A Treatise On The Principles And Practice Of Harbour Engineering

154 HARBOUR ENGINEERING. from it at any desired level. It acts as an aid to the alignment of the work, and it allows appliances of a more powerful and efficient character than floating plant to be employed. Some of these features are equally character- istic of the third or low level system, but we shall see that there are also corresponding defects in the latter which are not applicable to staging. Staging, as usually practised, takes the form of a series of piles in one or more rows of double line driven at regular intervals (say from 15 to 50 feet apart) and connected by longitudinal runners (or, for the longer spans, by wrought iron or steel girders), and bracing, with side strutting, cross bearers, etc. It thus forms a track, or a number of parallel tracks, for wagons, travelling gantries, and cranes; and, in order that as little surface as possible may be presented to wave action, these roads or tracks should be located well above the highest sea level, say not less than 20 feet, and preferably 5 or 10 Frø. 127. —Rail Track, Holyhead Breakwater Staging. feet more, though, of course, any increase in the height is made at the expense of stability. Furthermore, the solid strutting, which characterises much land staging, is best replaced by slender tension members—chains and wire rope stays attached to secure moorings; or, if the surging of these under wave action be deemed undesirable, second-hand railway metals will be found eminently useful for the purposc. The piles, wherever possible, are driven into the ground by a pile-driver rigged up on a barge or floating platform, or supported on a carriage which projects over from the land or the staging previously completed. The floating pile-driver (or rather, a number of such appliances) can, in still water, construct the road at a much quicker rate than the stage pile-driver, which is limited in the scope of its operations. In ordinary firm ground, the above is the usual course. If the ground, however, be of a very soft and yielding character, it will be desirable to