A Treatise On The Principles And Practice Of Harbour Engineering

ërëakWatër Construction. 151 On the other hand, it must be pointed out that the progress of work carried out by floating plant is very much at the mercy of the wind and waves ; so that long spells of unfavourable weather resuit in an almost complète cessation of operations, the period of which is therefore materially protracted. During the winter months, in particular, these interruptions are sure to be frequent and prolonged. Accordingly, in many cases, the method of staging is to be preferred, especially when it is necessary to complète the undertaking witli despatch. The loading of the barges is usually performed at the pier of an adjacent quarry by the ordinary means of tipping through a shoot, the atone being conveyed to the quay edge in wagons running on rails, the gauge of which is generally small. Tn the case of large blocks, cranes are necessary, both for loading and unloading. The loading crane is situated on the quay; the other is usually mounted on an attendant barge. A pair of sheer legs may take the place of a crane. On arriving at its destination, each hopper barge, containing random rubble, is adjusted in position with the aid of suitable sight-lines fixed on the shore, or of any convenient landmarks. It is difficult to niake satisfactory use of floating objects for this purpose, as they are necessarily moored in a flexible manner, and changes of tide and current may make sensible alterations in their positions, the exact estent of which depends, of course, upon the length of the moorings. Satisfactory adjustment having been achieved, the hopper doors are released and the material falls througli the bottom of the hopper. It may then be necessary to trim it, especially if the deposit forms part of the upper layers. In tidal situations this may be done at periods of low water; other- wise, the services of divers are required. Care should be taken both by accurate alignment and judicious deposit to reduce the labour of trimming to a minimum, as it adds considerably to the cost of the undertaking. In shallow water the trimming and levelling of a rubble bed may be not unsatis- factorily achieved by supplementary hand-tipping, the inequalities in level being indicated by a sounding-lead. Rubble should be evenly and systematically distributed over the entire width of base which the breakwater is intended to occupy, as also, where possible, over the entire length. Broken ridges and isolated heaps of stone give rise to currente and so to scouring ; and although any effects of this action may be rectified by subséquent deposits, yet an additional supply of material is entailed, as well as loss of time and labour. At Cette, excavations ranging from 3 to 5 feet in depth were found to have been produced by scour alongside rubble deposits which had been irregularly made. The Staging’ System.—The use of staging, though primarily more expensive than any other method of procedure, is attended by many direct and indirect benefits. It promotes, to a very great extent, the unbroken sequence of operations, which is, perhaps, the highest desideratum from every point of view, and it affords greater protection to those engaged upon those