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FORMING A ROAD.
In marking out the line of a road, a great deal of expense in cutting and embanking for forming the bed on which the road materials are to be placed, may be avoided by a judicious selection of the high and low ground which the surface of the country affords.
The chief care, where a road must be carried over a high elevation, is to lay it out so that it shall not have any fall in it from the point from which it departs till it reaches the summit. The lowering of heights, and the filling of hollows, should be so adjusted as to secure gradual and continued ascending inclinations to the highest point to be passed over.
It is a most important part of the business of a skilful engineer to lay out the longitudinal inclinations of a road with the least quantity of cutting and embanking.
He must do this by measuring and calculating the quantity of earth to be removed in cuttings, and taking care that it shall exactly make the embankments for raising the hollows to the required heights j a proper allowance being made for the
subsidence of the soil according to its quality, without leaving an overplus to be carried to spoil.*
When it is necessary to make a deep cutting through a hill, the slopes of the banks should never be less, except in passing through stone, than two feet horizontal to one foot perpendicular; for though several kinds of earth will stand at steeper inclinations, a slope of two to one is necessary for admitting the sun and wind to reach the road. The whole of the green sod and fertile soil on the surface of the land cut through should be carefully collected and reserved, in order to be laid on the slopes immediately after they are formed.
If a sufficient quantity of sods cannot be procured in the space required for the road, the slopes should be covered with three or four inches of the surface mould, and hay seeds should be sown on it; by this plan the slopes will soon be covered with grass, which will be a great means of preventing them from slipping.
When stones can be got the slopes should be supported by a wall raised two or three feet high at the bottom of them. These walls prevent the earth from falling from the slopes into the side channels, and add very much to the finished and workmanlike appearance of a road.
In many cases it may be advisable, particularly if an additional quantity of earth be wanted for an embankment, to make the slopes through the cut
i See Mr. Macneill's Work on Cutting and Embanking. Published by Roake and Varty.
tings on the south side of an inclination of three horizontal to one perpendicular, in order to secure the great advantage of allowing the sun and wind to reach more freely the surface of the road.
In districts of country where stones abound, expense in moving earth and purchasing land may be avoided, by building retaining walls, and filling between them with earth. In rocky and rugged countries this is generally the best way of obtaining the prescribed inclinations.
In forming a road along the face of a precipice, a wall must be built to support it. The difficulty in such a place is not so great as is imagined, for the face of a precipice is seldom perpendicular, and an inclination of half a foot perpendicular to one foot horizontal will admit of a retaining wall being built. By building such a wall, say thirty feet high, and cutting ten feet at that height into the rock, and filling up the space within the wall, a road of sufficient breadth will be obtained, as shown in Plate II. fig. 1.
In forming a road along the face of a hill that is indented with ravines, in place of carrying it over the natural surface of the land, in order to keep it level, the projecting points should be cut through, and the level obtained by laying the earth across the hollows, as shown in Plate II. fig. 2., where the road, instead of following the sinuosities of the hill, as represented by the dotted line a a a, takes the line b b b.
In forming the bed for the materials care should be taken (except where cutting into the surface is wholly unavoidable in order to obtain the proper longitudinal inclinations,) to elevate it with earth two feet at least above the natural surface of the adjoining ground: by following this course the road will not be affected by water running under or soaking into it from the adjoining land. In arranging the inclinations, they should be obtained by embanking, when that is practicable, in preference to cutting, in consequence of the better exposure of the surface on embankments, and of the risk in all cuttings of the slopes falling down.
Almost all old roads are sunk below the adjacent fields: this has arisen from their continued wear, and from carrying away the mud. No improvement is more generally wanted than new forming these roads, so as to raise their surfaces above the level of the adjoining land. This would greatly contribute to their hardness, to economy in keeping them in repair, and to enable horses to work with the advantage of having sufficient air for respiration.
Great care is necessary in making high embankments. No person should be intrusted with these works who has not had considerable experience as a canal or road maker; for, if the base of an embankment is not formed at first to its full breadth, and if the earth is not laid on in regular layers or courses not exceeding four feet in thickness, it is almost certain to slip. In forming high embankments the earth should be laid on in concave courses, as represented in Plate II. fig. 3., in order to give firmness and stability to the work. It is not at all uncommon in many parts of the country to see embankments formed convexly, as represented in Plate II. fig. 4., the consequence of which is, that they are for ever slipping.
There have been but few attempts to make embankments by turnpike trustees that do not afford illustrations of this defect, and of a want of knowledge of the proper rules by which these works should be managed. No doubt, a chief reason for making cuttings and embankments, as is frequently the case, with slopes of one to one, has been to save expense in the purchase of land, and moving earth. But the consequence of making such slopes is that the earth is constantly slipping; so that, in the end, the expense is always greater in correcting the original error, than it would have been had proper slopes been made in the first instance.
In forming embankments along the sides of hills, or what is called side-forming, the rule that should be followed is, that the slope to be covered should be cut into level steps to receive the earth, otherwise it will be very liable to slip down the hill: in such cases, the earth should be well compressed, and great care should be taken to intercept all the land springs about it by proper drainage. For this purpose, a drain should be cut on the upper side of the road, and open drains should be made on the side of the hill above the road, to catch the surface water.
The figure 5. in Plate II. explains the manner in