TUNNELS, Great Modern. The comple tion of the boring of the Simplon tunnel on 24 Feb. 1905, thus assuring of its being opened to travel during the latter part of that year, marked the culmination of one of the greatest events in civil engineering during the last dec. 'ade, and the successful termination of the greatest tunnel boring enterprise of any age, ancient or modern.
At the present time, four great tunnels pierce the great Alpine barrier between north ern and southern Europe. The Mont Cenis and the Saint Gotthard connect France and Italy; the Arlberg places Austria in communi cation with Italy, while the Simplon forms a direct connection between Italy and Switzer land, bringing Geneva and French Switzerland into closer communication with Milan and the Adriatic railways. It also shortens the dis tance from Calais to Milan by 95 miles, as com pared with the Mont Cenis route, and by 80 miles over the route passing through the Saint Gotthard.
The feasibility of a transalpine tunnel was contemplated about the middle of the 19th cen tury, and work was begun on the Mont Cenis tunnel in 1857, at a point near Modane France, from which it passes under the eleva tion, Col de Frejus, about 18 miles west of Mont Cenis, and emerges into Italian territory at a point near Bardonecchia, 24 miles from Susa. The exact length of the tunnel between portals is 7.6016 miles, but as the railway, in stead of entering the tunnel at the portals, joins it through special curved sections at each end, the total length of the borings amounts to about eight miles. It was bored simultaneously from both ends on a rising gradient, with its summit at the middle point. The grade from Modane is about 1 in 2,000. The rock forma tion traversed as the boring advanced south ward from Modane was characterized by car bonaceous schist 1.3027, quartz 02414, limestone 02210, and calcareous schist 2.0357 miles; while the entire distance traversed from the Bardo necchia end to the summit amounting to 3.800 miles, was through calcareous schist. The alti tude of the tunnel is about 4,248 feet above sea level, and 5,428 feet below the crest of the mountain. At the Modane end the tunnel is 25 feet 31/2 inches wide at the bottom, 26 feet 24 inches at the point of maximum breadth; semi-circular in form, and 24 feet 71A inches in height. At the Bardonecchia portal, an elliptical arch is introduced to resist the greater strain caused by the different inclination of the strata, and the height is 104 inches greater than that of the Modane section. The side walls are eight
feet six inches in thickness throughout the en tire length of the tunnel, and with the excep tion of 300 yards on the north side, are lined with brick and stone, while the side paths are paved with flagstones 20 inches in width. Dur ing the first three years, the operations con sisted of hand labor exclusively, but in 1861 and 1862 power drills were installed at the Bardonecchia and Modane ends, respectively. The drills employed were designed by Som meiller, the chief engineer. They were of the percussion type, the operative power being com pressed air. Gas factories and machine shops were installed at each end. During the boring process, many springs were tapped, the water from which found an outlet through the tunnel. This water was finally utilized to furnish the power for the air compressors operating the drills — the Italian engineer profiting by the suggestion of the invention by Bartlett in 1855, of a rock drill operated by air compressed by a steam engine. The air compressing apparatus was installed at the Italian end, the compres sion approximating six atmospheres, derived from the hydraulic pressure from mountain streams and the water from the tapped springs. The same apparatus supplied fresh air at the rate of 2,000 cubic feet per minute, while the ventilation was accomplished partly by the atmospheric drills, and partly by special venti lating pipes inches in diameter, the blow ers and exhausting bells being operated by powerful turbines. Gunpowder was exclusively used in the blasting operations, the charges be ing fired by a magneto-electric apparatus in front of a movable bulkhead, which was ad vanced as the work progressed, the detritus being subsequently removed by hand, by gangs of men 900 feet apart, working on scaffolds at various heights. The hauling was accom plished by horses and small trucks. The credit of the work belongs to the three Italian en gineers, Sommeiller, Grandis and Grattoni, whose genius surmounted the many obstacles which daily confronted its advance. The boring of the tunnel was finished in 1870, and it was opened to traffic in 1872, equipped with a double-track railway and cost $1,100 per linear yard, a total expenditure of $15,000,000, and required 131/2 years for the accomplishment of an engineering task, which was the greatest in its time. The entire undertaking was financed by the Sardinian government. It is an example of tunnel construction by the drift method.