Compressed Air Work And Diving 1909

?6 COMPRESSED AIR WORK. above it and also by the weight of its iron lining. Probably this is chiefly clue to the weight of the iron lining as it happens with tunnels in mud or silt as well as with those in rock. In mud or silt, which tencl to be of a viscous fluid nature, one might expect the pressure to be greater at the sides than at the top. This, however, does not seem to be the case. 1 he result is that the lining will be subjected to bending stress and the flanges will be in tension at their inside edges at the crown and invert of the tunnel and at their outside edges at the sides. To get over this clifficulty turnbuckles are used which pull in the sides of the lining and thus take the flatness out. This shoukl be done before grouting. Once the tunnel has got set in its proper shape it will not get out of shape again. The flanges of the lining are usually machined except for the last i or !-§■ in. on the inside. This is cast so as to leave a space of about il in. between the flano-es. When the grouting has been finished, this is caulked with iron borings to form a rust joint, or, better still, with |-in. leacl wire. This insures the lining being quite air and water tight. The shield is directed by means of long rods to fixed points measured off 011 the tunnel lining. These have to be carefully watched as the shield is being “shoved.” If one side shows signs of gaining, some of the jacks oii that side will have to be shut off. It is made to point up or downwards in the same way by shutting off jacks on the top or bottom. Sometimes a concrete cradle is used. A cradle is a conerete invert which is carried forward well in advance of the shield. It is shaped to the radius of the shield and usually has two rails embeddeel in it, the tops of