I9I0.] DAVIES— TUNNEL CONSTRUCTION. 181 



treatment will relieve the condition so as to allow the shield to be 

 pushed past the blow hole. Sometimes, however, the hole will in- 

 crease in size, often quite suddenly, and get beyond control, in which 

 case all doors in the shield are closed up and the men are removed 

 before the tunnel fills with water. In one famous incident which 

 occurred in one of the East River tunnels, a small blow had de- 

 veloped and a man was in front of the shield stuffing bags of hay 

 in the hole, when the pressure sucked the bag into the hole suddenly, 

 and the man, failing to let it go quickly enough, w^as carried, hanging 

 to his bag, through the ground into the waters of the river, when he 

 rose to the surface and was picked up alive by a passing boat. When 

 the condition is reached that the tunnel is completely flooded, the 

 steps to be taken to recover it must be from outside. Soundings are 

 then taken very carefully and accurately to determine the locality 

 and extent of the blow hole in the bed of the river. A large 

 quantity, usually several hundred cubic yards of good stiff silt or 

 clay, is loaded in a dumping scow, and at " slack water " the scow 

 is towed into position over the hole, being located by instrumental 

 observation from triangulation points on shore, and on signal from 

 the two observers of simultaneous correct position the scow is in- 

 stantly dumped. Soundings are then again taken to ascertain if 

 the hole is completely filled, and if not, scow after scow is dumped 

 at the same point. Following that, air is then pumped into the 

 tunnel, forcing the water out through the blow pipes ; as the pres- 

 sure is gradually raised it soon becomes evident whether or not the 

 hole is plugged. In the construction work on the up-town tunnels 

 of Hudson and Manhattan, there occurred fully a dozen blows, 

 necessitating dumping clay, although only in four or five cases was 

 the tunnel flooded. In one of these most persistent cases, which 

 had been occasioned by gross incompetence on the part of the 

 night foreman, who advanced the shield with too many doors open, 

 dumping was carried on until half the tunnel was full of silt, and 

 then the leak was only stopped by spreading a sail sheet over the 

 hole and dumping additional material on top. When the tunnel 

 was recovered and cleaned out, the sail covered the door opening. 

 The increased skill and experience attained by the workman before 

 the down-town tunnels were built was shown in the fact that only 



