Embankments to serve as Roads are generally mounds without cify walls, carried through coun tries -liable to be overflown, without reference to protecting any part from water, through lakes or marshes, or across straits of the sea, &c. The earth, in such mounds, is generally allowed to take its own slope on both sides, which is commonly from 40° to 450, and the width at top is regulated by that of the intended road. The materials, when the mound is formed in a country merely liable to be overflown, as in many parts of Lincolnshire and Huntingdonshire, are excavated from ditches, or taken from the surface on each side of the mound. In Holland, the roads formed on such mounds are bounded by rows of trees ; a practice which, it is to be regretted, has not been more attended to in Eng land, where accidents not unfrequently happen in the night, and particularly on the Boston and Wise beach roads; both of which are formed in great part on such mounds, unprotected by hedges, rails, or trees. In passing through part of a lake, or strait, or marsh, the earth must, of course, be taken from the firm ground on the shores ; and here the ground being generally soft below, the first operation is to lay a foundation, three or four feet thick, of branches or faggots of copeswood, in order that the mound may sink in a body. The next thing, the direction of the mound being marked out by a line of poles placed along its centre, is to begin at one end, and wheel, or cart on earth, throwing it down in the di rection of the bank, and raising it to its proper height and width; leaving the slopes at the sides to adjust themselves, either by the gravity of the mate rial alone, or jointly with the loosening and spread ing operation of the water.
The noblest attempt of this sort ever made in Britain was that of W. A. Madocks, Esq. M. P., to unite the counties of Merioneth and Cardigan by a mound across an estuary and embouchure of the Glasslyn two miles wide. Mr Madocks had, in 1802, succeeded in protecting from spring tides, by a wall or bank of the first species,• 1800 acres of "good alluvial soil, which he let at from SOs. to 50s. per acre; and his enterprising spirit induced him to contemplate the idea of gaining the whole bay or mouth of the stream, extending to nearly 4000 acres of alluvial and sandy earth, overflow.. ed in great part by every tide. Besides the mere gaining of the land, this patriotic improver had another object in view; that of uniting two ma ritime points in two counties which at that time were separated by a day's journey ; and, by effect ing this, he would, at the same time, have render ed practicable a new line of road from Worcester along the top of this embankment, through a crea tion of his own, called the town of Tre Madoc, to the newly formed-harbour of Porthdynlleyn ; by which 40 miles would have been saved to the public between Dublin and London, and 50 between Dub lin and Bath, &c.
After consulting various engineers, the first opera tion was begun in 1807, and-consisted in forming an immense bridge of flood-gates in the solid rock of the shore ; as such a bridge and gates could not be formed in any part of the mound. The use of this was to ad mit the exit of the river. This done, the mound was commenced from both shores, and rocky, sandy, and clayey materials thrown down in the direction of the mound, and left to take their own slope. The greater part of these materials were argillaceous rock broken into small pieces, which being mixed with clay the mound would have been of the strongest texture. As the work proceeded, an iron rail-road was laid along the top of it, and extended to the quarries and excava tions, by which means much labour was saved. In the course of three year the work was brought with in fifty yards of meeting in the middle, but was found extremely difficult to close from the rapidity of the influx and reflux of the tide. This difficulty, how ever, would have been overcome, and the proposed improvement effected at little more than the estimat ed cost, L.20,000, had not the various and extensive projects in which the proprietor was at that time en gaged, led him into pecuniary difficulties which put an end to the undertaking, and, as is usual in such cases, called forth popular clamour against the plan. It is but justice, however, to those concerned to state, that the very plan now put in execution was con templated above a century ago by Sir Hugh Middle ton, who then wrote to a friend, that, if he were not so deeply engaged in the scheme of bringing water to London, he should certainly engage in it.
Very little has been written on the subject of em bankments, as a separate branch of art, by British authors. Dugdale's work is entirely historical and topographical. But the writings of Smeaton, Young, Gregory, &c. contain the general principles on which is founded the art of embanking, and every other operation connected with water.