Compressed Air Work And Diving 1909

72 COMPRESSED AIR WORK. order. When the air pressure is over 20 Ibs., a second bulkhead similarly equipped with locks will be an advantage. The space between them will be kept at about half the absolute pressure for “purgatory” lock working. In laro-e tunnels two lines of tram rails are used for O bringing in the cast-iron lining and other material, and for taking away the excavated material. These start from each material lock and have short lengths of rail which have to be taken up each time a waggon eliters the lock. In the early tunnels mules were used to drag the waggons, but the best modern practice is to have an endless wire bonel worked by an electric motor. To this the wagvons are hitched, and the motor starteel by signalling with electric lamps. When a tunnel is being starteel under a river, a shaft must first be sunk near the bank. This is usually done by means of a Steel caisson, and when water is likely to be met with, compressecl air must be used. The caisson has a double shell filled between with concrete. Those for the Blackwall Tunnel were 58 ft. external and 48 ft. internal diameter. About 9 ft. from the bottom is a stron»' roof above which water can be filled. in for o kentledge when neæssary. At the point in the caisson where the shielcl will start, a circular opening is left, pluggeel with a single shell well stiffened. When the caisson has reachecl the level intended for it a strong water-tight concrete floor is laid at the bottom. The air is then taken off and the roof above the working chamber removed, and the caisson lowered into position or built in situ in the open. The roof is then put back again higher up, the air pressure put on again, the plug cut out and the shielcl started on its journey. When the shielcl can be started in the open, all this is rnueh simplified.