ABSTRACT OF PROCEEDINGS. XXXI. 



crete to make the tunnel water tight, 7 inches would 

 suffice. 



I quite agree with these gentlemen that it will be abso- 

 lutely necessary to make very careful inquiry into the 

 capacity of the Hawkesbury Sandstone to resist the pres- 

 sure it will be subjected to before adopting my proposal. 



We know the Water Board of New York followed the 

 plan of locating their pressure tunnels at a depth not less 

 than J 50 feet below the surface of the rock, the character 

 of which, however, has not been stated. On an average 

 the rock was found at about 50 feet below the ground 

 surface, this giving an average depth of 200 feet. This 

 depth was selected for the purpose of obtaining a thick, 

 solid bed above the tunnel, well able to resist the hydro- 

 static pressure. 



Great care was taken to test the rock when crossing 

 the river Hudson. Many borings were made, and they 

 showed that in the middle of the river rock occurred at a 

 depth of not less than 800 feet. As many as 14 cross 

 sections of the river gorge were taken, requiring about 

 300 borings, and it was ultimately decided to cross the 

 Hudson at Stormking, where the river runs between two 

 mountains. Subsequent investigations at this place resulted 

 in the tunnel being located at a depth of 1100 feet below 

 the surface of the river at L.W., the hydrostatic pressure 

 in the tunnel being 750 square inches. Here the tunnel 

 was lined with 17 inches of concrete, but elsewhere it 

 was less, varying with the pressure, the least thickness 

 being about 9 inches. Very little timber was used in the 

 tunnel while the excavation was being carried ou, the 

 supports being of light steel trussing, which were allowed 

 to remain, and were buried in the concrete, in which pipes 

 were inserted at frequent intervals, and after allowing 

 about three months for the concrete to harden, grouting 



