176 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



Street, travelled a total distance of 4,525 feet, of which amount 2,075 

 feet was constructed with rock over the partial cross-section of 

 the tunnel, and overlaid with wet sand. 



During the progress of this shield and for the purpose of blast- 

 ing and removing the rock, there were exploded 26,000 sticks of 

 dynamite in front of the cutting edge, causing very great damage 

 to the structure of the shield, so that at the time it arrived at 12th 

 Street and 5th Avenue, the shield was in such condition that it was 

 with considerable difficulty that the tunnel lining could even be 

 erected. 



As the shield advances, the permanent lining must be erected in 

 the rear. In Brunei's day brickwork was used, and it has recently 

 been used to some extent, but even with the use of quick setting 

 Portland cement neither brickwork nor concrete is successful for 

 subaqueous work, as the materials cannot reach any strength, in 

 the time during which it is feasible to leave the shield, before 

 advancing it again after construction of a ring. Structural iron, 

 imbedded in concrete at the points where jacks react, has been tried 

 without satisfaction, and the use of wood as a temporary lining, to 

 be backed up after advance of the shield by brick or concrete, while 

 satisfactory for small tunnels, is never likely to be much used for 

 any large sized tubes as needed for railroad tunnels. It is essen- 

 tial, for any tube tunnel construction with a shield, to have a 

 permanent lining which can be erected as the shield advances and 

 rapidly put in place, and which, as soon as erected, is strong enough 

 to take all permanent stresses and to permit of the reaction of the 

 jacks of the shields. A metal lining is the only solution of this 

 problem. Cast iron or cast steel has almost exclusively been used, 

 as the forming of the segments is so accurate, the joints are the 

 least in number, and the material is such as to give the longest life 

 with the least depreciation. If an internal concrete lining is after- 

 wards used, then undoubtedly a satisfactory structural steel lining 

 could be designed, but probably with no economy in cost of con- 

 struction. We have used this to a small extent for experimental 

 purposes. For any such metal lining, the complete ring has to be 

 erected from within the tail of the shield, which is only a couple 

 of inches larger inside than the external size of the finished tube. 



