TUNNELING 



sometimes been employed to advantage. Fig. 33 shows the 

 construction of one of these rings, which, it will be seen, is com- 

 posed of four segments connected to each other by means of 

 bolted flanges. The holes shown in the circumferential web of 

 the ring are to allow for the seepage from the earth side walls. 



The method of placing this 

 cylindrical strutting is to 

 start with a ring having a 

 cutting-edge. By means 

 of excavation inside the 



ring, and by ramming, 

 the ring is sunk into the 

 ground a distance equal to 

 its height. Another ring 

 is then fastened by special hooks on top of the first one, and 

 the sinking continued until the second ring is down flush with 

 the surface. . A third ring is then added, and so on until the 

 entire shaft is excavated and strutted. As in timber shaft 

 strutting, the solid iron ring strutting is carried down only to 

 the top of the tunnel section, and below this point there is an 

 open timber or iron supporting framework. 



FIG. 33. Cast-iron Segments! Strutting for 

 Shafts. 



