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Underpinning - Minimum Distance without Doing So

Underpinning - Minimum Distance without Doing So

(OP)
Hey All, Happy Friday.

Client is deepening their basement. 2 story building. Adjacent building has a basement. Each building has its own 12" wide foundation wall, but no spread footings, and they are directly next to each other. I'm underpinning my foundation wall, and when all is said and done, the bottom of my new footing will be 10" below the adjacent existing foundation wall.

Question: Do i have to underpin their existing foundation wall (10") so both are at the same depth?

Thanks,

RE: Underpinning - Minimum Distance without Doing So

You need to underpin only the adjacent building if your new building will be deeper. If the foundation wall to be underpinned is not a jointly owned party wall, you will need permission from the owner to go onto his property to underpin his building. Now, if your new building is going to be only 10 inches deeper, you may want to change your design so that you do not go deeper and so you do not need underpinning or permission to underpin. Ten inches is generally too little depth for underpinning, especially when you need 2.5 to 3 inches of dry packing beneath the existing foundation wall.

www.PeirceEngineering.com

RE: Underpinning - Minimum Distance without Doing So

(OP)
Thanks for the reply PEinc.

We have permission from other building owner, but I'm just trying to figure out if underpinning the adjacent wall is really necessary. I can't go any less than 10" per the client's request. I understand that you're saying that constructability may require me to go even deeper so they can underpin wall and add the drypack, but again, does a 10" difference in footing elevations require it in the first place.

Thanks,

RE: Underpinning - Minimum Distance without Doing So

I would think that something would need to be done. If the walls are adjacent to one another, I don't see how you'd excavate that extra ten inches without undermining/disturbing the bearing strata beneath the existing wall and messing with the flow of ground stresses in that area.

Given that your walls have no footing, I'm guessing that you're bearing on some pretty robust material. If that's the case, perhaps a geotechnical engineer would give you the green light to do forgo underpinning the neighboring foundation. I wouldn't be making that call on my own however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Underpinning - Minimum Distance without Doing So

Digging 10" below and continuously along a 12" thick foundation wall could cause the soil beneath the foundation wall to shear off. This would drop the wall and possibly collapse the roof and floor slabs. Please see my web site presentation on Philadelphia's Recent Building Collapses - Causes and Prevention.

www.PeirceEngineering.com

RE: Underpinning - Minimum Distance without Doing So

(OP)
Thanks for sharing. We would definitely be sequencing the underpinning, so as not to ever have the "continuous" condition, 4ft at the most at one time.

Thanks for your input everyone.

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