588 ANNUAL, EEPOET SMITHSONIAN INSTITUTION, 19 3 



inrushing water. Fire lines were installed in the compressed-air 

 chambers. Fire is a real danger in compressed-air work on account 

 of the increased amount of oxygen present. As an indication of the 

 fire hazard, a candle, if still glowing when extinguished, will again 

 burst into flame. 



The starting of the shields out of the caissons at the New York 

 land shafts was difficult because of the large diameter of the shields 

 and the shallow cover overhead. The material at this point was 

 granular, consisting largely of fine sand, which if undisturbed, held 

 air fairly well. As the shields were under the city streets, it was 

 impossible to increase the cover overhead. To avoid blow-outs at the 

 face with the consequent inrush of water, it was necessary to regulate 

 the air pressure carefully and to protect the face during each suc- 

 cessive step in excavating. 



As a preliminary step to shoving the shields out of the caissons, 

 the circular steel bulkheads in the caissons were burned out in front 

 of the shields. The work was done by removing the steel in hori- 

 zontal layers, each layer carefully protected as the steel was removed 

 to avoid exposing a great area of the face to air leakage, especially 

 when the air pressure sufficient to dry out the bottom would be heavy 

 enough to cause a blow-out at the top. 



Removal of the steel bulkhead was started, with the steel above 

 intact and with air pressure sufficient to dry out the bottom. After 

 the lower third of the steel bulkhead had been removed, a wooden 

 bulkhead was built in front of the shield, and the space between this 

 bulkhead and the ground ahead was packed with clay. The air 

 pressure was then reduced until it balanced the water pressure at the 

 top of the shield, and work was begun at the toj), removing the top 

 plates and proceeding downward. 



As these plates were removed, breast boards packed front and back 

 with clay were inserted to cover the exposed excavation. This work 

 proceeded down to the point where the bottom plates had previously 

 been removed, while at the same time the air pressure was raised step 

 by step to balance the water pressure. The shield was then advanced 

 against the wooden bulkhead at the bottom, compressing the clay 

 which was removed as the shield advanced, with the jacks reacting 

 against the cast-iron tunnel lining temporarily erected in the shaft. 



In order to prevent the leakage of air around the hood of the shield, 

 an annular pocket was excavated ahead of the hood the full length 

 of a shove, and this pocket was packed with clay. This served a 

 double purpose : First, the hood, as the shield advanced, cut into this 

 clay and made a thorough seal in front against air leakage; and 

 second, by exploring the full length of the shove, assurance was had 

 that the shield would not pick up and drag timbers in front of it, 

 leaving open channels behind them through which air could readily 



