Frost heave in saturated gravel below foundation

[naturewhim]

My project requires building pier foundations on a property with a very high water table (0–30 cm from the surface) and potential for frost heave (frost line is 45 cm). As such, I am thinking of excavating roughly 60 x 60 x 60 cm holes where the piers will go, lining the holes with filter fabric, filling the holes to grade with clean gravel and embedding a pre-cast concrete pier in the gravel (below grade but above the frost line).

Given that the gravel will most likely always be saturated with water at least 30 cm below the surface, what sort of frost heave should I expect? I understand that gravel is typically considered non-frost susceptible, but it seems that water freezing will expand and press up the foundation pier in these circumstances. Or would ice expand into the cavities within the gravel?

Other solutions I considered were helical piles, but they are exorbitantly expensive in my area. I could also pour concrete piers below the frost line (e.g., with a sonotube), but with the amount of water in the ground it would be a very messy job and easier if I could utilize the above method instead (if it is viable). Any advice would be most welcome. This is the first time for me to post here so I hope I am doing this the right way.

 

[CarlB]

If you think the groundwater can be controlled sufficiently to perform earthwork to that depth, why not instead just place the pier below the frost level and forego the subgrade replacement with gravel?

The gravel replacement idea should provide adequate foundation support, as the chance of heave/expansion should be minimal as you describe. But there is still some risk of frost movement, as a saturated gravel still might expand some, depending on the pattern of freezing.

 

[naturewhim]

If you think the groundwater can be controlled sufficiently to perform earthwork to that depth, why not instead just place the pier below the frost level and forego the subgrade replacement with gravel?

The gravel replacement idea should provide adequate foundation support, as the chance of heave/expansion should be minimal as you describe. But there is still some risk of frost movement, as a saturated gravel still might expand some, depending on the pattern of freezing.

Thanks so much for the response and advice. That’s good to hear that the gravel should work reasonably well with the caveats you mentioned. Actually, I was intending to just excavate (with an excavator) without pumping out any water since I just need a rough hole to dump the gravel into (which would displace the water). However, with the other method (concrete piers below frost depth), I think I would have to pump out water to make the work manageable, hence my preference for the gravel + pre-cast (small) pier approach. Also, with winter fast approaching, it seemed like it would be easier/faster since I don’t have to mix and pour concrete and wait for it to cure, speaking nothing of creating or setting the molds for the piers.

However, from what you are saying it sounds like the custom concrete pier below frost depth would be the safest approach…

 

[dik]

You try to drain it if you can.

 

[HTURKAK]

I understand that gravel is typically considered non-frost susceptible, but it seems that water freezing will expand and press up the foundation pier in these circumstances

The gravel is not frost susceptible as long as not submerged . In your case , gravel will be submerged and will be frost susceptible . I will recommend that the bottom of PC pier shall be below frost depth. Extending the excavation level below the frost depth should not be big issue.

 

[naturewhim]

Thank you everyone for your advice. Since the consensus seemed to be that the safest method would be to pour concrete piers below the frostline, and I was able to source a pump to make the work more manageable, I have decided to do that. Thank you very much again.