A Treatise On The Principles And Practice Of Dock Engineering

356 DOCK ENGINEERING. arranged in pairs. The working parts are open to inspection by means of a pneumatic shaft leading to the chambers in which the axles are placed. The buoyancy chamber is also accessible by means of a similar sliaft. Watertightness at the abutting surfaces of the caisson is established by greenheart facings. The caisson is designed to stand a head of water from either side. Its displacement is about 420 tons ; its own weight, 196 tons; and the ballast, 273 tons; leaving some 49 tons excess weight to insure stability during movement. The general framework of the structure comprises eight large vertical girders, placed at intervals of 8 feet, and extending to the full height and width of the caisson. The flanges of these girders serve as bearing surfaces for the plating; they are formed of 6 by 2| by 3 inches channel iron. The horizontal struts are similarly composed, but double. Six tiers of horizontal joists, 14 inches deep, connect the vertical girders on each face. These joists are spaced at varying distances apart, according to the intensity of hydrostatic pressure. Between the vertical girders are three rows of intermediate bearers, only the centre one of which is prolonged above the watertight deck. These bearers are of channel iron of the same section as the vertical flanges. The watertight decking is carried by the horizontal struts of the main girders, with deck joists between and at right angles to them. The thickness of the plating varies from 1 to 4 inch. Floating Caissons may either be of the box or the ship type. In the former case, they are generally rectangular in plan and similar to the examples of box caissons already described. Their distinction lies simply in the fact that they are moved entirely by flotation, without guides or rollers. Figs. 336 to 338 illustrate a floating caisson used at Blackwall, London.* The caisson has only one meeting face, and that of teak, 14 by 7 inches. There is a lower air-chamber extending the whole length of the caisson, formed by a watertight deck at a height of 11 feet 6 inches above the bottom. Above this level, the caisson is divided into three compartments by vertical bulkheads, which are also watertight. The ballast at the bottom of the air chamber consists of cast-iron kentledge, set in Portland cement concrete. Three sluices, each 3 feet in diameter, allow water to be transmitted through the caisson, the valves being controlled by spindles passing through the air-chamber to the upper deck. The following are the sizes and general dimensions of the framing : —Angle irons at sides, 3 by 3 by f and 18 inches apart ; angle-iron cross beams, 4 by 4 by 4 inches to high- water level, and 3 by 3 by ^ inches above; centre uprights, 4 by 4 by f inches, also 18 inches apart; deck beams, 3 by 3 by -| inches. The plating is ^ inch thick up to the watertight deck, and above that level, fand ^ inch thick. Rock elm fenders, 10 by 10 inches, and a decking of English * Vide Maoalister on “Caissons for Dock Entrances,” Min. Proc. Inst. C.E., vol. Ixv.