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Underpinning a stone foundation party wall

Underpinning a stone foundation party wall

(OP)
I have the task of deisgning the underpinning of a stone foundation wall under a 2-story urban rowhouse. Test pitting reveals that the stone wall is on a loose stone foundation just below the existing basement floor. The owner wants to dig out another 12" for headroom in the basement.

I have some questions: Can I expect significant arching action in the stone wall? Is it reasonable to expect I could install the underpinning with 2', 3', or even 4' sections without the need to shore the wall? If not, what would you recommend for attaching a shoring beam to the random stone wall?

RE: Underpinning a stone foundation party wall

When you describe the stone as 'loose' it tells me that you shouldn't rely on predictable arching action. It seems like the wall needs some pointing first, or reduce the footprint of the excavation by keeping the lowered portion away from the base of the wall.

RE: Underpinning a stone foundation party wall

(OP)
To be more clear, the stone is, in fact mortared, but the stores are not in any conventional bond pattern; just stones fit together as their sizes dictate.

RE: Underpinning a stone foundation party wall

What you're describing is properly termed "irregular" and unfortunately it is the very regularity which you are lacking that makes arches form reliably in block work.

Arches may form, but cannot rely upon them in my opinion.

RE: Underpinning a stone foundation party wall

In cases like that, I use 2' wide sections for the underpinning. Any less just isn't practical, and you don't lose the whole wall should a section crumble. Make sure you alternate sections - leave a new section time to cure before working directly next to it.

RE: Underpinning a stone foundation party wall

Similar to Slta I use 3ft long sections labeled A, B, and C that alternate around the perimeter. I will excavate the A sections then drive rebar into the soil each side. Once that concrete has set I will do similar to the B and then C sections.

RE: Underpinning a stone foundation party wall

Is the stone wall in good shape? Mortar still in place and in decent condition? Rubble foundation walls are not very forgiving. But for 12" excavation below, I agree with SLTA in using 2' wide sections. Max size would be 4' wide... spaced at a minimum of 12' clear apart at each lift. Leave 2" gap so you can shim and drypack.

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