A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER DESIGN. 133 becoming a mere foundation layer for a wall of massive and substantial proportions, such as is exemplified at Ymniden and Zeebrugge; The advantages and disadvantages attaching to each of the two principal l.w.o.s t. ■MB™™™ »®®æ«®æ ■^^^^?s^^m^— ■^^^^^^^^1 ■ 0i%%^ %%^^^%2 ^1 ■^^^^^w^^wl «■■«®»®i«i i^^s^s«^^«^^' 'Rubbie Fro. 114. —Section of Battery Pier, Douglas, I.O.M. types may be considered under the heads of (1) Cost of Construction, (2) Cost of Maintenance, and (3) Efficiency. Cost Of Construction.—As regards the first point, much, of course, dépends upon the locality of the breakwater and its coastal environment. Where stone is plentiful and quarries lie conveniently adjacent to the site, the rubble mound will commend itself on account of the facility with which it can be formed, and the comparative economy resulting from the use of undressed stone, with its attendant unskilled labour. Such was the case at Portland, where there was not only an abundance of stone, but also a practically unlimited supply of convict labour. In the absence, however, of these essential conditions, and provided the depth of water be not great and the foundation be sufficiently firm, the wall, involving, as it does, a much less quantity of material, will be found preferable. Especially will this be the case where skilled labour happens to be plentiful and cheap. Even the difficulty of a defective foundation may be overcome by one or other of several expedients without perceptibly altering the relative positions. But where the sea bottom lies at a great depth, the superior economy of the pure wall cannot be maintained. Taking (merely for comparative purposes) the cost of rubble stone at the quarry at say eighteen pence per ton, and allowing 1J to 1J tons to the cubic yard of stone, in situ (after deducting 20 to 30 per cent, for interstices), the total cost of obtaining and depositing a rubble mound under favourable conditions may be stated at from 3s. to 3s. 6d. per cubic yard of volume.1 This, of course, applies to natural rubble deposited at random in the body of, 1 The cost of the rubble mound at Holyhead ranged from 2s. 3d. to 2s. 7d. per ton, deposited in place. The cost of quarrying was 9d. per ton. At Sandy Bay, U.S.A., Is. 9d. per ton was paid for ordinary rubble deposited in situ ; larger blocks, averaging 5 tons, were rated at 4s. lød.