The New York Rapid-transit Subway

Proceedings.] PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. 21 States Government, of 20 feet, and a width of 400 feet between retaining-walls on each side. It is crossed beneath by the East Side line (Fig. 1, Plate 5). As the amount of natural cover was too small to tunnel beneath, the contractor decided, instead of making an artificial bed of the river with clay, which would afterwards have to be removed, to construct a roofed coffer-dam and to build the tunnel in sections beneath this coffer-dam, taking first one half of the stream and then the other, the Government permitting such temporary obstruction to the channel. For this purpose a structure was designed as shown in Fig. 10, Plate 5, consisting of two sectors of circles with a common vertical chord, all of cast iron and surrounded by concrete. The cast-iron lining had a shell 1 inch in thickness and ribs 6 inches deep. As the lining was to be put in place under a roof, it was not necessary to use a small key as ordinarily adopted in tubular linings. The plates were, therefore, made as large as could be conveniently handled, with a lengtli of about 6 feet longitudinally and a trans- verse length of about 5 feet 11 inches each. The contractor’s plan for the first half of the river consisted of two rows of sheet-piling of 12-inch by 12-inch timbers, each timber being grooved, and the grooves being filled with 3-incli by 4-inch pine keys. This heavy sheet-piling was necessary to withstand the head of about 45 feet. The rows of piling were set far enough apart to clear the side walls of the structure. To the tops of the sheet-piles was bolted a heavy timber roof, through which there was an ordinary air-lock. When this structure was in place, the water was pumped out, air-pressure was applied, and the permanent tunnel was constructed in the caisson-chamber thus formed. On the second half of the river the contractor decided to dispense with tlie temporary roof by ingeniously using instead the permanent roof of the tunnel. Rows of exterior sheet-piles were driven as already described, with interior transverse rows of piles at 8-foot intervals. On the interior piles were laid caps so arranged that their tops and the top of the sheeting would be at the plane of the centre of the tunnel when finally in place. The upper half of the iron lining was then erected in the open and covered witli concrete, according to the plans, but was reinforced with rods to take up any unequal stress in the moving. To the lower part of the plates external lugs had been cast, projecting 2 feet beyond the exterior surface. This roof was then floated into place and sunk on top of the sheeting and transverse caps, the exterior lugs resting on the former. When the roof had been bolted down, the water was pumped out and air was admitted, the excavation was completed, and