A Treatise On The Principles And Practice Of Harbour Engineering

15° HARBOUR ENGINEERING. finished mound, upon whicli the roads are continuously extended as the work proceeds. The Barge System.—The first method is best adapted to sheltered situations—the difficulties of discharging from vessels in a rough sea must be sufficiently obvions; but it necessitates the existence of sufficient depth of water for the loaded barge or scow, together with the additional clearance required, when, as is generally the case, the latter is fitted with hopper doors and the material is dropped through the bottom of the vessel. Certain barges permit of lateral discharge, but the type is not common; and in cases where there are no doors for the purpose, it is attended by some risk of capsizal, with attendant danger to the men engaged upon the work. A corollary to the foregoing restriction is that hopper barges depositing Fia. 124.—Hopper Barge discharging load. material through bottom doors cannot be employed for the entire construction of a breakwater consisting of a rubble mound only. Even though there be considerable tidal fluctuation, admitting of the higher stages of the work being carried out during periods of high water, yet it is evident that the mound cannot be brought to surface level, and must, indeed, cease at depths below it, which may be anything from 10 to 15 feet, or even more. One advantage attaching to the employment of barges is the opportunity afforded for depositing rubble uniformly and simultaneously over the whole site of a breakwater. This advantage is shared, but not to the same degree of freedom, by the second method. With floating plant there is no restriction whatever, and the work may be prosecuted over a very extensive area without incurring any higher expenditure or any greater risk of misadventure. As will be seen when we corne to deal with the question of settlement, this con- sidération has a very important bearing on the permanence of breakwaters.