BIGGEST CAISSON TO LIE UNDER HUDSON |
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The New York Times
December 24, 1922
The largest steel caisson ever used in bridge or tunnel construction was launched last week at the Staten Island Shipbuilding Company's plant, Mariners Harbor, S. I., and after being tested for water tightness and having its cutting edge concreted will be towed to its final resting place near Pier 35, Hudson River. Here it will form a link of the New York and New Jersey underwater motorway which is receiving attention from engineers all over the world. This caisson was built on ways like a ship and slid off into Kill van Kull in the same manner as a pleasure boat or man-of-war is launched. Completed, it will weigh about 15,000 tons. It will fill the double capacity of working chamber and ventilating shaft for the new tunnel which is to relieve the congestion of crowded ferries by taking care of motor traffic crossing underneath the Hudson River.
This caisson, which looks like an oblong drygoods box, is now 35 feet high, but its final proportions when it reaches its resting place beneath the bed of the river will be 108 feet high, 93 feet long and 37 feet wide. There will be a steel box within a steel box and the five feet between the the outer box and the inner one will be filled with concrete. On the New Jersey side two caissons for the river shafts will be placed, instead of another monster caisson similar to the one on the New York side. In all, seven of these oblong steel and concrete drygoods boxes will be used in constructing the tunnel, and will form part of its ventilating system. No one of the seven, however, is nearly so large as the big one at the head of Pier 35. The caisson will draw 25 feet of water when being towed to Pier 35, as much as all but the largest ocean liners.
As excavation of the bed of the river progresses the caisson keeps sinking and the top of it is built up and up. This construction will continue until the caisson, which will ultimately rest in about 37 feet of water, drives its way through 40 feet of clay and almost the same depth of rock; in fact, until it finds its permanent place 12 feet or so below the bottom of the tunnel. Slowly the big steel creation forces its way down through the rock and the clay as the material below it is excavated and carried up through its length to the top.
The men who are working inside this caisson have a dangerous job. Although statistics show improvement in the safety of tunnel construction, the work inside this huge caisson is not devoid of risk. Every week these men have a longer climb to reach the bottom of the shaft as day by day the caisson stretches out toward its ultimate 208 feet of length, and their working hours grow shorter as they sink further and further from daylight and natural air. The "sandhog" gets used to compressed air and finds it hard after a long job under ground to again work under ordinary conditions. At the top two doors or locks open outward to permit egress and ingress for men and melateria.
There are three things which make the big caisson unique: first, its size, second, the fact that it is the first one to be constructed and sunk before the shield of the tunnel has been driven through, and third, it is the largest ventilating shaft ever constructed. The tunnel which is of cast-iron, will be embedded in a clay blanket and thousands of cubic yards of clay and "rip-rap" are being dumped along the pathway of this tunnel to make that blanket. This will protect it from sunken ships, sharp pieces of rink and other dangerous objects which drop with considerable force to the bottom of a river.