OPEN-CUT TUNNELING. When a tunnel or rapid transit subway has to he constructed at a small depth below the surface, the excavation is generally performed more economically by mak ing an open cut, building the lining inside it, and filling in the space outside the lining. than it is by subterranean tunneling proper. The neces• nary condition of small depth which makes open cut tunneling desirable is most generally found in constructing rapid transit 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. The two methods of open cut tunneling commonly practiced may be classed as the longitudinal-treneh method and the verse-trench method. The simplest manner by which to construct open-cut tunnels is to open a single cut or trench the full width of the tunnel against the strutting planks. The lining is built in this trench and then filled around and above with well-rammed earth, after which the surface of the ground is restored. Generally the trench is opened in short lengths and each length is completed ready for surface traffic before the ad jacent length is opened. Another form of longi tudinal trench method is to open two narrow parallel trenches in which are built the side walls. The soil between these walls is then removed to a sufficient depth to permit the roof to he built and covered over, and the final process is to take out the core of earth inclosed by the side walls and roof. Generally one side wall and one-half of the
roof arch are completed and covered over before the other side wall and half arch are begun. By this arrangement of the work one-half of the street is always unobstructed.
strengthened and sharpened to form a cutting edge. The rear end extends backward so as to overlap the completed lining of the cast-iron rings. Around its inside hydraulic jacks are at tached at frequent intervals so that their piston rods bear against the front edge of the completed lining. By applying power to the jacks their piston rods pass with great force against the immovable lining and thrust the cylinder forward into the soft earth. Generally the cylinder has a vertical position or diaphragm near the front end, which prevents the soft earth from rushing hack into it except as it is allowed by opening suitable doors. In operation the cylinder is thrust forward until its sharp cutting edge pene In the transverse-trench method a trench about 12 feet wide is opened across the street, and in this trench a short length or 'slice' of the subway is built. Owing to the small size of the opening, it is possible to cover it with a timber platform which carries the street traffic during construc tion. This method was extensively used in con structing the subway at Boston, Mass., and at York.