A Treatise On The Principles And Practice Of Dock Engineering

INSTANCES OF WAVE ACTION. 273 At Cherbourg, the force of waves in storms has been found to vary from 600 to 800 Ibs. per square foot. Instances of Wave Action.—The following are a few recorded instances of the feats performed by waves :— During a summer gale, in the year 1869, fourteen stones, each 2 tons in weight, part of the structure of the Dhu Heartach Lighthouse, which had been laid in Portland cement and fixed in their places by joggles, at a level of 35 feet 6 inches above high water, were torn up, and eleven of them swept off the rock into deep water.* During the storms of December, 1896, and January, 1897, blocks, weighing 40 tons each, used in the construction of Peterhead breakwater, were displaced in courses bedded respectively at the levels of 17 feet 1| inches and 23 feet 7-^ inches below low water of spring tides. One of these blocks lodged on a concrete platform, 30 feet 7 inches below low water, and was washed away during a storm in the following March,t The destruction of the outer extremity of the breakwater at Wick, in December of the year 1872, is described in a report by Messrs. Stevenson to the directors of the British Fishery Society.J The end of the work was protected by a mass of cement rubble work. It was composed of three courses of large blocks of 80 to 100 tons, which were deposited as a founda- tion on the rubble. Above this foundation there were three courses of large stones carefully set in cement, and the whole was surmounted by a large monolith of cement rubble, measuring about 26 feet by 45 feet by 11 feet in thickness, and, at 16 feet to the ton, weighing upwards of 800 tons. This block was built in situ. As a further precaution, iron rods, 34 inches in diameter, were fixed in the uppermost of the foundation courses of cernent rubble. These rods were carried through the courses of stonework by holes cut in the stone, and were finally embedded in the monolithic mass which formed the upper portion of the pier. Incredible as it might seem, this huge mass, weighing not less than 1,350 tons and presenting an area of 496 square feet to the sea, was gradually slewed round by successive strokes until it was finally removed and deposited on the rubbie inside the pier, having sustained no damage beyond a slight fracture at the edges. The lower or foundation course of 80-ton blocks, which were laid on the rubble, at a depth of 15 feet below low water, retained their positions unmoved. ne second course of cement blocks, on which the 1,350 tons rested, was swept off after being relieved of the superincumbent weight, and some of the blocks were found entire near the end of the breakwater. The displaced mass was succeeded by a still more enormous block, weighing no less than 2,600 tons, which, after remaining undisturbed for Stevenson on “ The Dhu Heartach Lighthouse,” Min. Proc. Inst. G.E., vol. xlvi. tShield on “The Effects of Waves on Breakwaters,” Min. Proc. Inst. G.E., vol. cxxxviii. + Vide Min. Proc. Inst. G.E., vol. xliii. 18