The term "earthwork" is applied to all the operations per formed in the making of excavation and embankments. In its widest sense it comprehends work in rock as well as in the looser materials of the earth's crust.
Balancing Cuts and Fills. In the construction of new roads, the formation of the roadbed consists in bringing the surface of the ground to the adopted grade This grade should be established so as to reduce the earthwork to the least possible amount, both to render the cost of construction low, and to avoid unnecessary marring the appearance of the country in the vicinity of the road. The most desirable position of the grade line is usually that which makes the amount of cutting and filling equal to each other, for any surplus embankment over cutting must be made up by borrowing, and surplus cutting must be wasted, both of these operations involving additional cost for labor and land.
Inclination of Side Slopes. The proper inclination for the side slopes of cutting and embankments depends upon the nature of the soil, the action of the atmosphere and of internal moisture upon it. For economy the inclination should be as steep as the nature of the soil will permit.
The usual slopes in cuttings are: Solid rock. 1 to 1 Earth and Gravel 32 to 1 Clay 3 or 6 to 1 Fine sand 2 or 3 to 1 The slopes of embankment are usually made 11 to 1.
Form of Side Slopes. The natural, strongest, and ultimate form of earth slopes is a concave curve, ih which the flattest portion is at the bottom. This form is very rarely given to the slopes in con structing them; in fact, the reverse is often the case, the slopes being made convex, thus saving excavation by the contractor and inviting slips.
In cuttings exceeding 10 feet in depth the forming of concave dopes will materially aid in preventing slips, and in any case they will reduce the amount of material which will eventually have to be re moved when cleaning up. Straight or convex slopes will continue to slip until the natural form is attained.
A revetment or retaining wall at the base of a slope will save excavation.
In excavations of considerable depth, and particularly in soils liable to slips, the slope may be formed in terraces, the horizontal offsets or benches being made a few feet in width with a ditch on the inner side to receive the surface water from the portion of the side slope above them. These benches catch and retain earth that may fall from the slopes above them. The correct forms for the slopes of embankment and excavation are shown in Figs. 21 and 22. Covering of Slopes. It is not usual to employ any artificial means to protect the surface of the side slopes from the action of the weather; but it is a precaution which in the end will save much labor and expense in keeping the roadways in good order. The simplest means which can be used for this purpose consists in covering the slopes with good sods, or else with a layer of vegetable mould about four inches thick, carefully laid and sown with grass seed. These means are amply sufficient to protect the side slopes from injury when they are not exposed to any other cause of deterioration than the wash of the rain and the action of frost on the ordinary moisture retained by the soil.
A covering of brushwood or a thatch of straw may also be used with good effect; but from their perishable nature they will require frequent renewal and repairs.
Where stone is abundant a small wall of stone laid dry may be constructed at the foot of the slopes to prevent any wash from them being carried into the ditches.
Shrinkage of Earthwork. All materials when excavated increase in bulk, but after being deposited in banks subside or shrink (rock excepted) until they occupy less space than in the pit from which excavated.
Rock, on the other hand, increases in volume by being broken up, and does not settle again into less than its original bulk. The increase may be taken at 50 per cent.
The shrinkage in the different materials is about as follows: Gravel. S per cent Gravel and sand... 9 " " Clay and clay earths.... 10 "' Loam and light sandy earths 12 " " Loose vegetable soil. 15 " " Fuddled clay..