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Some locations in Illinois have narrow shoulders, often with outdated and deteriorated guardrail in place. Upgrading these locations with new roadside barrier is problematic because of the combination of narrow shoulder, and steep front slope. Often the front slope begins at the nominal shoulder width.
Crash testing at MwRSF has developed an option for cases where the steep slope begins at the back of guardrail post. However, the condition we are encountering is more severe. This photo is representative.
In some cases, depending on right of way, depth of culvert, etc, it may be possible to improve the roadside by placing a grated culvert end. However, in many locations the right of way is also narrow and we are faced either with do-nothing, remove the roadside barrier entirely, or provide an improved roadside barrier.
With the face of guardrail at the edge of shoulder and hinge point to the 2:1 slope, what guidance can we consider in placing a guardrail (MGS)? Is it reasonable to consider placing the posts on the 2:1 front slope? The elevation of the ground at the back of these posts would be approximately 11" lower than at the face of guardrail.
In some cases the slopes may be steeper, perhaps as much as 1.5:1, with resulting elevation difference of the ground line of 14" between the face of rail and the back of post.
In crash testing the more severe angle of departure is usually taken as the critical case. What about this instance? A car or pickup departing the road at a flat angle might be more likely to drop a tire(s) over the hinge point and begin snagging posts.
We understand that the MGS without blockouts has passed crash testing. This would put the post much closer to the hinge point. Would this be a preferable/allowable application with the posts onto the front slope (back of post 9" from the hinge point, and approximately 5" lower than the ground at the face of rail) for this case? What about using the longer 9' posts with the MGS without blockouts? Can we make that translation from one system to the other?
|Date||February 21, 2012|
From the attached photograph, it appears that the rock/gravel shoulder may be 1 to 2 ft wide and maybe conform to a 10:1 slope. The corrugated beam guardrail appears to positioned on a steeper slope, which in some cases may reach 2:1, or even 1½:1. Am I understanding the roadside geometry correctly?
|Date||March 5, 2012|
The location is between Huey Road and Boulder/Ferrin Road (CH 787) on US 50 east of Carlyle.
I checked in our roadway inventory and find that the shoulders are listed as 2' wide, aggregate, except the reverse curve area has bituminous paved shoulders 3' wide. However, I suspect the shoulders were originally constructed to the width where the face of guardrail, and culvert headwalls are. Over time, resurfacing and aggregate wedge shoulders have reduced the usable width by increasing the cross slope.
A couple of additional photos are attached for a better overview. Right of way is narrow, and there are also concrete box culverts for entrances.
I have asked the district to take cross-sections of the roadway. My thought was to regrade and stabilize the shoulders (if necessary) and the foreslope and backslope with erosion control blanket. They could remove the existing guardrail completely and replace where necessary for things like culverts. My thought was where we needed to place guardrail we could place a no-blockout system, essentially such that we are NOT reducing the roadway width (due to farm equipment). The one question I had was what impact did that have on the post length requirement. At this point, I think that this approach would provide significant safety improvement over what is out there and would reduce liability to the agency. Any recommendations you have would be welcomed.
|Date||March 16, 2012|
Thank you very much for the additional clarifications, information, and sample photographs. I like the options that you have proposed below.
Over the course of this discussion, I have received three photographs which appear to represent one particular site and both sides of the two-lane highway. Based on the information forwarded thus far, it is evident that the real-world scenario poses some difficult challenges, including whether to treat potential hazards within the clear zone, allow the current barrier system to remain in place, upgrade the overall barrier system, remove and replace all or segments of the barrier system, perform site grading of varying degrees, etc. Of course, a determination of an appropriate and/or a reasonable clear zone will likely help guide whether to shield or treat various roadside hazards, such as transverse culvert openings, parallel culvert openings which cross driveways or secondary roads, steep slopes and ditches, trees, utility poles, fences, etc. In addition, the existing W-beam guardrail systems more closely resemble prior ILDOT standards and technologies and were likely installed many years ago, including a non-blocked variety of guardrail with W-beam attachment bracket, steel-blocked W-beam guardrail option, and turned-down end terminations.
In terms of the current guardrail placement, there appears to exist two different sloped regions in front of the W-beam guardrail systems. The second steeper slope would appear to exceed 6:1. With that in mind and assuming that an improved guardrail is to remain in place to shield various hazards, then one may consider modifying the approach grading to provide a wider and flatter region between the roadway edge and guardrail face. If possible, one would provide 10:1 approach slopes; however, we have demonstrated that 8:1 approach slopes are acceptable for the standard MGS with blockouts and 31" mounting height. Depending on the grading behind posts, the guardrail configuration may vary. For example, if 2-ft of generally flat soil exists behind the posts, then a standard design can be used. Unfortunately, this may not be the case in your situation. Therefore, it may seem necessary to consider using a non-blocked version of the MGS when steep slopes exist behind the posts, say up to 2:1. In this scenario, I would consider re-grading the ditch to obtain a reasonably-flat approach slope in front of the W-beam rail (MGS) and then cut the ditch to 2:1 or as needed where you may desire the posts to be placed.
The MGS for 2:1 fill slopes was crash tested and successfully evaluated under MASH when installed with a 12" deep wood blockout and with the rail positioned at 31". However, the system was not successful when tested at 27¾". As such, we do not know the lower bound for this system variation when used with a blockout. As you know, the standard blocked MGS has a "recommended" lower bound at 27¾" for new construction based on the successful MASH testing of the modified G4(1s). Note that we recommend that it be installed at its nominal height of 31".
MwRSF previously tested a non-blocked MGS under MASH when placed on wire-faced MSE walls with 6-ft long steel posts and backup plates at the slope break point of a 3:1 fill slope. MwRSF has also successfully tested a non-blocked, steel-post version of MGS with backup plates on level terrain under TL-3 of MASH.
Based on the testing noted above, combined with engineering judgment and the best available information, we believe that a non-blocked version (with backup plates) of the MGS for 2:1 fill slopes could likely meet the MASH impact safety standards when installed at the 31" mounting height. However, it should be noted that no crash tests have been performed on this exact variation and that the lower bound would likely be affected, as was the case for the blocked version placed on 2:1 slopes with 9-ft long steel posts.
As a result and if deployed, it would be highly suggested that the MGS system only be installed with a 31" minimum mounting height when used in a non-blocked version on 2:1 fill slopes using the longer steel posts, backup plates, and an approach slope possibly ranging between 8:1 and 10:1. Of course, the proven crashworthy system which utilizes the blockout would provide the safest alternative out of the two options.
In summary and based on the best available information, we believe that a non-blocked, 31" MGS with back up plates, 9-ft long steel posts at the slope break point of a 2:1 fill slope, and a mostly flat approach slope (somewhere between 8:1 and flat) should also meet the TL-3 MASH impact safety standards. However, the MGS within this scenario has not been crash tested nor have we obtained FHWA acceptance for this variation. In addition, consideration for additional grading should be considered around the crashworthy guardrail end terminals.
Some situations may exist where guardrails can be removed if driveways and parallel drainage or transverse culvert structures are modified with flattened side and cross slopes in combination with any required grating to cover hazardous openings.
These are my initial thoughts for your scenario. I would welcome additional feedback regarding my comments contained above and am willing to continue this discussion. Thanks again!
|Date||March 21, 2012|
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