Chapter XVI

OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS; BOSTON SUBWAY AND NEW YORK RAPID TRANSIT.

OPEN-CUT TUNNELING.

When a tunnel or rapid-transit subway has to be constructed at a small depth below the surface, the excavation is generally performed more economically by making an open cut than by subterranean tunneling proper. The necessary condition of small depth which makes open-cut tunneling desirable is most generally found in constructing rapid-transit subways or tunnels under city streets. This fact introduces the chief difficulties encountered in such work, since the surface traffic makes it necessary to obstruct the streets as little as possible, and has led to the development of the several special methods commonly employed in performing it.

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,