EXCAVATION AND EMBANKMENT. Side Slopes. The angle of the slopes of the cuts and fills is designated by the ratio of the horizontal to the vertical distance. Thus, if the face of the fill has an inclination of 11 feet horizontal to 1 foot vertical, the slope is designated as 11 to 1.
The slope of the excavations varies with the nature of the soil, being for economy as steep as its tenacity will permit. Solid rock may be cut with a slope of to 1. Common earth will stand 1 to 1, or 11 to 1—the latter being safer and more usual. Gravel requires 11 to 1. Some clays will stand 1 to 1, while some require a much flatter slope—in extreme cases 6 to 1. Fine sand requires a slope of 2 to 1, or 3 to 1.
The slope of embankments has less range than that of excava tions, since there is less variety in the nature and the condition of the materials, and is usually 11 to 1.
In both railroad and wagon-road work, it is customary to establish all earthwork slopes as planes intersecting each other in right lines. The original form is never maintained, since it is not a form of equilibrium and stability. Storm water soon washes away the angle formed by the intersection of the two plane surfaces at the top of the embankment, and the water flowing down the slope soon rounds cut the angle carefully formed at the foot. Such construc tion violates one of the fundamental principles of stability, and it is a needless expense to build laboriously a form of construction which nature will inevitably destroy.
The transverse contours of the embankment and excavation shown in Figs. 13 and 14 are designed to meet the above objections to the ordinary forms of construction. These sections are signed in accordance with forms of railroad excavations and bankments recommended by D. J. Whittemore, the distinguished chief engineer of the Chicago, Milwaukee and St. Paul Railroad, which forms have met with the unanimous approval of leading engi neers.* It is customary in railroad construction to make the top of the earth embankment wider than the base of the gravel or broken stone ballast, which gives a berm between the base of the ballast and the outer edge of the earth embankment. This berm has been omitted in Figs. 12 and 13, since with an earth surface there is nothing corresponding to the ballast.
If the natural slope above the cut is long or steep, a catch water drain should be constructed along the upper edge of the exca vation slope to prevent the surface water from above from washing down over the face of the cut; but the catch-water should be well back from the edge of the excavation, to prevent the water in the drain from softening the upper angle of the slope.
The slopes of both excavations and embankments should be sowed with grass seed. Sometimes the material of the embank ment is such that grass seed will not grow, in which case it may be necessary to lay sod; but of course this is very expensive. The roots of the grass will hold the earth from slipping, and prevent the face of the slope from being gullied out and washed down.
There is a tendency for workmen to leave the side slopes of embankments hollow and those of excavations rounding, to de crease the amount of labor required. In inspecting the work, this tendency should be borne in mind.