Flowable Fill for Subfooting in Shrink-Swell Soil 5

[rtimg]

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?

 

[gandersen]

rtimg:

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

 

[geotekker]

We have had good luck using 150 psi flowable fill as sub-foundation bearing material in Alabama, where we have some high plasticity clay. Our approach it to use the flowable fill (which has greater strength than the surrounding soil)from the footing bottom to below the zone of typical seasonal soil moisture change, which in central Alabama is on the order of 3 feet. I agree with ganderson, open-graded crushed stone beneath footings in a high plasticity clay environment should be avoided. Cement, fly ash and water are the typical components used in our local flowable fill mixes.

 

[Focht3]

Avoid the stone at all costs. A "low permeability" flowable fill IS a superior answer. And it's cheaper, too.

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?"

 

[rtimg]

Thanks for the advice. I decided to go with flowable fill. When digging the footings, soft/moist soils were encountered and had to be undercut, requiring extensive use of flowable fill- 32+ inches in some areas to support 10" x 24" continuous concrete footing- considerably more flowable fill than I planned on using. Any worries here? I am not comfortable with extensive use of "low strength" subfooting material, but didn't seem to have many reasonable (affordable) alternatives.

 

[cvg]

assuming 150 psi compressive strength, your "low strength" flowable fill is much stronger than any type of clay soil

 

[Focht3]

Agreed.



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?"

 

[rtimg]

Thanks again. I guess what I am struggling with is the use of flowable fill between the foundation footing and the bearing soils. It seems logical that placing the footings at the bottom of the trench would have been a better approach to building the foundation. The big question is, how much better?? Gatorcock has described the same method in reply #2 so I know others are doing the same thing. There is very little information on the internet regarding the use of flowable fill for structural applications so I must assume that it is a relatively new practice.

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.

 

[cvg]

even a half sack mix will have more than enough strength.

1/2 sack 75 psi at 28 days
1 sack 150 psi
1 1/2 sack 425 psi

 

[rtimg]

Thanks for putting the "low strength" designation of flowable fill into perspective for me. Very sound reasoning. No one seems too concerned about placing the footings on top of fill versus directly on the bearing soils so I guess I need to take a Valium and relax. My biggest concern is what will happen to the flowable fill over time. No worries now, but will I run into problems 10-15 years from now. It is a relatively new material, and uniform standards have not been clearly established.

Any learned opinions on how well "flowable fill" holds up over the long term?

Tim

 

[Focht3]

Well, it depends.

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?"

 

[cvg]

when I speak of flowable fill, I speak of a standard product specified and used widely in this area for Controlled Low Strength Material (CLSM). Just portland cement, aggregate and water, no fly ash. Aggregate meets ASTM C33 specs and the mix design must be approved by the Engineer.

 

[rtimg]

Thanks all. I was told the components of the fill were sand, cement, fly ash and water. It looks like they may have mixed some air in the fill too as it appears to have very tiny bubbles entrapped- like a dried foam material. (I was able to inspect a small sample that was spilled on site).

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?