A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 306 and exceed those of gates with iron framework and planking by 25 per cent.” Owing to the variability of local practice, there is no absolute standard of comparison. 5. Durability.—As regards this point, the advantage, on the whole, lies with wooden gates. Salt water, especially if in any way contaminated with sewage, is extremely deleterious to ironwork. As has already been pointed out in Chapter iv., the metal, if unprotected, is speedily reduced to a condition resembling graphite or plumbago in structure. Painting, the preservative agency most usually adopted, is merely a temporary expedient calling for constant renewal, while the more expensive process of galvanising adds but a few years to the natural life of a gate at the expense of some reduction in the strength of the material. The life of an iron gate, under normal circumstances, can scarcely be expected to exceed thirty years, and the following are actually recorded instances of the rate of decay :—A pair of iron gates at a lock on the Dedemsvaart Canal in Holland,* constructed in 1880, were removed for repairs in 1894-, when it was found that the frame- work was covered with a layer of rust which had to be scraped away, while the sluice paddles and their grooves were completely worn out so as to need replacing. The galvanised sheeting was intact, but it was deemed advis- able to coat it with black varnish. A pair of gates at Glückstadt Harbour, on the Elbe, built in 1874, were condemned in 1902. Dock gates at Bremerhaven, erected in 1852, were removed in 1900 as completely worn out, the plates being eaten away below low water to a depth of | of an inch, and the rivet heads either badly decayed or entirely destroyed. f Naturally, the life of an iron gate depends very largely on the amount of care devoted to its maintenance, and, in order to keep such gates in proper condition, they should be scraped, cleaned, and painted annually, or at intervals not exceeding three years. The lock gates at Terneuzen and Ymuiden are thus treated. Apart from the attacks of sea worms (and some ports are apparently exempt from these pests), wooden gates, more particularly those of oak and greenheart, are extremely durable and need no attention. Mr. Blandy J mentions the case of the old Waterloo Dock gates at Liverpool, constructed of oak, which, when removed on account of alterations and taken to pieces, were found to be in a perfect state of preservation after forty years’ exposure to tide, wind, and weather.§ The 100-foot greenheart gates at the Canada Lock of the same port were in active use for a like period, 1856 to 1895, and, when removed on similar grounds and taken asunder, were found to be in an absolutely sound condition and as good as on the day * Min. Proc. Seven/h Int. Nav. Cong., Brussels, 1898, p. 325. + Brandt and Hotopp on “ Iron, Steel, and Wooden Gates,” Min. Proc. Ninth Int. Nav. Cong., Düsseldorf, 1902. J Blandy on “ Dock Gates,” Min. Proc. Inxt. C.E., vol. lix. § The gates lay on the beach for several ybars prior to being broken up.