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usually built by one of the cut-and-cover methods ; deep 

 tunnels, or those built at a great depth, beneath the surface 

 are constructed by any of the various methods of tunneling 

 described in this book, the choice of the method depending 

 upon the character of the material penetrated, and the local 

 conditions. 



In building tunnels under city streets the first duty of the 

 engineer is to disturb as little as possible the various existing 

 structures, and the activities for which these structures and the 

 street are designed. The character of the difficulties encoun- 

 tered in performing this duty will depend upon the depth at 

 which the tunnel is driven. In constructing shallow tunnels 

 by the cut-and-cover method care has to be taken first of all 

 not to disturb the street traffic any more than is absolutely 

 necessary. This condition precludes the single trench method 

 of open cut tunneling in all places where the street traffic is at 

 all dense, and compels the engineer to use the parallel trench 

 method employed in Paris, as previously described, or else the 

 transverse trench or slice method employed in the Boston 

 Subway. 



Both of these methods have to be modified when the work 

 is done on streets having underground trolley and cable roads, 

 and in which are located gas and water pipes, conduits for 

 wires, etc. Where underground trolley or cable railways are 

 encountered, a common mode of procedure is to excavate 

 parallel side trenches for the side walls, ami tin n the roof arch 

 until it reaches the conduit carrying the cables or wires. The 

 earth is then removed from lieneath the conduit structure in 

 small sections, ami the arch completed as each section is 

 opened. As fast as the arch is completed the conduit struc- 

 ture is supported on it. Where pipes are encountered they 

 may be supported by means of chains, susj>ending them from 

 heavy cross-lx-ams. or by means of strutting, or they may be 

 removed and rebuilt at a new level. Generally the conditions 

 require a different solution of this problem at different points. 



