180 TUNNELING 



CHAPTER XVII. 



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 tun- 

 nels under city streets. This fact introduces the chief difficul- 

 ties 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. These methods may be classed as 

 follows : (1) The longitudinal trench method, using either a 

 single wide trench or two narrow parallel trenches ; (2) the 

 transverse trench 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 longitudi- 

 nal trench method of open-cut tunneling is that it permits the 



