204 



TUNNELING 



1:1 the Blackwall shield 24 were used. The mechanical 

 construction of the jacks for tunnel shields presents no features 

 out of the usual lines of such devices used elsewhere. The 

 jacks used on the East River tunnel shield are shown by Fig. 

 118, and those for the St. Clair River tunnel by Fig. 133. 



Part Transverse Section. Longitudinal Section. 



FIG. 133. Cast Iron Lining, St. Clair River Tnnnel. 



Two general methods are employed for transmitting the 

 thrust of the piston rods against the tunnel lining. The 

 object sought in each is to distribute the thrust in such a 

 manner that there is no danger of bending the thin front flange 

 of the forward lining ring. In English practice the plan 



