A Treatise On The Principles And Practice Of Dock Engineering

2l6 DOCK ENGINEERING. consisted of a stout shield of nearly 70 feet surface, made of strong logs, abutting against a smooth vertical face in the clay. As a further précaution, the soil behind the quay was excavated to below low-water level, and the void filled with broken brick, which gave a backing of a lighter character, while, at the same time, it resulted in more efficient drainage. The arches between the counterforts having been destroyed by unequal settlement, a light concrete wall was formed behind them, to take the surface pressure and transmit it to the bearing piles at the base of the wall. The quay line of rails is now carried on iron joists 6 feet apart, bedded in concrete, and spanning the space intervening between the two walls. The work took eighteen months to carry out, cost about £30 per lineal foot, and has proved satisfactory, in every way, since the quays were re- opened to full traffic in 1892. Elevation of Vertical Section Anchorage. of Anchorage. 012345 . . 10 Feet Figs. 155 and 156. In the instance above recorded, the landslip occurred above the clay. The South-West India Dock, London, built in 1868, furnishes an example of a slip within the clay. Some portions of the dock wall were founded upon a hard bed of natural concrete, composed of gravel and shells, resting upon a layer of London clay. When the wall came to be backed up, it slid forward. In the course of excavation, for the purpose of rebuilding the wall from a deeper foundation, two disconnected surfaces of clay were found, one having slipped on the top of the other, showing that the slip had actually taken place some distance below the bottom of the wall itself.* Another well-known instance of sliding, due to the same kind of founda- tion, is that of the walls of the Empress Dock, at Southampton, built in 1888. A section is given, in fig. 157, showing the position taken up by the east wall of the dock after movement. It will be noticed that the earth in front of the toe has been heaped up above its original level. The buttress shown in the figure is one of a series, each 20 feet long, 15 feet wide, and 12 feet deep, set at about 30 feet apart, with the intention of strengthening the wall after a previous experience of its weakness.f The walls of the Kidderpur Dock, at Calcutta, have already been men- tioned (p. 182, ante) and a section given. In one case there was a central * Min. Proc. Inst. C.E., vol. cxxi., p. 120. + Ibid., p. 127.