Lateral resistance and frost depth 1

[JDvmi03]

I'm working on an analysis of an existing drilled shaft in SM soils (frost susceptible), where overturning is the governing criteria, not end bearing.

The existing drilled shaft terminates 9 feet below ground surface and frost depth is 5 feet below ground surface. When I neglect lateral resistance below frost depth this foundation is failing, big time.

I'm afraid to recommend removing any existing soils to place an "insulating" layer of materials less susceptible to freeze-thaw/seasonal variations, because 1) there is an existing structure adjacent to the one I'm considering. 2) I'm concerned that removing any existing soils would worsen the conditions and reduce my lateral resistance.

All of my resources specifically say that you should bear below frost depth, which this criteria has already been met. Does anyone have any reliable/credible resource(s) that includes soils above the frost depth for lateral resistance?

Intuition tells me that there should be some resistance, though it should probably be reduced. Thanks.

 

[CarlB]

I assume you menat "when you neglect lateral resistance ABOVE frost depth"...

When thee soil is frozen, strength and lateral resistance is generally much greater than when thawed. After thawing, and any excess pore water pressure dissipates, the soil returns to essentially the same strength (possibly some decreased density/strength in upper foot or so.) The worst case (for a particular layer/depth) is as the soil is thawing, if it had formed ice lenses when it froze, as it can temporarily lose a lot of strength. Hard to say the worst case for lateral resistance, depends on the rathe of thawing versus moisture dissipation, and there is some incresed capacity from the soil that remains frozen.

My experinece (in Alaska) is that reduced lateral capacity is rarely a concern. Frost jacking is what usually causes problems for posts/piles. You might consider placing board insulation (say at 1'-3' depth), which would reduce frost penetration, help lateral somewhat, and help resist jacking.