I 847.J 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



210 



end in tolal failure; and when to this I shall have added what I have yet 

 lo say upon the subject of Cherbouri;, Plyrnoudi, and Delaware break- 

 waters, works actually constructed ou the principles which the new theory 

 would abandon, and shall have adduced the opinion of the most eminent 

 and enlifjhteiied engineer of France, I trust I shall be considered to have 

 made a good case in suppoit of this dissent. 



There is no part of the Report of the :;8th January I84C from which I 

 more decideJly dissent than that which refers to Cherbourg Breakwater as 

 a failure, and as " au attempt which may serve as a warning to those who 

 may have to decide upon the construction of such works in this country," 

 that they avoid eniirely the principles upon which that work has been cou- 

 strncled. 



In the Annex (M.) is a brief historical account of Cherbourg Break- 

 water from the commencement, together with an extract of the Report of 

 tlie Co[nnii6sion of the Institute of France, of which Prony and Cliarles 

 Dupiti Here members, and Girard rapporteur. These eminent men, after 

 a careful inspection of that work, and after having investigated the whole 

 process of its execution throughout, reported that the fadures which had 

 taken place arose, not from its having a sloping face, but that the slopes 

 were not long enough to resist the action of the waves; that no constant 

 degree of slope is calculated lo resist the dill'ereut actions of the sea at dif- 

 ferent depths; that these actions reduced gradually the masses of stone 

 forming the origmal djke, to a protile having ddlereut degrees of slope, and 

 that this necessarily diminished the height of the work at dilVerent times. 

 'I'hey ailded that the whole mass was thus, at lengih, brought into a slate 

 of the most perfect stability ; and all this was verified by the United Slates 

 Commission. 



The first great lesson really taught by the work in Cherbourg Bay, as a 

 warning what to avoid, is that the system of caissouing should not be 

 adopted ; the next lessou is lo avoid the use of small stones deposited li 

 pierre perdue ; the third lesson is, not to coustruct the sea-face of break- 

 waters in one unilorni slope from the bottom, but to form the profile with 

 two slopes, and to make the slope far longer than that which was originally 

 designed for the work. The result of this extensive experiment demon- 

 strates first, how iusuHicient and iuconipeleut mere theory and speculatiuu 

 are, to fix within precise limits the degrees of resistance which should be 

 given to a work exposed to the violent eti'orls of the sea. 



We find that the mass of materials originally deposited in Cherbourg Bay, 

 was heaped up so as to form too steep a slope, and that the agency of tem- 

 pestuous waves has disposed of them by reduction to a form which secures 

 their permanent stability : 



That the part of a breakwater which is above the highest level of spring 

 tides, is so little exposed to the action of waves (which must have lost by 

 their ascent a portion of their momentum ere they arrive there), that it may 

 be more sleep than the part below. 



We learn also that the purt of the breakwater between low water and 

 high water, spring tide level, is exposed to the greatest violence of the 

 waves during the whole of the rise and fall of tides ; and that there the 

 slope should be longest, or the inclination of the face to the borizou should 

 be the least. 



Captain Washington states, in his report on the breakwater at Cher- 

 bourg, " that the long slope of ten to one, formed by the action of the 

 waves, from low-waler mark upwards, has not varied, not even in the gales 

 of 1S08, 1824, and 1S36, the most memorable on record." There cannot 

 be better evidence of the stabilily of the long slope. 



That the part of Ihe breakwater for a certain distance below the lowest 

 spring tide, is exposed only to the shock of waves tovvaids the termination 

 of the fall and tiie commencement of the rise of tide ; that there Ihe slope 

 maybe steeper, or the iuclinatiou to the hoi izon greater ; whilst at the 

 lowest part of all, or that which remains permanently submerged, the slope 

 may be still more steep, or have the greatest inclination to the horizon. 



With respect to the magnitude of materials, we find that small stones 

 have not suliicient stability to withstand even a moderate action of waves. 



That stones of from one and a half to two tous weight, are sufficient to 

 resist the effects of a moderate sea. 



That blocks considerably larger are required to withstand violent seas. 



That when small materials are used, it is indispensable to cover them 

 with blocks of large dinieusions. 



That very large blocks should be placed towards the top of the work, to 

 compensate by llieir weight the loss of stability caused by the total immer- 

 sion of the materials beneath, for these lose as much of their weight in 

 water, as is equal to ihe Wr-ight of water displaced. 



The last fai t to be noticed respecting the work at Cherbourg constitutes 

 a very decided warning against the n?e of blocks of concrete, which was 

 proposed by Captain Deuison, November 21, 1843 ; for Ihe application of 

 this material on a large scale has entirely failed ; the blocks of concrete 

 having broken to pieces. 



The imperfections of the original project being corrected, the breakwater 

 at Cherbourg is now proceeding rapidly lo completion ; and far from being 

 a warning that those who have to tiecide upon the construction of a Har- 

 bour of Refuge in Dover Bay, or elsewhere, should avoid the principles 

 and reject the form winch has been observed in its construction, it demon- 

 btrates in the most forcible manner, that the theory of the upright wall 

 should be rejected, and that in its place should be adopted the well-tried 

 slope, or rather a combination of ditl'erent slopes ; while a nearly upright 

 wall may be formed above, to serve for ihe facing of a parapet like that 

 which crowns the work at the French port. 



Now, persons who read cursorily that part of the Report to which I 



have referred, may imagine that Ihe old dyke at Cherbourg had been taken 

 down, and that the vertical wall which has recently been built, is raised 

 from the natural bed of the sea, to the exclusion of Ihe slope ; whereas it 

 is, in fact, merely a parapet with a nearly vertical face placed on the ori- 

 ginal breakwater, to prevent the waves from rushing over the terre-plein, 

 after their force had been expended or greatly diminished, in ascending the 

 long slope or glacis in its front. 



I repeat now, on the authority of the very highest, the most experienced 

 civil and miliiary engineer of France, or probably that the world ever knew, 

 that "all the enlightened engineers of France do continue to adopt, and 

 will continue to construct breakwaters with inclined slopes, and do reject 

 the theory of the upright wall ; that the only alteration they would make 

 if the work were to do over again, is in the degree of slope, which they 

 would make variable according to the nature, specific gravity, and magni- 

 tude of the materials used ; that the walls now being erecleJ at Cherbourg, 

 are not upright from the naked bottom of the sea, hut built as a parapet, 

 upon a well-consolidated basis; this being the breakwater previously 

 (uimed i> pierre perdue, whose slope has diflerent degrees of inclination to 

 the horizon, according as the action of the sea has reduced the original 

 mass." That great work now stands in the form of a combination of the 

 slope with the upright face for the superstructure ; a profile which Reodel, 

 Renuie, Cubitt, Vetch, Stuart, Colonel Harry Jones, Vignoles, and others 

 recommend, but which Professor Airy says, speaking of an entire break- 

 water so for'iied. is, theoretically, " without doubt ihe worst of all. 



That there may he no mistake upon this important matter in reference to 

 Cherbourg Breokwater, unquestionably the greatest piece of hydraulic archi- 

 tecture that has ever been executed, I annex a profile, showing a combina- 



tion of the long slope with the vertical parapet and its fore-slope of stones . 

 and I add the reasons which induced the French engineers 14 years ago to" 

 recommend such a superstructure. This combination was proposed for the 

 completion of the breakwater, by Mons. Duparc, director of hydraulic works 

 at Cherbourg, and sanctioned without modifications by the Minister of Ma- 

 rine in April 1832, on the advice of a special commission, to which that pro- 

 position had been referred; but so far from pulling down the ancient dyke, 

 as stated in the Report, it was raised from the level to which it had been re- 

 duced from not having slope enough, by depositing large blocks of rough 

 stone up to the height of low-water spring tides ; and on" it there was laid a 

 mass of concrete, about 3 feet thick, on which a wall or quay is built to the 

 height of 12 feet above high-water spring tides. The exterior side of this 

 quay or wall, is protected by a fore-shore of great blocks of stone, extending 

 in a slope of 120 feet to the depth of 21 feet below low-water mark. The ob- 

 ject of these blocks is stated by Mr. Virla to have been two-fold. The in- 

 clined surface of this fore-slope makes, with the face of the wall, a re-enter- 

 ing angle which might have been avoided in part by adopting the concave pro- 

 file of Mons. Emy, but which in this case was not thought necessary, inasmuch 

 as the artificial beach of great masses of stone the principal object of which 

 was to give to ihe slope of the dyke perfect stability, produced in addition 

 an iiiipuitant efl'ect in resisting the action of the waves at low water. It is 

 found, in fact, that the waves which break on the surface of a long slope, 

 have time to deaden their force against the asperities of the blocks which 

 form the slope, before they strike the re-entering angle of the foundation ; 

 and as the sea rises, and the time of high water approaches, the slope in 

 front produces the effect of an ordinary beach in turning and throwing up 

 the waves, which would otherwise break against the wall with extreme -viol- 

 ence at the moment of their maximum of intensity. 



Lieut.- General Sir Howard Douglas concludes by giving his dissent to the 

 statement made by the Commissioners in their Report, "that they do not ap- 

 prove fully of any of the plans sent in." 



And he objects to the use of blocks of concrete, or of brick set in cement, 

 or to any other artificial material, as substitutes for stone, for the formation 

 of national works, which, if not to be constructed on sound and well-tried 

 principles, with materials ot the best and most enduring descriptions, should 

 not be attempted ; and he also dissents to the recommendation of a majority 

 of the Commission, for the adoption of masses of brick, as proposed by Mr. 

 llendel. 



And lastly. Sir Howard dissents from any extension of the area to be com- 

 prehended bv the breakwater, as recommended in the Report of 1844 ; and 

 more especially from the suppression of the eastern opening; without such 

 opening, the proposed Harbour of Refuge would he deprived of an essential 

 condition which all such harbours should possess, that of facility of egress or 

 escape at all times and tides, and in all weathers; and he is convinced that 

 by omitting to form this opening, the proposed enclosure would become to 

 such a degree a close harbour, as greatly to increase and aecelerate the pro- 

 gress of the evil to which all close harbours are liable, that of rapiuiy silting 

 up. 



