178 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



weight of the plate, revolves until the plate reaches the position in 

 the circle which it is permanently to occupy, when the arm moves 

 forward, shoving the plate into position, and the bolts are inserted 

 and secured. Plate after plate is erected, until finally the insertion 

 of the key completes a ring, when the shield is again advanced. 



By subdivision of all bolt holes absolutely uniformly around the 

 pitch circle in the circumferential joint, it is feasible to shift the 

 position of the key at every ring and thereby stagger the longi- 

 tudinal joints, increasing materially the rigidity and strength of the 

 tunnels. 



For subaqueous tunnelling in silt or sand, this method of the 

 tube construction is essential to safety to employees as well as to 

 safe and certain construction of the work itself ; and with it and 

 with competent labor employed, subject to certain limitations as to 

 depth, it is difficult to imagine any condition which would prevent 

 successful execution of a piece of work. 



In the tunnel work of the Hudson & Manhattan Railroad there 

 is a greater length constructed under the land than under the water, 

 although every portion, except the Sixth Avenue Subway, north 

 of 1 2th Street, is below tide level. A portion of this work has been 

 in solid rock, though most of it is built through sand and gravel 

 formation. Under these conditions the use of a shield has not at 

 all times been necessary, and a very large portion of the tunnels 

 could, for economic reasons, be advantageously built with concrete 

 lining, under pressure but without the shield. The function of the 

 engineer is to determine when this can properly be done, and when 

 it can, the saving by substitution of concrete for metal lining 

 is very great, though the metal lining can be more easily made abso- 

 lutely water tight in wet ground than a concrete lining. 



A considerable amount of tunnel construction in solid rock has 

 been carried on with a shield, using iron lining, on account of the 

 likelihood of troubles arising, owing to fissures or thin cover in the 

 roof, which, in the absence of the shield for protection and support, 

 might be troublesome to execute. 



The whole art of tunnel construction in soft ground by any 

 process depends on the proper support of every hole exacavated, 

 and as excavated. A small hole in any kind of soil may be safely 



