TUNNELING 



of the material which they are designed to penetrate. In the 

 first class he places shields designed to work in a stiff and com- 

 paratively stable soil, like the well-known London clay ; in the 

 second class are placed those constructed to work in soft clays 

 r nd silts ; and in the third class those intended for soils of an 



Longitudinal Section. 



Cross Section. 

 FIG. 131. Semi Elliptical Shield for Clichy Sewer Tunnel. 



unstable granular nature. This classification will, in a general 

 way, be kept by the writer. As a representative shield of 

 the first class, the one designed for the City and South London 

 Ry. is illustrated in Fig. 127. The shields for the London 

 Tower tunnel, the Waterloo & City Ry., the Glasgow District 

 Subway, the Siphons of Clichy and Concorde in Paris, and the 



