[1849. 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



267 



was not used to support the wall, but merely to protect the foun- 

 dation, the wall suffered no injury in consequence of its removal. 

 In fig. 2, where the pitching was laid on the natural level of the 

 sand, the stones displaced were deposited near their original site; 

 but in fig. 3, where the slope was comparatively steep, they rolled 

 down with the receding oscillation. 



The walls (figs. 2 and 3) being vertical, or nearly so, approach 

 the form of section which gives at once to the particles of water 

 the least amount of displacing power, and to the masonry the 

 greatest amount of the resistance, arising from the pressure of the 

 superincumbent materials, and the cohesion of cement and mortar; 

 and they afford an illustration of the stability which may be 

 attained with such a form of section, in a structure built of com- 

 paratively small stones. 



Owing to the extremely small elevation of the wall (fig. 2) 

 above the surface of the water, it was necessary that the coping 

 should be sufficiently massive to resist, by its weight alone, any 

 force tending to displace it; but in fig. 3 such a coping was not 

 necessary, as the top of the wall was above the reach of tlie action 

 of the waves. 



The author has endeavoured, in drawing general conclusions 

 respecting the construction of sea walls, to avoid going beyond 

 the limits warranted by the facts he has stated. His conclusions 

 may be received with caution, or with dissent; but he trusts that 

 the facts will be considered wortli recording. 



Remarks made at the Meeting after the reading of the foregoing Paper, 



Major-General Pasley referred to an opinion expressed by him at a 

 meeting of the Institution in 1842,- when he stated, that from •' his ob- 

 servations of the action of the sea upon various parts of the coast, he con- 

 ceived, that a perpendicular wall, constructed of large ashlar work, well 

 cemented, would assume the character of a rock, and all the prejudicial 

 action of the receding wave would be avoided." The correctness of this 

 opinion had been confirmed, in his mind, by the result of an examination he 

 had made of a sea wall, built by Mr. Brunei, along a portion of the South 

 Devon line, upon which he had reported, when he was Inspector-General of 

 Kailways. The wall to which he alluded was nearly vertical ; it was so 

 situated, that it was only exposed to any very violent action of the sea for 

 about three days at a time, during spring tides, and then only when a heavy 

 gale of wind blew at the same time. This however had once occurred, and it 

 was then reported that extensive damage had been done. He found, how- 

 ever, that although at the part within the estuary of the Exe a portion of 

 the parapet had been destroyed by a barge striking against it, the only 

 serious damage which had occurred was to the earthwork withinside the 

 wall, which had not been protected by masonry; the waves had been thrown 

 over in such masses, as to plough up the earth and to wash it away. That 

 portion of the sea wall which was constructed of ashlar work, was not 

 injured at all; but a part which was constiucted of dry stone masses, or 

 boulders, of considerable weight, was entirely destroyed. The damage was 

 soon repaired, by covering the earth slopes withinside the wall with stone 

 pitching, and substituting good sound ashlar work for the dry stone wall, 

 and since that time no injury had been sustained. His previous opinion, 

 in favour of vertical sea walls, was thus, he contended, fully confirmed. 



Mr. Brunel confirmed General Pasley's account of the extent of injury 

 sustained by the sea walls on the South Devon line, on the occasion alluded 

 to, and begged to renew his thanks for the assistance afforded him by the 

 General, in refuting ej^/^ar/e and unjust statements, which had been made 

 at the time, and were calculated to produce prejudicial effects. With respect 

 to any comparison between vertical and sloping walls for sea defenses, he 

 must repeat his objections to drawing general conclusions from one class of 

 evidence, or to laying down general rules to suit all cases. In this particular 

 instance, upon the South Devon line, a nearly vertical wall was more ap- 

 plicable, from the position, the depth of the water, and the general circum- 

 stances. His intention had been to build the wall with a perfectly vertical 

 face; a slight hatter was however given to it; the coping, also, which pro- 

 jected from 2 feet to 2 ft. Gin., was found effectually to turn the wave down 

 again ; the recess of 6 feet, or 8 feet, caused by setting back the parapet 

 wall that distance from the face of the main wall, received the mass of the 

 wave, and the high parapet finally prevented any considerable quantity of 

 water being projected over the wall. The principal injury had, therefore, 

 been received by the slopes withinside the wall, and where no parapet was 

 built, as within the estuary of the Exe the waves had destroyed the slopes; 

 but no other damage was done. A.s it was always useful to give an account 

 of even comparative failures, he would state, that wiih respect to that part 

 of the work which was composed of dry stone, it had been formed of blocks 

 of a hard conglomerate, which were found in large quantities on the shore ; 

 the wall was about 25 feet high, by about 10 feet thick, and the blocks, 

 when used, were only roughly shaped, so as to give them a kind of bed, that 

 they might be laid together, without cement, like cyclopajan masonry. The 

 whole had been destroyed, as General Pasley had' described, while a wall 

 built close beside it, of small ashlar masonry set in mortar, resisted perfectly 

 the action of the same storm. Mr. Brunei ascribed the destruction of the 



2 See ' Journal,' 1842, Vol. V., p. 319. 



dry stone wall to the roughness of the surface, and the want of cement 

 between the blocks, allowing the sea to enter and to dislocate and destroy 

 the mass. It was necessary to have pitching at the base of sea walls, in 

 order to counteract the action of the receding waves, which was always 

 powerful where the water was only from 4 feet to 6 feet deep. On the 

 southern coast he had constructed bulwarks in advance of the wall, to act 

 as breakwaters, and as groynes, to collect the beach, in order to prevent the 

 scour, which so frequently undermined and destroyed walls, whose founda- 

 tions were even carried down to a considerable depth. A rock foundation 

 would not always save them, for he had seen the surface completely rubbed 

 away by the scour of the shingle, so as to render it necessary to underpin 

 the wall, and to build an apron upon the rock, to prevent the recurrence of 

 the same action. 



Mr. Green said, that as a somewhat analogous instance, he might mention 

 the Dymchurch wall, to which so much damage was done by the sea in 

 the year 1803, that he was subsequently sent there, at the recommendation 

 of the late Mr. Rennie, to carry out improvements in that extensive era- 

 hankment, which protected from the sea a tract of not less than 50,030 

 acres of marsh land, lying between the town of Hythe and the port of Rye. 

 The wall, or embankment, was constructed entirely of earth ; the face next 

 the sea was defended by "arming," which eonsisti-d of a facing of coppice 

 wood, held down to the earthwork by oak stakes and laths, in a very in- 

 genious manner; but the front of the embankment had been laid at slopes 

 of different inclinations, and in very irregular longitudinal directions. la 

 some parts, the front slope formed nearly an angle of 45° with the horizon, 

 or at an inclination of 1 horizontal to 1 perpendicular, varying in other 

 parts between that slope and 3 horizontal to 1 perpendicular. The top 

 of the embankment was in many parts at least 30 feet wide, and 20 feet 

 above the level of extraordinary spring tides, and a great portion of it 

 formed the public road from Hythe to Dymchurch and New Rumuey. It 

 was customary to make large depots of brush and coppice wood, for the oc- 

 casional repairs of the embankment, on the top of the more elevated and 

 widest parts of it; but occasionally, the force of the sea was so great, at 

 those parts having the steepest slopes, that the waves broke over the top, 

 and carried the large stacks, of 200 wagnn loads of coppice wood, from the 

 top of the embankment down into the marshes behind it. In executing 

 the repairs and the reconstruction with which he was entrusted, Mr. Green's 

 first object was to make the line of the face of the embankment as nearly 

 as possible that of the general line of the shore, and to remove all sudden 

 projections into the sea ; and secondly, to bring the front slope of the em- 

 bankment to an uniform inclination of 7 horizontal to 1 perpendicular, 

 the top of the embankment being only 6 feet higher than the level of extra- 

 ordinary spring tides. The general inclination of the beach on the shore, 

 from high water downwards, was, in that part of the shore, about 8 

 horizontal to 1 perpendicular ; but the face of the slope was defended by 

 the coppice-wood arming, as before, as the expense of facing it with stone 

 would have been beyond the means of the proprietors of the lands. The 

 plan pursued was found to answer extremely well, as, in the greatest storms, 

 the surge of the sea never reached the top of the embankment. This and 

 other experience, convinced him of the propriety of so forming sea em- 

 bankments, as to offer the least possible resistance to the direct force of the 

 sea. He had since successfully constructed many other sea embankments, 

 on the same principle, and had been fully confirmed in his opinion. 



Mr. BoRTHwicK said, that in the year 1833, Mr. Walker reported upon 

 the state of the Dymchurch wall, anil recommemled a general amendment 

 of its condition, which, he believed, was partly carried into effect, by the 

 construction of a sloping pitched bank, with a vertical wall at the top. Mr. 

 Elliott's paper, which was read at the Institution in the session of 1847,^ 

 stated, that even that plan had been modified, and a general line had been 

 adopted, with a pitched slope, at an average incli[iation of about 8 hori- 

 zontal to 1 perpendicular. 



Mr. Rendel knew the position where the walls described by Mr. Rankine 

 were situated, as well as those on the coast of Devonsliire, mentioned by 

 General Pasley and Mr. Brunei. In his opinion there could be no com- 

 parison between the two cases. The former were situated in an estuary, 

 sheltered by Incbkeith and the land of Fife, having, at the same time, a 

 long shallow foreshore; whilst the latter were placed in exposed positions, 

 with considerable depth of water, and heavy waves breaking upon them. A 

 substantial wall, built of proper proportions, of good smoth-faced material, 

 with strong hydraulic lime or cement, would stand well if it were nearly veiv 

 tical; and a slope would also stand, if well packed with dry stones. The sur- 

 face, however, must be such as to prevent the stones from offering such op- 

 position to the waves, as to permit them to be loosened and torn up. In 

 practice this was generally a question of relative cost, and in most cases a 

 nearly vertical wall, well built in mortar, was found to be the cheapest. 

 The great difficulty was to protect the toe of the wall, and to prevent it 

 from being undermined ; for even when the foundation was carried down to 

 the rock, the beach, which had previously accumulated to a considerable depth, 

 was not unfrequently carried away, and the surface of the rock was abraded 

 to such an extent, by the travelling of the shingle, as to loosen the lower 

 courses of the wall. In all such eases it was necessary, either to place a 

 paved apron, or to construct groynes, for the purpose of collecting the 

 shingle. The latter was perhaps the most effectual mode, if the litoral cur- 

 rents were well examined and taken advantage of, in settling the direction 



3 See 'Jouiual,' 1S47, Vol.X.. p 261. 



3S* 



