Engineering Wonders of the World
Volume I

328 ENGINEERING WONDERS OF THE WORLD. SECTIONAL VIEW OF CAISSON. Showing working chamber at the bottom, {Photo, J. Valentine and Sons ') with men using hydraulic spades, shafts for men and materials, air-locks, platforms, etc. the caisson. The operation took nearly ten months, but it proved a great success. The caisson floated once more, and reached its final position nearly a year after its three fellows—namely, in February 1886. When excavation under the Filling- __ caisson had been completed, the Air- air-chamber was cleared chambers. materials used during the sinking. The smiths then provided the bottom of the material shafts with flap doors opening downwards, and re- placed the large locks at the top by smaller ones. At a given signal the men below closed the flap of a shaft. Those above opened the cock and filled the tube up with freshly mixed concrete. Then the cock was closed and the first party admitted air into the shaft above the concrete, and, when the pressure there was the same as that in the chamber, released the flap, allowing the contents of the shaft to pour out. The con- crete was then rammed hard into every hole and cranny, from floor to roof, work proceeding from the circum- ference towards the shafts, which, after the men had finally withdrawn, were filled with cement grout. The next operation was to fill the caisson space above the air-chamber with con- crete up to low - water level. Whßn the material The Granite Piers. had set, granite courses were built on it to a height of 18 feet above high-water level. Three stout iron rings, incorporated into the work, reinforced the granite, in which were enclosed forty-eight steel bolts, 25 feet long and 2J inches in diameter, having a 2-foot square anchor-plate at the bottom, and a long screw thread cut on the top, where the diameter increased to three inches. The greatest care was needed to keep this large array of bolts in the exact positions corresponding to the holes in the massive bed-plates which crowned the pier. A bed-plate was 37 feet long and 17 feet