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We happened upon the following issue on one of our MGS construction projects last week (see photo).  The existing guardrail posts are bolted into the "retaining wall" below the pavement (plans of the wall are attached).  This was designed to allow for a narrow ditch section directly behind the edge of pavement, with drainage allowed through openings in the curb. 

This situation exists for approximately 200 feet, and MGS is being installed continuously on both sides of this location.  This is in an area of shallow bedrock, which the retaining wall is keyed into. 

I am seeking your opinion on how to address this situation.  Would you recommend bolting the MGS posts to the wall, as was done previously?  Or would we be able to place the posts independent of the wall?  Would thrie-beam be appropriate here, or would standard W-beam be sufficient?

At this time, I cannot recommend attaching the MGS to the headwall. as our MGS bridge rail project has not been completed. Actually, we are only beginning the Phase I part of that effort in the Year 18 program. The Pooled Fund supported a project leading to the design and full-scale testing of an MGS bridge rail using S3x5.7 posts in sockets which met MASH TL-3 impact conditions. If the reinforcement in the retaining wall and road structure are sufficient to support the guardrail posts and would be reasonably comparable to the deck reinforcement shown in the below report, you may be able to use the MGS bridge rail design in this location:

https://mwrsf.unl.edu/reportResult.php?reportId=53

 As an alternative you could attach the MGS using an adaptation of the guardrail culvert system that was previously developed for culvert slabs. We discussed this system in the first three topics of the MGS implementation discussions that occurred before I left on medical leave. Did you or Deanna receive those emails? I am not sure as many of the Pooled Fund State members have not commented on the noted topics.

 If that option is not possible, then I suggest using a thrie beam bridge railing detail that already has been crash tested. Also, one would then use the asymmetrical w-beam to thrie beam transition section in conjunction with approved transition designs. MwRSF is currently revises details for the transition to exisitng transitions as well.