1847.1 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



251 



CONSTRUCTION OF SEA WALLS. 



Opiiiimis of Evgineers oil the Construction of Sea Walls, referred to in 

 Sir Howard Douglas's Protest, giren in last month's Journal. 



Annex (B).— Sir John Rennie on the Mode of Construction 0/ the 



proposed Harbour. 

 Wilh regard to the last important consideration, namely, the particular 

 mode of construction, and the cost. Various plans may and have been 

 proposed for this purpose, such as founding the substructure below low 

 water in caissons, and raisins a superstructure of perpendicular walls of 

 masonry. upon them, carrying out frame-woiks of timber or iron, and form- 

 ing walls of masonry within them, filling the interior space between the 

 walls with chalk or concrete; another plan consists in throwing down 

 masses of chalk into the open sea, and covering them wilh stone ol harder 

 description. None of these, however, appear to me applicable to the pur- 

 pose, particularly for the great outer mole or breakwater ; the only similar 

 examples where caissons have been employed, are the memorable cases of 

 Cherbourg and Sheerness, where they signally failed. Wooden caissons 

 and hollow circular towers, composed of brickwork, masonry, and timber 

 combined, also woAlen floating breakwaters, were proposed by General 

 Uentham to be used in the constructiou of the breakwater at Plymouth, 

 hut after being fully discussed, these plans were abandoned as being inap- 

 plicable, and, looking to the particular circumstances of Plymouth Sound, 

 the reasons given for the rejection of those plans were unanswerable. In 

 such great and important works, where failure would be attended with 

 most disastrous consequences, none but those means which are best recog- 

 nised as certain of success should be adopted. This has been amply jusli- 

 lied by the result of the mode adopted in the construction of the break- 

 water at Plymouth, which has completely succeeded in every respect, 

 whether as regards desi^'n, construction, or economy, and I feel convinced 

 that under those circumstances, no other system would have answered so 

 well. Much has been said about the damage occasioned by storms dis- 

 turbing the rubble ; the fact is, storms form the principal agent in conso- 

 lidating the rubble and save manual labour, and, to use the late Mr. Ken- 

 uie's words, are the best workmen. 

 Annex (C). — Mr. George Rennie's Report on the Harbour in Dover 



Bay, and his Evidence. 

 With respect to the form and construction of the proposed breakwater. 

 Experience has proved the principles of Cherbourg, Plymouth, and 

 Kingstown Breakwaters. The destruction of the cones at Cherbourg, 

 and the failure of the brick masses at Sheerness, are sufficient arguments 

 against the adoption of caissons, or other expedients. 



If such a work as is now proposed be undertaken, it should be solidly 

 and properly done. The magnitude of such a work would not justify the 

 risk of a failure ; and, without entering into the question of the compara- 

 tive cost and effici"ncy of dilTerent systems, I have no hesitation in pro- 

 nouncing in favour of sloping stone breakwaters, similar to that of Ply- 

 mouth ; assuming, therefore, the same profile or section for the proposed 

 breakwater as that, from 1,800 to 2,000 square yards, and the same prices 

 which that work has cost, the probable amount will be about £3,500,000 ; 

 in consequence of there being no good materials in the vicinity, they must 

 be brought from elsewhere. 



Ques. You have spoken of the experience you have had in constructing 

 breakwaters, and of the observations you have made upon existing break- 

 waters ; you have studied fully also the theory upon which the construc- 

 tion of breakwaters depends with respect to the action of the sea? — Ans. 

 I have. 



You have said that you consider upright walls not so capable of resist- 

 ing the action of the sea as sloping ; do you form that opinion upon the 

 well-known theorem of the action of fluids in motion upon planes in terras 

 of their obliquity ? — I do. 



And that as the obliquity increases the effect upon the wall is diminished 

 in a very bigii ratio f — It is. 



Do not the advocates of the upright system predicate their theory upon 

 the supposition that waves in the open deep sea have only an oscillatory 

 motion, direct and indirect, but that they have no progressive motion; that 

 they do not act with any propelling or percussive force upon an erection in 

 the sea? — I believe they have that notion. 



And that the only motion is a vertical? — Yes. 



And that, consequently, the only effect that waves produce upon the 

 wall is by their statical pressure, or weight? — I believe so. But in answer 

 to that, some experiments have been recently made upon the horizontal 

 action of waves upon flat surfaces by Mr. Alan Stevenson. Mr. Steven- 

 son showed me an instrument when I was in Edinburgh in June last, 

 which consisted of a flat plane of a foot space, stuck upon the end of a 

 rod, just like a letter balance, placed vertically to the shock of the sea. 

 Jt had a spring behind it, and, of course, when a wave struck it, it indi- 

 < ated by the pressure against the spring the force of the wave. The 

 action of a wave moves horizontally, and when it strikes an object, it does 

 it with the mass of the wave put in motion, multiplied by its velocity. 

 Consequently, in order to repel that wave, we must have a mass which 

 shall be such that its weight shall be capable of resisting the shock. It 

 must be equal, both by its adhesion and weight, to overcome the shock of 

 the wave. 



Do nut you think that so far from a sloping breakwater creating the 



force (as the advocates of the upright wall state) with which the water 

 rushes up, it is the force with which the water does so rush up the plane, 

 being thus permitted to expend and exhaust itself, which diminishes the 

 effect in the horizontal direction upon the structure ; and that if it were 

 not so, as in the case of an upright breakwater, that force would act 

 wholly like a ram upon the perpendicular wall, to overthrow it? — It 

 would ; if we compare it with the friction of water in waves, which I 

 have found by experience to be something like a third of the pressing or 

 horizontal force, I should think by the same reasoning that the force of 

 waves would be diminished in the same proportion, only that the friction 

 of those large stones is much greater. I should say that the force by the 

 great angular inclination of those stones would be diminished in the ratio 

 of one-half the momentum or impulse of the shock. 



Is it not clear, then, that so far from waves having wholly an up and 

 down motion upon the face of an erection in the sea, so far from their 

 having no force in a horizontal direction, they do come in with propelling 

 and percussive force, which does act horizontally against the erection, 

 whatever it may be, wilh a force which varies according to the slope?— I 

 am quite of that opinion, and it is further confirmed by the forms which 

 beaches take. 



The advocates of the upright breakwater do not dispute the hydraulic 

 fundamental theory, that when fluids in motion act upon a plane, the force 

 of the motion upon the plane diminishes in a high ratio in proportion to 

 the angle of the plane, but they assert that this does not apply to the hy- 

 draulic construction which we are considering, because there is only an up 

 and down motion. Do you or do you not consider that the construction of 

 breakwaters, and their proper form, does depend upon that hydraulic 

 theorem, and that it would not be safe to proceed to the construction of 

 any breakwater upon the supposition that it is not governed by those laws, 

 for that there is no horizontal motion, but only a vertical pressure ? — I 

 think so. I think that the advocates of perpendicular walls are quite 

 wrong, with all due deference of course. 



Do you think it is practicable, or would it not be exceedingly diflicult to 

 build an upright wall in the open sea, in seven fathoms water .'—I think 

 it would be almost impracticable in deep water; I should be very sorry to 

 undertake such a thing. 



Do you think, if undertaken, it would he safe to use any artificial or 

 inferior materials, such as concrete or chalk, in any part of it ? — I do not, 

 decidedly. 



Do you know any case in which an upright wall has been built in modern 

 times in such deep water as that in which we propose to erect this break- 

 water '—I know the case of Sheerness, where the masses were sunk to 

 form an upright quay wall. 



Wculd not the erection of a breakwater, perfectly upright, in the open 

 sea, in Dover Bay, in seven fathoms water, be an experimental measure ! 

 — It would. 



Under all the circumstances of the case, confining yourself to the prac- 

 tical question, and especially considering the effect of failure, do you ihink 

 in the natural roiidstead and anchorage of Dover we should be justified in 

 making such an experiment under such circumstances as these?— I do 

 not. Of course I may be a partisan of a particular system, but I give 

 you my unbiassed judgment. 



Annex (D).— Mr. William Ciibitt's Plan for the Construction of a 



Harbour of Refuge in Dover Bay. 

 The most obvious mode of construction, and possibly after all the best, 

 is that of depositing large masses of rough hard rubble stone in the sea, 

 in the line of the intended work, in the same manner in which the harbour 

 of Kingstown and the breakwater at Plymouth were constructed. The 

 simplest is tumbling large stones into the sea, as Plymouth Breakwater 

 and Kingstown Harbour Pier ; another mode is building in water with 

 large stones, by means of the diving-bell; another by building caissons, 

 filling them up partially, and floating them into their berths, and sinking 

 them, and completing afterwards, either in or about them. 



On the whole, therefore, after a most careful consideration of the subject, 

 my recommendation to their lordships is, to form the harbour at Dover 

 Bay with piers or breakwaters, constructed by depositing the largest 

 blocks of either granite, Portland cap stone, or limestone, or all of them as 

 may be procurable together, with all the small stone that may be procur- 

 able together, with all the small stone that may arise in quarrying the 

 rough blocks ; and to form the breakwaters with circular heads at each of 

 the entrances 300 feet in diameter, brought up from the bottom with solid 

 facings of ashlar masonry by means of the diving-bell after the harbour is 

 enclosed, as is now being done at Kingstown Harbour. 



Ques. Will you have the goodness to state, after such strong expressions 

 in favour of an upright wall, why you now recommend a long slope 1—Ans. 

 The reasons I stated, that if in constructing a wall in the sea 72 feet high, 

 which is the height we calculated upon in deep water, we could be sure of 

 all our premises, the thing could be done, and would be the most perfect. 

 1 also stated, I think I recollect, in that report the reason I went from that 

 and recommended another plan was, because I wished to recommend that 

 which was perfectly certam to be effected with no contingencies, and at a 

 much more easily ascertained cost than a breakwater with upright faces, 

 which I certainly deemed the most perfect if done, but the casualties and 

 unknown circumstances attending which rendered it doubtful in sinie 

 points, and the expense of which would be greater. That in a thing of eo 

 large and important consideration as this harbour is, where millions of 

 money, to say the least of it, must be expended, to do anything efl'ectual, 



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