Compressed Air Work And Diving 1909

CHAPTER VII. Tunnelling. When compressecl air is usecl with a shield for driving a tunnel in water-bearing strata, and the exposed area is therefore vertical, instead of horizontal as in caisson work, there will be a difference in the head of water, which will have to be met by the compressed air, equal to the height from the invert to the crown of the tunnel. The result is that either the water will tend to come in at the bottom, or else a large volume of air will be escaping round the cutting edge at the top of the shield. When the tunnel is in hard rock free from fissures, this difference of pressure is of very little consequence, but when the ground is soft or sandy, it will be necessary to clo away with the open face. The shield is merely a hollow cylinder, or caisson placecl on its side, slightly greater in diameter than the iron lining. The front part is clivided up into compartments which give standing room to the workers excavating at the face, and also serve to give the shield the required stiffness. This front part is usually clivided off from the back by a diaphragm with cloors, through which the excavatecl material is passed. Some shields have a double diaphragm and two sets of doors, so that a clifferent air pressure may be employed at front and back. Hydraulic rams are fixed to the back of the shield