NEW LOCK AT BREMERHAVEN. 267 invert in the floor of the lock has been dispensed with as unnecessary, since no springs were likely to be found in the stiff clay on which the lock stands. The walls, which contain the levelling culverts, are founded on inclined piles, in rows, 4 feet apart. They are inclined alternately in opposite directions, an arrangement which secures a favourable distribution of the forces acting on the piles, and has the further advantage that the pile-heads are not so near together, and the piles can consequently be driven deeper into the solid ground. The inner end of the lock is closed by a sliding caisson, the outer end by a pair of iron gates. The former was selected on grounds of economy and utility as a movable bridge, the latter by reason of their greater strength, for during spring tides a strong current flows through the lock into the Kaiser Dock, which during southerly winds is considerably increased by the heaping up of the tide on the Bremerhaven shore. This current, aided by the force of the waves and the pressure of the wind, exerts a force which, it was considered, could not be so well resisted by a sliding caisson, supported at one end only, as by two strong gates. 268 CHAPTER VII. JETTIES, WHARFS, AND PIERS. DEFINITIONS-STRESSES-WAVE ACTION--FORCE OF IMPACT-RESULTS OF IMPACT— OBSERVED PRESSURES-INSTANCES OF WAVE ACTION-DESIGN OF JETTIES AND PIERS-CONSTRUCTION - CONCRETE MASS, BAG, AND BLOCK WORK - DRESSED MASONRY AND RUBBLE MOUNDS-FASCINE WORK-OPEN TIMBER FRAMING AND CRIB WORK-COLUMNAR STRUCTURES AND FRAMEWORKS OF IRON AND STEELMONIER AND HENNEBIQUE SYSTEMS TYPICAL EXAMPLES AT ABERDEEN, ZEEBRUGGE, 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. areas. 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 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 constituents 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 connected 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 DEFINITIONS. 269 acquired, and hence its application to maritime structures, the connection of which with the mainland is of a slight and restricted nature. This feature, however, is equally characteristic of jetties and moles. The word mole is evidently derived from the Latin moles, a mass, and is indicative of a large mound, or long ridge of material, heaped more or less regularly, in such a way as to constitute some protection from rough external seas. In this respect it fulfils the functions of a break water, with which it is closely allied, though, in later times, it has acquired the special significance of a break water provided with a broad superstructure capable of being used as an ordinary quay. Perhaps the position may be best summarised thus:Outlying works in exposed situations, used for protective purposes alone, are break waters. When joined to the shore, and equipped for commercial operations, they become, almost indifferently, piers, jetties, and moles. Accordingly, the latter terms will be employed, in the present chapter, as practically synonymous. A wharf may be defined as a continuous structure, occasionally acting as a retaining wall, along the open margin of a sea, or along the banks of a river, canal, or other waterway. The application of the word is somewhat loose, and it is sometimes taken as identical with quay, though its use in connection with dock and basin walls is rare. Wharfs have obviously provided the most natural sites for the berthing of vessels. from the earliest times, being employed for this purpose long before the ideas of outlying jetties and enclosed basins were conceived. In this connection, they are subdivisible into two classes-legal wharfs and sufferance wharfs. The former are certain wharfs, in all seaports, at which goods were required to be landed and shipped by Act 1 Eliz., cap. 11 (now repealed), and subsequent acts. Some wharfs, as at Chepstow, Gloucester, &c., are deemed legal from immemorial usage; others have been made. legal by special Acts of Parliament. Sufferance wharfs are places where certain goods may be landed and shipped-as hemp, flax, coal, and other goods by special sufferance, granted by the Crown for that purpose. These legal distinctions, however, have no bearing on the engineering aspect of the question. * From their close relationship to ordinary quays, much that has been said in Chapter v. is equally applicable to wharfs, but need not be repeated here. As part of a dock system, external jetties and piers serve a twofold purpose. In the first place, they act as protective works, by means of which vessels are guided and sheltered during their entry. Secondly, they serve as directive agencies for the deflection or regulation of currents. Whether intentionally on the part of the designer or not, this second function is one which must inevitably be performed by any artificial projection beyond the normal contour of a littoral. Hence it behoves the engineer to exercise great care in determining the location and disposition of a proposed jetty or Dr. Ogilvie. * |