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Flowable fill below frost line?

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milkshakelake

milkshakelake

Structural
Jul 15, 2013
1,125
The frost depth is 4'. My pile caps are 2.5' thick. Can I fill the bottom to frost depth with something flowable, like 3/4" crushed stone? I saw discussions here about using void forms, but this is a pile cap and it'll be pretty difficult to place styrofoam around a pile.

The soil itself under pile cap sucks. It's all fill (sand, silt, gravel, brick, debris) with blow count of 3 to 6, which is one reason why we're using piles in the first place. I don't know if this matters, but the other reason is because it's in a flood zone.

Edit: fixed some grammer
 
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This sounds plausible to me. The 3/4" crushed is certainly non-frost susceptible, and drains well. You could use other aggregate as well, such as pea gravel.

I assume that after driving piles, soils would be excavated below cap to -4' (and say 1' min beyond), backfilled with aggregate, then pile cap placed on that? If surrounding silts/clays could somehow work their way into voids in the rock, you may want to surround with a separation geotextile.
 
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Understood, thanks! Yes, the procedure you said is exactly what will happen. I don't think I have to worry about silts coming in because the water line is 4' lower than my bottom of excavation, but I'll use that if the site conditions are different.
 
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I agree. So long as frost susceptible soils are kept below the frost depth, the risk of frost heave is effectively mitigated.

DaveAtkins
 
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I generally locate pile caps and grade beams at the frost depth. I wouldn't be comfortable substituting ground cover to bottom of foundation with a layer of crushed stone unless it was specifically engineered by a geotech. In other words, I provide the same frost protection to deep foundation elements that I would for shallow foundation elements.
 
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@bones206 I do that, too. In this case, I'm superceding the job from an engineer who passed away and he didn't follow frost depth. When I brought that up to the owner, he didn't want to spend anything extra compared to the previous design, so I had to make the decision. It's not ideal, but it'll work and it's a reasonable amount of liability.
 
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I see. Reasonable approach then. When I first read the title, I assumed you meant a low strength flowable concrete fill. To me that would be an even better option than aggregate, and with less frost risk.
 
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I would suggest not... if water is trapped, it will freeze and expand.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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@bones206 Good idea, I haven't thought of that.

@dik That's a concern for soils but crushed stone has voids that allow water to mostly freely drain.
 
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as long as it drains...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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