I9I0.] DAVIES— TUNNEL CONSTRUCTION. 175 



consequent flow and displacement in silt, nor could it be used in 

 any stiffer material without probable damage to the shield. Sand 

 or gravel, which will not flow, must be excavated or removed, and 

 removed in advance of the forcing forward of the shield, or injury 

 will be done to the structure. In Hudson River silt the process is 

 very rapid, the complete cycle of operation involved in pushing this 

 shield, cleaning up the silt which leaks through the door joints, 

 placing and erecting a complete ring of plates for lining, bolting up 

 and being ready for the next cycle, requiring only thirty-seven 

 minutes, and in this way as much as seventy-two feet of finished 

 tunnel has been erected in a single day. 



In the design of a shield there have been numerous so-called 

 refinements introduced, some patented, some tested and discarded ; 

 but in our own experience, the greater simplicity the greater service, 

 and any addition to such a machine which is not necessary it is 

 better without. For example, working in sand or gravel, or when 

 the face consists in part of rock and in part of silt or sand, the 

 entire face of soil has to be excavated, and as this is done the 

 breast must be retained from caving or falling in. This is usually 

 done by skilled men placing boards, well and properly braced by 

 struts, through the shield doors back to the completed tunnel, so that 

 they are independent of the movement of the shield when it is next 

 shoved ahead. The addition to the shield of an elaborate mechan- 

 ically operated system of shuttering, like a glorified edition of 

 Brunei's idea, is no advantage in time nor labor, but enormously in- 

 creased cost to construct and additional trouble to maintain. 



The most difficult combination to deal with in shield tunnelling 

 is a partial face of rock overlaid by silt or wet sand. This con- 

 struction was first met with and successfully overcome in the 

 East River Gas Tunnel in New York. In this case the soft ground 

 overhead must be excavated and the exposed face securely sup- 

 ported by timber while the rock underlying is drilled and blasted. 

 This work, carried out in air pressure, necessitates very small 

 charges of dynamite and is very tedious and expensive. 



One of our shields with which the east-bound up-town tunnel 

 was constructed from under the Hudson River through Morton 

 Street and Church Street and into 6th Avenue as far north as 12th 



