Chap. XXVI RIVER AND SEA WALLS. 1525

Triangular baskets filled with stones are sometimes laid in rivers subject to floods, toprotect the starlings of the bridges.

Fig. 2800.

Fig. 2801.

Groins used on the coast for preventing encroachments of the sea are made of timber andconstructed across a beach, between high and low water, perpendicular to its general line :they are for the purpose of retaining the shingle or accumulating more, and are formed ofpiles, planking, land ties, blocks, tail piles, keys, and screw bolts. Their length dependson the nature of the beach upon which they are to be formed ; those on the Sussex coastyary from 150 to 250 feet, and have oak or beech piles from 12 to 25 feet long, shod withiron, their scantling being 8 inches by 6£.‘ The planking is 2| inches thick ; the land tiesare about 25 feet in length of rough timber, and the bars which secure them at the endsa re 13 feet 6 inches in length, and 12 inches by 5; the land tie bar blocks are 2 feet in length,and the tail blocks 2 feet 6 inches, their scantling 6 by 3 : the screw-bolts are of inch-round iron, 2 feet 9 inches long. Every 16 feet in length of these groins contains fourpiles, one land tie, with tail piles and keys, one land tie bar with two blocks 2 feet long,and two short bolts, 180 superficial feet of planking, and 140 6-inch spikes, the cost ofwhich is about 30 pounds.

To accumulate more shingle, the first pile is driven at the high water mark of neap tides,leaving its top level with that of spring tides; the next is driven at the point on the sands,beyond the bottom of the shingle to which the groin extends, leaving about 4 feet out ofthe beach ; the tops of these two piles formed the general slope of the groin, unless wherethe beach is curved, when it was slightly modified to meet the curvature.

From the high water mark of neap tides the piles are carried back nearly level to that ofspring tides, or further if necessary, and driven 4 feet apart from centre to centre, so as toadmit the planking to pass alternately between them; the land ties are placed about one-thirdfrom the top of the planking, and usually on one side, so as to resist the action of the mostprevailing winds. The groins are placed from 50 to 100 feet apart, and are raised as theshingle mounts.

Of River and Sea Walls -Before walls of masonry are commenced for this purpose,

the soil must be carefully examined, and the foundations laid in at sufficient depthsbelow the current or flowing in of the tide; the footings must also be protected byinternal and external rows of planking and piling; another matter for consideration isthe escape of the water from the land, which sometimes, if not properly drained off, willeither wash away the soil on which the foundations are laid, or thrust the whole out of aperpendicular direction ; and when river walls are built upon the heads of piles notproperly secured, they will often press the latter out of the position in which they weredriven, and destroy the strength of the work. Sea walls, from being exposed to a greatforce, must be so set out that the necessary resistance is obtained, and the slope next theocean should always be as flat as possible, that the waves may break easily upon it: inthose instances where a slope of 1 in 30 has been given, and the surface paved with stone,

Fig. 2802. SEA walls.

there has been less expenditure to keep it in repair than in others made more steep.Bock walls require that the interior should be puddled with clay, as it is found that at low

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