Slab on Grade Construction over demolished (lowered) existing foundation wall

[Jc67roch]

To save money and disturbance, I am looking to construct a new slab on grade over existing foundations and foundation walls. I would like to leave the existing foundations and walls in place, but demolish the walls to some distance below the bottom of the new slab, install compacted granular base material (over the remaining wall), and then construct the new slab on top of the base. My question is, what is the recommended minimum depth of demolition for the foundation walls below the bottom of proposed new slab on grade to avoid cracking issues (the remaining foundation wall providing a rigid or pivot point below the slab)? Is 12 inches sufficient? 16 or 18 inches? Or more?

 

[JAE]

I think I'd be comfortable with 12".

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[SuperSandman]

...and maybe add extra reinforcing at the pivot point(s)

 

[Ron]

A lot depends on the use of the slab. If you'll have forklift traffic or similar on it, the pavement will "feel" the hard spots. At those locations you'll likely get some reverse bending and potential cracking. Pay attention to the stability and compaction of the materials you place over the existing foundations. As JAE noted, 12" is fine; however, I would add a stability and compaction requirement to that.....for stability, I would require a CBR of at least 40 to 50 and compaction to 98 percent of the modified proctor maximum dry density.

 

[moe333]

12" would make me a little nervous. I would go with 24" along with Ron's suggestions in the lower 12".

 

[Jc67roch]

Thanks All! Great suggestions!

 

[cvg]

so the failure mode is differential settlement or alternatively, differential modulus. the surrounding area will ultimately settle more than the existing concrete wall. It also has a lower modulus which could result in some bending moment in the slab. placing granular material and compacting it extra well where the concrete wall is removed reduces future settlement and increases the modulus, however it might create a new "hard spot", so I am not sure if that improves things.

one solution is to reduce the settlement of the surrounding area to match that at the foundation wall. That might mean overexcavation, backfill and recompacton in order to improve the subgrade so that it all settles uniformly and has a similar stiffness. Very expensive option

alternatively, might be better to not over do the subgrade improvement and compaction in this one area and attempt to match settlement and subgrade modulus to that of the surrounding area. Removing the concrete structures to a deeper depth will accomplish this possibly without the need for granular backfill and special compaction requirements.

Or just reinforce the slab and allow it to bridge the small soft spot.