A Treatise On The Principles And Practice Of Dock Engineering

504 DOCK ENGINEERING. Tilbury Graving Docks, London. There are two graving docks lying parallel to one another at the entrance to the Tilbury Docks, London * (fig. 506). They are also capable of acting as entrance locks, and for this purpose they are provided with caissons at both ends. In addition to this, there are three central positions in each dock fitted for the reception of a caisson. The result of this arrangement is that, apart from the use of each graving dock in its entirety of 875 feet, there are virtually four graving docks, each complété in itself, two being entered from the tidal basin and two from the main dock. And, by means of a variation in the position of the central caisson, each pair can be adjusted to any of the following lengths in the clear—viz., 450 and 400 feet, 350 and 500 feet, and 300 and 550 feet. The large graving docks have a width of 70 feet across the bottom and a depth of 35 feet below Trinity high-water mark on the sills. The width of the small graving docks is 60 feet, and the depth on sills 30 feet (figs. o07 to 509). The walls of the large graving docks have a thickness of 16 feet 3 inches at floor level and of 5 feet at the coping. The backs of the walls are verti- cal, except where it was necessary to increase the width for culverts, and the internal faces of the walls have a batter of 1 in 20 for a height of 22 feet 6 inches. The upper part of the walls is stepped to form six altars. The thickness of the invert varies with the depth to which it was required to excavate to reach the gravel foundation, but the normal thickness is 15 feet. The walls of the small graving docks are 13 feet 6 inches thick at the base, and their other dimensions are, in general, smaller than those of the walls of the large graving docks in the same proportion. The invert is, however, relatively thicker, on account of the necessity of excavating the foundations to the same depth in both cases. A portion of the invert of the small docks is, owing to a dip toward^an old channel, carried upon short whole-timber bearing piles, spaced at 4 feet centres, in each direction. The invert is entirely of 9 to 1 concrete, with a stop- water course, 3 inches thick, of fine 3 to 1 concrete. Upnor clay-puddle, 1 foot 6 inches thick, is carried down the backs of both graving dock walls from Trinity high-water level to below the stop-water course. The floors are of pitchpine planking, 4 inches thick, spiked down to pitchpine sleepers, 14 inches square, which are bedded in concrete. Teak keel-blocks are laid along the whole length of the docks, and are fixed down to the floors by dogs. The altars are paved with 6-inch hard York stone, the copings being of teak, 12 inches square, furnished with eye-bolts and secured to teak cross-timbers, 3 feet 6 inches long, bedded in brickwork at the tops of the walls. The docks are provided with five hydraulic capstans of 2^-ton and 5-ton power and cast-iron bollards, the latter having perforated caps and being * Scott on “The Construction of Tilbury Docks,” Jilin. Proc. Inst. G.E., vol. cxx.