A Treatise On The Principles And Practice Of Dock Engineering

PIERHEAD AT MADRAS. 295 jetty is 938 yards long, and 42 caissons were employed, generally 68 feet long, with widths ranging from 15 feet 6 inches to 21 feet for the jetty and 31 feet for the pierhead. These caissons were sunk into the fine sand of the beach to depths of 16 to 26 feet below zero. The joints between the caissons were at the most 20 inches wide : they were simply closed by wood panels. The superstructure consists of an open timber framework adjoining the mainland, 650 feet long, a half-filled-in framewoik, 490 feet long, and a solid breakwater for the remaining length, bordered on the inner side by a stockade and on the outside by a mole, having a hearting of sand protected by a facing of masonry. In the open-work jetty the masonry base is carried to a level of 8 feet above datum and to a level of 16 feet 6 inches in the half solid portion. The platform is constructed throughout at a height of 29 feet 6 inches above zero, or 6 feet above high water of equinoctial spring tides. River Jetties at Tilbury.* The jetties at the entrance to the tidal basin are 45 feet in width, and project in the tideway into about 45 feet of water at high water of spring tides, or 48 feet below Trinity high-water mark. The centres of the rounded ends of the jetties were formed of cast-iron cylinders, 15 feet in diameter, sunk to a chalk foundation at about 75 feet below the level of the deck. These cylinders were afterwards filled with concrete. Immediately around the cylinders, and hooped at intervals to them, was driven a double row of piles, from which radial and cross strutting was carried to the outer piles. The straight portions of the jetty were formed by a double row of piles on each side in 10-foot bays, with four horizontal struts, and cross strutting extending the full width of the structure. The whole of the piles and main timbers were of sawn pitchpine logs, averaging about 14J inches square and 65 feet in length. The decks were formed of 3-inch planking in 4|-inch widths, laid upon 11-inch by 2-inch bearers. The shore end of the west jetty was similar in construction to the outer ends, and the corresponding end of the east jetty was connected with the solid knuckle formed by the return of the south wall of the tidal basin. Pierhead at Madras.t The pierheads at Madras Harbour are formed of cylindrical monoliths, consisting of a plating of iron with a concrete interior (figs. 242 and 243). For each pierhead a watertight iron caisson was provided, with outside diameters of 42 feet and 41 feet 5J inches at the base and summit respec- tively, and 53 feet in height. The bottom and sides were covered with J-inch plating, the latter being built up in a series of tiers or horizontal * Scott on “ The Tilbury Docks,” Min. Proc. Inst. G.E., vol. cxx. + Thompson on “ The Caisson at the North Pierhead, Madras Harbour,” Min. Proc. Inst. C.E., vol. cxxv.