Cut and Fill Sections
Cut Sections
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.
Fill Sections
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.
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