1847.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



125 



March 2. — This evening: the discussion upon Mr. Jackson's paper was 

 renewed, and was extended to such a length as to preclude the reading of 

 any paper. 



The comparative advantages and disadvantages of vertical and sloping 

 sea-walls were discussed, and instances were given of the eflect of seas 

 upon the former when walls of a certain batter or curved face surmounted 

 by an overhanging coping of such extent as to deflect the curling wave 

 outwards, and throw it back upon itself rather than allow it to fall bodily 

 inwards, as in the case of the Penmaenmawr wall mentioned at the last 

 meeting. The manner in which the waves were driven up long slopes, 

 acquiring force as they travelled along, was contrasted with this. On the 

 other band, the action of the various kinds of waves was shown upon sec- 

 tions of the beach at Madnis, where the surf was so notoriously bad, and 

 where it appeared tiiat by the clawing off of the waves the bedch was 

 washed away into natural steps, of a level and then a small slope of 4.'>°. 

 A breakwater had been formed oil' that beach by throwing in loose masses 

 of rock forming their own slope; this, when carried up to within ten feet 

 of the water-level, stood well. 



In Knootka Sound the same effect of the drawback of the waves was 

 noticed. Sections of the Mole of Venice were shown. That mole, which 

 js nearly IG miles in extent, had a section of a sloped foreshore with a 

 nearly vertical wall, then a slopf at another angle, and above high-water 

 mark another nearly vertical wall. \\'hen tlie seas rolled in upon the 

 mole they partially curled over against the hrst wall, and were projected 

 with augmented force against the upper one. The consequence was, that 

 the mole was partially destroyed, and in the repairs, which had beeu exe- 

 cuting for some time, it had been reduced to one uniform sloped face, at an 

 angle of about \6^. The destrnction of the nearly vertical walls of Port- 

 patrick was also noticed. Those walls, although constructed of the finest 

 Anglesea limestone, well dressed, dove tailed, aud lied down vertically and 

 horizontally by iron chain-bonds, were completely overthrown ; and, until 

 the thickness of the wall was increased to 80 feet of solid material, it 

 could not be made to stand. The situation was extremely exposed, and 

 the sea frequently sprung 50 feet above the top of the light-house, which 

 was Itself 60 feet above the level of high water of spring tides, 'the 

 causes of the peculiar action of the draw-back of the waves, as exempli- 

 fied by the removed shingle from the beach when the wind was on shore, 

 and its accumulation when the wind blew off the shore, were also dis- 

 cussed; and it appeared to be the received opinion that, in these cases, 

 the upper part of the waves being acted upon by the wind, a pecular roll- 

 ing motion in a counter direction was imparled to the lower wave, which 

 acted upon the shingle in the manner alluded to. This action appeared, 

 however, only to extend to a depth of about nine feel, which it seemed to 

 be agreed was the ultimate depth of detrimental action of all waves. The 

 forts of Boulogne were givau as further examples of the relleclion of waves 

 from nearly vertical walls ; but it was shown that the darting over of the 

 waves there was caused by their falling within the re-entrant angles of the 

 fortification. 



The effect of advanced groynes in protecting sea-walls was exemplified 

 by the concrete walls at Brighton and Dover, which were extended merely 

 for retaining walls ; and such was the eflect of the groynes, that siiue they 

 had been put down the shingle had accumulated to such an extent that the 

 sea did not approach injuriously to within 100 feet of the base. Our 

 limits will not permit a greater detail of this interesting discussion, which 

 will, however, appear entire in the proceedings of the Institution. 



March 9. — The paper read w as " On the practical forms (if engineering- 

 ji'Orfcs exposed to the action of the leaves of the sea, and on the adcaiitages 

 and disadvantages of certainforms of co7istruction for l^reakivaters and sea 

 walls." Ky Mr. John Scott Russell, 



Although agreeing as a general proposition, with the truth of the obser- 

 vation, that it was impossible to lay down any one undeviating rule for a 

 form of sea wall vvhicli should suit all cases, — the author had, from long 

 and careful experiment, and examination of various localities, endeavoured 

 to classify certain forms of artificial constructions, and to adapt them to 

 certain cases, having reference in each case to the action of the waves to 

 which they weie to be exposed. His li st process was to examine the ac- 

 tion and character of the several kinds of waves, deducing as a given 

 axiom, that, — First, the common form of waves is cjcloidal. Second, the 

 motion of the waves in a disturbed state is circular, and in a vertical plane. 

 Third, the water near the top of a wave moves the same way as the wave 

 itself. Fourth, the water in the hollow between the waves is receding. 

 Fifth, the power of a wave is exactly in proportion to the height of its crest 

 above the hollow between the waves. Sixth, the greatest power a wave 

 can exert is at the moment of the crest breaking over into the hollow. 

 Seventh, waves in the Biitish seas have rarel} been seen of a greater 

 height than 27 feet above the hollow , and 32 feet may be taken as their 

 greatest unbroken height; those of the Atlantic being stated to range 

 higher. Eighth, waves have never been seen of the full depth of the water 

 forming them, hence it is deduced that the greatest force waves can be ex- 

 posed to may be determined by the depth of the water they are placed in. 

 Ninth, there two or more classes of waves, — wind-waves, short, high, and 

 superficial ; and storm-waves, which are long, low, and deep. Tenth, the 

 depth of agitation caused by a wave is in the ratio of its height and length 

 conjointly. Reasoning upon these data, the paper then proceeds to ex- 

 amine two classes of hydraulic^ works. First, those which are designed to 

 act upon the waves ; and, secon^ those whose structures are exposed to 



the sea without any design of controlling it, but only to guide it under 

 particular circumstances. 



Of the first class are sea walls, piers, and other sea defences intended to 

 restrain the action of the waves; for the forms of which a number of de- 

 signs were given, ranging between the flat slope, with a foreshore, and the 

 vertical wall. Of all these the preference was given to a wall having a 

 concave or cycloidal curved face, to carry the wave up without breaking; 

 overhanging coping curved on the underside to return the wave upon itself, 

 and a recessed parapet on the outside to prevent the wave from being 

 thrown inside. For breakwaters, whose object it was to resist the waves 

 and produce still water within side, the best mode, under all circumstances 

 of locality, variety of malerials, and cost appeared to be the depositing of 

 the large and small materials, and allowing them to find their natural slope 

 under the action of the waves. 



Of the second class, are works designed to direct the scour at low water, 

 but which are quite covered at high water ; the foundations of lighthouses, 

 &c. — the object being to oppose the least possible resistance to the waves, 

 and to suffer the least from them. Groynes, embankments, and other 

 works intended to be under high water, also coming under this class ; the 

 best form is the parabola with the foot curved outwards on each side — the 

 apex being raised or lowered, and the base proportioned to its application. 

 This form being extended upward approximates to that of the Eddystone, 

 FJell-rock, and Skerry-vore lighthouses, which have withstood the action 

 of heavy seas so successfully. 



The vertical wall was condemned for many satisfactory reasons ; the 

 cost of workmanship, the expensive character of the materials, the liability 

 to destruction, if a breach be made, and the unsatisfactory action in conse- 

 quence of the waves making a clear breach over them in heavy weather. 



In the discussion that ensued, many interesting illustrations were given 

 of the truth of these positions, derived from the works of Whitehaven, those 

 oil the .Sonlh Devon Coast, and those at Hartlepool, in which latter case 

 the strong red marl, dry punned, mingled with small stone, and fared with 

 pitching, had been satisfactorily employed at a very small cost for the con- 

 slructiou of piers. 



March 16. — The paper read was " A description of the method adopted 

 in Preparing the Foandation, a'ld in Building the Bridge over the Polder- 

 vaart^ on the line of the Amsterdam and Rotterdam Railway " By the 

 Chevalier Conrau, M.Inst. C.E., compiled by Mr. C. Manby (secre- 

 tary), from documents furnished by Mynheer Wenckeback, 



This bridge derived its importance from the peculiarly treacherous na- 

 ture of the ground upon which it was constructed, for, although in Holland 

 bad foundations are the rule rather than the exception, the difficulties were 

 in this case so peculiarly great, as to demand particular notice. The Pol- 

 dervaart, is a canal encompassing and conveying away the waters from the 

 Polders, or spots of drained laud in the commune of Kethel. The railway, 

 traversing it at a considerable angle, rendered a skew bridge, of three 

 openings, necessary — the centre one 13 feet space for the navigation, and 

 the two side arches 21 feet space each, for tlie drainage water. The pro- 

 ceedings were commenced in the usual manner, with the intention of hav- 

 ing separate foundations for each pier ; tins was by shooting in large quan- 

 tities of sand, to form dams, within which, when pumped dry, the founda- 

 tions would have been excavated. After a length of about 70 feet of sand, 

 a dam 10 feet deep had been filled in, without exhibiting any signs of sink- 

 ing; a heavy thunder-storm occurred, during which the whole mass of 

 sand dam was suddenly engulphed to a depth of 29 feet; whilst there 

 arose simultaneously, at a short distance down the canal, to above the 

 water level, a mass of bog-earth, of an area of 4489 square feet — this mass 

 increased at subsequent periods of the proceedings to the area of 9G28 

 square feet. It was evident, that an extensive subterraneous shifting of 

 the bog-earth had occurred, and there was reason to fear for the safety of 

 the adjacent dykes and other works. Piling and fascine works were tried 

 without success — piles of 70 feet in length, when driven and tied together 

 by waling pieces, swerved bnilily from their po^iiiou, and became useless ; 

 fascines equally failed in producing stability. The engineer, therefore, de- 

 termined, after directing the canal water into a side cut, to surround the 

 site of the intended foundation with mounds of sand, allowing for their 

 subsidence iulo the gulph below, and then squeezing up the bog-earth 

 around and within the spot. Tliis was at length cnmpletciJ, and the foua- 

 daiiou pit was --naGled to be pumped dry. It then became necessary to 

 remove all the bog-earth from within the space for the foundation, which 

 was accomplished by digging out spaces of a yard square, and filling them 

 in with sand as they proceeded, nutil, by commenciug at the exterior, and 

 working inwards to the centre, all the bog-earth was removed, and a bed 

 of sand had been formed in its place. The piles for the ordinary founda- 

 tion, used in Holland, vveie then driven through the made grimnd, and the 

 structure was completed wilh jierfect success — the sand dams, and the 

 masses of upraised l)Og earth outside, being subsequently dredged up in 

 the ordinary manner, to restore the canal to its original bed. In this de- 

 scription, the circumstance most deserving attention, appeared to be the 

 sudden rising of the bog-ear(h during a thunder-storm. This is, however, 

 of frequent occurrence in Holland ; and it would appear as if the adhesion 

 of the masses of bog-earth to the bottom was so slight, that the vibration 

 communicated to the water by the thunder, sufficed to destroy the equili- 

 brium, and the bog-turf, which, from its slight specific gravity, will float 

 even when wet, instantly rose to the surface. When, therefore, as in this 

 case, a heavy mass of sand was placed in the vicinity uf such bog-earth. 



