Flowable fill below frost line?

Question

Flowable fill below frost line?

milkshakelake (Structural)

(OP)

13 Oct 23 20:35

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

CarlB (Civil/Environmental) · Answer 1 · 2023-10-13 23:02:50

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.

milkshakelake (Structural) · Answer 2 · 2023-10-14 00:03:14

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.

DaveAtkins (Structural) · Answer 3 · 2023-10-14 11:58:03

I agree. So long as frost susceptible soils are kept below the frost depth, the risk of frost heave is effectively mitigated.

DaveAtkins

bones206 (Civil/Environmental) · Answer 4 · 2023-10-16 20:50:57

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.

milkshakelake (Structural) · Answer 5 · 2023-10-16 21:00:15

@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.

bones206 (Civil/Environmental) · Answer 6 · 2023-10-16 21:31:35

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.

dik (Structural) · Answer 7 · 2023-10-17 05:38:59

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

milkshakelake (Structural) · Answer 8 · 2023-10-20 18:30:03

@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.

dik (Structural) · Answer 9 · 2023-10-20 23:14:48

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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Flowable fill below frost line?