It will be remembered that the plans of the Catskill aqueduct include the construction of a huge siphon for carrying the water below the Hudson River at a crossing near Storm King Mountain. The aqueduct reaches the western bank of the river at an elevation of several hundred feet above water level. The original plans contemplated the sinking of a vertical shaft from this level to a point about 400 or 500 feet below the river bed, where it was believed that solid rock, absolutely free from fissures, would be found. Through this rock a tunnel is to be driven horizontally, until the opposite shore is reached, where it is to meet a vertical shaft sunk from a level above the river, which would correspond to the established grade of the aqueduct. Geologists had predicted that solid rock would be encountered at a depth of 500 feet below the river, and borings were carried on through the past year to ascertain how far this estimate was correct. Much to the surprise of everybody concerned, no solid rock has yet been encountered, although the borings have been carried down to a depth of nearly 700 feet. Although the drills operated from scows in the river have been sunk for hundreds of feet below the river bed, nothing has been found thus far but mud, sand, and bowlders. In view of the fact that the pressure in the 14-foot circular tunnel will reach about 15 tons to the square foot, it is imperative that the tunnel be driven in absolutely sound rock; otherwise, under this pressure, there might be very serious leakage. Investigations of other possible points of crossing have shown there is no likelihood of finding a more satisfactory location than that at Storm King Mountain; for it is believed that the old bed of the river lies everywhere many hundreds of feet below the present bottom. Comparative estimates have been made of the cost of carrying the water across the river by means of steel pipes laid on the present bottom, or by an aqueduct built at high level; but the engineers of the Board have concluded that a deep tunnel through solid rock will, all things considered, prove to be the most satisfactory method of crossing. Apart from the fact that when such a tunnel is once built, there will be no subsequent expense for maintenance, a tunnel siphon would be absolutely safe from injury, a most important consideration where the health and safety of such a great city as New York is concerned.
This article was originally published with the title "Difficulties in Constructing the Great Catskill Aqueduct Siphon" in Scientific American 97, 25, 454 (December 1907)