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Different underpinning techniques

Different underpinning techniques

(OP)
The project is a a row house dig-out - from about 3 feet of crawl space to an 8 foot basement. The common shared foundations are of rubble stone on this three story home.

I am looking for comments - theoretical and practical - on the attached 3 different underpinning techniques, assuming that the new underpinnings are designed and sized properly.

Option A seems to be preferred by most engineers, most labor intensive

Option B was suggested by several experienced underpinning contractor and at least one local engineer endorses this method.
  • Pro option B: since half the shared wall is always supported much less risk of stones falling into the underpinning pit, faster / cheaper than option A
  • Con option B: shared wall is supported by two different conditions and may be subject to differential settlement; eats up more space than option A
Option C was suggested by an experienced underpinning contractor
  • Pro option C: ??? faster / cheaper than option B???
  • Con option C: eats up more space than option B

RE: Different underpinning techniques

Option A is most stable and least likely to fail. It should be set on a spread footing though in the detail.

Option B has induced eccentricities due the vertical load

Option C has lateral instability due to a knuckle joint at the wall intersection, and increased lateral load to the lower wall from the surcharge load of the wall above.

You are very correct with engineers liking option A, and for very good reasons.

Mike McCann
MMC Engineering

RE: Different underpinning techniques

Option A is conventional and recommended. It works and is tried and proven.

Option B is something I've never seen before. Mostly because it doesn't work for most conventionally designed foundation walls and underpinning methods. You can do this with helical pile underpinning brackets, but not with concrete piers.

Option C can work if you design the wall to act as the SOE. That means braced cuts, braced wall, braced everything. The wall simply cannot move more than 1/4" in any direction... and that's usually the reason why it's not conventionally used. Go with option A.

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