Flowable Fill for Subfooting in Shrink-Swell Soil
Flowable Fill for Subfooting in Shrink-Swell Soil
(OP)
Is flowable fill a good option to use as undercut fill for a continuous trench concrete foundation in moderate shrink-swell soil (residential construction- large house)? I wanted to use #57 stone, but the contractor insists on flowable fill (to save on labor). The trenches are 24" deep. Subfoundation material will take up the first 6" and rebar-reinforced concrete will be poured to make up the remaining 18".
Any specific ratios for the flowable fill mixture used for this application?
Any specific ratios for the flowable fill mixture used for this application?





RE: Flowable Fill for Subfooting in Shrink-Swell Soil
With expansive soils, I would not use any type of stone in an undercut situation becuase it can potentially act as a bath tub and bring the water directly to the expansive soils.
I have seen people use nonexpansive silty clays that have a high enough permeability to limit the amount of water that can reach the expansive soils.
If the flowable fill cracks with seasonal shrinkage and swelling of the underlying expansive clays, could the cracks be conduits for water to get to the clays.
The name of the game in expansive soils is to try to maintain the water content at a constant value.
Glen
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
Glen is absolutely right - constant moisture contents is "the name of the game" in expansive soil environments -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
I don't know the the compressive strength of the fill material used. Should I be concerned about this?
The soil engineer that made foundation recommendations did not specify the compressive strength for the flowable fill material to be used.
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
1/2 sack 75 psi at 28 days
1 sack 150 psi
1 1/2 sack 425 psi
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
Any learned opinions on how well "flowable fill" holds up over the long term?
Tim
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
If the flowable fill is composed of well understood materials: sand, cement, fly ash and water then the long-term performance should not be a problem unless you are in an area with low pH soils or high sulfate content soils. These would require expert review, but can still be done successfully with flowable fill.
If the materials are "exotic" and new to construction, particularly if they are a waste product ('phospho' gypsum comes immediately to mind as an excellent example) then I'd avoid them at all cost...
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
RE: Flowable Fill for Subfooting in Shrink-Swell Soil
I know that vertical rise and fall is the principle concern for shrink-swell foundation problems. What about lateral swelling of the soils in contact with the flowable fill column under the footings? Lateral movement is not a consideration in solid clays, but would a column of unshrinkable material embedded in expanding clays be "squeezed" and weakened over time?