A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER RESIGN. 145 1899, the substructure of the jetty includes a solid wall of concrete blocks brought up from a depth of 18 feet below the water level. These blocks are not laid in bond, that is, breaking joint as at Genoa or Cette, on account of the highly compressible foundation, which is sand and mud. They are super- imposed in such a manner as to form a series of piers, disconnected except for the masonry crown, which, though fairly continuous, is jointed every 25 or 30 feet. So far, the jetty has stood satisfactorily, but its construction is of too recent a date to admit of any definite pronouncement of its value. The cost is stated by M. de Joly to amount to £350 per lineal yard, or about the same as the Marseilles breakwater extension, which, however, is of much greater sectional area and in water of much greater depth. Breakwater at Sandy Bay, Mass., U.S.A.1—“The subject of an extensive harbour of refuge at Sandy Bay has been under consideration since 1882. The project submitted to Congress was for the construction, at a cost of $4,000,000, of a breakwater 9000 feet long in the location shown on plan in fig. 7. The proposed breakwater was to be a rubble mound surmounted by a masonry superstructure founded 15 feet below low water. The mound was to be 40 feet wide at the top. The superstructure was to be trape- zoidal in section, to rise 8 feet above high water, and to be 15 feet wide at the top. Below low water, it was to be laid ‘dry’; above low water, in mortar.” No work was ever done upon the superstructure above described, and, in faet, no project for the construction of a superstructure was adopted until 1892. In 1884 the plan for the substructure was changed to that of a mound 40 feet wide at the top, rising to 22 feet instead of 15 feet below low water. The depth of water at mean low water varies from 6 feet at Avery’s Ledge, the extreme southerly end of the breakwater, to about 89 feet at the extreme westerly end, and averages about 45 feet along the southerly arm and about 65 feet along the westerly arm. The bottom along the line of the work is nearly all ledge, except at the westerly end, where it is sand and shells. In the anchorage area, the holding-ground is excellent, being sand mixed with mud. The work done prior to 1892, up to which time $450,000 had been appropriated, consisted in the placing of about 500,000 tons of stone in the substructure. In the early part of 1892, a board was appointed to recommend à project for the superstructure and any changes that might be desirable in the existing project for the substructure. The section adopted is shown in fig. 120. By 1898, 600 feet at the northerly end of the southerly arm had been completed to full section, 1200 feet more had been carried up to low water, and 2800 feet more had been founded. The 600 feet of superstructure was formed of stones weighing not less than 4 tons each and averaging 6 tons, 1 McKinstry on Breakwaters, Trans. Am, Soc. C.E., vol. liv. ; Int. Eng. Cong., 1904. 10