188 TUNNELING 



of the Tremont St. line, where the traffic was very dense, 

 since it was possible to construct them without opening the 

 street Much of the wide arch line was constructed by the 

 use of the roof shield, which is described in the succeeding 

 chapter on the shield system of tunneling. 



Methods of Construction. Several different methods were 

 employed in constructing the subway. Where ample space 

 was available, the single wide trench method of cut-and-cover 



FIG. 110. Double Barrel Section, Boston Subway. 



construction was employed, the earth being removed as fast as 

 excavated. In the streets, except where regular tunneling was 

 resorted to, the parallel trench or transverse trench cut-and- 

 cover methods were employed. 



In the transverse trench method, trenches about 12 ft. wide 

 were excavated across the street, their length being equal to 

 the extreme transverse width of the tunnel lining, and their 

 depth being equal to the depth of the tunnel floor. These 

 trenches were begun during the night, and immediately roofed 



