National Geographic : 1918 Jul
THE NATIONAL GEOGRAPHIC MAGAZINE A lofty masonry aqueduct was open to danger of destruction; pipe-lines were both expensive and liable to leaks; the tunnel was decided upon. In some places these tunnels go very deep, so deep that the water exerts as much as three and a half tons pressure per square foot. When the aqueduct builders came to the Rondout Creek Valley they met with a discouraging situation. They found a very poor quality of rock under the valley, with many faults. Their drills slipped through into caverns of unknown depths, and at one place they encountered a spring deep in a rock fissure, which had a flow of 2,000 gallons a minute. There was sulphur present, also, and its fumes seriously inconvenienced the workmen. However, enough grout was put into the spring to drive it out of their way; with channel rings and concrete the rock was made strong where Nature had made it weak; and they steered clear of the caves. This tunnel was only 727 feet below grade, and little attention was paid to it by the public. CARRYING WATER BENEATH THE HUDSON When the Hudson crossing was reached, where the tunnel goes down so deep that the Woolworth Building placed on top of the United States Capitol and sur mounted by the Washington Monument would leave only the capstone of the latter showing above the water surface, New York was gloomy and fearful. Fail ure of the undertaking was freely pre dicted. Nevertheless, the construction of the spectacular Hudson tunnel was a far easier task than the unnoticed one at Rondout. The Hudson is tunneled 45 miles below Ashokan Reservoir. In the building of this under-the-river tunnel, it was first necessary to ascertain exactly the lay of the solid rock below the river. First, an effort was made to drill down to the surface of bed-rock with drills mounted on scows. But that was a failure. Drills were lost and all sorts of hindrances-encountered. Then it was de termined to dig a shaft at each side, about 300 feet deep, and from these to' dril two V's under the Hudson, one with a broad and the other with a sharp angle. In this way the engineers gained the necessary data about the rock formation. Diamond drills were used. These drills are circular tubes, the lower ends of which are studded with seven black diamonds, costing about $1oo each. The drill cuts through the rock like an apple corer through an apple and brings the core to the surface with each stratum in the posi tion in which it was found. Thus the position of the rock was re vealed and its density determined. The engineers ascertained that they could put their tunnel under the river nearly t,400 feet beneath the surface of the water. AQUEDUCT BUILDING, ANCIENT AND MODERN Engineering has progressed in great strides since the days when the Romans were building aqueducts. When the Aqua Claudia was under construction a tunnel under a mountain three miles long was necessary, and the chief engineer started work at both ends. He was cap tured by brigands, and when he was finally liberated he found that his two digging parties had missed one another entirely and were excavating two tunnels instead of one. When the Hudson tunnel was bored the two parties met and were not half an inch out of the way, although the undertaking required the sinking of two pits 1,114 feet deep, from the bottoms of which the tunnel operations were started. The Hudson crossing is about half way between West Point and Newburgh, at Storm King Mountain. Thirty miles nearer the city, on the east side of the river, is Kensico Reservoir, with a ca pacity of thirty-eight billion gallons-22 gallons for every inhabitant of the earth. i,This reservoir is four miles, long and from one to three miles wide. The water is impounded by a dam thrown across the Bronx River, one of the finest struc tures of its kind in existence. A third of a mile long, and 307 feet high, it is 235 feet thick at its base and 28 feet thick at its crest. The exposed portion of the down-stream side is made of huge granite blocks. Above Kensico there is what is known as a coagulation plant. Here provision is made for treating muddy water with a harmless coagulant. In flood periods silt is brought down from the watersheds.