1842.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



191 



the breakwater, which, although squared and dovetailed into the 

 structure, and embedded in excellent cement to the extent of their 

 •whole depth, and thus forming a solid mass, were torn from their 

 positions, and projected over the breakwater into the Sound. 



" Mr. Walker attributed this to the hydrostatic pressure exerted 

 beneath the stones, at the moment when the atmospheric pressure 

 above had been disturbed by the masses of water suddenly and rapidly 

 thrown upon the surface of the breakwater; blocks of stone were 

 thus often carried to a great distance, not so much by the waves lifting 

 them as by the vacuum created above by the motion of the water, 

 ■which exerted at the same time its full pressure from below." 



And as additional evidence that the formation of a partial vacuum 

 is sometimes a consequence of the envelopment of sea works by high 

 •waves, Mr. Walker further stated, " that during a storm in the year 

 1S40, the sea door of the Eddystone Light-house was forced out- 

 wards, and its strong iron bolts and hinges broken by the atmospheric 

 pressure from within. In this instance ne conceivecl that the sweep 

 of the vast bodv of water in motion round the light-house had created 

 a partial and momentary, though effectual vacuum, and thus enabled 

 the atmospheric pressure within the building to act upon the only 

 yielding part of the structure." 



As both the above remarkable instances came professionally under 

 the notice of Mr. M'alker, we are bound to place the utmost reliance 

 ■upon the inferences derivaljle from his statements; and they indicate 

 conclusively that no lateral connexion by dovetails, or otherwise, will 

 compensate for a want of depth of sohd stone ; but that we must 

 rely for stability in the coping of sea works, chiefly upon the weight 

 of a large mass of materials, so connected as to gravitate as one body ; 

 ■we must oppose pressure by weight, and counteract by gravity the 

 action of forces resulting from a disturbed equilibrium. 



The pressure of the atmosphere is about the same per superficial 

 foot as would be produced by the gravity of Hi ft. depth of stone, 

 ■weighing 175 lb. per cubic foot ; therefore, if breakwaters were 

 liable to be assailed by waves but 20 ft. high,* at the same time that 

 the atmospheric pressure mas wholly removed, their smnmits would 

 require coping with a depth of stone wrought into a solid mass, 

 equal to ll!i+7i, or 19 ft. measured vertically. 



In the nature of things, however, we cannot suppose that the wlole 

 pressure of the atmosphere would ever be removed from any point 

 of the summit of a breakwater wliilst a wave of the maximum height 

 ■was acting beneath ; and though this is very much a matter of con- 

 jecture, we may probably infer that under no circumstances would 

 more than two-thirds of the atmospheric pressure be removed from 

 any point ; this would be countervailed by a depth of eight feet of 

 stone, weighing 175 lb. to the cubic foot, and as we h-jve shown that 

 a depth of near seven and a half feet is necessary to withstand the 

 hydrostatic pressure of a twenty-feet wave, we may finally infer that 

 ihe summits of breakwaters should never consist of less than 15/1. 

 average depth of stone firmly bound iiito a solid mass, by clamps, dowels, 

 and cement, so as to gravitate as one body.f 



* The maximum height attained by the waves of the most violent storms, 

 at the sites of breakwaters, is such an important element in forming the plan 

 of the work, that it ought always to be ascertained experimentally, (which 

 would not be difficult,) by attaching a machine upon the principle of a self- 

 registering tide gauge, to a pile well driven at the site, and properly braced 

 against the sea. 



t This principle of constructing sea works was adopted in Rudyerd's Ught- 

 house, built upon the Eddystone, in 1709, and which, after successfully with- 

 standing the storms of half a century, was destroyed by fire iu 1759; re- 

 garding it, Sraeaton states that Rudyerd " judiciously laid hold of the great 

 principle of engineering," that " weight is the most naturally and effectually 

 resisted by weight," and accordingly formed bis light-house, near its foun- 

 dation, solid, and mainly of stone, for such a height as he conceived would 

 enable the gravity of the mass to resist the upward hydrostatic pressure of 

 the waves, in case the water insinuated itself beneath the building. 



This idea was not lost upon the able and sagacious Smeaton, who, in 

 erecting the famous stone hght-house which succeeded Rudyerd's, built the 

 first 30 ft. high from the foundation, solid, and so proportioned the walls and 

 lantern above the fundamental solid that if their mass were reduced to a 

 cylindrical shape it would add another sohd column of about 20 ft. in 

 height ; so that in opposing the action of the sea, the Eddystone Light-house 

 is equivalent to a solid column of stone 50 ft. in altitude. 



Now from the above reasoning, 50 x 2 •^, or 135 ft., is the altitude of the 

 wave, whose upward hydrostatic action this building is, by its gravity alone, 

 competent to resist ; and as the atmospheric pressure is never removed from 

 its summit, whilst the utmost altitude of the jet of water which is sometimes 

 thrown over the lantern is short of 100 ft., its superabundant stability must 

 be manifest. 



The courses of this celebrated construction being dovetailed and joggled 

 together, so as to prevent movement laterally ; as long as its materials are 

 proof against decay, the immutable laws of gravitation will retain it in 



The fact that continual repairs are rendered necessary, by the blow- 

 ing up and sweeping away of portions of the pavements of existing 

 breakwaters, during violent storms, strongly sustains the views taken 

 in this paper, and shows but too clearly, that the mass of stone usually 

 combined upon their summits, is deficient in the requisite stability. 



The form in which the stoue ought to be laid and connected to- 

 gether should (the writer believes,) be that of a semi-cylinder, the 

 axis lengthwise of the work, and the base laid upon such an inclina- 

 tion seaward as may counteract sliding, and prevent the possibility 

 of its overturning upon the harbour angle, for we know that on a 

 level plane, one half the amount of force will overturn a body which 

 is necessary to lift it. 



In the case, then, of a breakwater similarly situated to that of the 

 Delaware, stone weighing 175 lb. to the cubic foot, and exposed to 

 the assault of waves not exceeding 20 ft. high, it would seem that the 

 section shown in the following Figure 2 would be a proper one. 



Fig. 2. 



a, the high water line of spring tides, above which storm tides rise 

 three feet or more ; b, the plane of low water ; c, semi-cylindrical mass 

 of stone at least 32 ft. in diameter, thoroughly cemented and bonded 

 together; d, fore shore securing the angle of the sea slope, to be 

 mainlv formed of cubical blocks of rough stone, weighing at least 

 ten tons each ; e, the general foundation raised to the low water plane 

 by rubble stone thrown promiscuously into the water from the decks 

 of vessels;*/', cemented foundation prepared for the semi-cylinder, 

 and having a top slope seaward of about six feet base to one foot rise. 



The advantage of a semi-cylindrical solid (which must average 

 15 ft. deep vertically,) is that if in the construction it is properly 

 bonded on the diameter, with thorough courses of dressed stone, 

 whilst the blocks which form the curved surface are cut like arch 

 stone, no one sfone could be by any means extracted from its place if 

 properly doweled laterally, and the whole woidd resist motion by its 

 gravity as one solid mass ; the interior backing or hearting of the 

 semi-cylinder would be composed of rubble stone well set in cement 

 mortar, and grouted full in low courses, divided into sections for the 

 purpose. 



It is a general idea that the forces acting against a breakwater are 

 augmented by a great rise and fall of the tide, but from the above 

 reasoning it would appear that such a rise and fall as will allow a 

 wave of the maximum height of 20 ft. to exert its greatest energy ; 

 or a difference of 10 ft. between the top water of storm tides and the 

 low water plane, (as exists at the Delaware breakwater,! loill enable 

 such loaves to act with as much power upon sea works as any other 

 variation of tidal surface, and hence a greater rise and fall than 10 ft. 

 will not increase, whilst a less one would certainly diminish, the effects 

 of the assailing waves. 



The section of the Delaware breakwater, as planned by the Com- 

 missioners appointed by the President of the United States, in 1S29, 

 under the act of Congress cf May 24th, 1824, was trapezoidal in its 

 general outline, the sea slope having a base of lOSif ft. to a height 

 of 39 ft., and being profiled after the curvilinear figure to which the 

 waves of storms had reduced that of the breakwater at Cherbourg; 

 the top was fixed at 22 ft.,t and the internal or harbour slope at one 

 to one, or 39 ft. base, the entire base being 166^ ft. to a hei|ht of 

 39 ft. ; the base of the section which we have proposed as sufficient 

 for a similar work, is IGO ft., and the transverse area would be nearly 

 the same as that of the breakwater in the Bay of Delaware. 



Philadelphia, Nov. 1st, 1841. 



position, and enable it to defy, as it has for 82 years defied, the utmost force 

 of the Atlantic storms; unless, indeed, it should beassaultei.by waves more 

 than 135 ft. high, which is not within the range of probabdity. 



* Experience at the Delaware breakwater proved Ibil by tbi.s process alone 

 a ri)ugh slune work could be brought up with precision to a plane of two.feet 

 under the high water of neap tides. 



f With the design of increasing it to 30 ft., if subsequently found necessary. 



