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 1 inch. Washers should always be the full width of the pile, and are best made of cast iron.

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CHAPTER V.

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

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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.

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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.1

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.

and stretchers. The wall is 22 feet high, 16 feet wide at the base, and 5 feet at the top. The face is curved to a radius of 17 feet. The coping is 13 feet above H.W.S.T.

In heavy gales the waves break with great force on this wall, the water being thrown over the top and falling on the roadway.

Soon after the completion of the wall the wave-action scoured away the beach, and within a year the shale in places was scooped out to a depth of 3 feet. An apron 15 feet wide and 3 feet 6

inches deep, with a toe 5 feet deep, was added in 1891. This apron consists of pitched stones on a layer of concrete.

The cost of the wall, exclusive of the apron, but inclusive of 115,770 cubic yards of filling at the back, was £18,777. The illustration is taken from a section supplied to the author by Mr. H. W. Smith, the borough engineer.

Westgate. For the protection of the chalk cliffs on the estate of the Bethlehem Hospital, a sea-wall 16 feet high has been constructed. The face is curved to a radius of 12 feet at the bottom and 20 feet at the top. The base of the wall was carried 2 feet below the hard chalk of which the beach consisted, the surface of which, at the foot of the wall, is 9 feet 6 inches above low water of spring tides, the rise of ordinary spring tides being 15 feet