Part 43

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 8-3/5 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.

Superstructure below low-water level.

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; 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.

[Illustration: FIG. 8.--Havre Breakwater.]

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 30-3/4 ft. below low water (fig. 9).

[Illustration: FIG. 9.--Colombo North-West Breakwater.]

The breakwater for sheltering Peterhead Bay, where the rise of springs is 11-1/4 ft., was begun in 1888, and designed to extend into a depth of 9-1/2 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 36-1/2 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.

Construction of the superstructure.

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 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.

sloping block system.

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 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 deg. 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, 4-1/2 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 3-1/3 ft.), and raised 3-1/2 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 28-1/2 to 37-1/2 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 deg. instead of 76 deg. 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 3-1/2 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.

[Illustration: FIG. 10.--Colombo North-West Breakwater with Titan Crane.]

At Colombo the superstructures of both the south-west and north-west breakwaters were built on the sloping-block system in sections 5-1/2 ft. thick, and built at an angle of 68 deg. shorewards (fig. 10); and the blocks, from 16-1/2 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 30-3/4 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.

[Illustration: FIG. 11.--Marmagao Breakwater.]

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, 24-1/2 ft. wide, protected on the sea side by a promenade wall, 10 ft. high and 12-1/2 ft. wide at the top, raised 19-2/3 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.

[Illustration: FIG. 12.--Naples Harbor Extension Breakwater.]

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 16-1/2 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 14-3/4 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 16-1/4 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 42-2/3 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 6-3/4 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.