GROIN, sometimes spelt GROYNE, a kind of jetty built across the beach at right angles to the line of the shore from high to low-water mark. Groins are used particularly on the southern and south-western coast of England, to retain the shingle already aeeumulated, to recover it when lost, or to accumulate more at any particular point; also to break and check the action of the waves. The following deseription of a groin is taken from Mr. Weale's very useful little work, 'A Dictionary of Terms of Art:' " The component parts of a groin are piles, planking, land ties, land-tie bars, blocks, tail-piles, and keys and screw-bolts. The length of a groin depends on the extent, and the requisite strength of its component parts on the nature of the beach on which it is to be constructed.
"Those at Eastbourne, on the coast of Sussex, of which the following is more particularly a description, are from 150 to 250 feet in length, and the beach at that place being very rough, consisting of coarse heavy shingle and large boulders, they require to he composed ofproportionably strong materials to resist its force.
"The piles are from 12 to 25 feetlona, and 8 by 6+ inches scantling, shod with iron. The planking is in length of 8, 12, and 16 feet, 2+ inches thick, and with parallel edges. The land-ties are of rough timber from 20 to 25 feet long, and large enough at the butt-end to receive the bars. The land-tie bars are 13 feet 6 inches long, and 12 by 5 inches scantling. The land-tie-bar blocks are about 2 feet long. and of the same scantling as the piles. The land-tie tail-keys are about 2 feet 6 inches long, and 6 by inches scantling. The above materials are of oak or beech. The screw-bolts are of inch round iron, 2 feet 9 inches, and 2 feet inch long, in equal proportions.
"The relative proportions of the component parts are, four piles, one land-tie with tail-piles and keys, one land-tie bar with two blocks, two long and two short bolts, about ISO square feet of planking, and about 140 six-inch spikes for every 16 feet in length ; and the expense of a groin, con structed with materials of the above dimensions, may be calculated at about £30 for the same length.
"General rules observed in the construction.
"When the object, in constructing a groin, is to recover shingle, or accumulate more. 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 at, the point on the sands, beyond the bottom of the shingle, to which the groin is to extend, leaving about four feet of it out of the beach. The tops of these two piles may be taken for the general slope of the groin, unless the beach should be very steep. and much curved, in which case it becomes necessary to follow its cur vature in some degree. From the high-water mark of neap-tides, the piles are carried back nearly level to that of spring-tides, and as much further as may be considered necessary. The piles are driven four feet asunder from centre to centre, and so as to admit the planking between them alternately, and they should be sunk about two-thirds of their length. The longest piles are placed between the high-water mark of neap-tides and the bottom of the shingle, particularly from to 40 feet below the former point. The planking is, if possible, carried down to about two-thirds from the tops of the piles, and kept parallel with them. The land-ties are placed about one-third from the top of the planking (supposing the latter to commence from the tops of the piles,) and their tails are sunk to the level of the bottom of the planking or as nearly so as possible."