Question about frost depth.

[stevenspm]

Hello,
I am working on putting a new recessed loading dock between two existing buildings. The local frost depth is 4'-0" below grade. Our design called for the existing footings to be underpinned with concrete for the full width of the existing footings in order to keep the bottom of the footings 4' below the new paving. The owners rep/contractor while excavating the site encountered fairly stiff clay. He wants to do the attached while we are still maintaining that he should underpin the full width. What I am curious about is what his proposed compromise, underpinning only 6" of the footing, work. I realize that there may be failure due to part of the footing resting on hard concrete vs relatively softer clay. I am interested in your thoughts about the attached. Currently we are still maintaining the position of full width underpinning.
Thank you

 

[BAretired]

I don't think that detail works. The existing footing would still be subjected to frost heaving. I think you must underpin the entire existing footing.

BA

 

[BAretired]

Another option which could be considered is to support the lower slab on piles or pedestals to the left of the existing footing. The slab would need to rest on void form and the existing footing would have to be suitably insulated to prevent frost heave.

BA

 

[JAE]

And build in stages to avoid the existing footing from settling?

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[TLHS]

Insulating under the new slab and up the wall would solve a number of your problems. You'd need an isolation joint in there somewhere to deal with the possibility of differential settlement.

If you're okay with the slab heaving, you could insulate under it far enough to ensure you don't get heave under the footing. Once again, you'd have to carefully consider isolation joints.

 

[oldestguy]

How about insulating the exposed old foundation wall? Heaving does not have any rule to go only upwards as long as there is an easier push sideways, into the exposed area formerly buried. Also, a haunch extending to the full coverage of the old wall width avoids eccentric support there.. With insulation, as described by TLHS and as indicated here, that old footing is not likely to heave.

 

[stevenspm]

Thanks guys. What we are doing is telling the contractor that they need to underpin the whole width of the footing in sections. We may change if a geotech engineer can recommend or say that the detail as shown will be OK, but I am doubtful that will happen. What I was really curious about is how frost would penetrate in the soil with this type of detail as most of the work I have done has either been with new buildings or renovations to existing in frost free areas such as the Southeaster and Southwester US.