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Existing Guardrail on Slope

State IL
Description Text

In this contract, we are widening the Tri-state Tollway embankments for the fourth lane to be constructed in 2008 (Southbound) and 2009 (Northbound). The work included topsoil removal, embankment and retaining wall construction. No work was performed on the existing pavement or shoulder. The intent of this first contract was to leave the existing guardrail in place. In 2008 or 2009, the guardrail would be removed and replaced with new.


During construction,the contractor designed mechanically stabilized retaining walls required longer tie-backs than assumed in design. This required additional excavation that extended to the guardrail/back of shoulder and resulted in a need to support the guardrail when the shoulder is reopened to traffic at the end of this contract.


A solution was suggested to drive a new post behind the existing to provide necessary support. Will this work? If not do you have any other options to consider.


They have removed the embankment from around/behind the guardrail posts to a depth of about 2' below the original grade.  The existing guardrail is our former strong post system with 6'-9" long posts, and a top of rail height of 27.5", using 6" blockouts.


Does their suggestion to drive additional posts to back up the existing appear feasible?  What would we need to determine an appropriate post length and size?


It does not seem likely to me that the back up posts would work.  It appears that a vehicle deflecting the rail over the 2' vertical drop might roll or snag on the posts.  My suggestion was that they re-establish some fill around and behind the posts.  Another thought might be to add a rub rail below the w-beam.


Any comments or other ideas would be welcomed.
  • Guardrail
Other Keywords Slope
Date August 21, 2007


It is my understanding that your current situation involves the placement of guardrail posts at the slope breakpoint. However, it is unclear to me in the drawing as to whether the side slope is a 2:1 or flatter. If the roadside embankment is 2:1 or flatter, MwRSF has developed two different TL-3 guardrail systems for this situation. The first system was developed several years ago and utilized metric height (27 ¾-in.), strong-post W-beam guardrail with 8-in. blocks and 7-ft long steel posts spaced 37 ½-in. on center (half-post spacing). The second system was recently developed using the MGS technologies. For this MGS W-beam guardrail, a 31-in. mounting height was used with 9-ft long steel posts, 12-deep blockouts, and a standard post spacing (6 ft " 3 in.).


As an alternative, it would be acceptable to re-establish the 2 ft of compacted roadside fill behind the posts in order to allow for the installation of standard, strong-post W-beam guardrail systems. However, it would seem to me to be much more costly to redo the earthwork than to place one of the two recommended guardrail systems described above.


Finally, I am not in favor of utilizing the second backup post behind another one as we would not know how this combination would perform under full-scale testing.


P.S. " If your other guardrail system is already in place, it may be possible to drive intermediate posts measuring 7-ft long in order top account for the half-post spacing posts. Then, the rail would likely need to be raised to the metric rail height of 27 ¾ in. along with the replacement of 6-in. blocks with 8-in. blocks. With this system, the only non-standard part would be every other post would be 3-in. shallower than designed using system no. 1 noted above. I will see whether Bob or Dean have anything to add to this comment or if they disagree with this last possible alternative.

Date August 27, 2007

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