A Treatise On The Principles And Practice Of Dock Engineering

354 DOCK ENGINEERING. constructed in a side wall at right angles to the axis of the waterway, and in a direct line with the path along which they travel to close the entrance. These caissons are almost universally of the box type (hence sometimes called box caissons), consisting of a floor, side and end plating, and a water- tight deck, the whole being divided into compartments according to the requirements of buoyancy and the mutability of design. Sliding Caissons are provided with keels or rubbing plates on their undersides, by which they are hauled over sliding ways set in the floor of the caisson berth. This method gives rise to a certain amount of friction, which may be diminished to some extent by suitable flotation adjustment. Sliding caissons have been constructed at Malta, Portsmouth, Milford, and ■elsewhere. The following is a brief description of one used to close the Hamilton Graving Dock at Malta,* (see figs. 330 and 331) : — “ The rectangular sliding caisson, made of mild steel, is 40| feet high and 16| feet wide, exclusive of the keel and stem timbers, and is strengthened by two watertight decks, and bracing and framing. As the position of the entrance precludes heavy traflic passing over the caisson, the roadway deck could be placed low enough to pass under the cover- ing of the camber, 1| feet below the coping, connection being made with the quay by a hinged flap. The caisson can be floated out from its normal position to the outer stop, thereby adding 38 feet to the available length of the dock, The air-chamber, 92 feet by 16^ feet by 8| feet, in the middle of the caisson, is reached through two shafts. The caisson is ballasted by concrete blocks on the floor of the air-chamber, and by water in the tanks under the roadway deck at each end. Without any ballast, the caisson would float with the top of the air-chamber 2 inches above the water, but the concrete ballast more than balances the flotation, producing a normal pressure on ! DECK A. Fig. 331. the sliding ways of 10 to 20 tons. The water ballast is adjusted by means of a three-way stopcock in the 4-inch pipe connecting the tanks, enabling the water to be run from one tank to the other, or one or both tanks to be emptied. The caisson can be hauled in or out of the camber in five minutes, by two steel pitch chains connected with the hydraulic hauling gear, and exerting a pull of 30 tons on the two projecting arms of the caisson to which they are attached. The caisson is guided into the camber by the keels and granite rubbing pieces below, and by the fenders and * C. and C. H. Colson on “Hamilton Graving Dock, Malta,” Min. Proc. Inst. C.E., vol. CXV.