590 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 19 3 



should be left in place permanently, and the concrete walls of the 

 permanent structure placed immediately, being increased in thick- 

 ness to enable them to withstand the pressure from tunneling 

 operations. 



Preparatory to tunneling under the river bulkhead, clay and other 

 material to prevent the escape of the compressed air from the tunnel 

 were deposited in the slip between the piers and on the landward 

 side of the river bulkhead to fill such voids as might remain around 

 the tops of the piles supporting the timber platform of the bulkhead 

 construction. Not only were the voids around the piles filled, but 

 the soft mud in the slip was displaced by the heavier clay, a firmer 

 material and better adapted to resist air leakage. 



In this section great care was taken in excavating ahead of the 

 hood to be sure that all piles within the area of the tunnel section 

 were cut off before coming in contact with the shields. This was 

 done to avoid pushing the piles through the ground and leaving back 

 of them an open channel for air to escape. These piles extended down 

 to the springing line of the tunnel excavation, and as many as 30 had 

 to be cut off at one time in advancing the shield the length of one 

 ring. In this manner both shields passed under the river bulkhead 

 without accident. 



The tunnels then entered the Hudson Kiver silt. The front of the 

 shield was completely bulkheaded. Some of the lower pockets in the 

 shield were opened to allow a part of the material to enter the tunnel 

 as the shield was advanced. The balance of the material in excava- 

 tion was displaced bodily. At once it was noticed that there was a 

 tendency of the tunnel lining to rise behind the shield. This rising 

 always accompanied the movement of the shield ; whenever the shield 

 was stopped the rising ceased. The difficult feature at this point was 

 that the shield was so heavy that it settled while the cast-iron tunnel 

 lining behind the shield rose, so that the shield at all times was below 

 grade while the tunnel lining a short distance back was above grade. 



The bulkheads in the shield were moved forward to reduce weight 

 by lessening the amount of muck in the shield. This aided somewhat 

 in keeping the shield from settling and then more material could be 

 taken in through the shield. This procedure lessened the pressure on 

 the tunnel behind and reduced its tendency to rise. As the contract 

 required that a second tunnel bulkhead should be constructed in this 

 vicinity, the south shield was stopped after passing through 218 feet 

 of silt and the bulkhead was built. This bulkhead, which is typical 

 of all the bulkheads, is a concrete wall 10 feet thick, equipped with 

 the usual muck, man and emergency locks, and adds temporarily con- 

 siderable weight to the tunnel. 



With this additional weight, the rising of the tunnel was some- 

 what checked and after tunneling a distance of 121 feet farther 



