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Standard MGS W-Beam with Weak Soil

State IN
Description Text Would it be acceptable to place standard MGS w-beam guardrail (6 ft posts at 6'-3" post spacing and 2 ft of 10:1 slope behind the post) in weak soil?  We have projects where the soil is predominately peat.  After reading Q&A #1132, the standard system should be acceptable but we should assume a working width of 60 inches.  Do you have any additional thoughts. Thank you
  • Guardrail
Other Keywords weak spoil, peat
Date November 27, 2018


The MGS system has not been evaluated in weak soils at this time, so a definitive answer on its performance in weak soils cannot be given. However, there is evidence that it may perform satisfactory with reduced soil strength.


Previous testing of the standard MGS with 6' long posts installed at the slope break point of a 2:1 slope found that the MGS could safely redirect a vehicle adjacent to a steep slope with reduced soil forces under MASH TL-3. However, the dynamic deflection of this system increased to 72.9" as compared to 44" for the standard MGS in strong soil. Thus, there would seem to be potential for the MGS to work in weak soils.


Similar comparisons can be made to W-beam systems tested to MASH TL-3 with larger deflections such as the G3 weak post W-beam, which had a dynamic deflection of 103.2", and the MGS long span, which had a deflection of 92".


Based on these tests of previous systems, it is reasonable that the MGS has the potential to function with weaker soil. However, it is not known what the strength/resistance levels of the peat soil in your state are or what the dynamic deflection or working width of a system installed in that soil would be.


Thus, we would anticipate that the MGS would redirect errant vehicles under MASH TL-3 impact conditions, but that system deflections would increase significantly. Quantifying the increase in deflection would be difficult without further study. These comments are also limited to the LON of the MGS system. Anchorages and/or end terminals and approach guardrail transition performance may be affected as well. We would also not recommend the use of the weak soil for MGS special applications without further study.


Date December 4, 2018

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