Airolo toward a mast which he set up at the highest attainable point along the axis of the tunnel. He could not go toward this point from Goeschenen, so he went backward in the direction of the continuation of the tunnel-axis, ascending the flanks of the mountain till he could observe his mast. Then, having directed his glass toward Goeschenen, he raised it vertically to the level of the mast, when he saw it almost in the center of his field of vision. The direction within the tunnel was verified by means of field-glasses fixed within the observatories, so far as they would answer, then by means of lamps hung on the line of the axis. The direction was, moreover, carefully verified from the observatories two or three times a year.
The borings were made almost entirely by machines, and it was the policy of M. Louis Favre, the contractor for the tunnel, to dispense with hand-boring as far as possible. The machines were driven by water-power transmitted into the tunnel by means of the air apparatus of Professor Colladon, of Geneva, which had already been used at Mont Cenis. Water was obtained in great abundance and of strong fall at Goeschenen from the river Reuss. The supply of water at Airolo was not so abundant. The river (the Ticino) has not a rapid descent, and a very long canal would be required to procure a sufficient fall from it. A torrent, the Tremola, was chiefly relied upon, but the supply from this fell short at times, and had to be supplemented by an auxiliary supply from the Ticino. The force of the stream was applied to turn four turbine wheels which made three hundred revolutions in a minute, and exerted a force of about two hundred horse-power. These wheels were made to turn an horizontal axis with cranks revolving eighty-five times a minute, which kept the compressors in operation. The air, subjected to a compression of from four to eight atmospheres, became very hot, and had to be cooled by special applications of water circulating in cold currents around the pumps and in the pistons and piston-rods, and by injections of fine spray. After being further cooled and freed from water in suspension by passing it through reservoirs, it was conducted into the tunnel by tubes which were of considerable size at the beginning, but were diminished to correspond with the diminishing expenditure of air as the work was advanced.
The borings were begun by first cutting out a gallery about eight feet square at the top of the intended excavation. When this advance boring was completed, it was enlarged on the right and left. The arches of the roof were then built, and a trench nearly ten feet wide was dug to the level of the tunnel's base. This was called the "Cunette de Strosse." All that remained on the right and left of the
Fig. 2.—System of Excavation: 1, advance pillory; 2, side-workings; 3, 4, "Cunette de Strosse"; 5, "Strosse."
trench, called the "Strosse," was next removed. Thus, most of the digging was done downward. The work was interfered with at times by the infiltration of water, which, as it did not affect dynamite and soon stopped, was not considered serious; by rocks of exceptional hardness; and by a bed of loose material in the Goeschenen end, in which it was dangerous to work for fear the mass would fall and bury the workmen, and cut off their retreat. Communication was opened between the two galleries on the 29th of February, 1880. The chief miner on the southern (Airolo) side had pierced an horizontal hole about ten feet long, and had caused the attacks from the side of Goeschenen to be suspended on his penetrating to the northern gallery. Proceeding with a boring of moderate depth, he reduced the thickness of the remaining mass to about four feet. Preparations were made for the final attack by piercing four holes in the center of the boring, and eleven other holes at equal intervals around it and not very far distant from it. The explosion opened a passage of a little more than thirty inches in diameter, through which the engineers and some of the workmen were able to go over from one gallery to the other. When the communication was first opened, at eleven o'clock in the morning, the barometer stood .156 of an inch higher at Goeschenen than at the southern end of the tunnel. A current of air was immediately produced in the gallery, which blew at the opening at the rate of a metre and a half (nearly five feet) a second. A few hours later the relative pressure was reversed, and the barometer stood .039 of an inch lower at Goeschenen than at Airolo. The direction of the current of air was consequently changed; it blew from south to north, but at the rate of only about a foot in a second. The actual length of the tunnel was about twenty-five feet shorter than the calculated length. The difference in the level of the two galleries was not more than four inches, and their lateral deviation was not more than eight inches. According to a statement made by M. Colladon to the Academy of Sciences at Paris, the most efficacious means adopted to speed the work of excavation were the diking of the torrents and the application of water collected in aqueducts as a moving power to turbine-wheels requiring high falls, the adoption of air-compressors which worked with great rapidity, the cooling of the air in the compressors, at the moment of compression, by the injection of water in a fine spray, the use of dynamite, and the determination which was adopted from the beginning to carry on the excavations from the top of the tunnel. By the aid of these improved methods the advance through the hard rocks was made with double the speed that the engineers in charge had been able to attain in boring the Mont Cenis Tunnel. It is estimated that, notwithstanding its greater length, the tunnel of St. Gothard when completed will have cost from twenty-five to thirty per cent, less than that of Mont Cenis.
The provisions for conducting compressed air into the galleries, involving a system of pipes upward of sixteen thousand feet long in each gallery, afforded excellent opportunities for studying the flow of compressed air through metallic conduits. The loss of air in passing through the pipes was an important factor. The absolute pressure of the air, which was equivalent to six and a half atmospheres at the Goeschenen mouth, was diminished till it became, toward the end of the work, no greater than one atmosphere and an eighth at the front of the cutting. At Airolo it was necessary to enlarge the diameter of the perforators, and a much larger volume of air had to be spent to do the same work. The temperature at the front of the excavations rose to 91 during the last days of the operations, and greatly taxed the endurance of the workmen. Calculations have been made to the effect that, taking the work all through, each kilogramme of dynamite that was used corresponded with a cubic metre of rock that was removed. Among the gains to engineering which, it is claimed, have accrued from the enterprise, are the perfection of the machinery and tools for boring, and the training of a body of skilled workmen, who have become experts, able to determine, by merely inspecting a rock, how to deal most efficiently with it. The perforation of tunnels will in the future be a simpler, easier, and less costly operation than it has been heretofore. Since communication was established between the two galleries of the tunnel, a part of the mountain mass, 6,300 metres, or 20,475 feet from the southern entrance, has fallen in, killing and injuring several of the workmen.