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Re: Residential Foundation Tipping

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Dave,

I presume the house has a large aspect ratio; say 2:1 or greater, and there is a vertical crack in the basement wall near the center, or between beam points if the joists run parallel, and diagonal cracks in the corners? (If not, I've misread your post...and you can ignore the rest) The footing has almost zero resistance to rotation, of course, but given a typical FS of 2.5 to 3.0 on soils data, and the likelihood that the site was excavated and re-filled (a possible 9'x110psf of overburden compression which may not have completely relaxed if it is clay), you could see as much as 5000-6000psf before failure instead of a prescriptive 1500psf (geo may have like given 2500-3000 given the report uscs).

I've seen a couple of cases where the floor just couldn't handle the load at the top of the wall. In one case, the band joist started to roll inward, in others it appeared that the bolt/concrete interface failed. I don't remember a concrete break-out, but with brick veneer you might not see it. In one I'm thinking of, the center of the basement wall bowed in about an inch, actually leaving the brick in place. The brick slipped at the concrete ledge, held in place by the wall above.

It's not too hard to show a failure on paper. With the most non-conservative clay backfill, you'd be close to 800plf at the sill plate interface, and with code levels something like 1200.  Around here (low seismic and wind), houses get 1/2" bent anchors at 6' on center whether they need them or not ;-)  Put a layer of sill sealing foam and you can kiss most of your friction interface goodbye. I keep wanting a floor diaphragm to fail just to say "I told you so," but between the drywall and the nails they use for temporary support it just doesn't fail as often as it should.

The walls I've seen haven't been really bad, so we haven't done a full up mitigation. Tie backs are one solution, and replacing the backfill with stone might not be a bad idea (and may simplify the tie back design significantly).

Good luck...let us know how it turns out.
Jordan


David Maynard wrote:

Got this call earlier this week.  Like to hear what everyone has to say.

 

It’s a residential foundation, 9’-3” wall on top of 8” by 16” continuous footer.  Reinforcing unknown.  The wall is actually tipping into the building.  Anchor bolts on the outside have broken the concrete out at several locations.  Soils report says it’s a sandy-clayey-silt.  Took samples from the surface and brought to another dirt-engineer who, at sight and touch, believes it to be a lean or fat clay.  *shrug*  Jury is still out on this one.  Current landscaping is exposed backfill, or NO landscaping at all.  There is a perimeter sub-drain (one of those perforated pipes surrounded by Styrofoam peanuts in a mesh sock) around the base of the foundation.  House, and wall where damage exists, is about 100 feet away from the base of a hill.  There appears to be positive drainage away from the house.

 

I have my own suspicions as to what could have happened, but I am curious if anyone has run into this on their end.  Anyone?  Anyone?  Foundation damage that I have typically seen is either settlement and heave.

 

Dave Maynard, PE

Gillette, WY

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