Compressed Air Work And Diving 1909

R O THERHITHE TUNNEL. 83 were sent out of the tunnel full, and they were used in this Wciy until the shield had advanced sufficiently far under the river to allow a bulkhead to be built and the shaft opened to the atmosphere. There was no separate entrance for men, but the same entrance chamber was used for men and material. Buckets were not used, as is generally the case during the sinking of caissons, for removing the excavated material, but this was instead filled into waggons and the waggons hoisted by a lift worked by a hydraulic ram. This arrangement is clearly shown in the drawing, Fig. 14B. The caisson was sunk until the cutting eclge was about 16 ft. 9 in. below the invert of the tunnel, and this 16 ft. 9 in. was then filled up with Portland cement concrete, with the air-tight floor embedded in the con- crete in order to make a perfectly water-tight floor. The air-lock and shafts were then removed and the shield built in the open. An air-tight floor was then built at a higher level above the shield, and the air- lock conn jcted directly to the floor without the shafts, as the floor was high enough to make these unnecessary. The ping was then cut out and the shield started under air pressure. Before the large shield was started, however, a small heading tunnel with temporary cast- iron linino- was driven under the river. This heading- was useful as it sliowecl what was the nature of the strata which would be met with, and it also served to some extent to support the working face. The front portion of the shield (Fig. 14c) was built up of cast-steel rings in segments breaking joint. These segments were bolted together and were prevented from sliding by Steel dowels. The First ring, 2 ft. 6 in. wide, formed the cutting eclge. The other two rings were 3 ft. 6 in. and 4 ft. 9 in.