198 



TUNNELING 



driven by a wide heading, and one bench, since no strutting 

 will be required, and the masonry lining, even of the roof, may 

 be left far behind the front of the excavation. The masonry 

 lining will consist of concrete walls and brick arches. The 

 tunnel under Central Park being driven through a similar 

 rock, the same method of excavation and the same manner of 

 lining will be used. 



The tunnel under the Harlem River is to be driven through 

 soft ground ; and it will be constructed as a submarine tunnel, 

 according to the coffer dam process. The tunnels will be 



FIG. 115. Harlem River Tunnel, New York Rapid Transit Railway. 



lined with iron made up of segments, with radial and cir- 

 cumferential flanges. Concrete will be placed inside and flush 

 with the flanges. 



The tracks, both in the subway and tunnels, are an inti- 

 mate part of the concrete flooring. The rail rests on a con- 

 tinuous bearing of wooden blocks, laid with the grain running 

 transversely with respect to the line of the rail, and held in 

 place by two channel iron guard rails. The guard rails are 

 bolted to metal cross-ties embedded in the concrete. 



