Breakwater, an obstruction of any kind raised to oppose the action of the waves, and make safe harbors and roadsteads. The outer mole of the harbor of Civita Vecchia was built by the emperor Trajan for this purpose; and the piers of ancient Piraeus and of Rhodes are of the same class of structures. Josephus says that Herod, in order to form a port between Dora and Joppa, ordered mighty stones to be cast into the sea in 20 fathoms water, to prepare a foundation; the greater number of them being 50 ft. in length, 9 ft. deep, and 10 ft. wide, and some even larger. In the use of such immense blocks of stone, the true principles of constructing a permanent barrier to the waves appear to have been better understood than they were 17 centuries afterward. In modern times, the great breakwaters are those of Cherbourg in France, of Plymouth in England, and of Delaware bay in this country. From the experience acquired by their construction and history, principles before little understood have been established, upon which such works must be built to withstand the enormous forces opposed to their permanency.
These were so little known in the last century, that one of the commissioners appointed by direction of Louis XVI. to report upon the best locality for establishing, opposite the English coast, a port and naval arsenal, recommended the construction of a dike of 2,000 toisesin length, in water 70 ft. deep, in front of the harbor of Cherbourg, by sinking a vast number of ships filled with masonry as a nucleus, and covering these with heavy stones to within 18 ft. of the surface. And when at last four of the ablest naval officers and engineers of France were appointed to execute the work, which was regarded as one of the most stupendous operations, certainly the greatest piece of hydraulic architecture, ever undertaken by man, the plan they adopted was one which proved impracticable after having been prosecuted from the year 1784 to 1789, at enormous expense. This plan was the construction • of huge truncated cones of timber, which, of the reduced size at which they were actually built, measured 36 ft. in height, with a circumference of 472 ft. at base, and 339 ft. at top, the angle of the slope being 60°. This was strengthened by an interior concentric cone, 5 ft. 10 in. within the outer one. The frame of each was made of 80 large upright timbers 24 ft. long and 1 ft square.
On these were erected 80 more of 14 ft. in length, making 320 of these uprights for the two exterior and two interior portions. The structure was then planked, hooped, and firmly bolted together. The first cone was built and floated at Havre, then taken to pieces, transported to Cherbourg, and floated off and sunk on June 6, 1784; the second on July 7, in the presence of 10,000 spectators; but before the cavity of this one could be filled with stones, its upper part was demolished in August by a storm of five days' continuance, and the stones it contained were spread over the bottom, interfering with the placing of the next cone. The original plan was to set 90 of these cones, of 150 ft. diameter at base, 60 at top, and 65 ft. in height, in succession, and fill them with loose stones or masonry, and the spaces between them with a network of iron chains, to break the force of the waves. The number was afterward reduced to 64. After the second cone went to pieces, the government directed that the remainder should be set 192 ft. apart. This distance, by a new order, was increased to 1,280 ft., the spaces to be filled in with loose stones.
At last, when 18 cones had been sunk at enormous expense, and with serious damage to many of them, the plan was abandoned, and the tops of those left standing were cut off down to low-water mark in 1789, and the system of construction by sinking rocks was recognized as the only process sure to succeed. The filling in of stone was continued till, at the end of the year 1790, the quantity sunk was estimated at 5,300,-000 tons; and the total expenditure, by the estimate presented to the legislative assembly in 1792, was about 31,000,000 francs, or $5,800,-000. The commission appointed in 1792 by the departments of war, marine, and the interior, reported, after careful examination of the dike and of the partial protection it already afforded at different stages of the tide, that its stability could not be depended upon except by the use of larger blocks of stone as a facing than had before been employed, these stones to be at least of 15 to 20 ft. cube; and they recommended that the dike be raised 31 ft. above the level of the lowest tide, which would make it about 9 ft. above that of the highest tides.
But the revolution succeeding, further work was interrupted, In 1802, by advice of a new commission appointed two years previously by a new government, it was determined to raise the central portion of the breakwater to the height before recommended, for 195 metres (640 ft.) in length, and to give it a breadth at top of 19.5 metres,'in order to construct upon it a battery of 20 pieces of the heaviest artillery; and it was proposed to finally complete the two extremities in the same manner. At that time the old work, which had originally been raised to low-watermark, was reduced by the action of the sea to 15 or 18 ft. below it, and the profile imparted to it was regarded as that-of greatest stability with least expenditure of material. The interior slope was one of equal height and base, 12'5 metres. The slope exposed to the sea had at bottom a height of 6.3 metres to a base of 9, succeeded by one of 6 2 to a base of 47.5; its original form was a uniform slope of 1 in height to 3 of base. The sea washing over the top tended to move the stones from the outside to the inside; and it was essential to oppose this action by raising the top above the surface of the water.
In 1803 the central portion was completed to low-water mark, and a superstructure or parapet, of blocks of 60 to 80 cubic feet each, was raised along the south or inner side to the height of the highest tides, along which the smaller stones used in the construction, pressed upward by the great waves in the winter storms, collected and formed a solid and compact surface, at a new slope, of which the base was about quadruple the vertical height. It was observed that the lateral movement of the small stones by the storms, driving obliquely along the outer face of the dike, caused them to collect at each extremity in a conical mound of the precise configuration traced for the proposed terminal batteries; but to prevent their extending into and obstructing the passes, it was found indispensable to face the whole exterior with blocks large enough to resist these oblique impulsions. In May, 1805, the battery on the central portion was armed with 20 pieces of heavy ordnance. In February and May, 1807, occurred two great storms, the effects of which upon this portion, as also of the unprecedent-edly severe storm of Feb. 12, 1808, are described in the " Memoir upon the Dike of Cherbourg, compared with the Jetty or Breakwater at Plymouth,'1 by the baron Oachin, inspector general of roads and bridges.
In the last named storm the battery was submerged, the parapet was upset, and the barracks and garrison, with 60 men, were swept away. The large blocks of stone with which the dike was faced were by this storm arranged in new positions, and so closely stowed that they appeared as if placed by the hand of man in positions of the most perfect stability. As thus arranged, the outer side presents four slopes. At the upper part, reached only by the tops of the waves, the height is to the base as 100 to 185. Beneath this is the space between the high and low-water marks, which is exposed at all times of tide to the most violent action of the sea; its slope is the most inclined, the height being to the base as 100 to 540. Below the lowest spring tides is a space but little exposed to the action of the waves; the height of this slope to its base is as 100 to 302. The lowest part,which is always submerged, has a height of 100 to a base of 125. The slope on the inner side is of 45°. From the experience of these two breakwaters, incomparably the greatest of their sort which have ever been undertaken, M. Cachin concludes with the observation, that if man be strong enough to heap together rocks in the midst of the ocean, the action of the sea alone can dispose them in the manner most likely to insure their proper stability.
This, it may be added, will necessarily vary in form with the specific gravity and size of the stones used. In 1830 it was decided to raise the dike by building up a wall of rubble masonry faced with granite to the height of 6 ft. above highest water. This is protected by a foreshore of great blocks of stone on the outer side, which extend in a slope of 120 ft. to the depth of 21 ft. below low-water mark. This nearly vertical wall (the slope of its sides being 1/6 to 1) is 36 ft. 3 in. wide at base, and 29 ft. 3 in. wide at top. A parapet is raised to the height of 6 ft. upon its outer edge, which is 8 ft. 3 in. thick; at top 8 ft. 6 in. wide. The altitude of the breakwater is given by the United States commission of engineers and naval officers, who examined it in 1829, at 72 30/100 ft., the base of its sea slope being 228 50/100 ft.; and they state that similar proportions were adopted at the Plymouth breakwater, the altitude of which is 57 ft., and base 180 ft. The inner slope of this, however, was built at an angle of 32°, although that of Cherbourg had stood perfectly well at 45°. The adoption of the general plan of this work by the English and American engineers sufficiently proves the correctness of its principles, though by some English authorities the work is alluded to as a failure. - The breakwater at Plymouth, England, to protect the inner harbor from the heavy sea that is driven in by southerly storms, was commenced in 1812. Its dimensions are only about one fourth of those of the breakwater at Cherbourg, its total length being 1,700 yards, made up of a central portion of 1,000 yards, and a wing bending in from each end, at an angle of 120°, of 350 yards.
Its profile is 993 sq. ft. It was designed to have a base of 210 ft., breadth at top 30 ft., and height in the middle 40 ft. Its actual height exceeds this, but it is only about 3 ft. above the highest tides. It is built of large blocks of limestone, some exceeding five tons in weight, brought in vessels from the quarries at Catwater, about 2 1/2 m. up the harbor. The convenience of position of these quarries for loading the vessels, the facilities of quarrying the stone, and the judicious arrangements introduced, made the work of comparatively light expense. After some experience was had, the stone was quarried by contract at 2s. 5d (58 cents) per ton, and transported for 34 cents; and the total cost of the stone laid, including land purchases, salaries, buildings, etc, was estimated in 1816 at about 8s. l 1/2d. per ton. In 1841 it was calcufated that 3,369,261 tons of stone had been laid, at a cost of nearly £1,500,000; and the work was considered as being essentially completed. The 15 vessels kept employed in transporting the stone were furnished with two railways laid along in the hold, upon which were run the loaded cars from the quarries, entering through two stern-ports. These could be tightly closed when the vessel was loaded.
On each side were arranged eight trucks of the extreme capacity of five tons each. In discharging, these were drawn out by a windlass on deck, and upset as they passed out of the ports, each one being drawn up on the deck and run forward to make room for those behind. At the quarries they left the deck, and the track on which they descended over the stern being raised up, the loaded cars were run under it, into the hold. The usual cargo of 45 to 65 tons could thus be discharged in less than an hour. On Jan. 19, 1817, the work was tried by one of the most severe storms ever known. The breakwater, though in an unfinished condition, caused perfect protection to the inner harbor, where without it the damage would have been immense. Previous gales had had no effect upon it; but this caused the upper stratum of the finished part, 200 yards in length and 30 in breadth, to be stripped, and the huge stones of two to five tons weight to be carried over from the outside, and deposited upon the northern side of the breakwater. The quantity thus removed was estimated at 8,000 tons.
Since that time the outer slope has been "cased with regular courses of masonry, do welled, joggled, dovetailed, and cramped together; the diving-bell being brought into requisition for placing the lower courses, which were of granite, and were laid horizontally on their natural beds, and dovetailed, lewised, and bolted together." This work was reported by Mr. Stuart, the superintendent of the breakwater, to have been done on a slope of 5 to 1, as the sea had left it. The foot of the outer slope has also been extended further out with loose stones, to give protection to the courses of masonry. The additional cost since 1841 for this and similar repairs has been about £200,000, making a total expense of £1,700,000. - The new breakwater at Dover, in process of slow construction, is formed by an outer and inner wall of ashlar masonry, with a course of granite on the face, and blocks of concrete made with Portland cement and shingle in the core of the work to the level of high water, above which it is filled with liquid concrete. The masonry commences from the chalk bottom of the bay, the blocks being placed by means of diving apparatus. Both the inner and outer walls deviate from a perpendicular about three inches to the foot, in steps.
A parapet 15 ft. above the level of high water surmounts the work on the sea side. All the operations are carried on from timber staging. The water at Dover is very deep, 42 ft. at low tide, and the construction of the breakwater is upon the principle that the motion imparted by waves to water much below the surface is vertical, and that a vertical wall is best calculated to resist their action. There is a difference of opinion on the subject, and more time than has elapsed since such structures have been commenced will be required to allow a definite conclusion to be reached. - The construction of an important breakwater was commenced at Portland, on the southern coast of England, in 1849. On July 25 Prince Albert laid the foundation stone, and ten years later the works were so far advanced as to afford safe anchorage to vessels. The breakwater commences with a pier which starts from the island of Portland, near the point where it is connected with the mainland. The pier runs due E. for about 1,900 ft., at which point there is an opening 400 ft. wide, with a minimum depth of 45 ft., to admit vessels.
On the other side of this opening the outer mole of the breakwater commences, and, after proceeding a short distance in the same direction as the pier, turns, and extends in a N. E. direction for a distance of 6,000 ft. The pier, or inner mole, consists of a rubble mound composed of stones of all sizes, from masses of eight tons weight down to small chippings. After the mound had been well consolidated a trench was excavated in it down to the level of low spring tides and a wall of masonry erected. The face course of this wall is formed of large squared blocks, the body consisting of heavy rubble work set in water cement. The face courses, up to 6 ft. above high-water level, are of hewn granite, and the rest of the face is of the best stone from the neighboring quarries. The outer mole, or breakwater proper, is a rubble bank with a width at the base of 300 ft., at low-water level of from 90 to 100 ft., and at the top of 60 ft. The slopes on the sea face, from the bottom to a height of 12 ft., are 6 to 1; on the harbor face they are 1 1/4 to 1. The sheltered area is about 2,100 acres up to low water line. Connected with the works are two forts, an inner one on the end of the pier, and another at the further end of the outer mole.
The inner fort is 100 ft. in diameter, mounts 8 guns, and stands in 9 1/2 fathoms of water. The outer fort has a rubble base 45 ft. high, and containing 140,000 tons of stone. Its diameter is 200 ft., and it stands in 10 fathoms of water. The completion of the work was celebrated with great pomp, Aug. 10, 1872, when the prince of Wales laid the top stone and declared the work finished. The execution of this magnificent work is interesting as" affording an example of the economical use of the force of gravity alone as a motive power in moving loads to and from a given point. The quarries from which the stone is obtained are about 380 ft. above the sea level. Three inclined planes, each having two tracks, extended down to the breakwater. A wire rope passing over a drum connected two trains of trucks, one train descending with a load while the other ascended empty. A temporary staging was constructed along the line of the breakwater on which the trucks were propelled, and from which their contents were dumped into the water. The quantity of stone deposited per day varied from 2,000 to 3,000 tons, the total quantity used in the work being about 6,000,000 tons. It was quarried and landed mostly by convict labor, from 600 to 1,000 prisoners having been employed.
About 100 men were engaged in the actual work of construction. The cost of this extensive work has been only about £1,013,000, for which the British nation possesses one of the most magnificent harbors in the world. - In 1828 a commission consisting of Commodore Rogers of the navy, Brig. Gen. Barnard of the engineer corps, and William Strickland, architect and engineer, was appointed by the United States congress to ascertain the most eligible site and to prepare plans and estimates for a harbor near the mouth of Delaware bay. In their report, made Feb. 2, 1829, they selected Cape Henlo-pen as the site of a breakwater. They said: "The objects to be gained by an artificial harbor in this roadstead are to shelter vessels from the action of the waves caused by winds blowing from east to northwest round by the north, and also to protect them from injuries arising from floating ice descending from the northwest." They proposed two works, a breakwater proper, to secure the first object, and an ice-breaker, as an auxiliary to the breakwater, but chiefly to protect vessels against the ice. The breakwater was designed with a length of 1,200 yards, and on a course N. N. W. drawn from a pitch of the cape.
The ice-breaker was designed with a length of 500 yards, on a course W. by S. 1/2 S., and so placed that the line of the breakwater would meet its S.W. extremity if extended, leaving an opening between the breakwater and cape of 500 yards, and between the breakwater and ice-breaker an opening of 350 yards, with a depth of water of 24 ft. The area protected against all the most dangerous winds, with a depth of 3 to 6 fathoms, is estimated at 360 acres.
Fig. 1. - Section of Cherbourg Breakwater.
Fig. 2. - Section of Plymouth Breakwater.
Fig. 3. - Section of Dover Breakwater.
Fig. 4. - Section of Delaware Breakwater.
The work was commenced in 1829, under direction of Mr. Strickland, and in 1834 it was so far advanced that vessels found protection behind it. Blocks of rubble from the nearest quarries were thrown in to form their own slopes for a foundation. The outer covering to within 6 ft. of low-water mark was of blocks from 2 to 3 tons weight; from this to low-water mark they were of 3 tons; thence to high-water mark, 3 to 4 tons; and above this, 4 to 5 tons, to a height of 4 ft. 3 in. above highest water. The ordinary rise of tide is nearly 5 ft., equinoctial tides 7 ft., and extreme tides 10 ft. As the breakwater was built, its exterior slope for the first 16 ft. from bottom was at an angle of 45°, thence to summit 28°, or 3 to 1. The inner slope was 45°. The surfaces of both slopes to the level of low water were paved with rough blocks set at right angles to the slope, and well wedged together, thus presenting as little surface as practicable to the action of the waves. The stone used in this work was obtained from a variety of sources, some trap rock from the Palisades on the Hudson river, greenstone from the northern part of Delaware, and gneiss from different quarries in Delaware. These rocks, though averaging a weight of 175 lbs. to the cubic foot, and employed of the dimensions named, were insufficient to withstand the action of the sea in the course of the construction of the moles.
During the winter season, those upon the surface of the work were more or less displaced, and a large piece of 7 tons weight was moved in one storm 18 ft. to the inner slope of the ice-breaker, down which it was lost. At the same time about 200 tons of other heavy stone, that had been thoroughly wedged and compacted together, was torn up and swept over to the inner side. In 1839, according to the report of Major Bache in 1843, the breakwater was in course of construction for 862 yards, and the icebreaker for 467 yards. Gen. (then Major) J. G. Barnard, in a report of the works made by him in 1853, says: "The last stone was deposited ' on the work under former appropriations in the year 1839; and since that date a work costing already nearly two millions of dollars, a work in every sense national, has remained in its half finished condition." As long ago as 1843 various plans were proposed for completing the work and rendering the harbor more commodious. Major Bache, in the report before referred to, says: " In order to remedy the defects of the harbor which are caused by the rolling of the sea entering between the works, three modes have been suggested: 1, to cover the gap by extending the ice-breaker; 2, to close the gap by extending the breakwater proper; and 3, to cover the gap by a detached work." He estimates the cost of each of the plans to be as follows: " 1, for closing the gap by extending the breakwater, $551,635; 2, for covering the gap by extending the ice-breaker, $815,341; 3, for covering the gap by a detached work, $959,664. To these estimates must be added severally the sum of $108,921 for raising the existing works to their proper level, and for filling holes in the bottom at their ends." The report of Lieut. Col. J. D. Kurtz, of the engineers, for the year ending June 30, 1870, contains the following: "From July 1, 1869, to the completion of these works in conformity with the present design, Nov. 4, 1869, the work was carried on in furnishing stone by contract as heretofore, the United States performing the labor of placing the large stone in position by days' work.
During the fiscal year 10,698 tons of large stone for the superstructure of these works were received and placed in position, completing 120 running feet of the breakwater and 175 feet of the icebreaker. In addition, 122 tons of old superstructure have been reset and 2,198 perches of small stone deposited in holes at the extremities of the works and on the sea side of the breakwater opposite the lighthouse. The length of the breakwater on the top is 2,558 ft.; about low-water line the length is 2,603 ft.; length at base, 2,763 ft. The length of the ice-breaker as completed on the top is 1,353 ft.; about low-water line, 1,389 ft.; length of base, 1,601 ft." Col. Kurtz in his report for 1871 says: "The completion of this harbor according to the original project was reported in last year's annual report. This project was devised more than 40 years ago. It is the case here, as in many other government works, that the growth of the country has far exceeded the provision made for its probable wants. Last year's report shows that 15,000 vessels were recorded as visiting this harbor. Adding those entering and leaving without being noted, 20,000 or 25,000 may be taken as the number that used the harbor during the year.
Its present capacity is determined by the space that is sheltered by the breakwater proper. This is a straight line nearly half a mile long, and may be taken as the diameter of a half circle behind it, the area of which will represent approximately the sheltered harbor. Northeastwardly of the breakwater is the ice-breaker structure, a quarter of a mile in length, but separated by a gap of nearly equal extent, through which the sea rolls in nortlnvestwardly weather without hindrance. If this were excluded, the sheltered area would be increased three or four fold." Col. Kurtz then presents the plans of Major Bache enumerated above, but is obliged to greatly increase the estimates of cost, viz.: for the first plan, $1,314,000; for the second, $1,944,950; and for the third, $2,278,000. The existing works have cost $2,127,403. A special board of officers, composed of Gens. Woodruff, Wright, and Newton, and Cols. Kurtz and Craighill, appointed for the purpose of considering the question of further improvements to the breakwater, met in November, 1871, and again in the summer of , 1872. At the latter meeting a report was agreed upon favoring the plan of connecting the two moles by extending the breakwater proper until it meets the ice breaker. - Breakwaters are now in course of construction on our northern lakes, which are made of a crib-work of strong timbers filled with stone, and are found to be very effective, and to possess the advantage of economy and facility of construction.
It has been found by experience that the cribs will keep in position better if the bottom is formed of latticework, sufficiently open to allow the stones to sift through when the crib is stirred by the waves. The cribs are usually made from 30 to 40 ft. in width, from 60 to 80 ft. in length, and of a depth suited to the depth of water. They are successively sunk and placed end to end and filled with stone until the work has attained the desired length. There being no tide in these lakes, the top of the crib need not be more than 8 or 10 ft. above the mean water level. Such breakwaters, more or less approaching completion, are in process of construction at Buffalo on Lake Erie, at Oswego on Lake Ontario, at Plattsburgh and Burlington on Lake Cham-plain, and at other places. Aside from not possessing sufficient strength, these structures would not be practicable on the seacoast on account of the destruction to which the timber would be exposed from attacks of marine worms; but in our fresh-water lakes this objection does not exist. Timber which is placed below the action of the air in fresh water has been found to resist decay for an indefinite time, for centuries at least.
If from any cause, however, the framework of the cribs should become weakened, new cribs can be placed on the inner or outer line of the first row, or on both sides, and thus a permanent stone foundation of rubble for a stone breakwater of the ordinary description may be gradually constructed. - The experience acquired by all these breakwaters, and by the action of the waves upon coasts exposed to their greatest violence, establishes the principle that blocks of stone of large dimensions only can be depended upon to retain their places. Mr. James Walker, president of the British institution of civil engineers, advanced the opinion in 1841 that a partial vacuum is created by the action of the waves, and the atmospheric pressure being taken off for an instant, the mass of stone is the more readily influenced by the forces which at the same time solicit it. ("Civil Engineer and Architect's Journal," September, 1841.) If the whole atmospheric pressure were taken off the surface, it would be equivalent to the removal of a weight represented by a column of rock 11£ ft. deep, weighing 175 lbs. to the cubic foot.
Under such circumstances, and exposed to the action of a wave 20 ft. high, which is capable of moving masses of rock 7 1/2 ft. deep, stability would be insured only by the addition of this amount to the 11 1/2 ft. But as it is not probable that a large proportion of the atmospheric pressure is ever thus removed, and as 22 ft. is regarded as the maximum height of waves, a depth of solid stone of* 15 ft., used as a coping, would probably resist all action of the waves. - The construction and history of the principal breakwaters are fully treated in the great work of Sir John Rennie, president of the institution of civil engineers, upon British and foreign harbors (2 vols, fol., 1854).