A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER DESIGN. I 43 just under Is. 9d. per lineal yard per annum. The remaining and later portion of the breakwater costs practically nothing for upkeep. Apart from the parapet wall and the quay, the cost of the breakwater is stated by Baron Quinette de Rochemont1 to have been as follows, according to the depths of water in which it was founded :— In depths of 33 feet, £39, 13s. per foot run. „ 65 „ 71, 16s. „ 100 „ 118, 4s. According to M. de Joly,2 however, the cost of the original breakwater, including the parapet and quay wall in a depth of 60 feet, was £127 per linear foot, a figure which is evidently somewhat in excess of those quoted above, even when allowance is made for the additional work covered. M. de Joly’s cost for the extension, however, is in accordance with Baron de Rochemont’s figure for the same depth, viz.: 100 feet. t( 10 90 30 W 50 60 7Ç 8p 90 Kp 9^0 Scale of Feel:. Fro. 118. —Digue de la Joliette, Marseilles. A. Rubble deposited after construction of quay wall. B. Stones from 2 to 25 cwts. a-piece. C. Blocks above 25 ,, Of its class, the Grande Jetée is an efficient example. Only one objection can be laid agaiust the design, and that is the narrowness of the uppermost outer slope flanking the masonry apron. The existing width of 27 feet seems to be insufficient to prevent the protection blocks from being occasionally rolled off by the waves into deep water. Settlements in the mass of the breakwater, though they have been by no means inconsiderable in themselves, appear not to have given rise to any serious dislocation of the parapet wall. Indeed, it is said that it is only possible to observe, on serutiny, a few vertical cracks here and there, with widths of mere fractions of an inch. The shelter wall and its apron are not bonded together : they are simply in contiguity. Separation was inevitable, since they rest upon distinctly different foundations, the wall upon material of smaller size and greater compactness than the apron. The Joliette Digue (fig. 118), constructed in about 38 feet of water, follows 1 Cours de Travaux Maritimes, lère Partie, 1896. 2 Report on French Breakwaters to Teuth Int. Nav. Cong., Milan, 1905.