9.3.2 Side Slopes

For safety, it is desirable to design relatively flat areas adjacent to the travel way so that out-of-control vehicles are more likely to recover or make a controlled deceleration. Design guide values for the selection of earth fill slope rates in relation to height of fill are shown in .
Particularly difficult terrain may require deviation from these general guide values. Where conditions are favorable, it is preferable to use flatter slopes to enhance roadside safety.
The slope adjacent to the shoulder is called the front slope. Ideally, the front slope should be 1V:8H or flatter, although steeper slopes are acceptable in some locations.
The back slope should typically be 1V:6H or flatter. However, the slope ratio of the back slope may vary depending upon the geologic formation encountered. For example, where the roadway alignment traverses through a rock formation area, back slopes are typically much steeper.
The intersections of slope planes in the highway cross section should be well rounded for added safety and increased stability of out-of-control vehicles. Where barrier is placed on side slopes, the area between the roadway and barrier must be sloped at 1V:10H or flatter.
Table 9-10: Earth Fill Slope Rates
Height of Fill
Preferable Slope Rate,
Vertical:Horizontal
Terrain
Level
Rolling
1
0 – 5-ft
1V:8H
1V:6H
5-ft and over
1V:6H
1V:6H
Notes:
  1. Slope rates should be flatter where practical. Deviation may be permitted for particularly difficult terrain conditions.