CHAPTER III. EARTHWORKS, DRAINAGE, RETAINING WALLS, CULVERTS, BRIDGES, AND PROTECTION OF ROADS. 40. Equalizing Earthworks.-Earthwork, as generally understood, ombraces not only the excavation of the looser materials of the earth's crust, but rock also. In selecting the final direction and gradients of a new road, it is obvious that due regard must be observed to equalizing the cuttings and embankments, so that the earth taken from the one may be sufficient to form the other, or, in other words, the earthwork should be redistributed along the line of road. Unnecessarily heavy works are thereby avoided, and the outlay of capital involved is a consideration of greater importance than the mere maintaining of a strictly direct line. In the generality of cases occurring in practice the engineer is obliged, in arriving at this, to resort to a system of approximation; for this purpose the line of road should be divided into several sections, to be treated separately in the first instance, and finally in connection with the adjoining sections. It is advisable, in many instances, in carrying out the adjustment of the earthworks, owing to the distance apart of the cutting from the embankment on a line of road, to construct the embankment when possible from side-cutting, thus avoiding the great expense necessitated by having to transport the material an excessive distance. In like manner, transferring earth from a cutting situated a considerable distance from an embankment on the line of works, especially when of an unsatisfactory nature, may in some cases be deposited on the adjoining ground contiguous to the works. This method of disposing of the earth from the cuttings is known as forming a spoil-bank. 41. Transverse Balancing.-Transverse balancing in hilly parts, or in sidelong ground, as already indicated, is accomplished by locating the centre line of the road, so that the cutting from the one side will form the embankment on the other. The cost of transport compared with that of excavation and the value of land from which side-cutting is taken, or spoil-banks are formed, will determine the distance within which the balancing of the earthworks must be established. 42. Side Slopes.-The forming of the side slopes requires considerable attention, so as to ensure stability, and prevent slipping. The resistance to slip arises partly from the friction between the grains composing the soil and partly from their mutual adhesion. Friction is, however, the only force which can be relied upon for permanent stability, as the adhesion of the earth is destroyed by the action. of air and moisture, this being especially the case during alternate frost and thaw. The nature of the soil, its condition as to internal moisture and the atmospheric influence, therefore, combine in fixing the inclination of the side slopes. 43. The slopes of earthwork are generally described in the ratio of the horizontal breadth to the vertical height. The angle of repose, or, as it is generally termed, the natural slope at which different kinds of earth, by friction alone, will remain permanently stable, is shown in the following table given by Professor Rankine in his work on Civil Engineering. The slopes most frequently adopted for earthwork are those designated 1 horizontal to 1 perpendicular, and 2 to 1, corresponding to the angles of repose, 33 and 26° nearly. 44. Rock cuttings, the material of which does not disintegrate on exposure to the action of the weather, may be formed with perpendicular sides, but generally a slope of 1 horizontal to 2 perpendicular is given to admit of free action of the sun's rays on the road surface. 45. For a similar reason the side slopes on the south side of roads in northern latitudes are sometimes made flatter than the natural slope given to that part of the cutting on the north or opposite side. By this arrangement the surface of the roadway is exposed to the action of sun and air, causing rapid evaporation of the moisture on the surface. 46. Stratified soils and rocks, which have a dip to the horizon, are liable to slips caused by one stratum becoming detached by the action of frost or water and sliding on another, the worst combination being that of soils formed of alternating beds of clay and sand. 47. Embankments.-Embankments of any height should be formed with great care. They may be carried out by laying the material from the excavation in successive layers of from 3 to 4 feet in thickness, and thoroughly pounding each layer. This is, however, an expensive process, and seldom resorted to in ordinary works of any extent. The method usually followed is to run the extreme outsides of the embankment first, and then gradually fill up the centre portion, by which means greater solidity is secured than when the embankment is carried forward and the material tipped from one end only. By this arrangement the layers are formed with an inclination inwards, counteracting, so to speak, as a whole, any tendency to slip outwards. During the construction of an embankment, due consideration must be given to the settlement of the loose or excavated material which takes place. It is advisable to make the height and width of the embankment greater than the dimensions ultimately required; the amount allowed for settlement varies according to the class of material, but may be from one-twelfth to one-fifth of the total height. It is usual, as a precautionary measure, to form a slight excavation at the tail or toe of the slope to give the material forming the embankment additional stability. 48. Consolidating Embankments.-The compacting of embankments and road beds generally, until the material composing them forms a uniform and unyielding surface, is carried out in the United States and other countries by the use of a horse roller weighing about 3 tons. The roller is about 5 feet wide, and is ribbed or corrugated similar to corduroy. The formation level or foundation of a road is consolidated in this country by the aid of a steam roller. It requires, owing to its weight, very careful handling on newly-formed embankments of any great height. Depressions are made good by the application of material similar to that of which the formation of the road is composed. 49. Embankments on Sidelong Ground.-On sidelong ground different methods are employed in order to secure a solid basis for an embankment sustaining a road. Where the inclination of the natural surface of the ground is not great, and the formation of the road is partly in cutting and partly in embankment, the method of cutting the surface of the ground into steps, shown in fig. 15, is generally adopted. The natural surface of the ground is represented by a b, the steps or benches are shown at f,f,f, and the formation level at c d. The best position for these steps is perpendicular to the axis of greatest pressure. On sloping ground under similar conditions, but with the natural surface |