ISi7.] 



THE CIVIL ENGINEER AND ARCHITECPS JOURNAL. 



281 



CONSTRUCTION OF SEA WALLS. 



(Continued from page 25i.) 



BREAKWATER IN DELAWARE BAY. 



Annex (L.) — Report of Commission of the United States' Engineers and 

 iV«rfl/ Officers, on the Form which should be giren to Breakwaters, and Re- 

 port on the Construction of the Breakwater in Delaware Buy. 



Section of Breakwater in Delaware Bay, United States, 

 a. Top of breakwater, 30 feet wide. — *, Highest tide known. — c, Highest 

 spring tide. — d. Lowest spring tide. — e, Level 15 feet below lowest spring 

 tide.—./, Bottom 27 feet below lowest spring tide. — g. Average bottom 29-4 

 feet ditto. — The dotted lines on the top show a parapet to be made, if 

 needful, 22 feet wide. 



The following description of the Delaware Breakwater is compiled from 

 the Report of a Board of Commissioners to the Secretary of the Navy, 

 which WHS approved by the President of the United States in February 

 1829. The commission was composed of Commodore Rodgers, U.S. Navy ; 

 Brigadier General Bernard, U.S. Engineers; and William Stricklaodj 

 archilect and engineer: — 



With respect to these objects, upon which the solidity and durability of 

 the work so essentially depend, it must be acknowledged that theory and 

 mere speculation are utterly incompetent to fix, within precise limits, the 

 degree of resistance to be given to a work exposed to so many and such 

 incali ulably violent efforts of the sea. But valuable inferences may be 

 deduced from experimental results afl'orded by the construction of similar 

 works in Europe, and described in an able paper presented to the French 

 Institute by M. Cachin, general inspector of French Civil Engineers. Thus 

 the stupendous works erected in Cherbourg in France, and at Plymouth 

 in England, have been resorted to as guides in the investigation of the 

 leading principles upon which the breakwater under consideiatiou should 

 be coustructed. 



If the road of Cherbourg is of the highest importance to France, that of 

 Plymouth is probably of equal importance to Great Britain; as, among 

 other advantages, it enables her to assemble at one point the fleets destined 

 to watch the movements of her neighbours in the roads of Brest and Cher 

 bourg; added to which, the connexion of the road of Plymouth with an 

 extensive naval arsenal makes it a matter of much consequence that it 

 should be rendered perfectly secure. 



The works at Cherbourg fully answering the purposes for which they 

 were erected, and demonstrating their importance, the Governnienl of 

 Great Britain caused the erection of a breakwater to be undertaken in the 

 road of Plymouth, which was accordingly commenced in 1812. 



At Plymouth the interior slope has an inclination of i)7 feet altitude to 

 "JO feet base, making an angle of 32° with the horizon. At Cherbourg this 

 slope is of 45° inclination ; and, since it has stood firmly under an altitude 

 of mure tlian 70 feet, it may be inferred that at Plymouth the interior slope 

 mishi also have been kept at 45°. 



Tne B.iard was, therefore, uf opinion thai, as the Delaware Breakwater 

 must be 18 feet lower than that of Plymouth, and 30 feet lower than that 

 of Cherbourg, there should be no hesitation in adopting the slope of 45°. 



At Cherbourg, as at Plymouth, experience has taught that, if human 

 power was aide so to heap up materials as to fill up such a space in the 

 deep, it required the agency of tempestuous waves so to dispose of them 

 us to secure their permanent stability. On this score it would seem that 

 the results obtained at Cherbourg from vicissitudes in 1812 were but par- 

 tially known to the able projectors of the Plymouth Breakwater. Indeed, 

 the base of 180 feet of that work, and its altitude of 57 feet, have received 

 precisely the same ratio as that which the action of the sea had fixed be- 

 tween the base of 228,^^15 feet, and the altitude of 72,",^ feet of the work at 

 Cherbourg. The surface of the former work having been assumed to be a 

 plane, while at Cherbourg the eU'orls of battering waves have produced a 

 curvated surface, it is hence to be apprehended that at Plymouth it may 

 become necessary, in progress of time, to add new materials to the lower 

 part of the slope. 



The slope herein submitted has been framed out of the following facts 

 and principles afforded by the Cherbourg Breakwater. 



1. The part above the highest spring-tide having been for a short time 

 battered by the waves, which had lost by their ascension a portion of their 

 momentum, received from the action of the sea an inclination of nearly 

 2 feet base to 1 of altitude. 



2. The part comprehended between the highest and lowest spring-tide is 

 exposed, during the time of its rise and fall, to the greatest violence of the 

 waves. Thus permanently swept by the sea, this portion of the slope has 

 received an iuclinaiiou ot ii leei iiase to 2 oi iiiinuJe. 



3. The part comprised between the lowest spring-tide and a horizont 

 plane 15 feet below it, is exposed to the shock of the waves only during 

 the interval between the termination of the fall and the commencement of 

 the rise of tide : it has, therefore, to withstand the efforts of the sea under 

 a less inclination, viz., 3 feet base to 1 of altitude. 



4. The lowest part of the slope comprehended between the latter plane 

 and the bottom of the sea, remaining permanently submerged, and to a 

 depth at which the agitation of the waves has attained its minimum, has 

 assumed an inclination still less than the preceding, viz., 5 feet base to 4 

 of altitude. 



These experimental results show that the effect of water against loose 

 materials is to give to the mass in progress of time a slope, the inclination 

 of which will increase in proportion to the force exerted against it. 



It is on these data that the profile of the Delaware Breakwater has beea 

 delineated. 



This fact, with others not dissimilar which have happened at Cherbourg, 

 shows that the top of a breakwater must be elevated beyond the reach of 

 submersion, and loaded with the largest and heaviest materials that can be 

 procured, which should be laid in such a way that each shall present to 

 the action of the sea the smallest possible superficies, and to the lateral 

 materials the largest surface of friction. 



These considerations induced the Board to recommend for the Delaware 



Breakwater a profile, or transver.-al seclion,of the following dimensions: 



the iuward slope at 45°, the lop 30 feet in breadth, and at 5J feet above 

 the highest spriug tide ; the outward slope of 39 feet altitude, and of lOoJ 

 feet base ; both dimensions measured in relation to a horizontal plane pass- 

 ing by a point taken at 27 feet below the lowest spring-tide. The base 

 bears to the altitude nearly the same ratio as similar lines in the profiles of 

 Plymouth and Cherbourg Breakwaters. 

 The experience acquired at Cherbourg has taught — 



1. That stones of small size are not sudicieut to withstand even a mode- 

 rate action of the waves ; for, being constantly tossed about, they acquire 

 by attrition a round and smooth surface, which preveuts their assuming 

 any settled place in the mass. 



2. That stones measuring 18 or 24 cubic feet, and weighing 1 J to 2 tons, 

 present a suitable resistance to the efforts of a moderate sea. 



3. That larger blocks are required to withstand a violent sea ; and that 

 in the more exposed parts of the work their sizes should be still larger. 



4. That if small maierials were to be used, it would be indispensable to 

 protect them externally by others of larger size. 



5. That the smaller the external surface of a large block, the greater 

 will belts stability. 



6. That the largest blocks should be placed towards the top, in order to 

 compensate, by their greater steadiness, the loss of weight and of stability 

 caused by immersion to the materials located immediately under the w ater 

 line. 



The foregoing description of the Delaware Breakwater includes, with 

 occasional alterations, the Report of the Board of Commissioners. The 

 work has been executed so far in accordance with the views and plans 

 therein detailed. The dimensions recommended in tlie Report have been 

 adopted in its erection, with the exception of that portion designed for a 

 breakwater, which is 1,000 yards in length ; the length recommended was 

 1,200. 



The work may be considered now so far finished as to have accomplished 

 materially the purposes for which it was projected. Indeed, the plan of 

 commencing tlie work at the adjacent extremities of its two portions has 

 tended to yield a shelter to vessels during the whole progress of its con- 

 struction. 



CHERBOURG HARBOUR. 



Annex (M.) — Extracts from the Memoire of Mons. J. M. Cachin, on 

 the Breakwater of Cherbourg. 



At the close of the 16lh century, the French Ciovernment had resolved to 

 provide, by means of art, those advantages for France which nature has 

 conferred with such prodigality upon England, in the many ports she pos- 

 sesses in the English Channel. 



It was more particularly, however, after the battle of La Hogue, that 

 this necessity was more strongly felt, and the Government determined to 

 create at tJlierbourg a grand naval establishment, which in providing a safe 

 aud convenient harbour for their fleet, should give to the French nation the 

 degree of power befitting its maritime position. 



The American war had reanimated in the French mind the ambition of 

 disputing with England the empire of the seas ; and at the general peace, 

 immediate steps were taken for fortifying the roadstead of Cherbourg. 



The Bay of Cherbourg was selected as being opposite to the many almost 

 natural harbours which Great Britain possesses in the Channel ; as being 

 in a very advanced position upon the ordinary track of ships, aud offering 

 every desirable facility for watching the movements of the enemy, inter- 

 rupting his convoys, and for concentrating all the details of a grand mari- 

 time expedition. 



The War Departments, which had been charged with proposing the divers 

 projects for this graud enterprise, neglected to sound the roadstead, and to 

 observe its maritime properties ; and jud;;ing its extent as a harbour by its 

 apparent surface, proposed the adoption of a plan which had been sub- 

 mitted to the French Government in 1778, by the director of fortifications 

 at Cherbourg, for closing the roadsteaJ by meao:! of caissons tilled with 

 masonry, Jormiug ajelty or " digue" from Point Hornet to Pel^e Island. 



38 



