170 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



saw mills and machine shops. Air pressure was maintained con- 

 tinuously from these plants with no break or interruption for sup- 

 port of some part or other of the work under construction, from 

 September, 1902, to June 14, 1909, and during the construction 

 period over 250,000 tons of coal was consumed, while the value of 

 the plant exceeded $750,000. 



The plants installed for the Pennsylvania Railroad Tunnel con- 

 struction were much larger and more valuable than those of the 

 Hudson Company. 



The modern method of shield tunnelling is an evolution, and 

 except in the mechanical details and design, is in no respect new. 



Referring to the before mentioned essential factors in tunnel 

 construction, the conditions are met by provisions of the tools or 

 means of construction as follows : 



1. For the support of the soil, and the elimination of water, and 

 to provide for the safety of the workmen, we carry out the work 

 under air pressure. 



2. For the support of the soil, to provide safety for men, for 

 economical and rapid construction, and in certain cases also for the 

 removal of the soil, we install a hydraulic shield. 



3. For the permanent lining we require material strong enough 

 to stand the external pressures as soon as erected, and therefore a 

 metal plate lining is requisite. 



4. For putting in place this permanent lining there is installed, 

 either attached to the shield or operating independently, an erector. 



5. For water-proofing and protecting from corrosion . the iron 

 lining, where the same is erected in sand, or gravel, or partially in 

 rock, cement grout is pumped into the rear of the lining by a grout- 

 ing machine. 



The use of air pressure was a very early invention. About 

 1830 Admiral Cochrane, afterwards Lord Dundonald, took out 

 letters patent for use of air pressure applied to caisson or tunnel 

 construction. 



The diving bell had been in use at that time quite extensively 

 for laying foundations under water and for carrying on other engi- 

 neering works. This represents the simplest form of application 

 of compressed air due directly to the water depth, but obviously 



