Do you know if we need to protect the "cut slopes" steeper than 1:3 within the clear zone (everything I have seen is for "fill sections")? Could you please point me to some references on this topic?
The 2006 RDG provides limited discussion on the use on backslopes or cut slopes " see Section 3.2.2 and Figures 3.6 and 3.7. In general, the RDG states that the backslope may be traversable depending on its relative smoothness and the presence (or lack) of fixed obstacles.
Some time back, I recall briefly investigating this issue for Dave Little. His email (blue) and my response (red) is contained below, along with his original attachment. The noted references are NCHRP Report No. 158 and the 1996 RDG. From this, the guidance suggests the use of a maximum cut slope of 2:1.
What I see from the 1996 RDG is in Chapter 6, Subsection 6.4.1.9 Earth Berm (P. 6-8,9), which says that "slope rates should not exceed 1:2, although steeper slopes can be used if they are smooth and liberally rounded at the base."
But I don't see any such information in the 2002 RDG, so it apparently got removed in that revision. Also don't see that there are any references identified for this information in the 1996 RDG.
I have reviewed the results presented in NCHRP Report No. 158 which was also discussed at the spring Pooled Fund meeting. I have also reviewed the guidance in prior RDGs. Basically, the NCHRP authors do not recommend using slopes beyond a 2:1 back slope when the foreslope is flat. Front end bumper/vehicle snag into the slope was a noted concern. Dean and I are also concerned with a 1:1 slope as it would be the worst situation for causing vehicle rollover, especially for higher center of mass vehicles found on the roads today and as compared to the test vehicles used in the early 70s.
Therefore, we recommend treating the 1:1 back slope situation by one of the following options. First, as you mentioned, a reinforced concrete parapet could be installed close to the base of the back slope but actually cut into it to match the wall height slightly above the soil grade. A vertical parapet would be preferred, although single slope or other approved shapes could be used. Alternatively, a smooth MSE or block type wall could be constructed at the same cut back location, thus producing a smooth vertical parapet for redirecting vehicles. Both of the barrier options would be backed up (i.e., supported) with soil over most of the vertical height.
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