View Q&A

We have a pavement overlay project where the district wants to add 6 inches of pavement, which causes a sliver 1.5:1 slope at the guardrail.  The district is balking at expending additional funds needed to extend the shoulder is this fill segment.  See attached sheet, and refer to the outside shoulders.

The design currently calls for standard 6 foot post to be installed at the breakpoint of the slope, and not our standard installation of having 2 foot of shoulder behind the post before the break point. The existing guardrail would be raised 6 inches as shown on the sheet, but we are concerned about the sliver fill at that point and the 1.5:1 slope to catch the existing 2:1 embankment slope.  Since the 6" fill at the guardrail would probably be just dumped and not compacted, I want to ensure we have sufficient post embedment for the 27 guardrail height.

I've reviewed MWRSF recommendations for W-beam system on a 2:1 slope and was wondering if the use of 9 foot posts (at half post spacing) on this project would compensate for the sliver?  And could wood posts be substituted for the steel, even with the longer post lengths?

MwRSF has performed two studies involving W-beam guardrail placed on 2:1 fill slopes.

The first study involved the development and testing of metric height, W-beam rail (706 mm = 27-3/4") supported by W6x9 by 7' long steel posts spaced 37.5" on center. The center of each post was placed at the slope breakpoint for a 2:1 fill slope.

The second study involved the development and testing of the MGS, W-beam rail (787 mm = 31") supported by W6x9 by 9' long steel posts spaced 75" on center. The center of each post was placed at the slope breakpoint for a 2:1 fill slope.

Both MwRSF efforts, in cooperation with the Midwest States Pooled Fund Program, were successful.

Later and using available crash test data as well as engineering judgment, Dr. Dean Sicking and Mr. Bob Bielenberg prepared the preliminary guidance for guardrail placed on slopes, subject to refinement in the future. It was as follows:

For standard W-beam guardrail:

1.  Standard W-beam guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.

2.  For w-beam guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.

3.  For w-beam guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.

4.  For w-beam guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 7' W6x9 posts at half spacing are recommended.

For MGS guardrail:

1.  Standard MGS guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.

2.  For MGS guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.

3.  For MGS guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.

4.  For MGS guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 9' W6x9 posts at standard spacing are recommended.

In 2008, TTI researchers evaluated 27" tall, standard W-beam guardrail with half-post spacing placed 1ft off of a 2:1 fill slope using TL-3 of NCHRP 350. For this evaluation, the pickup truck rolled over in front of the barrier.

Based on the information provided above as well as the proposed OHDOT design detail, I recommend using one of two options. First and assuming a portion of the 6 in. of fill would be effective and remain in place over the years, it would be acceptable to use the Case 4 option for standard W-beam guardrail which consists of 7-ft long steel posts at half-post spacing although preferred to be at the metric height of 27-3/4 in. Alternatively, it would be acceptable to use the Case 4 option for MGS guardrail which consists of 9-ft long steel posts at full post spacing. The overall preferred option would be to use the MGS with 9-ft steel posts spaced on 75-in. centers. For the wood post option, MwRSF would need to evaluate longer posts in a 2:1 foreslope using dynamic bogie testing before recommending a suitable post length.

In the future, we recommend that the OHDOT begin to implement the MGS along with its crashworthy design variations, including systems for placement across culverts, adjacent to slopes, etc.