A Treatise On The Principles And Practice Of Dock Engineering

270 DOCK ENGINEERING. pier, lest serions or even disastrous conséquences ensue. The effect of a misplacement might be the shoaling of a hitherto navigable channel. Unfortunately, the conditions affecting fluvial, estuarine, and marine currents are too complex for anything of the nature of a brief and satisfactory resumê, and the subject, indeed, constitutes a brauch of maritime engineering which scarcely cornes within the purview of the present treatise. There can, however, be no doubt that the influence of training works, in the form of walls and dykes, upon the augmentation and maintenance of waterways is very powerful, and that, judiciously employed, they are a valuable means for increasing the accessibility of a port. There are several instances of such works in existence, notably at the mouths of the Tees and the Ribble, where training works have been recently constructed in order to afford a navigable channel, in the first instance to the ports of Middlesbrough and Stockton, and in the second to the town of Preston. External jetties are either detached or arranged in pairs. Of double jetties there are three forms—viz., parallel, convergent, and divergent. Parallel jetties are mainly used for training purposes, as at Leith and Ostend; convergent jetties enclose a sheltered basin or outer harbour, as at Rarry and Sunderland; divergent jetties afford guidance and direction to vessels entering a narrow waterway, as at the Alexandra Rock, Hull, the Canada Basin, Liverpool, and the Tilbury Docks, London. Jetties are also to be found in the interior of many docks, especially those of large size, with the object of increasing the proportion of quayage to water area. Thus the south-west India Dock, London, with an area of 26| acres, is furnished with 16 jetties, affording accommodation to 32 vessels. The Victoria Dock, at the same port, has 13 jetties in an area of 74 acres. The Alexandra Dock, at Hull, has 4 jetties in an area of 46| acres. Wide projections, of solid construction, into the interior of a dock are designated Tongues, such as the Canada Tongue at Liverpool. They really constitute an integral part of the dock outline. Short tongues or jetties used for coaling purposes are called Staiths. There are 13 staiths at the Penarth Dock and 31 at Barry Docks. Stresses in Piers and Jetties.—In Chapter vi. some consideration has already been given to the character and influence of various natural agencies in so far as they affect the navigability and usefulness of dock entrances. Tn the present section it will be necessary to supplement this information by some observations on the effect of these agendes upon the stability and durability of exposed structures. Wave action alone calls for serious notice. The effect of wind pressure upon the superstructure of a pier is trifling compared with that of the onset of waves upon its base. The only danger to be apprehended from currents is their tendency to undermine the foundations, and this can readily be guarded against by the exercise of the précautions indicated in Chapter v. The mathematical theory of waves is a physical question too purely academie for discussion in an engineering treatise. Students who desire