LOCATING ROCK UNDER HUDSON RIVER
A notable illustration of exploration drilling was furnished by the surveys for the Hudson crossing of the Catskill Aqueduct. To bring water from the Catskill Mountains to New York City it was necessary to cross the Hudson River. In order to furnish a permanent conduit it was decided to carry the water in an inverted siphon bored through solid rock. It was necessary to obtain a profile of the rock at the point where a crossing seemed most feasible. Borings were therefore made from a barge anchored in the river, but it was impossible with such an unsteady working base to carry on the boring to any considerable depth. Tides, the wash of passing steamers, floating ice, all combined to obstruct the work. Drills were constantly broken. Finally it was decided to do the boring from opposite banks of the river at such an angle that bore holes would meet or pass each other under the middle of the river. The exploration boring from the barges indicated that solid rock lay at a considerable depth below the river bed and that the bore holes would have to be set at a sharp angle to keep from breaking through rock. The river at that point is about 3,000 feet wide and a boring from the surface at the middle of the river had been driven to a depth of 768 feet without striking solid rock. Two shafts were sunk to a depth of about 250 feet and from these diamond drill borings were started at such an angle that they would cross at a depth of 1,500 feet. Excellent rock was encountered throughout the boring. Then a second set of borings was made which crossed at a depth of 950 feet without encountering any appreciable amount of water. It was accordingly decided to carry the aqueduct in vertical shafts, one at each side of the river, at a depth of 1,100 feet below water level, and then connect these shafts with a horizontal tunnel. It was important to have a good solid rock cover over the tunnel because the aqueduct reaches the river with a head of 400 feet which added to the 1,100 feet of depth of the inverted siphon gave a total head of 1,500 feet or a hydraulic pressure of about forty-two tons per square inch.
Although the diamond drill borings just mentioned were remarkable because of their inclination and because of the cramped quarters from which they were driven, they do not begin to compare in depth with some of the borings made in search of water and of oil. The deepest boring in the world to date is near Fairmount, West Virginia, where a hole six inches in diameter was driven to a depth of 7,579 feet or nearly a mile and a half. At that point an earth slide stopped further borings.