A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR ENGINEERING. 156 made, and any marked restriction of the working length tends to cripple progress and increase the cost. Low Level System.—The low level system of tipping, by means of wagons running along tracks laid on the solid breakwater structure as il advances, saves the cost of staging and reduces the amount of work done during the process of discharge ; but, at the same time, it is a method which is greatly restricted in scope, in that operations are limited to the immediate vicinity of the completed mound ; for, until the bank be raised at least above sea level, any extension of the railway track is impracticable. And allowing, under the most favourable circumstances, that the lines can be laid close to the water level, there is the added risk, the probability, the certainty even, that rough weather will cause frequent irruptions and settlements, so that there will be more or less constant relaying of the tracks—all entailing delay and expense. On the other hand, it may be urged that the use of the completed section of the mound as a roadway for the transport of materials assists to consolidate it and to reveal any sources of weakness which it may possess. This is no doubt true ; but whether it affords sufficient justification for the adoption of a system which is otherwise slow and restricted, is a point which must be determined by the special circumstances of the undertaking. It is a form of construction which is not generally suitable for works on a large scale. For small embank- ments, however, it may be considered convenient and economical if time be not a matter of importance, and it produces substantial and reliable work. Leaving the mound type of breakwater at this point, we pass on to methods of wall construction. Wall Construction.—The masonry wall, built with prepared blocks of ashlar or concrete, carefully bedded and laid in accurate alignment, mani- festly calls for more elaborate and less rudimentary appliances than are available for the formation of mounds. Other kinds of wall, such as those consisting of concrete deposited in mass in a fluid condition,1 or built up of sacks and bags laid in courses, also demand special apparatus. The methods of construction generally adopted may be ranged under the headings of :— (1) The Staging System. (2) The End-on, or Over-end, Low Level System. (3) The Caisson or Buoyant Monolith System. Floating plant, while useful enough as an adjunct, cannot be relied upon alone to carry out operations with sufficient exactitude. The Staging’ System.—As regards the staging system, there is little to add to what has already been written in connection with mounds. The same lines of formation are followed and the process of depositing is the same with the exception that, instead of being tipped in bulk, each block of stone is laid individually in position. Cranes or gantries are therefore an integral part of the system, and the tracks will be arranged to suit their requirements For concrete work, platforms may either be erected on the staging itself where 1 The deposition of concrete in a plastic or partially-set condition is a practice to be deprecated.r