Foundation design (footings placed at two elevations)

We are building a breezeway home (like photo attached). One side, the main living area, will have a finished crawl space for storm shelter, and the other side will be a slab on grade w/ bonus living space. The excavators plan to dig down 1 foot and then build up as we live in a wet area. Originally, we had planned to create one shared footing with delta-footing membrane and one shared stem wall. Our concrete contractor has recommended a 5’ wall for the crawl space side and a 4’ wall for the breezeway and bonus space to save on concrete. This would raise the footings on breezeway/bonus side by a foot while still sharing the same stem wall. A couple thoughts…
1. This would mean that the breezeway and bonus space side would only have 3’ below grade (the frost line is 4’ in our location). They assured me this is not a problem because 2’ of exterior foam would be on the exterior of the entire building and up to the top of the stem wall which qualifies the full 4’ (including the 1’ above grade) as having 4’ of frost protection even thought it doesn’t go down to the frost line. Is this correct?
2. With each side’s footing being at a different elevation, and the entire home being built up, this would mean one side may have 1’ fill underneath the footings where the other side might have 2’. Is this an issue for uneven settling?
3. Would this compromise the effectiveness of the footing membrane? If the footing membrane is 1’ lower on one side, would you have a place where water can wick into the lower wall where the two sides meet?
4. The concrete contractor is also concerned using the footing membrane would make the stem walls unstable and able to shift over time as the only thing holding the wall in place would be the key. His concern is the stem wall concrete would not be able to bond to the footing concrete. Has anyone had an issue with this?
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Replies
Emil,
I only have a problem with #1, which makes no sense. The frost depth is the frost depth. It is independent of what occurs above grade. Following your contractors logic you could continue the exterior foam up another three feet and not need any below grade.
#1: I agree with Malcolm. Your builder is misapplying legitimate rules for frost-protected shallow foundations. His reasoning is wrong but the system would work at the heated house. It would be useless at the breezeway.
#2: Properly specified and properly compacted fill under footings is fine. Note "properly." Ask what he plans to use and what his plans for compaction are. Terms vary regionally but I'd want angular 3/4" stone compacted to refusal with a flat plate ("jitterbug") in maximum 6" lifts. Or another fill graded for structural use. No bank run gravel. (Bank run means taken directly from a gravel pit with no processing.)
#3/4: Capillary breaks like the Delta membrane are still pretty new to the residential market and builders are often nervous about them. You do need something to prevent the wall from shifting on the footing; a lot of pressure is trying to push the wall inward. A structural engineer can design a rebar schedule that would prevent that from happening, and/or you can insist on a keyway--a groove cast into the footing that the wall locks into. Or both. Alternatively, you can get and apply Thoroseal, a paint-like product, to the footing surface. The contractors probably won't do it and you'll have limited time between footing and wall placement. Concrete cold joints have little strength so contractor concerns are largely invalid. You should have footing drains leading to open air to deal with groundwater; if you don't, a capillary break is worthless.
#4: