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Using existing stem wall as new basement wall?

Using existing stem wall as new basement wall?

Using existing stem wall as new basement wall?

As a preface, I'm sure we will be consulting a structural/getotech engineer, but I'd love any insight that folks can give us.
Our house has a 7 foot depth basement under one half, and rests on a slab for the other half. As far as we can tell (by driving stakes down til they hit the footing) the footings under the slab go down as deep as the existing basement space (1960s construction, cinder block wall in basement and cinder block stem wall under slab).

The proposed addition would extend the entire back of the house, including new basement space along both the existing basement and the existing slab.

Now the question: can the existing stem wall under the slab be used as one wall of the new basement addition? A comment on another web forum hinted that a stem wall under a slab might not have enough stiffness to act as a "retaining wall", since it was designed to have dirt piled on both sides and only carry the vertical load of the house, not the horizontal forces of having dirt on one side and basement on the other.

(And no, I have no fantasies about digging out under the existing slab to make basement space).

Any comments would be welcome, at least it will help us with what questions to ask the architect and structural engineer!


RE: Using existing stem wall as new basement wall?

You may use it if acceptable, reinforce it if not by suplementary works, or entirely discard its lateral strength to this purpose. All these things you will decide with proper exchanges about the options with your structural designer.

RE: Using existing stem wall as new basement wall?

I would not depend on the existing wall.  As previously stated, it was not designed to carry lateral/horizontal forces.  Pour a new concrete wall AND footing as specified by a reputable home builder contractor or structural engineer.  Also, be sure to have your new floor joists/structure bear on top of, and be bolted to the recommended new concrete wall. This is called a "restrained" wall or "basement" wall condition.  The concrete "basement" wall would be dependant upon the top support of the floor structure to prevent tipping or overturning which is a common tendancy of a true retaining wall, which is not "pinned" or fastened on top, and would thus, require a larger footing than that of a restrained basement wall.  Enough said.  Ask the "licensed" engineer or "reputable" bulding contractor.
And do not revisit the thought about the fantasy of digging out from under the existing slab, since that existing slab, also, was not designed to be self-supporting.

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