A Treatise On The Principles And Practice Of Dock Engineering

36° DOCK ENGINEERING. “The hüll is of wrought iron, framed and braced together. There are four decks, the three lower of which are open lattice work, connecting the deck stringer-plates of both sides. The keel- and stem-plates are 2| inches thick, and the keel-base is formed of two angle-irons rivetted in the wake of the garboard strakes. On both sides of the keel and stems, heavy pieces °f greenheart are bedded on and bolted to the ironwork, the timber on botli sides of the keel and stems being planed true to fit the polished masonry faces of the grooves, with which they form a thoroughly watertight joint. The frames of the caisson are of angle-iron ; below the level of deck, B, they are spaced at distances apart of 3 feet. The deck beams are of angle-iron. The skin plating ranges in thickness from f inch at the upper part, to § inch at the lower part of the caisson. The large plates are laid with their greatest lengths horizontal, in alternate inside and outside strakes, with vertical butt joints. Between decks, A and B, in the centre of the caisson, is a room with watertight floor and bulkheads, in which the engine, boiler, and pumping machinery are placed. Hatch- ways in the upper deck give access to the engine-room and other parts of the interior of the caisson. The roadway deck has strong angle-iron deck beams and stringers, and it is cleaded with 4-inch Dantzic oak planking, caulked and payed with marine glue. A horse-track, along the centre of the roadway, is formed of American rock elm slabs, spiked to the deck flat; and on each side of it, tracks of wrought-iron ^-inch bars are screwed down to the decking ; guides of angle-iron are fitted along their outsides ; and a hinged handrail of wrought-iron gaspipe is fixed along both sides of the roadway.” The caisson illustrated in figs. 345 to 347, forming one of a number of interchangeable caissons in the service of the Mersey Dock and Harbour Board, is mainly used for work of a temporary nature during the absence of, or in case of accident to, the dock gates. It consists of four decks, below the lowest of which is located the concrete ballast. It will be noticed that the upper deck, which is of wrought-iron plating, is not available for traffic. There are three bulkheads, two transverse and one running fore and aft. These caissons do not fit into grooves, as in the previous instance, but have a single plane bearing surface against a sill, and quoins arranged in the curved pierhead of an entrance, so that the same caisson, which is 100 feet long, can serve in several situations. Lowering Platforms. —The difficulty of entirely recessing a traversing caisson under cover of the quay, and of, at the same time, equipping it with a suitable deck at quay level for the purposes of traffic, has been overcome by the introduction of a lowering platform. The platform, which consti- tutes the roadway, is supported on a series of hinged verticals, in a manner more fully described in Chapter x. A caisson designed in this manner, by the late Mr. W. R. Kinipple, closes the entrance to the Garvel Graving Dock, at Greenock.* It is a rolling caisson, with the rollers attached to * Macalister on “Caissons for Dock Entrances,” Min. Proc. Inst. C.E., vol. Ixv.