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MnDOT is updating our Reinforced Soil Slope details and we
are looking for guidance relating to MGS (MnDOT Type
31) guardrail and reinforced soil slopes. 

Specifically, how much room is needed behind the post when
guardrail is in reinforced soil? 

Please see attached file:  reinforced_soil_slopes.pdf

• This pdf includes Standard Plans 5-297.646 thru
  5-297.649 which are MnDOT’s Reinforced Soil Slope Standards . 

• We are looking to update the  “Steel Plate
  Beam Guardrail Detail” on sheet 5-297.649.   (The crossed-out detail
  on 5-297.649 currently shows our old 28” plate beam guardrail.) 

Please see attached file:  Updated_Steel_Plate_Beam_Guardrail_Detail.pdf

• This is the new detail that shows the MnDOT Type
  31 guardrail in relation to the curb and reinforced soil.

• Our question is:   How much distance
  behind the 9’ post is required , from the back of the
  post to the PI of the reinforced slope?

Prior to recent testing of the MGS adjacent to slopes, guidance for W-beam guardrail adjacent to slope was typically given that standard W-beam guardrail (including the MGS at standard post spacing with 6' long posts) could be installed with a 2' offset from the back of the posts to the slope break point of a 2:1 slope.

More recent testing of the MGS has by MwRSF and TTI adjacent to slopes has shown that W6x8.5 or W6x9 steel posts ranging from 6'-9' in length and 6"x8" SYP posts ranging from 6'-7.5' in length can be installed at the slope break point of 2:1 slopes. I have attached a paper from the 2017 IRSC conference that outlines that research and the slope guidance. We are also currently working on a synthesis report on MGS adjacent to slope for the Midwest Pooled Fund, but the write-up is still in progress.

Slopes steeper than 2:1 have yet to be analyzed, but research on 1:1 slopes is currently underway at TTI.

If slopes near 2:1 or shallower were used, the reinforced soil slopes shown in your detail should perform similarly to the previously tested MGS configurations adjacent to slope. However, the details suggest slopes as high as 45-70 degrees. There is concern that the 9’ post length would be overly stiff for the MGS system if the offset is sufficient to develop the full-strength of the post at that large embedment. This could lead to rail pocketing and other concerns. Conversely, it is not known what the minimum offset should be for a 9’ post on a reinforced slope that steep. Determination of that offset would likely require component testing of the post at various offsets to determine an offset that meets the force vs. deflection performance range of previous MGS systems.

We would recommend using the standard 6’ post at an increased offset rather than the 9’ post. In order to determine an acceptable offset, we reviewed previous research we conducted regarding installation of the MGS on wire-faced MSE walls, which are similar reinforced soil structures. In that research, we recommended the use of 6’ long posts with the MGS with a 4’ offset from the back of the post to the edge of a vertical wire-faced MSE wall. We would recommend a similar 4’ offset with the standard 6’ long MGS post for this installation.

 https://mwrsf.unl.edu/researchhub/files/Report26/TRP-03-235-11%20(draft%20final%20to%20FHWA-CFL).pdf