Underpinning existing slab on grade with concrete wall footing

[Boozie]

Hi All,

I am working a design-build project, and we have reached an issue during construction.

We have a small building that we now want to structurally extend. As-builts are limited. During the design, we assumed that the CMU walls extended 2'-0" below grade and were supported by a concrete strip wall footing. Our goal was then to continue the CMU wall and wall footing system, by doweling into both the existing wall and wall footing and continuing the system.

However, after excavating around the structure, we now realize that the walls do not extend 2'-0" below grade, but instead bear on a slab on grade with a thickened edge. There is now a conflict between the as-built condition and the design drawings, and we won't be able to tie in the new wall footing into the existing slab/thickened edge.

The DOR has proposed to underpin the slab to concrete wall footing, as shown in the attached detail. I don't have too much experience with foundation underpinning. Are there any technical difficulties/considerations if we consider this?

Thanks for your time.

 

[KootK]

I mostly just question whether there's any real value in the underpinning when it's executed so locally like that. Could you tie the walls together rather than tying the footings together? I assume that the goal is to minimize differential settlement between the new and the existing.

 

[Boozie]

Yeah, my concern was the differential settlement that could occur.

We should be able to tie-in the walls just fine, but isn't the wall footing also helping to resist in-plane shear? If the designer accounted for the existing structure to help handle the in-plane shear loads, would we not also need to tie-in the footings together? Although I understand that underpinning like this does not "tie the footings together".

 

[KootK]

We should be able to tie-in the walls just fine, but isn't the wall footing also helping to resist in-plane shear?

It is helping but, as always, it'll be the stiffest path that helps the most. Because of the nature of your existing foundation, I would argue that the stiffest path is the wall and not the foundation. Tying the foundations together makes more sense to me when you have frost/stem walls in play and it is those things that you tie together (because they are relatively stiff).