. Shield and compressed air tunneling . ^ could lock extension is a means of escape in case of such an accident,and it is described on page 241 of the same paper. It consisted ofa short cjdinder about 4 ft. long and 4 ft. in diameter, open at oneend and having there a flange that could be bolted to the end of AM) 227 any of the locks in the Hudson tunnels. At the other end was anordinary lock door opening into the cylinder. If the regularlock door refused to work, this extension would be bolted on theoutside, the regular lock would be abandoned and the lockingout wo
. Shield and compressed air tunneling . ^ could lock extension is a means of escape in case of such an accident,and it is described on page 241 of the same paper. It consisted ofa short cjdinder about 4 ft. long and 4 ft. in diameter, open at oneend and having there a flange that could be bolted to the end of AM) 227 any of the locks in the Hudson tunnels. At the other end was anordinary lock door opening into the cylinder. If the regularlock door refused to work, this extension would be bolted on theoutside, the regular lock would be abandoned and the lockingout would take place in the lock formed by the extension, usingthe outer lock door as the inner door of the lock. It was usedat least once with success on these tunnels, but we know of nootlier tunnel where it has been Fig. 103.—Sectional view of model of compressed air tunnel (A-18) lookingtoward face of shield. (Courtesy oi Pennsylvania Railroad.) 140. Interior View of Tunnel.—Figures 102 and 103 are giwn toshow the general interior arrangement of a shield driven tunnelunder compressed air. These are photographs of a sectionalmodel made of the Pennsylvania Railroad Hudson River Tunnels(A-18). They give a clearer idea of what a tunnel looks likethan any photograph of an actual tunnel. Practically the onlything not shown is the safety screen. CHAPTER XISURFACE PLANT 1. Items of Plant.—The; siufaec plant for a shield driventunnel consists of: (A) The items which supply compressed air to the tunnelwhen driven under air pressure and the power for driving thevarious implements used in the construction of the tunnel. Thiswill be called the power house plant. (B) The items which deal with the receipt and distributionof materials for the tunnel, the disposal of spoil from the tunnel,the maintenance o
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