FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.A. Geometric Design. (continued)Table 9-8Desirable and Maximum SlopesCut and Fill Slope Ratios for Soil MaterialsHeight Slope Flat Rolling Mountainous(meters) Type Desirable Maximum Desirable Maximum Desirable Maximum0-1 Cut 1:6 1:4 1:6 1:4 1:6 1:3Fill 1:6 1:4 1:6 1:4 1:6 1:41-3 Cut 1:4 1:3 1:3 1:2 1:3 1:2Fill 1:4 1:4 1:4 1:4 1:3 1:33-4.5 Cut 1:3 1:2 1:3 1:2 1:3 1:2Fill 1:4 1:3 1:4 1:3 1:3 1:1.54.5-6 Cut 1:3 1:2 1:2 1:2 1:2 1:1.5Fill 1:3 1:2 1:3 1:2 1:2 1:1.5Over 6 Cut 1:2 1:1.5 1:2 1:1.5 1:2 1:1.5Fill 1:3 1:2 1:3 1:1.5 1:2 1:1.5NOTE:Cut and fill slopes steeper than 1:2 should be avoided in clay or silty soils subject toerosion. Fill slopes steeper than 1:1.5 may be used in critically tight areas with geotechnicalguidance when the fill material is composed of quality rock.g. Rock Cut Slopes. Generally, rock slopes vary from near vertical to 2:1, depending on the type andquality of rock, joint patterns, fractures, cross bedding, etc. Rock slopes dipping toward the roadway mayrequire flatter slopes.High cuts, particularly in weathered or weak rock, may require fallout ditches for stability and safety. Afallout ditch at the bottom of high rock cuts keeps falling rock from encroaching on the highway. Ageotechnical investigation will determine the need for fallout ditches, their width, and necessaryconfiguration.When soil or highly weathered rock overlays the solid rock, overburden benches at the top of the solid rockmay be desirable. The overburden slope should range from 1.33:1 to 1:2, depending on the type and depthof overburden and the steepness of the topography. When the rock surface is known, compound slopeswork very well.From a safety viewpoint, rock cuts should be vertical or nearly vertical if the rock will stand on theseslopes. Under these conditions, falling rocks seldom roll once they hit the ditch. Rock cuts on the insideof curves designed on 5:1 or flatter slopes prevent the appearance of an overhang to drivers.Figures 9-11 and 9-12 provide guidance for designing rock cuts and fallout ditches. However, the finaldesign shall rely on the recommendations in the geotechnical report. Typical sections for rock cuts shouldbe shown on the plans.9 - 44