182 TUNNELING 



used ; or to the level of its ceiling, if a flat roof is used. After 

 the completion of the side walls, the portion of the excavation 

 shown at No. 2, Fig. 106, is removed a sufficient depth to en- 

 able the roof arch to be built. When the arch is completed, it 

 is filled above with well-ram rned earth, and the surface is re- 

 stored. The excavation of part No. 3 inclosed by the side 

 walls and roof arch is carried on from the entrances and from 

 shafts left at intervals along the line. 



A modification of the method just described was employed 

 in constructing the Paris underground railways. It consists in 

 excavating a single longitudinal trench along one side of the 

 street, and building the side wall in it as previously described. 

 When this side wall is completed to the 

 roof, the right half of part No. 2, Fig. 106, 

 is excavated to the line AB, and the right- 

 hand half of the roof arch is built. The 

 space above the arch is then refilled and the 

 surface of the street restored, after which 

 the left-hand trench is dug and the side 

 wall and roof-arch masonry is built iust as 



FIG. 108. Side -Wall J J 



Foundation Con- in the opposite half. Generally the work 



is prosecuted by opening up lengths of 



trench at considerable intervals along the 



street and alternately on the left- and right-hand sides. By 



this method one-half of the street width is everywhere open 



to traffic, the travel simply passing from one side of the street 



to the other to avoid the excavation. When the lining has 



been completed, the center core of earth inclosed by it is 



removed from the entrances and shafts, leaving the tunnel 



finished except for the invert and track construction, etc. 



Transverse Trenches. The transverse trench or "slice" 

 method of open-cut tunneling has been employed in one work, 

 the Boston Subway. This method is described in the specifica- 

 tions for the work prepared by the chief engineer, Mr. H. A. 

 Carson, M. Am. Soc. C. E., as follows : 



