THE HOLLAND TUNNEL GRAY AND HAGEN 589 



escape. The nccessitj' of takino: this precaution is evident when it is 

 considered that at this point there were but 14 feet of cover above the 

 shield to the street surface, and only 8 feet from the top of the shield 

 to tlie under side of an ohl l)rick sewer, uliiih would readily allow 

 the air to escape from the tunnel heading. 



As the tail of the shield loft the caisson, <]:routincj was at once 

 started to fill the annular space which the shield left outside the 

 tunnel lininj^. Every effort was made to keep this space fully 

 grouted, even to the extent of stopping the shield in the middle of a 

 shove to keep the grout up with the shield. 



The method just described was later modified so that in the bottom 

 quarter of the shield, instead of pacldng ahead with clay, a fixed 

 wooden bulkhead was built in the shield, and the shield was advanced 

 into the fine wet sand with this bulkhead in place. This compressed 

 the earth, driving out the w'ater, so that the material was firm and 

 could be excavated during the shove over the top of the bulkhead, or 

 through small openings cut in the bulkhead itself. This prevented a 

 free run of wot material into the bottom which is the ordinary method 

 of tunneling under the river. 



The grouting previously described was continued, and not only 

 prevented an abnormal escape of air at the tail of the shield, but 

 also prevented settlement of the streets and adjacent buildings. 

 The buildings at the corner of West and Spring Streets settled 

 slightly, but at no time were they in need of shoring, nor were the 

 occupants disturbed at any period of the tunnel work. This was 

 the situation also with the New York Central tracks under which 

 the Canal Street tunnel was driven. The grouting was carried on 

 so effectively that it filled some of the old sewers in the vicinity 

 which later had to be cleaned out. 



The Canal Street shield passed very close to a cofferdam around 

 an excavation for a sewage treatment plant, and it was evident from 

 the first that great care must be exercised in driving the tunnel past 

 this location. At the nearest point the .shield was within 5 feet of 

 the steel sheeting of the cofferdam, with the bottom of the sheeting 

 at about the springing line of tunnel. On November 30, when the 

 shield was about 40 feet away, it was noticed that sand and water 

 were being forced through the slieeting into (he cofferdam by the 

 air |)ressure from the tunnel heading. In aijout 2 hours approxi- 

 mately 150 cubic yards of earth had been blown into the excavation 

 from behind the sheeting, and it was plain that not only was the 

 cofferdam in rjanger, but the continuation of tunneling operations 

 would be hazardous because the cavities left in the ground providi^l 

 open channels for the leakage of air, which might have resulted in 

 a tunnel blow-out. It was decided that tunneling operations should 

 be temporarily suspended, that the steel sheeting of the cofferdam 



