The New York Rapid-transit Subway

20 PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. [Minutes of and an external diameter of 16 feet 9 inches. The cast-iron shell varied in weight, according to the nature of the ground, between 3,990 lbs. and 5,130 lbs. per lineal foot. The heaviest lining had a thickness of shell of 1|inch, a depth of flange from front to back of 7^ inches, and a width of ring of 22 inches. The other tunnels, chiefly for two tracks, were in rock, the longest being the Fort George tunnel, with a length of about 2 miles between portals. This and the other similar tunnels, had a width in the clear of 25 feet, the arch being semicircular in form. The Murray Hill tunnel, whicli is on the four-track section, consists of two twin tunnels of two tracks each. The roof was built as a three- centred arch instead of a full semicircle, in order to avoid endanger- ing the foundation of the surface-car tunnels above. All these tunnels were in rock, which was generally of good character and required no timbering. There were places where the ground became locally heavy and where very heavy timbering of the crown-bar type had to be resorted to in order to maintain the ground overhead. The lining of all these tunnels was of concrete. Where the ground was heavy, the walls and roof were thickened proportionately, and were further stiffened by reinforcing rods laterally and longitudin- ally embedded in the concrete. Fig. 9, Plate 5, shows a reinforced section of the Fort George tunnel in very heavy ground. As it was not possible to waterproof the back of the lining, and as a considerable quantity of ground-water was encountered at several places, the tunnels were made dry by grouting. Two-inch pipes were embedded in the roof during construction, at intervals of about 50 feet. After the lining had been finished, force-pumps were connected to these pipes and grout was pumped behind the arched lining, until all the voids in the concrete and all the spaces between the concrete and rock were completely filled. The largest tunnels were for two stations on the Fort George section which had an extreme width of 53 feet from back to back of the ’walls, and a width in the clear of 48 feet. The roof-arch was struck from three centres, with a rise of 15 feet above the springing and a rise of 26 feet above the base of the rails. In order0 to produce the desired architectural effect, these arches were made of brick instead of concrete, and were given a thickness of 2 feet 6 inches at the crown. The cross section of these tunnels is similar to that of the Mott Avenue Station described already, and illustrated in Fig. 6, Plate 5., The north end of Manhattan Island is separated from tlie mainland by the Harlem River, a tidal connection between the Hudson River on the west and the so-called East River on the east. This waterway has a depth at low water, as fixed by the United