252 



TUNNELING 



thus regulating the action of the motive power the direction of 

 the shield could be altered at will. Work was abandoned on 

 the Broadway tunnel in 1870. In 1871-2 Beach's shield was 

 used in building a short circular tunnel 8 ft. in diameter in 

 Cincinnati, and a little later it was introduced into the Cleve- 

 land water-works tunnel 8 ft. in diameter. In this latter work, 

 which was through a very treacherous soil, the shield gave a 

 great deal of trouble, and was finally so flattened by the 



Longitudinal Section, 



Cross Section. 



Fm. 129. Shield for Blackwall Tunnel. 



pressures that it was abandoned. The obviously defective fea- 

 tures of this shield were its want of vertical bracing and the 

 lack of any means of closing the front in soft soil. 



With the foregoing brief review of the early development of 

 the shield system of tunneling, we have arrived at a point where 

 the methods of modern practice can be studied intelligently. 

 In the pages which follow wo shall first illustrate fully the 

 construction of a number of shields of typical and special 

 construction, and follow these illustrations with a general dis- 

 cussion of present practice in the various details of shield 

 construction. 



