A Treatise On The Principles And Practice Of Dock Engineering

268 CHAPTER VIL JETTIES, WHARFS, AND PIERS. Definitions —Stresses —Wave Action — Force ok Impact — Results ok Impact— Observed Pressures —Instances of Wave Action — Design of Jetties and Piers — Construction — Concrète Mass, Bag, and Block Work — Dressed Masonry and Rubble Mounds—Fascine Work—Open Timber Framing and Grib Work—Columnar Structures and Frameworks of Iron and Steel— Monier and Hennebiqub Systems — Typical Examples at Aberdeen, Zee- brugge, Havre, Kingstown, Algiers, Hook of Holland, Blyth, Liverpool, Newcastle, Soukhoum, Touapsé, Belfast, Dundee, Dunkirk, Tilbury, Madras, Sunderland, Greenock, and Hull. In one sense, and that perhaps the most important, jetties, wharfs, and piers may be looked upon as constituting the outlying or advance works of a dock system. It is quite true that they are by no means exclusive, or even indispensable features, being found at many ports which have no docks and absent from others where docks are numerous. Furthermore, they do not always, or necessarily, occupy outlying positions, being often located in sheltered basins and even within docks themselves. Seeing, however, that their most important functions are discharged in connection with exposed situations, we shall deal with them mainly from this standpoint, and afterwards consider their adaptation to more sheltered areas. And as to the strict propriety, or otherwise, of treating such structures as forming an integral part of a dock system, we need not concern ourselves too closely. The fact that they do play so prominent a rôle in many cases, and that they have indubitably demonstrated their ability as accessory features generally, is sufficient justification for treating the subject in its broadest aspect. Definitions.—Our first duty is a delimitation of the respective con- stituents of the group. It is no easy matter to draw a strict, or even a serviceable, distinction between the various types. A jetty is radically that which juts out or projects, and the term is appropriately applied to all structures which project from the general contour of any littoral. But it shares this signification in common with piers and moles, both of which are similar projections. The primary meaning of the word pier is apparently con- nected with the notion of support, and it is commonly used in engineering to indicate the intermediate props or supports of a series of arches. Probably from this association, an idea of isolation or detachment has been