Engineering Wonders of the World
Volume I

58 ENGINEERING WONDERS OF THE WORLD. BACK OF SHIELD. length, having flat ends in which were the openings for the doors. The latter were of steel, and, of course, hung so as to open towards the air pressure. Rails were laid through the air-locks for the wagons containing spoil to run along, and valves for admitting and releasing the air pressure were fitted. The principle of air-locks is de- scribed in the chapter on the Tube Railways of Lon- don. As the machinery for air- compressing, etc., was lo- cated near the site of No. 3 Shaft, tunnelling started from that point, and the portion under the river was under- taken first. Borings had been made as near as possible to the line of the H'nnel, but there was still some uncertainty as to how far the river-bed would exclude the river water. It was therefore decided to drive a trial heading, or small tunnel, across the For compressing the air there were six powerful engines, and when work was in full swing the pumps delivered 10,000,000 cubic feet of air per day. The air was taken into the tunnel in steel pipes 10 inches in diameter, after having been cooled by means of a water spray. The air-locks used for sinking the shaft and those fitted in the bulkheads in the tunnel were all of the same construc- Air-Locks, tion—a long cylinder with a door at each end of it. The cylindrical portion was about 6 feet in diameter, and varied from 10 to 36 feet in river, to ascertain the nature of the strata to be dealt with. This pilot tunnel was 12 feet 6 inches in outside dia- meter, and lined with cast Trial iron. It was constructed by “Pilot” means of a shield of the same Tunnel, pattern as was used in the tube railways of London, and fitted with a Price’s rotary excavator.* Compressed air was used and rapid progress made, the strata proving satisfactory, with the exception of a bed of rock which somewhat delayed the * A description of the rotary excavator is given in the chapter on London Tube Railways.