Page:Popular Science Monthly Volume 88.djvu/738

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��Popular Science Monthly

��each of the cyUnders next simultaneously turned wheels which opened a three- inch water-valve in the bottom of the center compartment of each cyliner and by carefully observing the rate at which the cylinders became submerged and testing the subsequent load transferred to the derrick, the nineteen tons of water to overcome the buoyancy was admitted, filling the center compart- ment to about one-half its capacity. Then just enough more was let in to hold the section in position when lowered, against the action of the tidal currents in the river. This total excess load never amounted to more than a few tons, which the derricks readily sus- tained. The section was lowered, until one of the diaphragms at or near each end, rested upon temporary timber frames, in the shape of an inverted "11." By means of the location cables attached to the ends and sides, the section could be shifted north or south, east or west until the masts (which projected about ten feet above the water) indicated that the- structure was in proper position. The control over this large steel struc- ture was very complete; the section could be raised or lowered, shifted at will, or could even have been brought to the surface again if conditions had made it necessary.

How the Sunk Tubes Were Joined

Each section after the first, had a positive anchorage to the section pre- viously placed; the ends were brought into perfect alinement by means of steel pins mounted on the end of one section, and guided into tapered holes in castings mounted in the same relative position on the other section. As the two sections were drawn together, the pins were started into the tapered holes and served to guide the ends to a positive junction, then a diver bolted them together. The complete operation from the time of opening the valves to admit the water to fill the tubes, to their final anchoring, required but three hours.

As soon as a section was placed, prep- arations were made to deposit the encasing concrete, the weight of which was necessary to keep the tubes from coming to the surface when their buoyancy would be restored in the un-

��watering, and the strength of which concrete, together with the reinforcing effect and waterproofing qualities of the steelwork, was to provide a safe working- chamber for the completion of the sub- way structure. The section, as far as described, might be considered to be a large box sunk in the bottom of the river, without top or bottom but having sides and ends, and divided by the diaphragms into a series of pockets which could be filled with concrete in any convenient order.

Pouring Concrete Through Pipes

The tubes, being surrounded with water, the problem resolved itself into displacing this water with concrete and without the loss of the cement which would occur in dropping the concrete through even a much less depth of water. This was accomplished by what is technically known as the "tremie" method of depositing which involves the use of long pipes which are kept nearly full of concrete and which are raised a little as the concrete is poured in at the top. A nearly continuous flow is main- tained. The concrete gradually dis- places the water but does not mix with it. Each pocket required an average of twelve hours for its completion by this method.

When all of the pockets had been filled, except those over which the buoyancy cylinders had been placed, these cylinders having performed their functions, were disconnected by forcing the water out of them; they floated to the surface there to be reclaimed for use on the next section.

With all of the sections in place and encased and with the extreme ends of the series closed by the heavy wooden bulkheads previously mentioned, four small steel shafts or wells attached to the tubes before sinking, were opened and the water pumped out. It was then possible to get inside of these sub- merged passages beneath the river, assemble the concrete forms and place the lining, thus completing the structure. There were no leaks in the tubes except where some of the bolts in the interior walls had not been tightened sufficiently, and by tightening these bolts, the finished work was, figuratively speaking, "dry as a bone."

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