268 



THE CIVJL ENGINEER AND ARCHITECT'S JOURNAL. 



fSEPTKMBF.R, 



niifl pnsltion of tlie groynes. No conclusions, as to tlip hcst forms for 

 hrpakwaters, could be deduced from examples of walls built like these, four- 

 fifths up the shore. The cases were not in any degree similar, and it was 

 necessaiy to be very careful not to assume any similarity of action, or of 

 etTect, between the long heavy deep waves to which breakwaters were ex- 

 posed, and the comparatively shallow and hrokeu-up waves to whose action 

 the sea walls near Edinburgh were subjected. 



Mr. Scott Russell contirmeil Mr. Rendel's view. The value of the ar- 

 rangements shown in the sections depended entirely upon the nature of the 

 foreshore, the depth of the water, the length of the reach for the sea to 

 rise upon, and the direction of the exposure. It appeared to him, that the 

 mass of masonry in the wall, (shown in figs. 1, and 2,) was so great, the 

 depth of the water was so small, and it was comparatively so little exposed 

 to the sea, that the form of the wall was of little importance, and the cir- 

 cumstance of its standing proved nothing. In the wall shown in fig. 3, 

 on the other hand, the perpendicular part was nothing more than a parapet, 

 being protected by a sloping wall, up to the level of high water. The thin- 

 ness of the wall in fig. 4, compared with that of fig. 2, rendered any com- 

 parison uninstructive, as it was manifestly insufficient, and the foimdation, 

 which was only a loose rubble wall, was evidently inadequate; there was 

 also a greater depth of water and more action of the waves, than in the case 

 of the wall in fig. 3. The wall shown in fig. 5, was precisely of the form to 

 endanger the stability of the parapet; and it bad given way, as might have 

 been expected. It was similar to that on the Dublin and Kingstown Kail- 

 way, of which a great part was thrown down by a storm, although it was 

 built of large blocks of granite. The general conclusions he was disposed 

 to draw from the facts stated, and from siniilar instances which bad come 

 under his own observations, were, that a vertical wall, if it was composed of 

 good masonry, united by strong hydraulic lime, or cement, not carried to a 

 height exceeding 30 feet, and not exposed to the action of a heavy Atlantic 

 swell ; but protected by a long foreshore, or shallow water, would answer 

 perfectly well, uidess the fonndalion gave way. lie might give as examples 

 the quay walls of Liverpool, which were all built nearly vertical; they were 

 also examples of excellent rubble work, which bad resisted perfectly all the 

 action to which they were subjected. In the case of exposure to a heavy 

 breaching sea, or deep water, with large waves, it was necessary to begin to 

 break the water as eaily as possible, and for this purpose a considerable 

 slope would be found safer, and eventually cheaper, especially with such 

 materials as were, usually, most conveniently to he found for such purposes. 



Mr. J. ThojMson wished that Mr. Rendel had extended his observations, 

 and had told them for what positions he considered each kind of wall was 

 liest adapted ; for although he thought that the promulgation of any em- 

 jiirical rules must be prejudicial, yet there were conditions under which cer- 

 tain forms of construction had been proved in practice to be bad, and it was 

 very desirable to have such examples brought before the meetings. Mr. 

 Thomson thought that the foundation upon which the wall or the slope was 

 10 be placed was a principal consideration ; if it was bard and sound, a ver- 

 tical wall would, under ordinary circumstances, and with proper precautions, 

 most probably stand well but if it was soft and liable to be washed away, 

 .1 slope, wiib a long pitched foreshore, must be preferable. The situation, 

 also, must be eoiisidered. If a sea defense was required to be built on the 

 beach, near low-water mark, where the half tide would set upon it with all 

 its destructive force, a vertical wall was not desirable, however well the same 

 wall might succeed if it were placed out of the direct itrfluence of the half 

 tide. With respect to the material, Mr. Thomson had found that coursed 

 rrdrhle formed an excellent durable vertical sea wall, if laid in strong by. 

 diaulic mortar, or any cement, so that a smooth face could be given to the 

 masonry, atfirrding no salient points tor the jrower of the water to be 

 exerted upon ; and even in slopes, it was ntore desirable to attend to having 

 close joints, and smooth-faced stones, than to selecting those of large 

 dimensions. 



Major-General Pasley thought that the depth of the water against the 

 foundation should be considered, as from the experience of the divers who 

 operated under his directions upon the wreck of the Royal Georr/e, it ap- 

 peared that at certain depths the water was comparatively still, when it was 

 niUi'b agitated at the surface. It might be received as a rule, that the waves 

 bad little or no force below the level of low water ; and even at a depth of 

 6 feet below the tide level, be thought that the force of the waves would be 

 innocuous. 



■ Mr. Rendel, V.P., said, it was well known that at a depth of about 12 

 feet below low-water mark, there was no injurious action of the waves, 

 however deep the action of the tidal current might be. 



Mr. Bateman agreed that the specific gravity of materials employed under 

 water should be considered ; yet be thought that their structure and qualities 

 were of more importance. Some stones tjecame softened, others readily 

 disintegrated, others were chipped away, or their surfaces worn away by the 

 travelling over them of sand and shingle and others again appeared to he 

 worn down by the action of the water alone. These were all to be avoided 

 for submarine constructions. He bad seen some sea walls built of basaltic 

 rock, which possessed great strength and solidity, standing well at an in- 

 clination of 3 to 1 ; hut the most remarkable example he recollected 

 was that of the Loch Foyle embankment, the face of which was pitched 

 with a clay slate stone. By the action of the waves, small laminated por- 

 lions were carried oft", and were forced like wedges into the interstices be- 

 tween the larger stones; the nbol^ face by this means became so smooth 



i that not a crevice coirld be detected, and the waves rolled over the surface 

 innocuously, having nothing to lay hold of in their passage. In fact, this 

 material gave naturally, as perfect a surface as engineers endeavoured to ob- 

 tain artificially, by laying large dressed blocks of stone, at a considerable 

 expense. 



Mr. Murray agreed to the preceding remarks as to the abrasion of ma- 

 terials ; it proceeded in some cases to such an extent, as to be a subject of 

 serious consideration to the engineer. The faces of several glacis, or sea 

 slopes, of 4, or 4 J to 1, which be bad constructed of hard limestone, had been 

 so worn down and scooped out by the action of the shingle when travelling 

 over it, that constant repairs became necessary. In the harbour of Walker, 

 there was a glacis faced with sandstone full 3 feet in thickness. During 

 the progress of the works, the small chippings of the stones formed a very 

 sharp shingle, which being carried by the waves over the finished portion, 

 wore it away, and injured it so extensively, that the sandstone pitcbtng was 

 obliged to be taken up and be replaced by wbinstone, and this, in spite of 

 men being employed to clear the shingle from the surface of the glacis. It 

 was of the utmost importance to the duration of masonry in such situations, 

 that the action of the sand and shingle, under the influence of the tides and 

 currents, should be carefully observed, as by the judicious erection of groynes, 

 it was practicable to accumulate masses of sand or shingle, which would act 

 etfectually as a protection for the works, or they might be made to deflect 

 the travelling material into the deep water. This principle had been adopted 

 with great success at the new harbour works at Sunderland, which he had 

 mentioned on a previous occasion.'' The coasts of Holland exhibited many 

 interesting examples of sea defenses of various kinds. There the groynes 

 were constructed of fascines, straw, and sand, and yet with these simple ma- 

 terials, which the Dutch engineers bad been compelled to use, from the 

 absence or great cost of more o'urahle substances, very effective structures 

 were raised, which protected the coast, and cost very little for repairs.^ 



Mr. Ranking wished to explain that he had not read Mr. Scott Russell's 

 paper on Sea Walls^ before he "had written the paper under discussion ; if 

 he had done so, he should have referred to some parts which were confirmed 

 by bis observations on the walls described. He begged it to be understood, 

 that in giving this description, be had not pretended to lay down universal 

 rules for the construction of breakwaters in deep water, from effects that 

 had been produced upon walls founded on the beach. He concurred in the 

 principle, that an eufc^ineer ought to be guided by circumstances in designing 

 works; but he conceived that cases sometimes occurred where the locality 

 permitted the engineer to create such circumstances as he required ; for in- 

 stance, at Cherbourg, the want of a natural beach, on which to found the 

 vertical f?ce of the seaward side of the breakwater, was supplied by the flat 

 summit of a stone embankment; it iiowever remained to be shown by ex- 

 perience, whether that artificial foreshore was sufliciently extensive for its 

 purpose. Mr. Rankine thought Mr. Scott Russell somewhat underrated the 

 power of the waves against the vertical wall at Trinity (fig. 3). The fact 

 that a fishing boat bad been thrown over and had been lodged upon the 

 road, which was 8 feet above the high water level of spring tides, showed 

 that the wares had acted on it throughout its whole height, and the ex- 

 posure had been still further increased at the points where the pitching bad 

 been carried away. He also pointed out that the vertical wall (fig. 3) was 

 not exposed to a less depth of water than the curved wall (fig. 4), but rather 

 to a greater depth, and an increased action of the waves. 



Mr. C. H. Smith saiil it appeared to him that in almost all engineering 

 works, the specific gravity or weight of the stone was of the utmost im- 

 portance, especially for low buildings which were occasionally under sea 

 water, and where there was, perhaps, a rapid current, or in other situations 

 subji'ct to the influence of powerful waves; such circutnstances would re- 

 quire a heavy quality of stone to he used, because the weight of all bodies 

 when submerged was reduced by that of the bulk of water displaced. The 

 lightest stone he had ever found in massses sufficiently large for building 

 purposes, weighed only 1031b. per cubic foot; and if this was used in sea 

 water, its weight would be reduced about 06 lb., which was the weight of a 

 cubic foot of sea water, and therefore it wouhl be like building on land, 

 with a material weighing but 37 lb. per cubic foot. The heaviest building 

 stones that he had met with, were the dark grey varieties of sandstone from 

 the vicinity of Swansea, from Abercarne, from the Forest of Dean, and from 

 Duiitlee ; some of these were even hea\ier than granite, weighing upwards 

 of 170 lb. per cubic foot, and in bis opinion, they would be quite as durable. 

 There was also a remarkably heavy stone found in some of the western 

 islands of Srotland, jiarticularly in the island of Tiree ; it was composed of 

 carbonate of lime, with a large quantity of hornblende in small nodules. 

 He conceived that such stones were peculiarly adapted for the building of 

 docks, harbours, breakwaters, sea embankments, and indeed for all purposes 

 where the violent action of water was to be contended against. In situations 

 where the stone was constantly or alternately under sea water, sandstone was 

 preferable to limestone, because it was not so likely to be acted upon by the 

 SoA'icava ruf/osa, the Photaa, or any other boring mollusca;, which frequently 

 pierceil cali:areous stones to the depth of several inches, thus changing a 

 smooth face to an extremely rough one, and consequently, by increasing the 

 friction, rendering the stones more likely to be disturbed by the action of 

 the waves. The sea walls or embankments in the neighbourhood of Leith, 

 were constructed with Craigleitb stone, which was a very good material for 



4 See 'Journal,' 1847, Vol. X., p. 1S9. s See • Jounial,' IS47, Vol. X., p. 83. 



6 See • Jo.iriTal,' 1S47, Vol. X., p. 125. 



