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Digging out a crawlspace

Digging out a crawlspace

Digging out a crawlspace

(OP)
I have a contractor who wants to convert most of his crawlspace into a basement (it is his own home). Enclosed is the detail we are tossing around. The soil is pretty stiff and he is doing this in small sections so he is not worried about stability during construction. My question is how to account for the surcharge load due to the existing house footing? Part of the footing load is being used to prevent overturning of the new retaining wall, but part of the load is adding surcharge. If the computed pressure under the existing footing is, say, 750 psf (service), what would you do?

Thanks

RE: Digging out a crawlspace

AASHTO has a bunch of info on surcharge loading from spread footings and similar. Most geotechnical textbooks have some info as well. The load can get pretty high but your detail should be feasible.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Digging out a crawlspace

(OP)
I imagine they do. But my case seems different as part of the footing is sitting on top of the wall.

RE: Digging out a crawlspace

Have done this often, but make the wall thickness the same as the footing width... no eccentricities... break into underpinning segments... I'll see if I can find my standard detail for this...

Dik

RE: Digging out a crawlspace

Like dik, my first stop here would be conventional underpinning. I'd want the footing all the way supported or not at all. Edge support seems like it would lead to weird settlement and/or transverse bending of the footing. A very recent thread here on brick wall shoring actually contained the usual, sequential detail.

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: Digging out a crawlspace

Yes, why not conventional underpinning ?

RE: Digging out a crawlspace

I retract my statement, I didn't notice that it's supported on the edge of the wall there. I agree about this causing lots of weird issues and not worth the headache. I also assumed that conventional underpinning was ruled out for some reason. If possible that would be easier to design.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: Digging out a crawlspace

I assume you're designing the wall for active pressure as a cantilevered element? If so, you have the lateral pressure on the existing wall being resolved up to the floor diaphragm and down to the top of the new wall (might not be much depending on how much soil you have there). You also have the lateral pressure from the soil below the existing footing that acts on the new wall.

If the existing footing rests on the edge of the wall, this is an eccentric axial load on the wall, which creates bending in the direction of the soil pressure.



RE: Digging out a crawlspace

(OP)
Thanks for all the responses. Sounds like some underpinning that can resist the lateral pressure is the way to go. This is commonly done around here with details much worse than what I drew and they seem to hold up fine.

RE: Digging out a crawlspace

Sorry, but I wouldn't consider your sketch to be "underpinning."

www.PeirceEngineering.com

RE: Digging out a crawlspace

I have investigated a number of similar basement 'digouts' similar to your project. The final, new basement walls have varied from 2' to 6' high and have supported everything from concrete footing/stemwall to old cinder block (real cinder block) to stacked brick footings supporting brick masonry to stone wall/footing to wood mud sill supporting wood framing. Most of the original construction is prior to 1920 and the 'digouts' were from 20 to 70 years old.
All worked for a while, None worked well unless either the new construction was under the original wall (underpinned) or a proper retaining wall was built.

My Advise ... Either use conventional underpinning and concrete wall placement COMPLETELY supporting the old foundation or step inside about 1-1/2 to 2' and build a PROPER cantilevered retaining wall.

RE: Digging out a crawlspace

(OP)
@dik - thanks for the detail. I will have to add a toe to it to resist the overturning. I believe the reason most of the ones I have seen are working well is due to the very stiff soil not realy needing any lateral support.

RE: Digging out a crawlspace

That detail assumes some form of attachment is likely between old wall and old footing. Very unlikely. Any connection from new wall (that form or no form against earth) is likely providing some support to a block of concrete (footing) resulting in a tipped footing and that whole upper wall possibly caving in. Go with underpinning.

RE: Digging out a crawlspace

I have seen this done quite a few times. It typically seems to have worked,but I would not be comfortable specifying this method.

RE: Digging out a crawlspace

OG: Most of the ones that I've been involved with had no footing... the foundation wall was between 24" and 30" wide, founded on native soil... sometimes the back excavation was carried down a little deeper... and occasionally, the hydraulic properties of the mortar had 'disappeared' and only sand was left.

Dik

RE: Digging out a crawlspace

(OP)
Thanks dik.

RE: Digging out a crawlspace

(OP)
Just for giggles, how do y'all feel about this detail? I have not run the numbers on it, but it seems like it might prevent the weird settlement issues and also balance the bending and overturning in the retaining wall. Would likely only be practical with light gravity loads.

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