Residential clay foundation "piers"

[lapstruct]

Has anyone seen this situation before:
Older home (50-60yrs), foundation consists of exterior load bearing cmu walls and interior brick piers that support floor joists and beams. The unusual part: about 10 years ago, the previous owners decided they wanted some storage space I guess, so they dug out around all the brick piers. Currently the brick piers are supported by what appears to be virgin clay soil "pedestals" about 3' high and 24" in "diameter". The system appears to be stable, and the interior walls/ceilings give no indication of any differential settlement. I have been told that there are several homes in this area that have been built and modified in this manner. How has this worked for the past 10 years, and what would make anyone think that this is acceptable as a foundation system?

 

[SlideRuleEra]

The "success" of this modification is probably due to a combination of fortunate factors:
1. The foundations had many years to reach "steady state" before the excavation was made.
2. Ground water must not be a problem, the clay certainly is a help on this issue.
3. The excavation is not "too deep", at least the clay pedestals are fairly "squat" (no where near being a column).
4. Residential live load is typically only a small fraction (maybe 20%) of the of what has been considered the historic design value (40 psf) for sizing foundations and structural members.
As to why someone would have done this - they probably just did it, unaware of the risk. Also once one home had be "modified" successfully, word would spread about this "simple" way to get extra space.

 

[RUSSRASCAL]

I strongly recommend reinforcement. Use a perimeter reinforced poured in place conctete walls a mimimu of 3.5" thick. 5.5" much better. This sounds like a "woops" waiting to happen.

 

[Focht3]

Yup. Better protect those clays, and reinforce them. They have a habit of failing suddenly, and catastrophically -



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