Engineering Wonders of the World
Volume I

THE ROTHERHITHE TUNNEL. two circular tunnel openings 32 feet in diameter, which were plugged while the caisson was being sunk, and not opened until the tunnel was ready to pass through them. Just above the cutting edge a steel air-tight deck or floor was fixed, right across the caisson. The floor was made up of steel plates and girders, and was strong enough to withstand the upward pressure of the compressed air confined beneath it. In the space below the air-tight floor the men worked while the caisson sank, excavating the ground over the whole area of the bottom, and sending up the spoil through a vertical shaft or large pipe to the air-lock ” at the top. This air-shaft was fitted with hoisting machinery, and a cage ran up and down it carrying a small wagon, which on reaching the top was run out of the cage into the air-lock, and thence into the open air. The process of building and sinking a caisson was as follows :— A level bed having been prepared on the site of the shaft, the plates forming the cutting edge were erected on timber supports and the lower rings of plating gradually Sinking added. When about 15 feet _ .the of the caisson’s height had Caissons. , ° been thus put together, the framework of the air-tight floor was fitted in, the plates being absent at this stage. All this steelwork was then riveted together and lowered gradually on to the ground, the sup- ports being carefully removed and the caisson let down by hydraulic jacks. As soon as the cutting edge was resting on the ground, into which it sank slightly, concrete was filled in between the skins of the caisson to add weight. Next, further erection of steelwork proceeded, and at the same time excavation of the in- terior was started, the excavated, material being hoisted out of the caisson by derrick cranes. Concreting then followed, and all these operations went on simultaneously until the work of the excavators was interfered with 55 RING OF TUNNEL LINING. by the ingress of water. As it was not thought desirable to use pumps to remove the water, owing to the presence near the shafts of build- ings which might be disturbed by pumping, excavation was then suspended. The erec- tion of steelwork proceeded, and the plating of the air-tight floor was put in place and made staunch. The air-compressing and hoisting machinery was prepared for work, and con- nections made for the supply of compressed air to the caisson. As much concrete as pos- sible was now filled in between the skins of the caisson in order to force the cutting edge well into the ground and so prevent the escape of the air. Compressed air was then admitted under the air-tight floor, a few pounds per square inch only at first; and the water being kept out by this means, excavation could be continued. These operations were repeated, and the caisson sank gradually, being constantly built higher and weight being added in the form of concrete. Any tendency to get out of level was counteracted by excavating most earth on the side which was highest. When everything went favourably the caisson usually sank at the rate of about I foot per day, work being carried on day and night.