174 DAVIES— TUNNEL CONSTRUCTION. [February i8. 



erect the permanent lining, and slides along overlapping the tunnel 

 tube as construction advances. 



If the shield is of large size, there are fitted in front sliding 

 tables, advanced in front of the cutting edge of the shield by 

 hydraulic power, for the double purpose of forming platform for 

 men to work upon while drilling, excavating or breasting the upper 

 face, or to shelter the men working below while doing the similar 

 bottom work, and also to push forward against the timbers used to 

 breast up the face, and to hold them in position during the 

 operation. 



The machinery with which the shields are equipped consists of 

 numbers of hydraulic jacks fitted at close intervals around the 

 periphery, the jack cases heeled against and pushing close to the 

 cutting edge by admission of water pressure to the jacks, forcing 

 forward the entire structure of the shield by reaction of the rams 

 against the last erected ring of permanent plate lining. The jacks 

 are controlled by an arrangement of valves, so devised that any jack 

 can be operated singly, or any group of jacks can be operated to- 

 gether by one man, from a convenient platform. By applying pres- 

 sure to either quarter of the shield, the proper direction is given to 

 advance the shield. The doors are very simple, and the design best 

 adapted depends partly on the character of soil in which the shield 

 is being used. In our work nearly every type of door has been 

 fitted, but the simplest and best adapted to the usual condition of 

 use and emergency is the loose slat. A shield is, needless to say, 

 a very massive piece of construction, as the strains put upon it are 

 enormous. One of the Hudson and Manhattan shields complete 

 weighs about 67 net tons, while a Pennsylvania North River shield 

 weighs about 195 net tons. Presuming that work is being carried 

 out in silt under the Hudson River with pneumatic pressure of 30 

 pounds per square inch and with all the shield doors closed tightly, 

 the shield is pushed through the clay as though one pushed a stick 

 into a heap of dirt. In that case, with sixteen jacks worked under 

 5,000 pounds hydraulic pressure, there is a total force of 2,500 tons. 

 This, on the Hudson and Manhattan shields, represents a pressure 

 on the soil equivalent to eleven tons per square foot. It is not often 

 that such a pressure as this is necessary to produce penetration with 



