"Flowable Fill" used as subfoundation

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Does anyone have experience with using "flowable fill" under foundation footing? I have received several opinions on the matter. Most informed opinions are positive, but I have encountered a few negative comments too.

My home is being constructed on moderate shrink-swell soils. The 24" wide continuous trench footings were dug from between 30 inches to 40 inches deep. Flowable fill was used to bring the level to about 12 inched from the grade, with 10" of concrete as the footing placed on top of the fill. The builder says he used 200 psi flowable fill (cement, sand, fly-ash and water).

I want to know how common this practice is, and if anyone has long-term experience with this method of foundation construction. I can't get past the reasoning of another builder that told me putting the footing on the bearing soils directly makes much more sense. He is no engineer, but his reasoning is irrefutable- the flowable fill layer between the bearing soils and the footing is a variable in the integrety of the foundation that is unnecessary (if willing to spend a little more money). Still, civil engineers have told me to stop worrying.

What do you all think?

[mjpliv]

I have seen this method used before with no long term ill effects. Its seems to me that this product would be much more stable than natural fill.

[Rich]

Flow fill is great for filling voids etc.. perfect for your situation. The problem I would have with it in your situation is where you mention moderate shrink/swell soils and what that will do to the flow fill layer and eventually your footing. If the engineer says it's fine I would tend to agree with them - as long as they're stamping the construction documents. :)

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Thanks mjpliv and Rich!

The rationale that the engineer gave me for using fill base was the undercut is sufficiently deep to minimize seasonal moisture variations at the level of the natural bearing soils. Still, I share Rich's concerns about how the "low strength" material will hold up over the years in this environment.