Subways are usually constructed under and along important streets where electric cars are running. The engineers have taken advantage of the presence of these lines to facilitate the construction of subways. In New York, for instance, the tracks of the electric lines were supported by cast-iron yokes 4 or 5 ft. apart and were surrounded by concrete, leaving only a large hollow space in the middle for the wires and trolleys. The rails from 40 to 60 ft. long formed almost a solid concrete structure for their entire length. The tracks and the street surface were supported by horizontal beams inserted underneath the tracks. These were the caps of bents constructed underground whose rafters were finally resting on the subgrade of the proposed subway.
The various methods for constructing the subways may be classified as follows: (1) The single wide trench method; (2) the single narrow longitudinal trench method; (3) the parallel longitudinal trench method; (4) the slice method.
Single Longitudinal Trench.
—The simplest manner by which to construct open-cut tunnels is to open a single cut or trench the full width of the tunnel masonry. This trench is strutted by means of side sheetings of vertical planks, held in place by transverse braces extending across the trench and abutting against longitudinal timbers laid against the sheeting plank. The lining is built in this trench, and is then filled around and above with well-rammed earth, after which the surface of the ground is restored. An especial merit of the single longitudinal trench method of open-cut tunneling is that it permits the construction of the lining in a single piece from the bottom up, thus enabling better workmanship and stronger construction than when the separate parts are built at different times. The great objection to the method when it is used for building subways under city streets is, that it occupies so much room that the street usually has to be closed to regular traffic. For this reason the single longitudinal trench method is seldom employed, except in those portions of city subways which pass under public squares or parks where room is plenty.
This method was followed in the construction of the New York subway, Section 2, along Elm St., a new street to be opened to traffic after the subway had been completed, and at other points where local conditions allowed it.
Fig. 112.—Diagram Showing Sequence of Construction in Open-Cut Tunnels.
A modification of this method was used in Contract Section 6, on upper Broadway. The street at this point is very wide, so by opening a trench as wide as the proposed four-track line of the subway there still remained room enough for ordinary traffic. The electric car tracks were supported by means of trusses 60 or 70 ft. long, which were laid in couples parallel to the tracks and which rested on firm soil. The soil under the car tracks was removed, beginning with transversal cuts to receive the needles which were tied to the lower chord of the trusses by means of iron stirrups. After the excavation had reached the subgrade, posts were erected to support the needles thus forming bents upon which the tracks rested. The trusses were removed and advanced to another section of the tunnel, and, in the clear space left, the subway was built from foundation up.
The Single Narrow Longitudinal Trench.
—This method was used on Contract Section 5, of the New York subway in order to comply with the peculiar conditions of the traffic along 42nd St. On this street, on account of the New York Central Station, there is a constant heavy traffic, while pedestrians use the northern sidewalks almost exclusively. A single longitudinal trench was then opened along the south side, and from this trench all the work of excavation and construction was carried on. At first the steel structure of the subway was erected in the trench and then a small heading was driven and strutted under and across the surface-car tracks. Afterward heavy I-beams were inserted, which rested with one end on top of the steel bents and the other end blocked to the floor of the excavation. These I-beams were located 5 ft. apart and they supported the surface of the street by means of longitudinal planks. The soil was removed from the wide space underneath the I-beams and the subway was constructed from the foundation up. When the structure had been completed, the packing was placed between the roof of the structure and the surface of the street, the I-beams withdrawn and the voids filled in.