Frost Depth Increase Due to Foundation

[JohnnnyBoy]

I came across some info from a screw pile manufacturer that was saying one of the benefits of screw piles is you don't increase the frost depth like you do when using a pilaster. See attached photo.

When I design strip footings, isolated footings, pilasters or any foundation system I solely use the frost depth from experience or from geotech reports. If typical frost depth is 4'-0" do most engineers design for deeper than this if using a pilaster? This example would assume an unheated structure, most likely for a deck.

 

[SteelPE]

Maybe I am not up with the times, but, the only time I have ever heard of increasing frost depth is for pipes under paved roadways (this has to do with the roadways being cleared and exposed to freezing temperatures vs the "insulation" of snow cover on top of the ground). If the jurisdiction requires a 4'-0" frost depth we indicate that depth on the drawings and then design the foundation accordingly using the 4'-0" specified. Also, I was always under the impression the frost depth was a conservative number.

 

[JStephen]

That sounds like salesman-talk, actually. The screw pile may be smaller, but is still very thermally conductive, certainly more so than any soil it displaces.

 

[Craig_H]

That seems like utter nonsense. This ignores the fact that the coefficient of thermal conductivity of steel is ~ 20x that of saturated concrete. The frost penetration around a pile is indeed slightly deeper than in the surrounding soil due to:
1) the surrouinding ground likely has turf and a snowpack sitting on it (which both insulate the ground surface from the air)
2) the coefficient of thermal conductivity for the pile material is likely higher than for the surrounding soil

Typically, the geotechnical reports that come across my desk recommend a specific type of foundation and provide guidance on the minimum depth for frost protection. If I were to consider a type of foundation not recommended by the geotechnical report, I would go back to the geotech consultant and ask for their input.

I personally find some helical pile suppliers to be approaching snake-oil levels of truthfulness in their sales materials. I recall once seeing a helical pile manufacturer who offered to fill the pipe section with spray foam to reduce the frost penetration depth. Similar nonsense and utterly useless.

 

[dik]

snakeoil...

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

-Dik

 

[JohnnnyBoy]

Thank you all for the replies, I am glad that we are all on the same page that this is a snake oil salesman rather than a design consideration that I should have been previously using.

 

[milkshakelake]

If the pile cap or grade beam is above the soil level, I actually could understand what they're saying. I haven't seen that before; the pile caps I've seen and designed are always in the ground. The building code here specifically says that the pile cap must adhere to the frost depth.

The picture looks like they're putting the foundation above the soil. So in that case, frost heave isn't an issue for pushing the concrete part of the foundation upwards. I personally wouldn't construct it that way, but there is some logic to it.

Just something that came to my mind: how do helical piles and tiebacks not corrode in soil, even if galvanized?

 

[phamENG]

Just something that came to my mind: how do helical piles and tiebacks not corrode in soil, even if galvanized?

The AGA has a decent primer....

 

[milkshakelake]

So it does corrode, but it's hard to predict the rate. Will keep it in mind. I hope these piles don't collapse in 100 years or so.

 

[JohnnnyBoy]

Milkshakelake, if you don't mind me asking where are you referring to when you say the pile caps need to adhere to frost depth. This seems a little wild to me since in some cases frost depth is roughly 10'-0". To make a pile cap go to frost depth then it would be impractical to design a piled foundation since you would need to excavate so deep anyway.

phamENG, yes the screw piles would corrode over time, the highest corrosion rate would occur at the surface interface where there is plenty of moisture and oxygen. The deeper you go the less oxygen and therefore at one point it would be considered to have no corrosion.

 

[milkshakelake]

My local building code:

This provision isn't exactly in IBC as far as I'm aware; it doesn't specifically mention pile caps in 2018 IBC 1809.5. It does say "foundations."

 

[JohnnnyBoy]

Very interesting and this might lead me to start another thread since up in Canada, we never put the pile cap below frost or at least none of the buildings I've seen or been a part of.

 

[dik]

...but we put voidform below them, at least that's common.

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

-Dik

 

[Craig_H]

As Dik has said, often the void form is considered to provide sufficient room for the ground to heave upwards without contacting the pile cap. If you have some sort of exceptional situation, you may need to consider increasing the depth of your void form, or taking other measures to protect the pile cap. I've never run across that, though. A bond break is also common in frost susceptible soil to prevent the upper (frozen) soil from adhering to the pile and "jacking" the building up. I seem to remember someone telling me of a building in Northern Alberta that was climbing every year on the order of inches due to frost jacking.

All of this of course depends on whether the building is protecting the piles from frost action (covering the ground, internal heat loss into the piles, etc), or whether the piles are isolated from the building's heat and are exposed to the environment. The current trend among architects is to pay more attention to preventing building heat loss into the foundation, making frost protection a growing concern.

 

[dik]

I use plastic foam voidform, and have for a few decades. I've done a couple of investigations where it is likely the cardboard type has disappeared and the void filled with water and caused the heaving...

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

-Dik

 

[dik]

The best way to do that is to extend the building envelope to include the foundations, if you can.

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

-Dik