Attaching new basement to existing slab stem wall?

[fognozz]

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!

 

[GeoPaveTraffic]

Sounds like you are on the right track by getting some information prior to talking with your engineer. It is possible, but not likely, that the existing stem wall could act as the basement wall. There are two conditions that must be addressed and that effect the wall very differently. The first is during construction when the wall has no support on the addition side. This is likely to be the worst case and when the wall will not be able to support the fill under the existing house. The second condition is after the new addition is constructed and the first floor has been installed. During this condition the existing stem wall might be adequate.

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.

 

[fognozz]

Thanks for the info!

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?

 

[AlohaBob]

You could put togethers plans based on exactly what is your first hope for how will it go.

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.

 

[GeoPaveTraffic]

fognozz, you are correct, that would result in the lowest long term risk. However, there may be a short term risk during construction depending on the exact existing conditions and structural details of the existing wall.

 

[Focht3]

The biggest risk is during construction - when the existing stem wall has to act as a retaining wall. Depending on the soil and groundwater conditions, this may or may not be a big risk.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[fognozz]

Thanks again for all the info!

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!

 

[ERV]

Core through the existing floor slab approximately 8' in from end walls (1/3 points), if the soil is reasonably dry it should not present much of an issue. If the soil contains excess moisture, additional investigation should be done prior to excavation.