FOUNDATION AT DIFFERENT ELEVATIONS

[Tstruct]

Please have a look at the attached sketch.
I want this type of foundation (as shown by "1" in the sketch) to be constructed, but want to get some answers first.
As shown in "2", some extra space (around 2 to 3 feet) would be needed to be excavated for the contractor to work. When the foundation on the upper level will be casted, there will difference of sub grade under it. I need your opinion on is there any way this extra need of space could be avoided, or any other option to construct this other than tapered excavation.

Thanks.

 

[HTURKAK]

The following detail is common in my zone and remember that it is defined in very old BETON KALENDER.

The numbers show the construction sequence.

....

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[canwesteng]

Just fill the excavation in with concrete and use the soil as formwork.

 

[Moxie_Valor]

Curious what the concern is here. They should be able to backfill behind the stub wall prior to pouring the upper slab. are you worried about the differential settlement between native soil and the backfill? I would think that to be negligible.

"Let me help you by not helping you."

 

[Tstruct]

Is it fine to pour concrete directly against soil?

 

[Tstruct]

Yes one of my concerns is differential settlement and upper foundation may act as suspended

 

[Tstruct]

Thank you Hturkak, you have been very helpful.

 

[Moxie_Valor]

Here's my two cents. The step will add some stiffness, so you're probably right that there will be some portion of the upper slab that doesn't really put a lot of bearing load down on the soil. I don't think that will be enough moment or shear for you to worry about as long as the slab is reinforced for temperature and shrinkage.

You could have them pour against a step in the soil. I think that is why they typically do a tapered step, to avoid having to form a wall. We usually tell contractors to include an extra 3" of cover to the rebar to account for variations in the soil surface if they pour against vertical soil surfaces.

As for the differential settlement, if the backfill is properly compacted you shouldn't have enough differential settlement to cause concern. Good Luck!

"Let me help you by not helping you."

 

[canwesteng]

I would probably just not run the bar in the tapered portion, use concrete only in the tapered part. Make 2 pours only and all the concrete is bearing against native soil (or well compacted fill), not poorly compacted fill dumped into the gap.