617 



PIERS. 



PIERS. 



513 



however the piers of such harbours may be placed, there must always 

 be so great a danger of a ship missing the entrance in a storm, and the 

 agitation of the waves in the outer harbour, at such periods, must pro- 

 duce such an amount of injury to the lock-gates of the inner harbour, 

 that it may be laid down as a general rule that no safe harbour, no real 

 harbour of refuge, can ever be formed on an open shore. 



Assuming, however, that the position selected for the erection, of the 

 piers of a harbour is properly sheltered from the violence of the open 

 sea, the direction of the piers themselves must be such as to allow the 

 current of the flood-tide to assist in carrying a ship into the fair way 

 of the channel ; and the general practice appears to be in favour of 

 their being placed so as to form an angle of from 1 2 to 20 to the 

 direction of the prevailing winds, although local circumstances may, 

 and often do, require that the latter law should be modified. If the 

 sea should be exposed to the action of the wind to such an extent as to 

 be very rough in stormy weather, the piers must not be built in 

 straight, or even in long, circular, or elliptical lines ; because in that 

 case the agitation of the open sea would be transmitted to the inner 

 harbour. But when the outlines of the piers, in plan, are made poly- 

 gonal, the agitation of the sea may be reflected from side to side, and 

 thus reach the inner harbour in perhaps a more dangerous manner 

 than even in the case of the straight walls, because the waves will be 

 more irregular, and will interfere greatly with the steering of the ship. 

 It would appear that the system adopted in the construction of the 

 jetties of Dieppe and of Havre,*in both of which ports wooden break- 

 waters are inserted in the line of the piers (the latter being polygonal 

 in plan), is the one most fitted for harbours in exposed situations ; 

 whereas, in sheltered positions, either straight or curved piers^tnay be 

 resorted to, 



One of the most important functions piers are designed to per- 

 form being to facilitate the movements of the shipping resorting to a 

 harbour, it is advisable in designing them to dispose their plans so as 

 rather to facilitate the entry than the departure of vessels ; because a 

 ship in harbour can always wait for a favourable condition of circum- 

 stances, whilst a ship entering may be compelled to do so under stress 

 of weather. Under these conditions, the entrances are disposed so as 

 to allow the current of the flood-tide, or the inshore currents, if no 

 marked tide? should exist, to carry the ship into the fair-way channel, 

 without exercising any influence likely to carry the ship against the 

 jetty walls. As to the length to be given to the piers, it must be 

 regulated, firstly, by the distance of the deep-water line from the line 

 of the shore ; secondly, by the set of the currents ; and thirdly, by the 

 means used in the particular port for hauling out the vessels. This 

 latter practical detail of navigation will be found especially to affect 

 the respective lengths of the piers, when two are used ; because when 

 the outgoing vessels are hauled by men, or warped by ropes, the pier 

 which serves for those operations must be made sufficiently long to 

 allow the vessel to make her first tack without any danger other 

 falling off against the opposite pier. As a general rule, it is preferable 

 to make the pier under the prevailing wind longer than the other one ; 

 but local circumstances may at times render it necessary to modify 

 the application of this law. 



The width to be given to the channel between the two piers of 

 harbours, where such double works are required, should be at least 

 such as to allow three ships to enter at the same time, under full sail ; 

 and this width may vary between 100 and 250 feet, according to the 

 clasa of vessels frequenting the harbour. At the heads of the piers 

 the widths of the channels should be increased, because the ships 

 entering require more room for their evolutions when under the 

 influence of the way they bring in from the open sea than they do 

 when they have followed the narrow channel for some time. The 

 general use of steam-tugs, by enabling several vessels to leave at the 

 same time, seems to have rendered it desirable to increase the width of 

 the channels. 



In river-piers, and in landing-stages, the only considerations which 

 affect the plan to be given to the structures are, the depth of the water, 

 and the possibility of the transmission of the agitation from the open 

 sea, or of the local agitation produced by the action of the wind upon 

 a large surface of water. The latter phenomenon, as is well known, 

 depends on the depth quite as much as on the extent of the water- 

 surface over which the wind blows ; and it is for this reason that, in 

 broad deep rivers, such as the Southampton Water, the piers are made 

 curvilinear in plan, in order to inclose and protect a species of port ; 

 whilst on the shallow shores of the Thimes, at such places as South- 

 end, a straight pier is found to be sufficient for the real necessities of 

 the locality. The peculiar range of the tides at any particular place 

 may render it necessary to vary the means adopted for facilitating the 

 loading and unloading of ships ; but a study of the various landing- 

 stages on the Thames will teach the student far more on this subject 

 th;in it would be possible to do within the limits of this article. 



2. As to the teclinical methods of constructing piers, they must 

 evidently be regulated by the nature of the building materials fur- 

 nished by the locality ; and those structures may either be formed of 

 wood, concrete, loose rubble stone, or solid masonry bedded in mortar ; 

 they may be either perfectly closed on the water-line, or open, so as to 

 allow any littoral current to sweep through them. 



When wood is used, the foundations intended to receive the upper 

 structure require to be executed with great care, and in such a manner 



as to allow the said upper structure to be repaired with ease, and with- 

 out affecting the foundations ; for it is essential to bear in mind, that 

 the shock, or the abrasion, produced either by vessels, by waves, or 

 currents, and that the ravages of the Teredo navalis, or of the Limnoria, 

 tenbrans, are most injurious to the parts of a wooden pier placed 

 between high and low water-mark. All those parts must, therefore, 

 be easily accessible, and renewable, either separately or entirely, aa 

 may be required. It is customary to execute woolen piers with piles 

 and whaling-pieces, fixed below the level of the ground of the fore- 

 shore, and upon these a series of trapezoidal frames, carefully braced 

 and bolted together, are placed. The lower part of these frames is 

 covered with loose rubble, concrete, or fascines, in order to protect the 

 foundations. 



In open wood piers the frames are fixed at distances asunder of from, 

 6 to 10 feet in the sea, according to the depth of water, and the 

 habitual agitation of the latter at the place considered. The frames 

 are made in the form of a trapezium, the iuclined sides being 

 respectively turned towards the open sea, or towards the sheltered 

 channel, and forming an angle of from 13" to 33", with a vertical line 

 passing through their points of junction with the sill pieces. According 

 to the width intended to be given to the roadway of the pier, the 

 frames are made with one or more upright posts, between the inclined 

 sides, and the whole are retained in their places by cross ties and 

 raking braces converting the parts of the assemblage into a series of 

 triangles, the sills and heads forming with the inclined sides the outer 

 lines of the frames. Longitudinally these frames are connected by 

 whales, stringers, and by the joists of the platform, and upon the 

 latter the planking of the road-way of the pier is laid ; this planking 

 is formed of rather narrow timber, about 4 or 5 inches thick, laid witii 

 intervals of about 1J or 1 inch. The planks are spiked down upon the 

 joists, but upon their ends they are clipped by a bridging joist, bolted 

 to the longitudinal sill fastened to the frames; sometimes this 

 bridging joist also bears a handrail of open tiinberwork. Tenon joints 

 should be avoided in the carpenter's work of the frames, and under no 

 circumstances whatever should the planking of the roadway be laid 

 water-tight. 



Should the wooden piers be filled in with rough stonework, the ' 

 latter material will be retained by means of close boarding laid hori- 

 zontally against the upright posts of the framing ; and when that 

 planking is carefully laid, the interior may be filled in with shingle, 

 sand, or clay, or with concrete made with a hydraulic lime basis. It 

 would appear that the best position for the planking, under these 

 circumstances, is upon the outside of the frames ; for although 

 unquestionably, the planks would resist the tendency to displacement, 

 from the weight of the filling-iu material, more efficaciously if they 

 were nailed upon the inside than if they were nailed upon the outside, 

 yet in the latter case the framing would be far more efficaciously 

 protected from the action of the waves, and it would be much easier to 

 repair any accidental damage. The essential condition in all these 

 cases is, in fact, that the substantial framing should be withdrawn as 

 much as possible from the direct action of the sea ; and that those 

 parts, which are exposed to it, should be susceptible of being easily 

 replaced. 



The advantages of wooden jetties consist in the economy of their 

 construction ; but in practice it is found that the repairs of such work 

 are so frequent, and so costly, that the use of wood should be only 

 resorted to when it is utterly impossible to resort to the use of more 

 durable, even though originally more expensive materials. Open wood 

 piers have, however, another occasional advantage, namely, that they 

 do not seriously interfere with the advance of any littoral current, and 

 therefore they are not so h'kely to produce any change in the outline of 

 the shore, by producing a deposit of alluvial matters, as a solid pier 

 would do. In the landing stage lately erected at Margate an attempt 

 has been made to unite the advantages of open piers with greater 

 durability, by the use of cast iron piles ; but it would be desirable to 

 watch the eifect of sea water on the iron during some years, before 

 passing any opinion on the merits of this system. It may be 

 added that the use of creasoted timber, for works exposed to 

 the action of the sea, seems to have removed some of the most serious 

 objections to the use of that material ; for not only has this process 

 increased the durability of the wood, but it would appear to 

 constitute also an efficient protection against the teredo. Whether 

 wood or iron piles be used, it is necessary to guard against the 

 percussive action of the ground swell upon their heads ; and it is for 

 this purpose that the layer of concrete, rubble, or fascines, over the 

 foundations is introduced. 



Stone piers are either executed of loose rubble thrown down and 

 allowed to assume its natural slope by the action of the waves ; or of 

 ashlar masonry external walls bonded together so as to form a series of 

 canes filled in with concrete, or with rubble masonry, or with a hearting 

 of earthwork protected by dressed masonry walls; the determining 

 motives for the selection of either of these modes of execution being, 

 of course, merely economical ones. The most difficult part of such 

 structures consists in their foundations when executed at some 

 distance from the shore, aad in deep water, where caissons, or coffer 

 dams cannot be used ; especially if any strong littoral currents should 

 exist in the particular locality. The very successful manner in which 

 the new graving dock of Toulon was entirely constructed of concrete, 



