BREAKWATER. When a harbour (q.v.) is proposed to be established on an exposed coast, whether for naval or commercial purposes, to provide a protected approach to a port or river, or to serve as a refuge for vessels from storms, the necessary shelter, so far as it is not naturally furnished by a bay or projecting headlands, has to be secured by the construction of one or more “breakwaters.” These breakwaters, having to prevent the waves that beat upon the coast from reaching the site which they are designed to protect, must be made sufficiently strong to withstand the shocks of the waves during the worst storms to which they are exposed. It is therefore essential, before constructing a breakwater, to investigate most carefully the force, periods and duration of the winds from the quarters to which the work will be exposed, the distance of any sheltering land from the site in the most stormy direction, the slope of the beach and the depth of the sea in the neighbourhood of the shore, and the protection, if any, afforded by outlying shoals or sandbanks. In a tidal sea, the height required for a breakwater is affected by the amount of tidal range; and the extent of breakwater exposed to breaking waves depends upon the difference in level between low and high water. The existence, also, of any drift of sand or shingle along the shore must be ascertained, and its extent; for the projection of a solid breakwater out from the shore is certain to affect this littoral drift, which, if large in amount, may necessitate important modifications in the design for the harbour.
Observations of the force and prevalence of the winds from the different quarters at the various periods of the year, and the instruments by which they are recorded, belong to the science of meteorology; but such records are very valuable to the maritime engineer in indicating from which Winds. directions, open to the sea, the worst storms, and, consequently, the greatest waves, may be expected, and against which the most efficient shelter has to be provided. Moreover, it is necessary, for constructing or repairing a breakwater, to know the period of the year when the calmest weather may be safely anticipated, and also the stormy season during which no work should be attempted, and in preparation for which unfinished works have to be guarded by protective measures. In the parts of the world subject to periodical winds, such as the monsoons, the direction and force of the winds vary with remarkable regularity according to the seasons; and even such uncertain occurrences as hurricanes and cyclones generally visit the regions in their track at definite periods of the year, according to the locality. Even in western Europe, where the winds are extremely variable, violent gales are much more liable to beat upon the western and northern coasts in the winter months than at any other period of the year; whilst the calmest weather may be expected between May and August.
The size of waves depends upon the force of the wind, and the distance along which it blows continuously, in approximately the same direction, over a large expanse of ocean. The greatest waves are, accordingly, encountered where the maximum distance in a certain direction from the nearest land, or, as it is Waves. termed, the “fetch,” coincides with the line travelled by the strongest gales. The dimensions, indeed, of waves in the worst storms depend primarily on the extent of the sea in which they are raised; though in certain seas they are occasionally greatly increased by the exceptional velocities attained by hurricanes and typhoons, which, however, are fortunately restricted to fairly well defined and limited regions. Waves have been found to attain a maximum height of about 10 ft. in the Lake of Geneva, 17 ft. in the Mediterranean Sea, 23 ft. in the Bay of Biscay, and 40 ft. in the Atlantic Ocean; whilst waves of 50 to 60 ft. in height have been observed in the Pacific Ocean off the Cape of Good Hope, where the expanse of sea reaches a maximum, and the exposure to gales is complete. The length of large waves bears no definite relation to their height, and is apparently due, in the long waves often observed in exposed situations, to the combination of several shorter waves in their onward course, which is naturally dependent on the extent of the exposure. Thus waves about 560 ft. in length have been met with during severe gales in the Atlantic Ocean; whilst waves from 600 to 1000 ft. long are regarded as of common occurrence in the Pacific Ocean during storms.
The rate of transmission of the undulation also varies with the exposure; for the ordinary velocity of the apparent travel of waves in storms has been found to amount to about 22 m. an hour in the Atlantic Ocean, and to attain about 27 m. an hour off Cape Horn. The large waves, however, observed in mid-ocean do not reach the coast, because their progress is checked, and their height and length reduced, by encountering the shelving sea-bottom, which diminishes the depth of water on approaching the shore; and the actual waves which have to be arrested by breakwaters depend on the exposure of the site, the existence of continuous deep water close up to the shore, and the depth in which the breakwater is situated. On the other hand, the height, and, consequently, the destructive force of waves, is increased on running up a funnel-shaped bay, by the increasing concentration of the waves in the narrowing width, just as the tidal range of a moderate tidal current is much augmented by its passage up the Bay of Fundy, or up the Bristol Channel into the Severn estuary, or by filling the shallow enclosed bay of St Malo. This effect is intensified when the bay faces the direction of the strongest winds. Thus at Wick a mass of masonry weighing 1350 tons, placed at the head of the breakwater projecting half-way across the bay and facing the entrance, was moved by the waves during a violent storm; and a portion of Peterhead breakwater, weighing 3300 tons, was shifted 2 in. in 1898, indicating a wave-stroke of 2 tons per sq. ft. Southwesterly gales, blowing up the Gulf of Genoa, cause large waves to roll into the bay, reaching a height of about 21 ft. in the worst storms.
Where outlying sandbanks stretch in front of a coast, as for instance the Stroombank in front of Ostend and the adjacent shore, and the sandbanks opposite Yarmouth sheltering Yarmouth Roads, large waves cannot approach the land, for they break on the sandbanks outside. Waves, indeed, always break when, on running up a shoaling beach, they reach a depth approximately equal to their height; and the largest waves which can reach a shore protected by intervening sandbanks, are those which are low enough to pass over the banks without breaking.
The force of the wind, as transmitted by degrees to the sea, is manifested as a series of progressing undulations without any material displacement of the body of water, each undulation transmitting its accumulated force to the next in the direction the wind is blowing, till at last, on encountering an obstacle to its onward course, each wave, no longer finding any water to which to communicate its energy, deals a blow against the obstacle proportionate to its size and rate of transmission; or on reaching shoal water near the shore, the undulation is finally transformed into a breaking wave rushing up the sloping beach. till, on its energy being spent, it recoils back to the sea down the beach. A breaking wave concentrates its transmitted force on a portion of the water forming the undulation, which, consequently, strikes a more powerful blow over a limited area against any structure than the more distributed shock of a simple undulation beating against a vertical wall. Moreover, the recoil of broken waves down a sloping beach or rubble mound produces a greater scour than the simple reflection of an undulation from a vertical wall, especially where the depth is sufficient to provide a cushion of water below the undulation, protecting the toe of the wall from the wash of recoil.
Types of Breakwaters.—There are three distinct types of breakwaters:—(1) A simple rubble or concrete-block mound; (2) a mound for the bottom portion, surmounted on the top by a solid superstructure of masonry or concrete; and (3) an upright-wall breakwater, built up solid from the sea-bottom to the top. The second type forms a sort of combination of the first and third types; and each type presents several varieties. In a few harbours, two different types have been adopted for different situations at the same place; but generally the choice of type is determined by the materials available at the site for the construction of the breakwater, the nature of the sea-bottom and the depth into which the breakwater has to be carried.
1. Rubble and Concrete-Block Mound Breakwaters.—A rubble mound consists merely of a mass of rubble stone, just as it is obtained from a neighbouring quarry, tipped into the sea along a predetermined line, till the mound emerges out of water. The rubble stone is deposited, either from barges, as Rubble mound.adopted for the construction of the detached breakwater sheltering Plymouth Bay, or from wagons, having hinged opening flaps at the bottom for dropping their load, run out from the shore along staging erected in the proposed line, according to the method employed for the outer breakwater enclosing Portland Harbour, and the north-east breakwater at Colombo Harbour. The mound thus deposited is gradually consolidated under the action of the sea; and a tolerably stable form is by degrees attained by continued deposits of stone. This system of construction is very wasteful of materials, and can only be resorted to where extensive quarries close at hand are able to furnish readily and cheaply very large quantities of stone, especially where, as at Portland and Table Bay, convict labour has been advantageously utilized in quarrying. When the site is very exposed, the large waves in storms, dashing over a rubble-mound breakwater, carry the stones on the top, if unprotected, over on to the harbour slope, and in recoiling down the outer slope, draw down the stones on the face, so that the top and sea slope of the mound need replenishing with a fresh deposit of stones after severe storms.
Under the action of the breaking and recoiling waves, the mound assumes a very flat slope on the sea side, from a few feet above high-water down to several feet below low-wafer level (fig. 1). The flatness of the sea slope depends on the exposure of the site, and the limited size of the stones covering the outer portion of the mound; and its extent increases with the range of tide, as a large tidal rise exposes a greater length of slope to the action of the waves. This flattening of the sea slope greatly increases the amount of stone required for a rubble-mound breakwater, in proportion to the exposure and the range of tide; and the amount is also affected, but in a proportionately minor degree, by the depth in which the breakwater is situated. In order to avoid the injuries to which an ordinary rubble mound is subjected by waves, certain methods have been devised for protecting the top and sea slope of the mound. For instance, the upper portion of Plymouth breakwater has been covered over by granite paving set in cement, to diminish the displacement of the stones by the waves. Frequently, on the continent of Europe, rubble mounds have been formed of materials so sorted that the smallest stones are placed in the centre of the lower part of the mound, and covered over along the slopes and top by layers of larger stones, increasing in size towards the outer part of the mound, so that the largest stones obtainable are deposited on the outside, and especially on the top and sea slope of the mound. This is, no doubt, theoretically the correct method of construction of rubble mounds exposed to the sea; but it involves a considerable amount of trouble and expense.
Practically the chief point of importance is to cover the outer slope and the top of the mound with the largest stones that can be procured, and where large stones are not readily obtainable concrete blocks furnish a very convenient substitute. These blocks are generally deposited as the outer covering Concrete blocks with rubble mound. on the top and sea slope of a rubble mound, as for example at the mound breakwaters in deep water sheltering Algiers harbour, and at the French parts of Cette and Bona on the Mediterranean; whilst they furnish the protection of the top and upper part of the sea slope of the rubble-mound extension of Marseilles breakwater down to 20 ft. below sea-level. At Alexandria, concrete blocks compose the outer half of the mound, sheltering the inner half consisting of small rubble (fig. 2); at Biarritz the mound breakwater is formed mainly of concrete blocks, with rubble stone filling the interstices and on the top; whereas at the outer end of the western breakwater at Port Said, protecting the entrance to the Suez Canal, a bottom layer of rubble is surmounted by concrete blocks. These blocks are generally deposited at random; but at Cette (fig. 3), and at the breakwater in deep water at Civita Vecchia, the concrete blocks covering the rubble have been laid in stepped, horizontal courses. This arrangement necessitates more care and better appliances in construction; but, in compensation, the blocks so placed are less exposed to disturbance and injury by the waves.
Concrete blocks possess the great advantages for breakwaters that they can be made wherever sand and shingle can be procured, and of a size only limited by the appliances which are available for handling them. In fact, in places where stone of any kind is difficult to procure at a reasonable cost, as for instance at Port Said, concrete blocks are indispensable for the construction of breakwaters. Large concrete blocks, moreover, by enabling a comparatively steep slope to be formed with them on the sea side of a mound breakwater, reduce considerably the amount of materials required, especially at exposed sites, and also for breakwaters extended into deep water, such as those of Algiers and Marseilles.
Occasionally, in the absence of suitable rubble stone, a mound breakwater has been formed entirely with concrete blocks; and of this the main portion of the western breakwater at Port Said furnishes a notable example (fig. 4). Sometimes, in exposed situations, the mounds of the composite type Concrete block mound. of breakwaters have been constructed exclusively with concrete blocks, such, for instance, as in the curved breakwater protecting the outer harbour at Leghorn, and in the central breakwater in deep water sheltering the harbour of St Jean de Luz, and directly facing the Bay of Biscay. These large concrete blocks are deposited by cranes from staging, tipped into the sea from a sloping platform on barges, or floated out between pontoons, or slung out from floating derricks. This last method proved so expeditious for the upper blocks at Alexandria, that, in conjunction with the tipping of the lower blocks from the inclined planes on the decks of barges and the deposit of the rubble from hopper barges, provided also with side flaps for the higher portions, the detached breakwater, nearly 2 m. long, sheltering a very spacious harbour, was constructed in two years (1870–1872). Sometimes, when a mound breakwater has been raised out of water, advantage is taken of a calm period of the year and a low tide to form large blocks of concrete within timber framing on the top of the mound, so as to provide a very efficient protection.
The large masses composing mound breakwaters give them great stability against the attacks of the sea; and, moreover, the wide base of the mounds enables them to be deposited on a sandy or silty sea-bottom, without any fear of settlement or undermining. A mound breakwater, however, has the disadvantages of requiring a large amount of material, and of occupying a wide space on the bed of the sea, more especially where the mound consists of rubble stone and is in deep water, so that the system, though simple, is costly, and is unsuited for harbours where the available space to be sheltered is limited. Nevertheless, a mound breakwater can be rapidly constructed by the employment of a large number of barges; and by the adoption of large concrete blocks, the quantity of materials and the space occupied by the mound can be considerably reduced. This form of breakwater, with its long outer slope exposed to breaking waves, particularly where the tidal range is considerable, is, indeed, more subject to frequent small injuries than the other types, but they are readily repaired; and a mound is not generally liable to the serious breaches which occasionally are formed in solid superstructures and upright walls in exceptional storms.
2. Breakwaters formed of a Mound surmounted by a Superstructure.—The second type of breakwater consists of a mound, composed of rubble or concrete blocks, or generally a combination of the two, carried up from the sea-bottom, on the top of which some form of solid superstructure is erected. This superstructure reduces considerably the amount of materials required (which, on account of the slopes of the mound, increases rapidly with the height) in proportion to the depth at which the superstructure is founded; and the solid capping on the mound serves also to protect the top of the mound from the action of the waves. In the case, however, of a mound breakwater, portions of the highest waves generally pass over the top of the mound, and also to some extent expend their force in passing through the interstices between the blocks; whereas a superstructure presents a solid face to the impact of the waves. A superstructure, accordingly, must be very strongly built in proportion to the exposure, and also to the size of the waves liable to reach it, which depends upon the height and flatness of the slope of the mound just in front of it on the sea side. Special care, moreover, has to be taken to prevent the superstructure from being undermined; for the waves in storms, dashing up against this nearly vertical, solid obstacle, tend in their recoil down the face to scour out the materials of the mound at the outer toe of the superstructure, and thereby undermine it, especially where the superstructure is founded on the mound near low-water level, and there is, therefore, no adequate cushion of water above the mound to diminish the effect of the recoil on the foundation.
The mound constituting the lower portion of the composite type of breakwater has been formed in the same varied way as simple mound breakwaters, namely, of rubble, sorted rubble, rubble protected by concrete blocks, and wholly of concrete blocks. The only differences introduced in the mound in this case are, that it is not carried up so high, that the top portion covered by the superstructure needs no further protection, and that special protection has to be provided on the slope of the mound adjacent to the outer toe of the superstructure.
The forms of the superstructures exhibit considerable variations, ranging from a few concrete blocks laid in courses on the top of the mound, or a paving furnishing a quay protected by a narrow parapet wall on the sea side, up to a large, solid structure, only differing from an upright-wall Super-structures. breakwater in being founded upon a mound, instead of on the sea-bottom. Notwithstanding, however, this great variety in design, these breakwaters may be divided into two distinct classes, namely, breakwaters having their superstructures founded at or near low-water level, and breakwaters with superstructures founded some depth below low water. The object in the first case is to lay the foundations of the superstructure on the mound at the lowest level consistent with building a solid structure with blocks set in mortar, out of water, in the ordinary manner; and, in the second case, to stop the raising of the mound at such a depth under water as to secure it from displacement by the waves. In fact, the solidity and facility of construction of the superstructure were the primary considerations in the older form of breakwater; whereas the stability of the mound and the avoidance of the undermining of the superstructure have been regarded as the most important provisions in the more modern form.
Well-known examples of breakwaters formed of a rubble mound surmounted by a superstructure founded at or near low water or sea-level, are furnished by Cherbourg and Holyhead breakwaters, the inner breakwater at Portland, and the breakwaters at Marseilles, Genoa, Civita Vecchia, Naples, Super-structures at low-water level. Trieste and other Mediterranean ports. The very exposed breakwater at Alderney was commenced on this principle about the middle of the 19th century; and the outer breakwaters at Leghorn and St Jean de Luz have superstructures founded at low water on concrete-block mounds.
The long, detached breakwater sheltering the series of basins formed by wide projecting jetties along the sea coast at Marseilles (see Dock), is a typical instance of a breakwater where a quay has been formed on the top of a sorted rubble mound, sheltered on the sea side by a high wall, or narrow superstructure, founded at sea-level, and protected on the sea slope of the mound from undermining by large concrete blocks deposited at random (fig. 5). In this case the quay has been rendered accessible for vessels on the harbour side by a quay wall, formed of concrete blocks deposited one above the other, providing a vertical face to a depth of about 223 ft. below sea-level; and a similar arrangement has been adopted at Trieste, and in a less effective manner at Civita Vecchia and Naples. At Marseilles, however, when the breakwater reached great depths, the quay was abandoned on account of the increased exposure, and the extension made of a simple rubble mound, protected on the sea side, from the top down to 20 ft. below sea-level, by large concrete blocks deposited at random.
The superstructures at Holyhead and Portland, being built on the old weak system of a sea wall and a harbour wall, with rubble filling between, are protected on the sea side by raising the rubble against them from low water up to high water of spring tides; whereas the superstructure of Cherbourg breakwater, being built solid and less exposed, is only protected on the sea side by large rubble and some concrete blocks, forming an apron raised slightly above low water. These three breakwaters are provided with a quay sheltered by a raised wall or promenade on the sea side; but as the mound on the harbour side is raised up to, or a little above low water, the quay is only accessible for vessels near high water. This, however, is of comparatively little importance, since these quays, though very useful for access to the end of the breakwater in fairly calm weather, are inaccessible in exposed situations with a rough sea; and quays for the accommodation of vessels are better provided well within the sheltered harbour.
The outer portions of the main breakwaters at Genoa and at Naples (fig. 6), extending into depths of about 75 ft. and 110 ft. respectively, have been provided with superstructures, similar in type, but more solid than the superstructure at Marseilles; and the sorted rubble mounds upon which the superstructures rest are protected on the sea slope by stepped courses of concrete blocks from a depth of 26 ft. below sea-level, covered over at the top by a masonry apron forming a prolongation of the superstructure. The outer extension of the main breakwater at Civita Vecchia furnishes an interesting example of a composite form of breakwater, in which the rubble mound has been protected, and greatly reduced in volume and extent in deep water, by stepped courses of concrete blocks carried up from near the bottom of the mound (fig. 7). The breakwaters in front of Havre, constructed in 1896–1907, for sheltering the altered entrance to the port, were formed of a sorted rubble mound, protected on the sea slope by concrete blocks, and raised a little above low water of spring tides, upon which large blocks of masonry, built on land, were deposited with their upper surfaces about 18 in. above low water of neap tides. As soon as settlement of the mound under the action of the sea appeared to have ceased, these masonry blocks were connected together by filling the spaces between them with masonry; and a solid masonry superstructure was built during low tide on this foundation layer, asshown in fig. 8.
The breakwaters constructed for forming harbours on the sea coast of the United States are almost all rubble-mound breakwaters. The two old detached breakwaters sheltering Delaware Harbour near the south-eastern extremity of Delaware Bay, were formed of simple rubble mounds raised about 13 ft. above low water; but in closing the gap between them towards the close of the 19th century, the rubble mound was stopped at low water, and a sort of superstructure, consisting of stepped courses of large rectangular blocks of stone on the sea and harbour sides, with tightly packed rubble between them and capped across the top for a width of 20 ft. with a course of large blocks, was raised to 14 ft. above low water, resembling, on a small scale, the upper part of the Civita Vecchia mound (fig. 7). A similar construction was adopted for the new breakwater formed in 1897–1901 for providing a harbour of refuge at the mouth of Delaware Bay; but in this instance the mound was made considerably wider at the top, and had to be protected along the toe of the superstructure on the sea side by large stones. The same form of superstructure, also, on a narrower base, was resorted to for a breakwater in deeper water at San Pedro in California with satisfactory results. When, however, a breakwater of the Delaware type was in progress for forming a harbour of refuge in Sandy Bay, Massachusetts, in front of Rockport to the north of Boston, the upper 13 ft. of the 600 ft. of completed superstructure were carried away during a severe storm in 1898 leaving only a portion about 5 ft. in height above low water, the average rise of tide there being 83 ft. The design was, accordingly, modified in 1902, by commencing the stepped courses of large stones at 12 ft. below mean low water on each slope, instead of at low water raising this kind of superstructure to 22 ft. above low water in place of 18 ft., and capping the stepped courses at the top by large blocks of stone, 20 ft. long and 5 ft. deep, laid across the breakwater, which thus presented a marked resemblance to the upper section of the mound at Civita Vecchia.
The breakwater at Sandy Bay just referred to, and the one at Civita Vecchia, which it somewhat resembles, approximate to that class of breakwater which has a superstructure founded below low-water level, so far as stepped courses of blocks can be regarded as forming part of a superstructure; Super-structure below low-water level. but as the protection afforded by these courses differs only in the arrangement of the blocks from that obtained by blocks deposited at random, it appears expedient to restrict this class to the more solid structures, resembling upright-wall breakwaters, founded on a mound at some depth below low water As the main object of this class of breakwater is to keep the mound below the zone of disturbance by waves in severe storms, it is evident that the depth at which the superstructure is founded should vary directly with the exposure of the site, and inversely with the size of the materials forming the mound.
The depth at which waves striking against a superstructure may affect a rubble mound near its toe by the recoil, has been only very gradually realized. Thus, in 1847, the Alderney breakwater, though fully exposed to the Atlantic Ocean, was begun with a superstructure founded at low water of spring tides upon a rubble mound; but within two years the foundations had to be carried down 12 it. below low water, and this was adhered to till close to the head, though the breakwater, completed in 1864, extended 4700 ft. from the shore into a depth of 130 ft. at low tide, the rise of springs being 17 ft. The great recoil of the waves in storms from the promenade wall on the sea side of the superstructure, raised 33 ft. above low water, disturbed the sea slope of the mound along the outer portion, situated in depths of 80 to 130 ft. at low water, out to a distance of 90 ft. from the superstructure and to a depth of 20 ft.; whilst the outer toe of the superstructure was only preserved from being undermined by frequent deposits of stone along the sea face.
The south-west breakwater at Colombo Harbour, constructed in 1876–1884, facing the seas raised by the south-west monsoon, extends into a depth of 39 ft. at low water, where the rise of tide is only 2 ft. at springs, and was built with a superstructure founded upon a rubble mound at a depth of 20 ft. below low water, but raised only 12 ft. above this level without any parapet, and protected along its sea face by an apron of concrete in bags. In this case, not only was the depth of the sea much less than at Alderney, but the small elevation of the superstructure above low water enabled a portion of the waves in storms to pass over it without materially impairing the shelter inside. These circumstances reduced the shock and recoil of the waves; and the greater depth of the foundations and the protection of the toe of the superstructure greatly diminished the danger of undermining. Consequently, the Colombo breakwater has been preserved from the injuries to which the outer part of the Alderney breakwater succumbed. Nevertheless, in subsequently constructing the north-west detached breakwater, less exposed to the south-west monsoon, but in somewhat deeper water (see Colombo), the experience of the action of the sea on the south-west breakwater led to the laying of the foundations of the superstructure on the rubble mound at 303 ft. below low water (fig. 9).
The breakwater for sheltering Peterhead Bay, where the rise of springs is 111 ft., was begun in 1888, and designed to extend into a depth of 91 fathoms at low water (see Harbour). It was built as an upright wall upon the rocky bottom for 1000 ft. from the shore; but owing to the increase in depth it was decided to construct the outer portion with a rubble base, surmounted by a superstructure originally designed to be founded 30 ft. below low water. As, however, during a storm in October 1898, the recoil of the waves from the breakwater, which is provided with a promenade wall rising about 35 ft. above low water, disturbed rubble to a depth of 361 ft., the superstructure has been founded 43 ft. below low water on the rubble base; and its outer toe is protected from being undermined by two rows of concrete blocks on the rubble mound.
Formerly, in constructing a large superstructure upon a rubble mound, it was a common practice to build a sea wall and a harbour wall several feet apart, and to fill up the intermediate space between them with rubble, so as economically to form a wide structure on the top of the mound, and provide Construction of the super-structure. an adequate width for a quay along the top. A sheltering wall was also generally erected on the sea side. This, for instance, was the system of construction adopted for the superstructures, founded at low water, of Holyhead breakwater, Portland inner breakwater, and St Catherine’s, Jersey, breakwater. Alderney breakwater, the Tyne breakwaters and Colombo south-west breakwater were also commenced with a precisely similar method of construction. The system, however, possesses a Very serious defect for exposed situations, namely, that if once the sea can force a small opening through the sea wall, the scooping out of the rubble filling, and the overthrow of the thinner harbour wall are rapidly accomplished if the storm continues or recurs before repairs can be effected. Experience soon proved at Alderney and Tynemouth the unsuitability of the system for very exposed situations; and the intermediate rubble filling was replaced by solid hearting down to a certain depth. At Colombo, after the first 1326 ft. of the south-west breakwater had been built with two walls and intermediate rubble for the superstructure, as the exposure proved greater than had been anticipated, and a slight displacement of part of the sea wall, 24 ft. wide, had occurred, the rubble filling was discontinued, and the two walls were united into a solid superstructure 34 ft. in width.
A difficulty experienced in constructing a solid superstructure on the top of a rubble mound consists in the settlement of the mound which takes place when the weight of the superstructure comes on it, in spite of the consolidation of the rubble under the action of the sea for one or two years
before the erection of the superstructure on it is undertaken. When the superstructure is carried out in long stepped-forward courses, irregular settlement is particularly liable to occur, as the weight is progressively imposed in an uneven manner on the yielding rubble, in proportion to the height of the rubble base and its deficiency in compactness. The open joints between the blocks laid below low water enable the air to penetrate, on the recoil of the waves at low tide, into any internal fissures resulting from settlement; and the following wave, on striking the superstructure,
compresses the air inside, which, on its expansion when the wave recedes, forces out any unconnected face stones. The hole thus formed is rapidly enlarged by the sea if the storm continues; and a breach is eventually formed. The sloping-block system was, accordingly devised to provide against the dislocation of superstructures by the inevitable irregular settlement, by forming them of a series of sloping sections, composed of concrete blocks laid at an angle, free to settle independently on the mound, as shown in fig. 10. In the first superstructure thus constructed, in 1869–1874, at the
Fig. 10.—Colombo North-West Breakwater with Titan Crane. entrance to Karachi harbour, founded 15 ft. below low water on a rubble mound and 24 ft. high, the blocks in each section, consisting of two rows of three superposed blocks laid at an inclination of 76° shorewards, were entirely unconnected; and, consequently, though the superstructure offered as little opposition as practicable to the waves by having its top slightly below high water, the waves in a storm forcing their way into the vertical joint between the two rows, threw some of the top 27-ton blocks of the inner row down on the harbour slope of the mound. This cause of damage was obviated in effecting the repairs, by connecting the top blocks with the next ones by stone dowels. The superstructures of the breakwaters forming Madras harbour, commenced in 1876, were similarly constructed in sloping, independent sections, 41 ft. thick, composed of two distinct rows of four tiers of blocks founded upon a rubble mound 22 ft. below low water (the rise of tide at springs being 31 ft.), and raised 31 ft. above high water. The blocks in each row were connected by a tenon, projecting at the top of each block, fitting into a mortise in the block above it. The retention of the vertical joint however, between the two rows led to the overthrow of the greater part of the superstructures of the outer arms at Madras, situated in a depth of 45 ft. and facing the Indian Ocean, during a cyclone of 1881. In the reconstruction of these superstructures, bond was introduced in the successive tiers of each sloping section; and the blocks of the two upper tiers were cramped together. Alter settlement on the mound had ceased, a thick capping of mass concrete was laid all along the top of the superstructure; and, finally, a mound of concrete blocks was deposited at random on the mound in front of the sea face of the superstructure to break the force of the waves and prevent undermining. A similar wave-breaker, with blocks somewhat specially arranged, was deposited in front of the sloping concrete-block superstructure of the breakwater sheltering the Portuguese harbour of Marmagao on the west coast of India, more particularly with the object of preventing the undermining of the superstructure founded only 18 ft. below low water of spring tides, on a layer of rubble spread on the muddy sea-bottom, the settlement in this case being occasioned by the yielding of the soft clay bed. This breakwater having been commenced in 1884, subsequently to the failure at Madras, the superstructure, formed of concrete blocks weighing 281 to 371 tons was built in accordance with the design adopted for the reconstructed outer arms at Madras, with the exceptions that the separate sections were given a slope of 70° instead of 76° shorewards to ensure greater stability, that the superstructure was made 30 ft in width instead of 24 ft., that the top tier of blocks in each section was secured to the next tier by two dowels, each formed of a bundle of four rails, penetrating 31 ft. into each tier, so as to enable the top courses to be more correctly aligned than with tenons and mortises, and that the outer side of the continuous concrete-in-mass capping was raised about 22 ft. above low water (fig. 11). The rise of spring tides at Marmagao is 6 ft.
At Colombo the superstructures of both the south-west and north-west breakwaters were built on the sloping-block system in sections 51 ft. thick, and built at an angle of 68° shorewards (fig. 10); and the blocks, from 161 to 31 tons in weight, were laid in bonded courses across each section, with four tiers of blocks in the south-west breakwater founded 20 ft. below low water on the rubble mound, and six tiers of blocks in the north-west breakwater, founded 303 ft below low water. Five oblong grooves, moreover, were formed in moulding the blocks, in the adjacent faces of each sloping section, extending from top to bottom of the sections. These, when settlement on the mound had ceased, were filled with concrete in bags which not only connected the tiers of blocks in each section together, but also joined the several sections to one another, and effectually closed the transverse joints between the successive sections, which were further connected together by a continuous capping of concrete-in-mass along the whole length of the breakwater.
These sloping blocks are laid by powerful overhanging, block-setting cranes, called Titans (see Cranes), which travel along the completed portion of the breakwater, and lay the blocks in advance on the mound levelled by divers, as shown in fig. 10. The earlier Titans, employed for the sloping-block superstructures at Karachi and Madras, were constructed to travel only backwards and forwards on the completed work, with sufficient sideways movement of the little trolley travelling along the overhanging arm, from which the block is suspended at the proper angle, to lay the blocks for each side of the superstructure. In later forms, however, such for instance as the Titan laying the 14-ton blocks at Peterhead breakwater in horizontal courses, the overhanging arm is supported centrally on a ring of rollers, placed on the top of the truck on which the Titan travels, so that it can revolve and deposit blocks at the side of the superstructure for protecting the mound, as well as in advance of the finished work. These Titans possess the important advantage over the timber staging formerly employed for such breakwaters, that, in exposed situations, they can be moved back into shelter on the approach of a storm, or for the winter or stormy months, instead of, as in the case of staging, remaining out exposed to the danger of being carried away during stormy weather, or necessitating loss of time in erection at the beginning of the working season.
Though composite breakwaters are still occasionally constructed with a superstructure founded on a rubble mound at, or above, low-water level, these breakwaters are now almost always constructed with the superstructure founded at some depth below low water, even at harbours on the continent of Europe, where formerly broad quays founded at sea-level, protected by a parapet wall and outer concrete blocks, were the regular form of superstructure adopted. The breakwater for the extension of the harbour at Naples provides an interesting example of this change of design. A solid superstructure, formed of large concrete blocks capped with masonry, about 50 ft. wide at the base, is laid on a high rubble mound at a depth of 31 ft. below mean sea-level, and provides a quay on the top, 241 ft. wide, protected on the sea side by a promenade wall, 10 ft. high and 121 ft. wide at the top, raised 192 ft. above sea-level (fig. 12). In view of the increased depth at which superstructures are now founded upon rubble mounds, causing the breakwaters to approximate more and more to the upright-wall type, it might seem at first sight that the rubble base might be dispensed with, and the superstructure founded directly on the bed of the sea. Two circumstances, however, still render the composite form of breakwater indispensable in certain cases: (1) the great depth into which breakwaters have sometimes to extend, reaching about 56 ft. below low water at Peterhead, and 102 ft. below mean sea-level at Naples; and (2) the necessity, where the sea-bottom is soft or liable: to be eroded by scour, of interposing a wide base between the upright superstructure and the bed of the sea.
The injuries to which composite breakwaters appear to have been specially subject must be attributed to the greater exposure and depth of the sites in which they have been frequently constructed, as compared with rubble mounds or upright walls. The latter types, indeed, are not well suited for erection in deep water, in the first case, on account of the very large quantity of materials required for a high mound with flat slopes, and in the second, owing to the increased pressure of air under which divers have to work in laying blocks for an upright wall in deep water. The ample depth in which superstructures are founded, the due protection afforded to their outer toe, the adoption of the sloping-block system for their construction, and the dispensing in most cases with a high sheltering wall on the sea side of the superstructure, render modern superstructures as stable as upright-wall breakwaters of similar height. Nevertheless, superstructures require to be given a greater thickness than similar upright walls, because the greater depth of water in which such composite breakwaters are built causes them to be exposed to larger waves under similar conditions.
The superstructures of composite breakwaters erected by the United States for harbours on the shores of Lake Superior were formerly in some cases composed of timber cribs floated into position and sunk by filling them with rubble stone. On account of the cheapness of timber several years ago in those regions, this simple mode of construction was also economical, even though the rapid decay of the timber in the portions of the cribs where it was alternately wet and dry involved its renewal about every fifteen years on the average. Owing, however, to the fact that the price of timber has increased considerably, whilst that of Portland cement has been reduced, durable concrete superstructures are beginning to be substituted for the rapidly decaying cribwork structures.
With the exception perhaps of the Alderney breakwater, which, owing to its exceptional exposure and the unparalleled depth into which it extended, had its superstructure so often breached by the sea that, owing to the cost of maintenance, the inner portion only has been kept in repair, the composite breakwater of Bilbao harbour has probably proved the most difficult to construct on account of its great exposure. The original design consisted of a wide rubble mound up to about 161 ft. below low water, a mound of large concrete blocks up to low water of equinoctial spring tides, and a solid masonry superstructure well protected at its outer toe by a projection of masonry, and raised several feet above high water, forming a quay sheltered by a promenade wall. The rise of equinoctial spring tides at the mouth of the river Nervion is 143 ft. In carrying out the work, however, the superstructure built in the summer months was for the most part destroyed by the following winter storms; and, accordingly, the superstructure was eventually constructed on a widened rubble base, so as to be sheltered to some extent by the outlying concrete-block mound already deposited, a system subsequently adopted in rebuilding the damaged portion of the North Pier at Tynemouth under shelter of the ruins of the previous work. The modified superstructure of the Bilbao breakwater was founded on the extended rubble mound at a depth of 161 ft. below low water, and formed of iron caissons partially filled with concrete and floated out, sunk in position, and filled up with concrete blocks and concrete. It thus consists of a continuous row of concrete blocks, each of them being 422 ft. in width across the breakwater, 23 ft. in length along the line of the breakwater, 23 ft. high, and weighing 1400 tons. These caisson blocks, raised 63 ft. above low water, form the base of the superstructure, upon which the upper part was built of concrete blocks on each face with mass concrete filling between them, forming a continuous quay, 24 ft. wide, raised 8 ft. above high tide, and slightly sheltered by a curved parapet block only 5 ft. high. The outer toe of the caisson blocks is protected from being undermined by two tiers of large concrete blocks laid flat on the rubble mound. This superstructure has successfully resisted the attacks of the Atlantic waves rolling into the bay. At this breakwater and at Tynemouth advantage has been taken of the protection unintentionally provided by previous failures, by which the waves are broken before reaching the superstructure and pier respectively; but instead of introducing a wave-breaker of concrete blocks, for a protection to the superstructure, as arranged at Marmagao (fig. 11) and the outer arms at Madras, it would appear preferable to increase the width of the solid superstructure, if necessary, as carried out at Naples (fig. 12). and to dispense with a parapet and keep the superstructure low, as being unsuitable for a quay in exposed situations, according to the plan adopted at Colombo (fig. 9).
3. Upright-Wall Breakwaters.—The third type of breakwater consists of a solid structure founded directly on the sea-bottom, in the form of an upright wall, with only a moderate batter on each face. This form of breakwater is strictly limited to sites where the bed of the sea consists of rock, chalk, boulders, or other hard bottom not subject to erosion by scour, and where the depth does not exceed about 40 to 50 ft. If a solid breakwater were erected on a soft yielding bottom, it would be exposed to dislocation from irregular settlement; and such a structure, by obstructing or diverting the existing currents, tends to create a scour along its base; whilst the waves in recoiling from its sea face are very liable to produce erosion of the sea-bottom along its outer toe. Moreover, when the foundations for an upright-wall breakwater have to be levelled by divers, and the blocks laid under water by their help, the extension of such a breakwater into a considerable depth is undesirable on account of the increased pressure imposed upon diving operations.
The Admiralty pier at Dover was begun about the middle of the 19th century, and furnishes an early and notable example of an upright-wall breakwater resting upon a hard chalk bottom; and it was subsequently extended to a depth of about 42 ft. at low tide, in connexion with the works for forming a closed naval harbour at Dover. This breakwater, the Prince of Wales pier of the commercial harbour, and the eastern breakwater and detached south breakwater for the naval harbour, were all founded on a levelled bottom, carried down to the hard chalk underlying the surface layer, by means of men in diving-bells. The extension of the Admiralty pier and the other breakwaters of Dover harbour consist of bonded courses of concrete blocks, from 26 to 40 tons in weight, as shown in figs. 13 and 14, the outer blocks above low water being formed on their exposed side with a facing of granite rubble. The blocks, composed of six parts of sand and stones to one part of Portland cement, moulded in frames, and left to set thoroughly in the block-yard before being used, are all joggled together, and above low-water level are bedded in cement and the joints filled with cement grout. The blocks were laid by Goliath travelling cranes running on temporary staging supported at intervals of 501 ft. by clusters of iron piles carried down into the chalk bottom. On each line of staging there were four Goliaths, preceded by a stage-erecting machine. The front Goliath was used for working a grab for excavating the surface layer of chalk, which was finally levelled by divers, the second for carrying the diving-bell, the third for laying the blocks below low water, and the fourth for setting the blocks above low water. This succession of Goliaths enabled more rapid progress to be made than with a single Titan at the end of a breakwater; but it involved a considerable increase in the cost of the plant, owing to the temporary staging required. The foundations were carried down from 4 to 6 ft. into the chalk bottom, the deepest being 53 ft. below low water of spring tides, and the average 47 ft. With a rise of tide at springs of 183 ft., the average depth is thus approximately 66 ft. at high tide, necessitating a pressure of 29 ℔ on the square inch, which is the limit at which men can work without inconvenience in the diving-bells. The breakwaters are raised about 11 ft. above high water of springs. The detached southern breakwater was finished off at this level; but the extended western breakwater, or Admiralty pier, is provided with a promenade parapet on its exposed side, rising 13 ft. above the quay; and the eastern breakwater also has a parapet on its exposed eastern side, raised, however, only 9 ft. above its quay. The breakwaters are protected from scour along their outer toe by an apron of concrete blocks, extending 25 ft. out from their sea face.
The levelling of the foundations for laying the courses of an upright-wall breakwater is costly and tedious, even in chalk; and the expense and delay are considerably enhanced where the bottom is hard rock. Accordingly, in constructing two breakwaters at the entrance to Aberdeen harbour Concrete-bag foundations. on a bottom of granite in 1870–1877, concrete bags were laid on the sea-bed; and these bags, by adapting themselves to the rocky irregularities, obviated levelling the bottom. They formed the foundation for the concrete blocks in the south breakwater; and by the deposit of successive layers of 50-ton concrete bags till they rose above low water, they constituted the whole of the submerged portion of the north breakwater. The 50-ton bags were deposited from hopper barges towed out to the site; and the portions of both breakwaters above low water were carried up with mass concrete. Subsequently, the breakwater at Newhaven was constructed on a foundation of chalk, with lop-ton concrete bags up to low water, and mass concrete above. Still later, the two breakwaters sheltering the approach to the river Wear (see Harbour) and the Sunderland docks were built with a foundation mound of concrete in bags, 56 to 116 tons in weight, on the uneven sea-bottom, raised slightly above low water of spring tides, on which a solid upright wall was erected, formed of concrete blocks on each side faced with granite, filled in the centre and capped on the top with mass concrete. The most exposed northern Roker breakwater, raised about 11 ft. above high water of springs where the rise is 14 ft. 5 in., is devoid of a parapet; but a subway formed near the top in each breakwater gives access to the light on the pierhead in stormy weather (fig. 15). These concrete bags are made by lining the hopper of the barge with jute canvas, which receives the concrete and is sewn up to form a bag whilst the barge is being towed to the site. The concrete is thus deposited unset, and readily accommodates itself to the irregularities of the bottom or of the mound of bags; and sufficient liquid grout oozes out of the canvas when the bag is compressed, to unite the bags into a solid mass, so that with the mass concrete on the top, the breakwater forms a monolith. This system has been extended to the portion of the superstructure of the eastern, little-exposed breakwater of Bilbao harbour below low water, where the rubble mound is of moderate height; but this application of the system appears less satisfactory, as settlement of the superstructure on the mound would produce cracks in the set concrete in the bags.
Foundation blocks of 2500 to 3000 tons have been deposited for raising the walls on each side of the wide portion of the Zeebrugge breakwater (fig. 16) from the sea-bottom to above low water, and also 4400-ton blocks along the narrow outer portion (see Harbour), by building iron caissons, Foundations with large blocks. open at the top, in the dry bed of the Bruges ship-canal, lining them with concrete, and after the canal was filled with water, floating them out one by one in calm weather, sinking them in position by admitting water, and then filling them with concrete under water from closed skips which open at the bottom directly they begin to be raised. The firm sea-bed is levelled by small rubble for receiving the large blocks, whose outer toe is protected from undermining by a layer of big blocks of stone extending out for a width of 50 ft.; and then the breakwater walls are raised above high water by 55-ton concrete blocks, set in cement at low tide; and the upper portions are completed by concrete-in-mass within framing.
Sometimes funds are not available for a large plant; and in such cases small upright-wall breakwaters may be constructed in a moderate depth of water on a hard bottom of rock, chalk or boulders, by erecting timber framing in suitable lengths, lining it inside with jute cloth, and then depositing Concrete monoliths. concrete below low water in closed hopper skips lowered to the bottom before releasing the concrete, which must be effected with great care to avoid allowing the concrete to fall through the water. The portion of the breakwater above low water is then raised by tide-work with mass concrete within frames, in which large blocks of stone may be bedded, provided they do not touch one another and are kept away from the face, which should be formed with concrete containing a larger proportion of cement. As long continuous lengths of concrete crack across under variations in temperature, it is advisable to form fine straight divisions across the upper part of a concrete breakwater in construction, as substitutes for irregular cracks.
Upright-wall breakwaters should not be formed with two narrow walls and intermediate filling, as the safety of such a breakwater depends entirely on the sea-wall being maintained intact. A warning of the danger of this system of construction, combined with a high parapet, was furnished by the south breakwater of Newcastle harbour in Dundrum Bay, Ireland, which was breached by a storm in 1868, and eventually almost wholly destroyed; whilst its ruins for many years filled up the harbour which it had been erected to protect. In designing its reconstruction in 1897, it was found possible to provide a solid upright wall of suitable strength with the materials scattered over the harbour, together with an extension needed for providing proper protection at the entrance. This work was completed in 1906.
Upright-wall breakwaters and superstructures are generally made of the same thickness throughout, irrespective of the differences in depth and exposure which are often met with in different parts of the same breakwater. This may be accounted for by the general custom of regarding the top of an upright wall or superstructure as a quay, which should naturally be given a uniform width; and this view has also led to the very general practice of sheltering the top of these structures with a parapet. Generally the width is proportioned to the most exposed part, so that the only result is an excess of expenditure in the inner portion to secure uniformity. When, however, as at Madras, the width of the structure is reduced to a minimum, the action of the sea demonstrates that the strength of the structure must be proportioned to the depth and exposure. In small fishery piers, where great economy is essential to obtain the maximum shelter at limited expense, it appears expedient to make the width of the breakwater proportionate to the depth. This was done in Babbacombe Bay; and in reconstructing the southern breakwater at Newcastle, Ireland, advantage was taken of a change in direction of the outer half to introduce an addition to the width, so as to make the strength of the breakwater proportionate to the increase in depth and exposure. In large structures, however, uniformity of design may be desirable for each straight length of breakwater; though where two or more breakwaters or outer arms enclose a harbour, the design should obviously be modified to suit the depth and exposure. At Colombo harbour, the superstructure of the less exposed north-west breakwater has been made slightly narrower than that of the south-west breakwater; and a simple rubble mound shelters the harbour from the moderate north-east monsoon. In special cases, where a breakwater has to serve as a quay, like the Admiralty pier at Dover, a high parapet wall is essential; but in most cases, where a parapet merely enables the breakwater to be more readily accessible in moderate weather, it would be advisable to keep it very low, or to dispense with it altogether, as at the southern Dover breakwater, the northern breakwater at Sunderland, and the Colombo western breakwaters. This course is particularly expedient in very exposed sites, as a high parapet intensifies the shock of the waves against a breakwater and their erosive recoil. Moreover, when a light has to be attended to at the end of a breakwater, sheltered access can be provided by a subway, as at Sunderland.
Structures in the sea almost always require works of maintenance; and when a severe storm has caused any injury, it is most important to carry out the repairs at the earliest available moment, as the waves rapidly enlarge any holes that they may have formed in weak places. (L. F. V.-H.)