Compressed Air Work And Diving 1909

WATÉR TRAR. 69 all the way round, and at the other end press against the cast-iron lining when pushing the shielcl forward. The rear part of the shield is called a tail. It is made of two or more plates riveted together witli countersunk rivets and overlaps the cast-iron lining. It is long enough to allow of an overlap when the shielcl has been pushed forward to allow the next ring of iron to be placecl in position. Behind the diaphragm and fixed to the shielcl is the erector. This is an arm which revolves round the centre line of the tunnel, and is usecl for picking up and placing in position the iron segments. It contains a ram, so that it can be shortened or lengthened to allow of the segment being swung round conveniently, and then shot out and presseel home when opposite its correct position. It is slewed round, in some shields, by a rack and pinion, and in others by a chain passing over a drum. In either case the power is hydraulic. Shields for tunnels of large diameter are very frequently fitteel with two erectors. Some shields are fitteel with a water trap. This was first done at the Vyrnwy Aqueduct Tunnel* at the suggestion of the late Sir Benjamin Baker. The front of this shield, a little way back from the cutting eclge, was entirely closed, with the exception of a small hånd hole, from the top clown to the centre. The remaining portion was fitteel with removable shiitters. A little way further back was a diaphragm starting from the bottom of the shielcl, and reaching up to and slightly overlapping the front closed-in half. The result was that any water coming in woulcl rise to the level of the top of the diaphragm at about the centre of the tunnel, and there * Sinim’s “Practical Tunnelling.