THE HOLLAND TUNNEL GRAY AND HAOEN 591 



in the silt the shield entered at the bottom of the sand layer 

 which overlies the rock, and thereupon all rising of the completed 

 tunnel during shield driving ceased. In the north tunnel, which 

 was driven through the same material after the south tunnel was 

 built, a larger amount of material was taken in through the shield at 

 the start, and while there was some rising of this tunnel behind the 

 shield, it was very much less than in the south tunnel. In neither 

 tunnel was the movement sufficient to endanger the structure. 



The excavation in tiie part-earth and part-rock section just east of 

 the New York river shaft caisson was carried on by driving a short 

 bottom heading in advance of the shield, in which was placed a con- 

 crete cradle with steel rails embedded in it upon which the shield 

 slid. After placing the cradle the rock was blasted out for one or 

 two advances of the shield and then the soft material on top was 

 carefully excavated and supported b}' poling and breast boards. 



The New York river ventilating shaft caisson was sunk by the 

 compressed air method in the river near the New York pierhead line. 

 It was built on launching ways, then launched and drydocked. After 

 concrete had been placed in the pockets surrounding the working 

 chamber, additional steel was erected, carrying it to a height of 55 

 feet. 



A platform supported on piles had been built on three sides of the 

 site (the south side being open ready to receive the caisson), and the 

 caisson was towed to its position on the work. The caisson at that 

 time weighed approximately 1,650 tons. Upon arrival, additional 

 steel was erected and concrete was placed in the walls, the caisson 

 sinking as the additional weight was placed. Care was taken to 

 keep the center of gravity as low as possible to maintain the 

 necessary stability. When it had reached a depth of 35 feet, the 

 cutting edge encountered the river bottom, into which it settled at 

 each low tide, and weight was added with sufficient rapidity to over- 

 come the tendency to float on the subsequent rising tide. 



No excavation was carried on in the working chamber until the 

 cutting edge had penetrated about 9 feet into the mud, as the weight 

 of the caisson displaced the material up to this point. Compressed 

 air was then introduced into the working chamber and the usual 

 shaft mucking operations started. At a depth of G9 feet below mean 

 high water, rock was encountered. This was taken out in lifts about 

 6 feet deep and the caisson was lowered by successive drops until it 

 reached its final position. 



The upper half of the outside of the caisson, or the part which is 

 exposed to open water, was covered with water-proofing, which in 

 turn was covered with an 18-inch layer of protection concrete. An 

 additional protection is afforded in the upper portion by a granite 

 facing where the shaft is exposed to tidal action. 



