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Is there any guidance on upward/positive slopes extending behind the MGS? In Ohio, we have always installed standard MGS with a 10:1 extending downward 1’ beyond the back of post, or variations with a 2:1 max. slope in conjunction with longer posts. But we have a situation where a positive 10:1 would need to extend upward for 1’ beyond the post and then break to a positive 6:1. Are there any concerns about the additional fill limiting post rotation? Is there a maximum slope allowed after the grade break (could we install a 2:1 instead of the 6:1 if we needed to)?

I don’t believe we have ever directly answered this question previously.

In general, placement of a steep upward slope directly behind a guardrail post would potentially cause concern. The increased soil behind the post would tend to increase the post/soil resistive forces. This may cause issues with the guardrail performance, including increased stiffness, rail pocketing, and increased post snag. These concerns can also lead to excessive rail loading and rupture. The slope may also prevent the posts and rail from deflecting in the normal manner. We have seen this type of behavior for guardrail installed in paving without mow strips or with excessive embedment.

There is also somewhat of an unknown how the vehicle traversing up the slope as the rail deflects will affect rail loading. Vehicles traversing curbs have increased rail loading and rail rupture potential due to wedging of the vehicle between the rail and ground. A similar effect could be observed due to the slope behind the rail.

To alleviate these concerns, we would conservatively recommend a 2’ offset between the back of the post and the initiation of the steep slope. The use of a 10:1 slope would not be considered a steep slope for guardrail. I think that you could use steeps slopes than the 6:1 if necessary. There is probably a trade off between the steepness of the slope and the offset that could be made, but I don’t have much data to base that on. Thus, the 1’ offset adjacent to the 6:1 slope shown may be reasonable. However, if the slope were to get steeper, then the offset should increase as well.

Thanks!