SUBMAUINK Tl NNKl.INd 265 



usually adopted, is to attach a shoe or bearing casting to the 

 end of the piston rod, which will distribute the pressure over 

 a considerable area. An example of this construction is the 

 shield for the City and South London tunnel. In the East 

 River and St. Clair River tunnels, built in America, the tail of 

 the piston rod is so constructed that the thrust is carried 

 directly to the shell of the lining. 



LINING. 



Either iron or masonry may be used for lining shield-driven 

 tunnels but present practice is almost universally in favor of 

 iron lining. As usually built, iron lining consists of a series of 

 successive cast iron rings, the abutting edges of which are pro- 

 vided with flanges. These flanges are connected by means of 

 butts, the joints being packed with thin strips of wood, oakum, 

 cement, or some other material to make them water-tight. 

 Each lining ring is made up of four or more segments, which 

 are profiled with flanges for bolted connections similar to 

 those fastening the successive rings. Generally the crown seg- 

 ment is made considerably shorter than those forming the sides 

 and bottom of the ring. The erection of the iron segments 

 forming the successive rings of the lining may be done by hand 

 in tunnels of small diameter where the weights to be handled 

 are comparatively light, but in tunnels of large size special 

 cranes attached to the shield or carried by the finished lining 

 are employed. The construction of the iron lining for the East 

 River tunnel is illustrated in Chapter XIX.. and that for the 

 St Clair River tunnel is shown by Fig. 133. 



