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In Belgium and Holland columnar basalt has been very largely used during recent years, both for upright walls and for the facing of sloping banks. The hexagonal shape of the basalt blocks lends itself to the keying of the stones in a much better manner than the random shape of flints or granite.
At Hastings it has been in use for some time past, for the facing of the granite blocks used for groynes, and for the aprons to a sea-wall, and has been found to answer more satisfactorily than granite or Kentish Rag, the stones previously employed. For the new sea-wall in course of construction at Southend-on-sea, as also that at Clacton, basalt blocks from 4 to 7 inches in depth have been adopted in substitution of Kentish Rag stone, the material formerly used. For the protective works in the Maas and the Rhine this stone is exclusively used, as it has also been for the sea-walls at Scheveningen, Norway, and other places. Granite has also been superseded by basalt for the sea-wall, and for pitching the great sea-banks at Petten and West Kapelle, as hereafter described.
Timber Walls.—For mere protective purposes, and where economy of outlay is a consideration, sea-walls may in some cases with advantage be constructed of timber.
Ten feet may be taken as the greatest height to which such walls should be built when dependence is placed on the strength of the piles used. Beyond this the strength of the timber is not calculated to resist the pressure, and ties have to be resorted to. When such a wall is at the foot of a cliff of unstable material, difficulty may be encountered in fixing the tie-piles in a reliable position. In any case, these should be well beyond the line of the angle of repose of the earth.
Pitchpine is frequently used for construction, but this timber is very uncertain in its strength and lasting qualities. Although due care may have been exercised in the selection of the wood, it will frequently be found, on examination, that in the exposed part, such as the walings and the tops of the piles, decay will have set in soon after the construction, and at the end of ten years this will have extended to such an extent as to materially impair the strength of the timber. Sound Memel fir, creosoted with 10 lbs. of oil, will be found a more reliable material, and its life may be placed at three times that of timber that has not been creosoted. Pitchpine will not absorb more than from 4 to 6 lbs. of oil.
A wall consists of main piles, walings extending horizontally between the main piles, and sheet piles or boarding. In some cases it is found more convenient to form the lower part with sheet piles and the upper part with boarding. The practice of not continuing the sheeting or boarding behind the main piles, although apparently saving timber, is not to be recommended, as it makes the fixing of the walings difficult, and unless greater care is exercised in the driving of the sheet piles than is usually practised in work of this kind, voids are left between the main and sheet piles which lead to the disintegration of the backing behind. The water is drawn through these voids by the waves, and the earth behind sucked out; or, when it falls on the top, works its way down through the earth and washes it away.
The failure of a timber sea-wall due to the pressure of the earth at the back takes place much more frequently from the yielding of the earth in front of the piles than by their fracture at the point where they enter the ground; it becomes therefore necessary, if the ground is soft and yielding, to place concrete blocks or large boulders in front.
There is little or no advantage gained in driving piles beyond a certain depth, as fracture will take place at the surface of the ground before the compression of the earth, beyond a few feet from the surface, allows the piles to move forward. Ten feet may be taken as the usual limit in ordinary cases for main piles, and half this for the sheet piles.
In determining the depth to which the piles are to be driven, and the strength of the timber, consideration must be given to the fact that the beach, unless proper protective measures are taken, may be considerably lowered after the piling is constructed, increasing the strain upon the wall.
The distances apart of the piles and of the other timbers will depend on the sectional size of these. There is, however, a convenience and economy in using the ordinary marketable sizes, as the price becomes greater as the size of the timber increases. There is also an advantage in the fact that where the timbers are placed nearer together, the strains due to the pressure of the earth are more evenly distributed. An average dimension of 5 feet in the clear may be taken as that generally found sufficient for the main piles.
Taking these dimensions, the following may be taken as an example of a sea-wall of this character with earth of good quality at the back.
Height from the normal level of the beach to the top of the wall, 10 feet. Main piles driven at a rake of 1 in 10, to average 13 inches square, distance apart in the clear, say 5 feet. Two walings, the lower one placed about 3 feet from the beach, 9 inches by 6 inches, and the upper one near the top 6 inches by 6 inches. This is stronger in proportion than the other timber, but as these walings are more exposed to wind and weather, it is not prudent to make the scantling less. Sheet piles, from 4 inches to 6 inches thick, and driven 5 feet into the solid ground, and planking the same.
If ties are required, these may be placed on every third pile. The tie-pile to be 9 feet long, with a breast piece in front, say 4 feet in length, 12 inches by 6 inches. Tie-rod 13 inch. Washers should always be the full width of the pile, and are best made of cast iron.
EXAMPLES OF SEA-WALLS.
The examples of sea-walls given in this chapter are intended to be typical of those generally in use for coast protection.
Hove.—The sea-wall erected here for the protection of the promenade about 1884, by Sir John Coode and by Mr. Ellice
Clark, is constructed of concrete, the front being formed with blocks of the same material, having large flints embedded 4 inches deep on the face. The height of the wall is 22 feet; the
base is 9 feet thick, and the top 3 feet, finished with a granite coping. The wall is vertical for 3 feet from the bottom, and then batters at the rate of 1 in 6. The top is 12 feet above ordinary high water of spring tides, which rise 20 feet above low water; the base rests on the shingle, and was carried 9 feet below the lowest level to which the beach had been known to scour, the chalk being 28 to 30 feet below the top of the coping. The height above the beach was therefore only 13 feet at the time the wall was built. The mean section of the wall is 6 feet. High water of spring tides reached to about the exposed part of the wall when it was built.
About a year after its construction, the beach was cut away within 10 inches of the foundation, and the front had to be protected with sheet piles. The wall is 2000 feet long.
Scarborough.—The cliff on the north side of the town rises from 100 to 175 feet above the sea-level, and consists of shale mixed with irregularly bedded sandstone, and on this is imposed a stratum from 10 to 30 feet thick of boulder clay, which has fallen from the cliff behind. The shore consists of shale, which dips seaward, the beach having a covering of sand and pebbles varying from 2 to 5 feet thick, which is frequently bared to the shale in rough weather. In calm weather the sand sometimes accumulates in front of the wall to the level of H.W.S.T. The length of beach to L.W.S.T. is from 250 to 300 yards, and the rise of tide 16 feet.
The beach near the foot of the cliffs is about 11 feet above low water, the depth of water against the cliff or sea-wall at H.W.O.S.T. being 5 feet, and in E.H.W. 10 feet. Owing to the action of the waves in undermining the cliff, aided by the soakage of rainwater, landslips were frequently taking place. In 1879 a slip occurred, carrying away a portion of the road on the top of the cliff, and placing the hotel and houses located there in danger. Between 1886-90 a sea-wall was erected, and the cliff benched and drained. The sea-wall extends from the Castle northwards for a distance of 3600 feet. It is founded on the hard shale, into which the base is sunk 2 feet. It is constructed of concrete composed of sea-shingle, sand, and cement, in the proportion of 8 to 1, with random blocks of stone interspersed, the face blocks being 6 to 1. The blocks are 2 feet deep, by 1 foot wide, and 2 feet long, grooved at the ends, and laid in alternate courses of headers
1 Min. Proc. Inst. C.E., vol. cv., 1891.