Modifications to an existing slab/footing system (supporting walls) 1

[LR11]

Has anyone had any experience with the following:
Converting an internal wall to an external wall, and modifying the foundation underneath.

Locally, what is normally done for a new dwelling is to place a reinforced concrete strip measuring 300x300mm to 400x400mm, underneath the slab. This supports a double brick wall.
But to cut and modify the existing slab seems like a lot of work.
Has anyone ever installed a footing above the slab, as shown in the second option?

 

[r13]

If freeze-thaw is not a problem, then consult with a geotechnical engineer for settlement assessment.

 

[LR11]

Thanks for the response.
Have you come across anything to estimate the settlement based on the standard soil tests.
Please bear in mind that I'm not familiar with the testing regime/s.

 

[milkshakelake]

Neither option seems feasible. You can't install a foundation under an existing wall with no foundation. And you can't install it above because the brick isn't floating.

I agree with r13 that frost depth is a concern for an exterior wall.

Other foundations on this property have settled over the years. This new foundation will settle more because settlement takes 5 years. Best to consult a geotechnical engineer.

 

[LR11]

Thanks for responding.
The walls will be relocated, this is not an issue. What I want to do is keep modifications the slab at a minimum.
With respect to frost, it's not an issue.
Settlement: I guess a large part of the query is to asses this.

 

[milkshakelake]

I think the people in the geotechnical forum can give a good answer. In my limited experience, a soil boring with the type of soil, blow counts, and loading analysis by you is enough to do a differential settlement analysis.

 

[structSU10]

I'm guessing this is a bearing wall? If it is, there should already be a footing below, or a thickened slab. In that case, underpinning the slab from the new outside face could be workable. The contractor would need to do it in small zones, using needle beams and shoring of the existing wall as required by the loading / geotech analysis.

If it's not a bearing wall it seems like you would want to remove the slab on grade to make a continuous foundation wall to define the perimeter of the building / provide required insulation and such, which would beam demo of the wall and rebuild (maybe using different materials) could be a better option.

 

[LR11]

Ok thanks I'll try the geotechnical forum.

 

[phamENG]

If that is a slab on grade, then I would go with cutting the slab and excavating to put the footing down where it should be.

If this is a basement slab, then I might be willing to consider the "inverted footing." I was actually going to use a similar detail recently. Assuming you've got frost depth and lateral stability worked out, your next problem is cracking of the slab below. The US DoD has a really useful (if somewhat dated) document: UFC 3-320-06A. You can use that to get a good idea of what the slab could likely take before cracking. Another option is to saw cut the existing slab where the block will be cast on top, forcing it to crack along the edge and preventing unsightly damage.

 

[LR11]

OK great, I think that will get me started.
Thanks phamENG.