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We have a Weigh-in-motion enforcement site being constructed along I-90.  The designer proposed essentially widening the asphalt shoulder by 30' for the State Police to use to pull overweight vehicles over to check with portable scales.  This area has tapers on each end and is several hundred feet long.  The State Police had requested that the area be "protected" with guardrail, so the designer proposed a run of guardrail parallel to the mainline, between the mainline shoulder and the enforcement area.  The pavement is 9" asphalt and they are proposing to drive the posts thru it.

questions:  will the guardrail react properly when placed in that thick of pavement?  I thought that the posts needed to be able to rotate in the soil to absorb the energy.  That is why we are telling all of the designers that the posts cannot be placed in concrete.  Wouldn't they just snap off or bend at the top of pavement?

If 9" of pavement is too much around the posts, how much is acceptable?  has this been tested?

Prior testing of W-beam guardrail systems with thick asphalt (or rigid concrete) surrounding the posts has been shown to degrade guardrail performance. Several years ago, TTI researchers developed a methodology for placing guardrail posts in a cutout to allow for adequate post rotation (Report No. 1 and ASCE Paper). Details for this method are contained in the attached FHWA acceptance letter (B64b.pdf). Within this letter, FHWA also included details for placing posts in situations where subsurface rock is encountered, per a research study by MwRSF (Report No. 2). In the MwRSF study, additional details were provided for the configuring the size of asphalt leave-outs.

More recently, TTI researchers have continued to develop leave-out alternatives for guardrail posts placed in mow strips. Although that research is continuing or recently completed, I will try to find either a recent progress report and/or draft report that summarizes the most recent findings and acceptable practices for posts placed in mow strips or over subsurface rock (Report Nos. 3 and 4).

You are correct in noted that it is desirable for guardrail posts to rotate in the soil and dissipate a portion of the vehicle's kinetic energy. When premature wood post fracture occurs, other behavior may occur, such increased barrier deflections, vehicle pocketing, or vehicle instabilities upon redirection. Similarly, steel posts may yield with limited displacement at the ground line, thus changing the loading to the rail as well as the rail movement while deflecting. For steel posts, rail rupture can occur as well as barrier override. For now, we must provide leave-outs in the rigid pavement in order to allow the posts to behave as they would in compacted soils. TTI has developed some alternative leave-outs that may be worth considering, as presented in the latter reports. Finally, you are correct in noting that 9-in. asphalt pads are excessive and would result in wood post fracture or immediate steel post yielding and twisting.

Several weeks ago you sent me a considerable amount of information on guardrail posts in concrete and guardrail when used in mow strips.

I have gone thru most of what you sent. There seems to be a range of values for the leave-out area around the posts.

The 2004 report by TTI recommends an 18" x 18" area, which only leaves 9" behind the post, but this was not the MGS. I think I saw somewhere else that it should be as much as 2 feet behind the post.

Using the MGS, what value are you comfortable with from the back of the post to the edge of the leave-out hole? I am thinking of using an 18" x 24" leave-out area, which provides 15" behind the post.

A distance of 15" behind the post would be more than adequate. I could comfortably live with 12" as well.