TUNNELING 



ring, beveled to form a chisel-like cutting-edge, and bolted to 

 the ends of the forward shell plates. This construction was 

 .first employed in the shield for the London Tower tunnel, and 

 has since been used on the City and South London, Waterloo 

 and City, and the Clichy tunnels. The second construction 

 consists in bracing the forward shell plates by means of right 

 triangular brackets, whose perpendicular sides are riveted 

 respectively to the shell plates and the diaphragm, and whose 

 inclined sides slant backward and downward from the front 

 dge, and carry a conical ring of plating. The shields for the 

 St. Clair River, East River, and Blackwall tunnels show forms 

 of this typs of cutting-edge construction. A modification of 

 the second type of construction, which consists in omitting the 

 conical plating, was employed on some of the shields for the 

 Clichy tunnel. This modification fs generally considered to be 

 allowable only in materials which have little stability, and which 

 crumble down before the advance of the cutting-edge. Where 

 the material is of a sticky or compact nature, into which the 

 shield in advancing must actually cut, the beveled plating is 

 necessary to insure a clean cutting action without wedging or 

 jamming of the material. 



Cellular Division. It is necessary in shields of large diam- 

 eter to brace the shell horizontally and vertically against 

 distortion. This bracing also serves to form stagings for the 

 workmen, and to divide the shield into cells. The following 

 table shows the arrangement of the vertical and transverse 

 bracing in several representative tunnel shields. 



