168 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



tainly represents every known type of tunnel construction. The 

 most spectacular portion of the work, but by no means necessarily 

 the most difficult, is the tunnelling under the rivers. 



The Hudson Valley is a deep and narrow gorge, having on the 

 west bank the red sandstone formation of the Newark series, and 

 on the east side the micaceous gneiss of New York. For the most 

 part, this valley — the floor of which is some 250 feet below sea 

 level — is filled with silt. This material at the depth of the tunnel 

 sections is a firm substantial clay. Its specific gravity, wet, is 

 about 1.65, dry, about 2.50; weight per cubic yard, wet, about 103 

 pounds, dry about 156 pounds. Its chemical analysis is as follows: 



Per Cent. 



Water 29.50 



Silica 47.52 



Iron oxide 2.69 



Alumina 1 1.66 



Lime 1.88 



Magnesia 1.18 



Soda 1.80 



Potash 1 .82 



Chlorine 0.30 



Sulphuric anhydride o.ii 



Carbon dioxide 0.88 



Phosphoric acid 0.13 



Organic matter 0.53 



100.00 



It will flow under pressure, but will stand very considerable pressure. 

 It is impervious to water, though water will convert it rapidly into 

 a demoralized condition. It is ideal material to tunnel in under 

 modern shield methods. 



In stating that the work involved all types of tunnel construction, 

 it is to be understood that portions of the work were in solid rock 

 — in certain locations Newark sandstone, and in others gneiss, and 

 later — in part below the waters of the Hudson River, where at 

 times, alongside the American Line Pier, there was a shell of only 

 five feet of rock between the tunnel roof and the river bed — portions 

 in sand and gravel saturated with water, portions in quicksand also 

 saturated, portions in made or filled ground — the most trouble- 



