TUNNELLING THROUGH MOUNTAINS Where a great thickness of rock overlies a tunnel through a mountain it may be necessary or advisable to do the work wholly from the two ends without intermediate shafts. The problem largely resolves itself into devising the most expeditious way of excavating and removing the rock. Experience and modern mechanical devices, such as compressed air drills, and mucking and loading machines, have led to speed and economy.
The Hoosac tunnel, in Massachusetts, on the line of the Fitch burg railroad, was the first prominent tunnel in America, and for many years the longest, being 4.73 m. long. It was begun in 1855 and finished in 1876, after many interruptions. It was memorable for the original use in America of compressed air drills and nitro glycerin, mechanical drilling being adopted in 1866.
In 1857 the first blast was fired in connection with the Mont Cenis work in the Alps, in 1861 machine drilling was introduced and in 1871 the tunnel was opened for traffic. It is located be tween Modane, France and Bardonecchia, Italy. It is a single tunnel 7.98 m. long, with a "horseshoe" section 26 ft. 3 in. by 24 ft. 7 inches. The material penetrated was granitic and the average progress 7.7 lin.ft. per 24 hours. The approximate cost was L75 per linear foot. With the exception of about 900 lin.ft. the tunnel is lined throughout with brick or stone. During the first four years, by hand labour, the average progress was not more than 9 in. per day on each side of the Alps, but with com pressed air drills the rate towards the end was five times greater.
In 1872 the St. Gotthard tunnel was begun and in 1881 the first locomotive ran through it. It lies between Goschenen and Airolo, in Switzerland. It is 9.3 m. in length and of the same dimensions as the Mont Cenis. The material also was granitic and the average progress 18 lin. ft. per 24 hours. The approxi mate cost was £48 per linear foot. Mechanical drills were used from the beginning. Tunnelling was carried on by driving in ad vance a top heading about 8 f t. square, then enlarging this side ways and finally sinking the excavation to invert level. (See de
tails in fig. 6.) Air for working the rock-drills was compressed to seven atmospheres by water turbines of about 2,000 horse-power. The two inclines leading to the summit, which total about 36 m., are 28% tunnel, including seven spirals forming almost complete loops within the mountains, in order to gain altitude and distance.
The driving of the Arlberg tunnel was begun in 188o, and the work was completed in little more than three years. It is a single tunnel, 6.36 m. long, between Innsbruck and Bludenz, in the Tirol. It is 25 ft. 3 in. wide and the average progress in 24 hours was 27.2 linear feet. The approximate cost was £36 per linear foot. The main heading was driven along the bottom of the tunnel and shafts were opened to the upper heading 75 to 210 ft. apart, from which smaller headings were driven right and left. The tunnel was enlarged to its full section at different points simul taneously in lengths of 24 ft., the excavation of each occupying about 20 days and the masonry 14 days. Ferroux percussion air drills and Brandt rotary hydraulic drills were used, the per formance of the latter being especially satisfactory. After each blast a fine spray of water was injected, which assisted the ven tilation materially. In the St. Gotthard tunnel the discharge of the air drills was relied on for ventilation. In the Arlberg tunnel over 8,000 cu.ft. of air per minute was thrown in by ventilators. To keep pace with the miners, 90o tons of excavated material had to be removed, and 35o tons of masonry introduced daily at each end of the tunnel, which necessitated the transit of 450 wagons. The cost per linear foot varied according to the thickness of masonry lining and the distance from the mouth of the tunnel. For the first 3,000 ft. from the entrance the prices per linear foot were L3 16s. for the lower heading; L2 for the upper one; LI() 3s. for the unlined tunnel; £15 for the tunnel with a thin lining of masonry; and L41 8s. with a lining 3 ft. thick at the arch, 4 ft. at the sides and 2 ft. 8 in. at the invert.