Compressed Air Work And Diving 1909

BLACKWALL TUNNEL. 79 in the manner described in the last chapter. At a point 9 ft. 3 in. back from the cutting eclge the front of the shielcl was entirely shiit off from the back by a bulkhead composed of two Steel diaphragms 3 ft. apart, so that a higher pressure could have been used in front of the shielcl than behind. This methocl of working was, how- ever, never actually adopted. Material shoots were proviclecl in each compartment worked on the air-lock principle, and there were four air-locks for men. There were twenty-eight ranis 8 in. in diameter, and several rouncl the lower half 10 in. diameter when the shield was entirely in ballast, and these with a water pressure up to 3 tons to the square inch, and neglecting the friction of the rams, gave a pressure on the back of the shielcl of 2,800 tons. A total pressure of 4,000 tons was, however, required at times. There were two hydraulic erectors at the back worked by rack and pinion. (Plate II.) This shielcl, together with the rest of the plant, was designed for Messrs S. Pearson & Son, Ltd., the con- tractors, by Mr E. W. Moir, now one of their directors. There were four skafts on the line of the tunnel, Nos. 1, 2, 3, and 4. Shafts 1 and 2 were on the north side, and 3 and 4 on the south side of the river. Driving was commenced in No. 4 Shaft and was continued through No. 3 Shaft, and from there northwards under the river to Shaft No. 2. The caisson for Shaft No. 4 was carried clown 13 ft. below the invert of the tunnel, and this 13 ft. was filled up with concrete with a wrought- iron water-tight floor built in below it. The shielcl was built on top close to the shaft, and a dock was then duo- in the ground underneath and rouncl it. Some of the plates of the caisson were then removed down to the level of the bottom of the dock, and both caisson and