Compressed Air Work And Diving 1909

8o COMPRESSED AIR WORK. clock flooded with water. The shield was then floatecl into the caisson, and lowered into position by pumping out the water. The plug was cut out in the open, and driving started without the use of compressed air. A good deal of water came in, but this could be dealt with by pumping until a point 67 ft. from Shaft No. 3 was reached, when it was found necessary to use air pressure, and from this point onwards, under the river and through Shafts Nos. 2 and 3 on the other side, and to within about 30 ft. of the cut-and-cover at the extreme north end, the driving was done with the help of compressed air. After leaving Shaft No. 1 and coniing out under the river, very little difficulty was experienced at first, as there was a layer of clay between the top of the tunnel and the bottom of the river. The clay, however, gradually died out, and for a distance of about 350 ft. there was noth- ing but ballast, the depth of which, between the top of the tunnel and the bed of the river, did not exceecl 10 ft., and in some places did not exceecl 6 ft. A clay blanket 150 ft. wide and 10 ft. thick was therefore tippeel over the tunnel, in order to prevent the air from escaping too freely, and to enable the pressure to be maintained. It also served to fill up any holes which might form at any time by the ground sinking. Such holes would some- times form and the clay fali and fill them up. Sometimes clay which had filled up holes in this way would come through the openings in the front of the shield and into the tunnel. It was at this point that the 10-in. diameter rams round the lower half of the shield had to be addecl, as it was impossible to get in front of the shield to exeavate, and therefore a very big pressure was required to force the shield forward. During this period a force of 4,000 tons was sometimes required. The 8-in. dia-