The New York Rapid-transit Subway

Proceedings.] PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. 19 spread and connected with the waterproofing course at the bottom. The most satisfactory material for this outer or guard wall was found to be square hollow tile, about 4 inches thick and about 10 inches on the side. The hollow spaces were set vertically so as to act as vertical drains. This wall, with the waterproofing, was set to clear the back of the flanges of the wall-beams by about 2 inches, which space was completely filled by the fluid concrete of the walls. The roof was finished in the same manner. After the roof-beams had been erected, and the arches between them were finished so as to cover the tops of the flanges and give a slight drainage-slope from the centre to the sides, the alternate layers of felt and asphalt were spread. It will thus be seen that there was a complete envelope of waterproofing material surrounding the whole structure. In order to protect the concrete on the roof from damage by subsequent street-excavation a layer of coarse concrete about 3 inches in thickness was spread over it. This method of keeping the subway dry was found to be entirely satisfactory against moderate hydrostatic pressure. Where a high hydrostatic pressure was to be encountered, say 20 to 40 feet layers of brick dipped in asphalt were used instead of the felt. For this purpose porous bricks were adopted, which, after being warmed to remove moisture, were dipped in liquid asphalt of the quality described above, and were then laid, usually in two layers, flatways, in both floor and roof ; but in the side walls, in a vertical wall either one or one-half brick in thickness. After the bricks were in place, hot liquid asphalt was poured over them so as to ensure the complete filling of all spaces. The concrete structure was then placed on top of this waterproof course. Tunnelling. Table III (p. 12) shows that there were 2-6 miles of single- track tunnels, and 3-9 miles of two-track tunnels. The word “tunnel” is used here to denote a subway constructed wholly by underground excavation, and not by cut-and-cover work. The single- track tunnels consisted chiefly of iron-lined tubes, built by means of a shield, and in a manner very similar to that employed in the construction of the London tubes, except that, being beneath the East River and the approaches thereto, and in water-bearing material the tunnelling had to be carried on in compressed air under à pressure ranging from 20 to 40 lbs. per square inch. The cross section of these iron-lined tubes is shown in Fig. 8, Plate 5. It will be seen that the tubes had an internal diameter of 15 feet 6 inches c 2