Engineering Wonders of the World
Volume I

114 ENGINEERING WONDERS OF THE WORLD. Fig. 15.—MEETING OF SHIELDS, PENNSYLVANIA HUDSON RIVER TUNNELS, NORTH TUBE. Illustrating the wonderful accuracy that may be attained in tunnel driving. The inside framing of both shields was removed after the junction, and the cast-iron lining of the tunnel was then continued through, the shells of the shields being left in place. The line of junction is indicated by the two arrow heads near the centre of the picture. was started after the Pennsylvania work be- gan, have done much to reassure every one as to the stability of a tunnel in the Hudson mud. No piles have yet been put down in the Pennsylvania tunnels, and it is likely that none will be put in. The tunnel shell itself, 23 feet in diameter, is to be lined with a 2-foot layer of con- crete, a new departure among the New York tunnels. This tends to equalize the weight of the tunnel and of displaced silt, and also increases the strength of the tube. The concrete would probably be strong enough in itself to stand all the strains if the iron shell were removed. There is one further use in this lining—namely, that it affords a means of adjusting irregularities in the alignment and grades of the iron lining due to divergences of the shield in driving. Slight divergences are inevitable, and where the clearance be- tween car and tunnel-shell is small, as is the case in most of the other New York tunnels, it may even become necessary to reconstruct (enlarge) some sections of tunnel after its completion. A large amount of such recon- struction was in fact needed in the Battery Tunnel, as mentioned farther oif. But where there is to be a thick inner lining, the shell may be considerably out of line, and it will still be possible to make the inner bore straight by simply putting more concrete on one side and less on the other. The shields used in the Pennsylvania Hudson River Tunnel were of interesting