206 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[^Septembeb, 



Tlie gross averafje cost of tlie wall was somewliat below twelve 

 shiHiiigx per enlAe yard. 



Aliniist inmiediately after the completion of this work, the most 

 violent iioitli-east jfale occurred whicli had been known on that 

 part of the coast for twenty years; but the wall, though exposed 

 to the full force of tlie waves, did not sustain the slig^htest damage. 

 Ten yeai's afterwards, on tlie tith of December, 184^7, a more violent 

 storm occurred, which damaged or pai-tially destroyed almost 

 every other sea defense on the neighbouring coast; yet the sea 

 wall of the Leitli Branch Railway escaped without injury. The 

 horizontal pitching at the foot of the wall was alone damaged to 

 a slight extent, by some of the stones being lifted out of their 

 places by the waves; but they were generally deposited near their 

 original sites, so that the necessary repairs could be made at a 

 trifling cost. 



Tlie sections (figs. .3, 4, and .5) represent sea defenses between 

 Newhaven and Granton, the foundations of which rest chiefly on 

 shale, or on sandstone, which strata crop out on that part of the 

 coast. 



Fig. 3. 



A, A, A. Level of Higlj Water Siiring Tides; 

 r, Hrcuch ; C. lij'iiiterlorl ; D, UuUoai of 

 Bieucb i 11, Huiidway. 



Fig. 5. 



Scale to Figs. 3, 4, and 5, Ifj to the Foot. 



Fig. 3, represents the cross section of a much older sea wall 

 and bulwark, on the turnpike road between Newhaven and Trinity. 

 As this wall was not breached at any point by the storm of the 

 6th of December, the author is not able to give its thickness, nor 

 the dejith of its foundation. The face is nearly perpendicular, 

 and it is built entirely of rubble, laid in mortar, with a pointing 

 of cement in the joints of the lower courses. The roadway is 

 about 8 feet above high water of spring tides, and the parapet 

 rises 4 ft. 6 in. liigher. The foundation of this wall is protected 

 by a dry stone pitching, sloping at angles of from 30° to 40°. The 

 only effect of the storm was, that the upper part of the pitch- 

 ing, at several points, was carried away, to tlie extent siiown 

 in the section. Tlie vertical wall was not damaged, except at one 

 point, where a fishing-boat being thrown upon the road by the 

 waves, overturned an iron railing, which at that spot filled a small 

 opening in tlie jiarapet. 



Fig. 4, is a section of the sea wall of a portion of the Edin- 

 burgh, Leith, Tind Granton Railway. This wall is 2 ft. 6 in. thick; 

 its section a])proaclies to tlie form of a hyperbola. Foi a depth of 

 about 7 feet below the base of the parapet, it is nearly perpen- 

 dicular, and has counterforts of the form shown in the sectitui. 

 Below the point marked />, it becomes a dry stone bulwark. The 

 effect of the storm of the 6th December upon this structure 

 was to form two long breaches, by wliich the building was en- 

 tirely demolished, with the exception of the lower portion of the 

 dry stone bulwark (marked by a darker colour in the section), and 



several of the counterforts, which were left standing alone. This 

 result was obviously occasioned by undermining; the- stones be- 

 tween the point A and the bottom of the breach being extracted 

 by the waves, the upper part of tlie wall, having no independent 

 foundation, fell into the sea. 



Fig. S represents the sea wall of the Edinburgh, Leith, and 

 Granton Railway, at and near Granton. This is a sloping bul- 

 wark of a nearly parabolic form. It is built dry, except the string- 

 course and parapet, and consists of stones, which are, as the sec- 

 tion shows, much larger and heavier than those employed in build- 

 ing any of the walls previously mentioned, most of those in the 

 lower part of the slope weighing full half-a-ton each. The stones 

 of the heavy string-course, on wliich the parapet rests, are con- 

 nected by means of a flat malleable iron bar, measuring 2| inches 

 by |-inch, laid along their upper surfaces, and attached to the 

 stones by iron spikes |-incli in diameter. The coping stones are 

 connected with eacli other, and with the dado of the parapet, by 

 T-shaped iron cramps. The damage done to this wall by the 

 storm, was comparatively trifling; it consisted in the overturn of 

 a few yards of the parapet and string-course, and of the dry 

 building immediately beneath; the iron connecting-bar being bent 

 and broken. 



The efficiency of the surface of a wall to resist the action of the 

 waves, obviously depends on two circumstances; first, the power 

 with wliich the moving particles of the water act on the stones at 

 the surface; and secondly, the force with which those stones resist 

 removal. The object to be attained is to render the moving 

 power of the water as small as possible, and the resisting force of 

 the stones as great as possible, relatively to each other. 



Without entering into the theory of waves, wliich involves the 

 highest branches of mathematical analysis, it is sufficiently obvious 

 to daily observation that the oscillation of each particle of water, 

 in a wave mo\ing freely, is partly vertical, and partly horizontal; 

 that when a sufficient depth of water exists in front of a wall, or a 

 line of cliffs, the mutual action of the direct and reflected waves, 

 produces a series of points of greatest agitation; and at those 

 points the horizontal oscillation is either null, or so small, as com- 

 pared with the vertical, that practically the motion of the particles 

 may be considered merely as an oscillation up and down. A ver- 

 tical surface is, therefore, that which offers the least possible 

 impediment to the natural motion of the particles of water, under 

 such circum.staiices, and upon which, consequently, they act with 

 the least power; and a horizontal surface, being perpendicular to 

 tlie motion of the particles, is that upon which they act with 

 greatest power. 



It is also obvious, that when waves encounter a sloping bulwark, 

 or a sloping beach, the vertical part of the oscillation is gradually 

 converted, as the waves proceed, into an advancing and retreating 

 oscillation, parallel to the slope; that being the only direction in 

 which the particles can move, without destroying the surface of 

 the beach or bulwark; and this oscillation has a powerful tendency 

 to overturn and to remove any obstacle which projects above the 

 line of the slope. Hence it is, that large stones, extracted during 

 storms, from the seaward slopes of breakwatei-s, have frequently 

 been sviept entirely over to the landward side; and from the same 

 cause it also ai'ises, that the coping and upper portions of a curved 

 bulwark, such as that in fig. 5, are liable to be overthrown, by the 

 concussion of the body of water directed against them, by the 

 lo«er part of the slope. 



The force with which a stone resists removal is composed of 

 three parts; the first arises from its own weight, and is obviously 

 greater the flatter the slope, and is greatest of all when the surface 

 is horizontal; — tlie second arises from the pressure of the super- 

 incumbent masonry; and this is as obviously greater the steeper 

 the slope, and greatest in a vertical wall; — the third is the adhesion 

 of the mortar, or cement; and as this depends, to a certain extent, 

 on the pressure from abo^■e, it also is greatest in a vertical wall. 

 These principles appear to be entirely in accordance with the facts 

 which have been narr.ated. 



In such structures as the pitching at the foot of the walls in 

 figs. 2, 3, and 4, and the lower part of the slope of fig. 5, the 

 resistance to the action of the waves arises, almost altogether, 

 from the weight of the stones, and therefore increases as the slope 

 apjiroaches tlie horizontal; but as the moving power, exercised by 

 the particles of water, also increases, it is clear, that the stability 

 of bulwarks so constructed depends altogether on the use of suf- 

 ficiently large and heavy stones, such as those employed in fig. 5. 

 Hence, in figs. 2, 3, and 4, the stones of the pitching not being 

 sufficiently heavy, were partially displaced. 



In fig. 4, the destruction of the slope occasioned the fall of the 

 wall which rested on it; but in figs. 2, and 3, where the pitcliing 



