THE HOLLAND TUNNEL GRAY AND HAOEN 687 



lines, electric cables, and telephone cables. Every facility had to 

 be provided, even an iiulependont telephone system connecting all 

 parts of the work with the public teleplione system. 



Canal Street Park was made available as a site for the air-com- 

 pressing plant and engineer's field oflice. Pier 35 and adjacent slips 

 were used for the storage of materials and for the disposal of ex- 

 cavated matter from the tunnel heading. Overhead gantries con- 

 necting the shafts with the pier permitted traffic to the water front 

 in connection with the tunnel to pass above the city streets. 



The first shield was erected in tlie Canal Street shaft. On Oc- 

 tober 2G, 1922, compressed air was introduced into the shield cham- 

 ber, and tunneling was begun. Each shield was 30 feet 2 inches 

 in outside diameter, IG feet 4 inches long, and the upper half was 

 equipped witii a hood projecting 2 feet 6 inches ahead of the shield 

 proper. Five vertical and three horizontal walls divided the shield 

 into 13 compartments, through which the ground in front was ex- 

 cavated. It was equipped with thirty 10-inch jacks, having a 

 combined thrust of 6,000 tons. A hydraulic erector was used 

 to build the tunnel segments into a complete ring. The weight 

 of the shield, with all equipment, was about 400 tons. 



The tunnel lining is composed of rings 2 feet 6 inches wide, 

 consisting of 14 segments, each approximately 6 feet long, with 

 a key 1 foot long, bolted together. Inside the lining is an inner 

 lining of concrete 19 inches thick. As the shield advanced and 

 the lining was erected behind it, the space due to the difference in 

 tlie diameter of the shield and the rings forming the lining was 

 filled by forcing a grout of cement and sand in equal parts into the 

 void under high air pressure. For this purpose each segment 

 was provided with a grout hole fitted with a screw plug. The lining 

 was made water-tight by placing hemp grommets soaked in red 

 lead around the bolts, and by caulking lead wire into grooves be- 

 tween the segments. 



Shield driving requires extreme care and exactitude to keep to 

 line and grade. The position of the shield fixes the location of the 

 tunnel, and no correction can be made afterward. It is absolutely 

 essential that the slightest deviation of the shield from its theoreti- 

 cally correct j)Osition be known at once, so that measures may be 

 taken to remedy the error during the next shove. The shield is 

 guided by the operation of the jacks distributed around its cir- 

 cumference, omitting the use of those jacks in the direction toward 

 which the shield is to move. 



Every precaution was taken to provide for the safety of the work- 

 men in the compressed-air chambers. A high emergency gangway in 

 the upper part of the tunnel led from the shield to the locks, for 

 escape In case of a blowout. Safety screens were installed to trap the 



