586 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 19 3 



Since there was a real hazard involved in carrying on operations 

 from a plant anchored in midstream, the shield method was clearly 

 called for. In addition, silt conditions in the Hudson River were 

 regarded as extremely favorable to this method. In a trench tunnel, 

 soft material greatly increases the volume of excavation, while in the 

 case of a shield tunnel this material is most easily excavated. If the 

 silt is not shoved aside by the shields, it is easily disposed of through 

 the tunnel. The shield may be closed with the exception of certain 

 openings through which the material is squeezed into the tunnel as 

 the shield advances. 



The first contract provided for the sinking of two land shafts, 

 one at Washington and Canal Streets and the other at Washing- 

 ton and Spring Streets, New York City. They were sunk by the 

 compressed-air method. 



The double steel walls of the caissons were filled with concrete as 

 the caissons were sunk. This added to their weight when sinking 

 weight was needed, and at the same time completed the structure of 

 the walls. In addition to this concrete, weight for sinking was ob- 

 tained by storing the excavated material from the working chamber 

 on the roof of the chamber as the caisson went down. This necessi- 

 tated handling the material a second time, but gave the desired 

 weight and permitted the lowering of the caisson without greatly 

 reducing the air pressure in the working chamber, thereby prevent- 

 ing loss of ground. 



Upon the removal of the compressed air, the bottom seals of the 

 caissons proved to be water-tight. The shafts were now ready for 

 the building of the shields preparatory to the beginning of shield 

 tunneling. Temporary bulkheads were provided in the west side 

 walls to permit the passage of the shields, and in the east side walls 

 to connect with the approach section which was to be constructed 

 by excavation from the surface. 



This work was followed by placing under contract the entire un- 

 der-river portion of the tunnel. Power plants had to be constructed 

 to produce low-pressure air for caissons and tunnel, high-pressure 

 air for the operation of grouting machines, air drills, and hoisting 

 engines used below the surface, and hydraulic pressure for operat- 

 ing the jacks used in driving the shield and for operating the erector 

 arm for building the tunnel lining. 



Overhead gantries and dumping platforms for the receipt and 

 disposal of materials and buildings for housing the workmen had 

 to be provided. Pipes, through which compressed air would be 

 supplied to the tunnel headings, had to be laid to the shafts. On 

 the New Jersey side this involved laying low-pressure lines as large 

 as 16 inches in diameter, high-pressure lines, hydraulic lines, water 



