Attaching new basement to existing slab stem wall?
Attaching new basement to existing slab stem wall?
(OP)
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!
Thanks!
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!
Thanks!





RE: Attaching new basement to existing slab stem wall?
Construction of a new basement next to an existing slab is always difficult. It can be done, however, there is a cost versus risk situation. For example, you could just assume that the stem wall will hold during construction and make the excavation for the new basement. This is the lowest cost but highest risk. On the other end of spectrum the risk can almost be eliminated by various construction techniques, however, the cost will be much more.
Good luck, I'm sure you will get more responses.
RE: Attaching new basement to existing slab stem wall?
Can I assume that the lowest risk approach would be to pour or build a new, full height basement wall on all sides of the new basement, even on the side abuting the slab stem wall?
RE: Attaching new basement to existing slab stem wall?
Then have an engineer review it.
It shouldn't take an hour or two.
Seeing clearly what you have he'll find something you missed and it will save you a fortune. Then you can share it.
RE: Attaching new basement to existing slab stem wall?
RE: Attaching new basement to existing slab stem wall?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Attaching new basement to existing slab stem wall?
I have directed our architect/engineer to design the basement addition with new walls on all 4 sides, and not to rely on the existing slab stem wall for supporting the new addition.
In regard to having to rely on the existing stem wall for support during construction, what sort of techniques might be used to reduce the risk? And would I direct my architect/structural engineer to investigate support/stabilisation methods, would this be a conversation I'd have with the GC or concrete sub, or do I need to bring in a consultant of some variety?
Thanks!
RE: Attaching new basement to existing slab stem wall?