THE HOLLAND TUNNEL GRAY AND HAQEN 593 



After the caisson was sealed to the rock ami waterproof, the east 

 and west shield bulkheads in botii the north and south tunnel cham- 

 bers were burned out and both shields wore driven throup;h the cais- 

 son. A timber and concrete cradle of suflicient strenfrth to carry 

 the shield was erected in each chamber and the shield jacked across. 

 After the shields had proj^ressed a suflicient distance west of the 

 river shaft to permit tunnel bulklicads, these were built in each tun- 

 nel and placed in operation. After this, tunneling ojierations were 

 carried on from the river shaft, releasinj^ the tunnels between the 

 land and river shafts for the placiufif of concrete lininj^. 



The caissons for tiie north and south land shafts on the New 

 Jersey side were assembled and sinkiiif]: started in the fall of 1922. 

 After the cais.sons had passed tiirough the cinder fill of the railroad 

 yard, a timber crib lilled with riprap was encountered which made 

 excavation extremely diflicult. The timbers had to be sawed or 

 chopped into short len<rths and some of the rock broken up. 



The distance between the tubes on the New Jersey side required 

 the sinking: of two separate river ventilatin^r shafts. This presented 

 a problem duo to depth of the bedrock, 250 foot as compared with 70 

 feet on the New York side. It was considered that the silt which 

 overlies the bedrock would not afford a satisfactory supj)ort. 



Accordin<jly, it was decided to support the shafts b}' means of 

 steel casinj^s 24 inches in diameter, filled with reinforced concrete, 

 extendins: from the bottom of the shafts to led*;e rock. They were 

 made in lonf]fths of 20 feet, threaded at both ends for couj)linirs. 

 Three len^hs were connected and one end lowered into the silt. The 

 silt inside the pipe was then loosened by churninrr with a 2,000-pound 

 bit, and the mud and water bailed out. Excavation was continued 

 in this manner to a depth of approximately 20 feet below the bottom 

 of the pipe. The material was firm enoufjh to prevent cavin^; into 

 the iiold. Another section of pipe was then added and the entire 

 section driven into the hole previously excavated. 



The north tunnel shield east and the south tunnel shield west were 

 builL first and started out from their respective caissons. After the 

 south tunnel shield west had progressed a sufficient distance to erect 

 a tunnel bulkhead, the face of the shield was bulkheaded and the 

 roof was removed from the south caisson and the south tunnel shield 

 east was erected. As soon as this shield was ready, the roof was 

 replaced on the caisson and the shield was started eastward, so that 

 at the close of 1923 two shields were tunneling eastward, and one 

 westward. 



The method followed in stalling these shields out of the shafts was 

 similar to that already described for the New York shields, except 

 that here it was not so difficult as there was adequate cover overhead. 

 After the roof of the working chamber had been replaced, the 



