Engineering Wonders of the World
Volume I

ENGINEERING WONDERS OF THE WORLD. 56 AIR-LOCK AT THE SURFACE. The strata passed through were clays, sands, and a hard bed of conglomerate rock. The air pressure gradually increased ^se with the depth, until about Compressed „ . . Ajr 20 lbs. per square meh was required. By the use of com- pressed air the surface on which the men worked was kept perfectly dry, and with careful regulation of the pressure little escape of air under the cutting edge took place. The total weight of the deepest caisson was about 7,000 tons. The procedure described above applies to Shafts Nos. 2 and 3, which were sunk with compressed air ; in the two other caissons this process was not used, the small quantity of water met with being pumped. As soon as each caisson reached its proper level, excava- tion ceased, and the whole space under the air-tight floor was filled with, concrete, form- ing an absolutely solid foundation for the shaft. When all the tunnelling was finished, the shafts were lined with brickwork, and the two deeper ones (Nos. 2 and 3) were provided, with spiral staircases giving access to the streets above, and covered in with domed roofs. The cast-iron-lined tunnel has a total length The Cast “iron Tunnel. a rinsf. The of 1,212 yards, and is in three sections—one under the river, and one on each. side. The tunnel is lined with cast-iron plates, having very strong internal flanges, by means of which the plates are bolted together. The plates, or segments, are built up in rings, each ring being 30 feet in diameter and 2 feet 6 inches wide. Sixteen plates and one special key-piece form key-piece has to be made in the shape of a wedge, so that it can be put in place from the inside. Each ring weighs nearly 19 tons, and there are about 25,000 tons of cast-iron lining in all. The flanges of the plates were machined on all sides, so that they might fit closely, except for a small groove or recess, formed at the inner edges of the flanges, for what is known as a “ rust joint.” The joint is made by filling the groove with a mixture of cast-iron borings and sal-ammoniac, driven in tightly by hammer and caulking tool. This material sets very hard, and if not disturbed forms an absolutely water-tight joint. The iron lining extends through the openings in the shafts, and is made staunch by specially prepared iron plates bolted to the end flanges of the lining and to the inner ends of the openings. AIR-LOCKS IN TUNNEL.