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I wanted to get your input on an application for the MGS.  We have a project in a mountainous region of Virginia the requires extensive use of retaining walls adjacent to the roadway.  This is a unique situation and the project team would like to use guardrail instead of concrete barrier.  To increase the offset from the roadway, they would like to use MGS 4’ from the retaining wall.  I wanted to see if you had any data on the deflection (at the groundline) of MSG with 9’ post (full encapsulated) and away from a drop-off.  We want to make sure that the post can fully rotate prior to hitting the retaining wall.  Of course, we have to address the retaining wall straps or nails too.

Thanks

We do have some information related to your situation.

First, we did research on installation of the MGS on MSE walls several years ago. I have attached the link below. It provides quite a bit of guidance related to a very similar type of installation.

https://mwrsf.unl.edu/researchhub/files/Report26/TRP-03-235-11.pdf

In that research, we recommended at minimum MGS offset from the edge of the wall of a little less than 5’ to limit damage to the retaining wall and soil anchors due to post displacement.

If you have a more conventional, fixed retaining wall (concrete wall), there may be other options for looking at the barrier offset. Typically, when we look at post movement during barrier impact, the post rotates about a point roughly 2/3 of the embedment depth in the soil. Thus, for a standard MGS post with 40” of embedment, the post rotates about a point 26.7” below grade. For a standard guardrail impact, we assume the posts move a maximum of 15”-20” at the center of rail height. This would correlate to roughly 7.75”-10.3” of displacement of the post at grade. In this case, 2’ of soil behind the posts would seem adequate to develop the required soil forces. Potentially one could consider 1.5’ of space if pressed. The MGS with 12” blockouts is roughly 21” wide. Thus, 4’ of offset from the face of rail to a fixed retaining wall seems reasonable. This all assumes that the post doe not yield. Post motion is often a combination of soil rotation and deformation in the posts which would limit the soil displacement of the post as well.

You denote the use of 9’ long posts (76” post embedment) in your email. I am not sure why the need for the much longer posts exists unless you have slopes involved. If the 9’ posts are installed on level ground and don’t deform, we would assume that the post moves around 13.5” at groundline. However, the high embedment of these posts would increase the soil resistance significantly, and the posts would likely yield and bend at some point slightly below grade (typically 6”-12”) rather that rotate in the soil. Thus, you would not likely achieve the 13.5” of post displacement at grade.

Thanks for your reply.  In our case we were considering the use of 9’ posts as a way to limit the deflection instead of using half post spacing due to the number of posts required (long runs of w-beam).  Wanted to see if the 4’ from face of rail to the retaining wall would be a reasonable offset (with 9’ posts) to prevent damage to the retaining wall (I think they are soil nail walls not gravity retaining walls).  Another option would be use the non-blocked MGS with 9’ posts, which would give us more space but the concern remains whether 4’ from the face of rail to the retaining wall would be adequate.

Thanks for the feedback.

We do not recommend using long posts to reduce deflection with the MGS. The stiffer post-soil response can limit post energy dissipation and create pocketing and increased rail loading. The effect would be similar to guardrail posts installed in asphalt or concrete without appropriate leave outs. We have full-scale testing that has demonstrated the potential pitfalls. As such, there would be concerns about the use of 9’ posts on level terrain.

I believe that the non-blocked option would be reasonable as well. Standard posts with a 4’ offset from the face of rail would provide plenty of room for post rotation. This is similar to what was done on the MSE wall project. We used a slightly higher offset to limit MSE wall damage.