HIGHWAY CONSTRUCTION 45 



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 .'U to 1 



Clay 3 or 6 to 1 



Fine sand , 2 or 3 to 1 



The slopes of embankment are usually made 1 ./ to 1 . 

 Form of Side Slopes. The natural, strongest, and ultimate 

 form of earth slopes is a concave curve, in 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 

 .slopes will materially aid in preventing slips, and in any case they will 



Fig. 21. Cross-Section for Embankment. 



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. 



