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AGT fill question

State OH
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The question is:  can we use our 617 compacted aggregate instead of standard embankment material for the attached bridge terminal assembly?





Approach Guardrail Transitions (AGTs)

Material Specifications/Alternatives

Date September 11, 2023
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Attachment MGS-3.1_2018-01-19.pdf

A few thoughts from me.  First, shoulder or embankment fill soil is typically recommended for guardrails because it is assumed to be a strong soil.  If the soil if strong enough to hold embankment/slope shape, then it is likely strong enough to support guardrail systems. Thus, I kind of works as a blanket assumption/recommendation in lieu of doing specific analyses on all the soil types from around the country.  That said, soils other than typical embankment/should fill soils may work just as well.

Guardrail transitions are more susceptible than typical MGS installations to negative performance effects resulting in weak soils.  For a typical MGS, a weaker soil would result in higher deflections, but little else should change.  Increasing system deflections in a transition region can easily lead to increased vehicle snag and pocketing, which increases the risk of excessive decelerations, excessive occupant compartment deformations, and/or vehicle instabilities. Thus, care should be taken to ensure guardrail transitions are always placed in strong soils.

The gradation windows for your 617 aggregate are similar to the gradations we get for our crushed limestone soils used for crash testing.  That said, we heavily compact our soils – I believe 95% density is the target for our compaction. I did not see a compaction requirement for your 617 aggregate, but I may have missed it.  If you have similar gradation and similar compaction, it would seem reasonable to expect similar strength (lateral resistance for the posts).  Of course, there would be many other factors at play (aggregate materials, plasticity, etc…), but gradation and compaction is a good start.

The best way to evaluate if a specific soil provides adequate strength would be to conduct tests to measure its strength.  I would likely run lateral tests on the posts in the selected soil to obtain force-displacement curves and compare them to the curves obtained from similar tests at crash testing labs.  There may be other tests to help determine a soil’s strength, but I am not a soils expert and would need help in identifying such tests.



Date September 12, 2023
Previous Views (56) Favorites (0)