A detailed engineering soils analysis of a proposed highway alignment is a crucial part of the highway design process. The results of the soils analysis are used to develop the design details of cut sections such as depth and slope of the cut. The engineer has to keep in mind that the volume of excavation increases significantly as the depth of the cut increases, and therefore usually tries to avoid excessive cut depths. Cut slopes are rarely steeper than 2:1 (2 units horizontal to 1 unit vertical or about 27 degrees from horizontal) except in very competent materials such as solid rock. AASHTO recommends that cut slopes steeper than 3:1 be evaluated with regard to soil stability and traffic safety.
The greatest amount of roadway construction in rural areas occurs on fill. In flat terrain, the highway pavement should be elevated several feet above the original ground surface to aid drainage. Slopes for fill should be determined in accordance with the guidelines discussed under Cross Section Elements. It is desirable to keep the height of the fill section to 30 feet or less, with 20 feet being a preferred maximum. With fill heights greater than 20 feet, it may be more economical to build a bridge, depending on the topography.