A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER CONSTRUCTION. 155 substitute screw piles with a broad-bladed screw at the foot to afford the necessary surface bear- ing, or the pile may simply be set upright upou a large iron base-plate in the form of a shoe. A very broad and fairly thick stone slab, care- fully set upon the surface of the ground, will often afford a sufficiently substantial base. This method, however, entails much cross strutting between the piles. Finally, if the sea bottom be rocky, the lower ends may be let into sockets drilled in the rock and steadied by concrete filling, or the pile may be shod with a stout iron spike, capable of being driven several inches, at least, into the solid. For rubble work, the tracks are such as will suit the wagons in which the material is con- veyed. The number of these tracks and their distances apart will depend upon the actual extent of the breakwater, but 25 feet or so seems to constitute a fairly average distance between track centres, and there are few breakwaters where the number of such tracks need exceed six, affording a width of 175 feet over all. The stone, having been conveyed directly on to the stage in wagons, is tipped either by hand or by automatic arrangement, the wagons being tilted at the ends or at the sides. The staging method is particularly convenient on account of its adaptation to an organised con- tinuous supply of stone, and the ease with which wagons may be marshalled and dis- charged. But it does not command the same extent of area as the barge system, unless the staging be erected from end to end in the first instance, which is unlikely, owing to the delay, risk, and cost. Under general circumstances, staging may be utilised several times over in different posi- tions ; in other words, it is not necessary to provide for a length of staging equivalent to that of the breakwater. As the work is com- pleted, the rear staging may be moved forward, connection with the ground level being main- tained by sloping ways. There is inevitably some interruption while the change is being Fie. 128. —Transverse View of Staging, Holyhead Breakwater.