Discount Frost Depth for Pole Foundation Design

[Brian0723]

I have seen it mentioned in some reports that when designing pole foundations, you cannot rely on the soil within frost depth to resist lateral bearing (any resisting earth pressure within frost depth must be discounted). This causes all pole/sign foundations to be quite deep since the moment arm increases as well.

Does anyone have a credible resource/reference that states that any resisting earth pressure within frost depth must be discounted?

 

[r13]

Because soil tends to be weaker when heavethaw starts. I think you can improve the surrounding soil to ensure the development of passive resistance, but I don't know the extent/how far the improvement is required. Another less mentioned reason for not to rely on upper few feet of soil for passive resistance is, I remember the inflection point of sheet piling is at a distance "a" below grade, thus the soil resistance is unreliable within that distance.

 

[phamENG]

Brian - this is how I've understood it. Hopefully one of the geotechs will step in and correct me if I'm wrong:

The frost depth, as I'm sure you know, is the depth to which it is likely that low air temperatures will cause ground water in the void spaces of the soil to freeze. The water freezes and expands within the voids, and the voids grow and the soil mass is pushed up (unless the voids are large enough to accommodate the expansion, but that's generally not the case in naturally occurring soils where you'd be drilling a pole foundation). I could imagine this stage being fairly strong - the soil is hard as ice - but I don't know if that's something anyone has bothered to quantify. The big problem comes after the thaw. The ice melts and the water is no longer taking up the expanded void space. Applying pressure to this soil will cause those expanded (and now empty) voids to collapse, and the soil will settle/consolidate. That's no good if you're relying on it to resist the lateral pressure of your pole foundation.

 

[JAE]

phamENG - that's how we view it as well.

 

[r13]

Thanks both of you. Yes, I made a huge mental mistake. Correction made.

 

[geotechguy1]

Yes, this recommendation is common in Canadian practice unless there is a system of insulation to protect the ground.

Some engineers make arguments about the impact of heat loss from buildings on the frost depth / thawing, but I'm not sure anyone has studied it in detail.

 

[msquared48]

Yes. Here we normally ignore the top foot of soil for any lateral resistance.

Mike McCann, PE, SE (WA, HI)

 

[dik]

I'm not sure that's common here (Winnipeg, Canada)... for small signs, anyway... they are embedded 6' to 8' and occasionally 4', normally and if you deduct the top 6' there's not much left.


Dik