180 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



tunnel engineer exercises his judgment and fixes on a pressure that 

 will not escape through this material and will exclude as much water 

 from the bottom of the tunnel as possible. Some escape of air 

 occurs in nearly all materials, excepting the most impervious, and 

 this escaping air, if pressure is too great, blows the particles of 

 materials away and enlarges the passages until they become so 

 large that the air escapes faster than it can be supplied, when the air 

 pressure drops and an inrush of water occurs. This is what is 

 meant by a " blow-out." 



For any railroad tunnel, in order to minimize the gradient of 

 approaches, the grade is necessarily established at the least depth 

 feasible below the bed of the river or below the surface ; and where, 

 as in the Hudson River, the water is sixty-five or seventy feet deep, 

 and man cannot, with safety, work under air pressure exceeding 

 forty-five pounds per square inch, which represents a hydrostatic 

 depth of salt water of one hundred and two feet, it is obvious that 

 the amount of cover over the roof of a tunnel and below the bed 

 of the river must be very small. If the soil is in any degree porous, 

 and pressure is allowed to drop, there is a grave liability of the 

 ground becoming "demoralized" by infiltration of water, and if 

 pressure is then raised to check the inflow, it sometimes happens 

 that the cover of soil cannot withstand the increase, and the roof is 

 blown off or a big hole blown out, and water then comes in in large 

 quantities, often beyond control, so as to flood the tunnel works 

 within the air locks. 



The instinct of an unskilled foreman is usually to raise pressure 

 in case of a leak commencing. If nothing happens it may be a 

 good thing to have done, but an expert will usually lower the 

 pressure in this case, putting up with the nuisance and difficulty of 

 having a good deal of water in his tunnel and thereby allowing the 

 greater external pressure to squeeze what soil exists into the 

 pockets of the shield and thereby choke the leak. There is far 

 greater skill and caution needed to raise air pressure than to lower 

 it, in case of any leakage occurring. In the event of a " blow " it 

 may be small, in which case it can be stopped from inside by stuffing 

 up into the hole bags of sawdust, hay, balls of clay, and in fact 

 anything handy and available to fill a hole. Usually this prompt 



