House with partial basement 1

[n3jc]

I have brick house with partial basement. What to do with foundations? What do you guys do/suggest in cases like this? I attached a picture.
I have no data on soil - there hasnt been any geotechnical research made to evaluate the soil bearing capacity.
I was thinking about designing a slab for ground floor and under basement too.

 

[dik]

I don't know what your foundations are, but, it seems like a simple temporary support of the structure and constructing foundation walls and a new foundation for them, unless I'm missing something.

You should have some geotekkie input.

Dik

 

[oldestguy]

Is this yet to be built or is it existing? If existing, is there any problem? Appears to be pretty standard as to foundations that work.

 

[BAretired]

I question the wisdom of using a partial basement. Better to keep it all on one level, either no basement or full basement. The characteristics of the underlying soil and depth of frost penetration would be helpful to know.

In central Alberta, assuming a clay soil and frost penetration of five feet, I would probably use drilled concrete filled piles or screw piles for the ground floor and spread footings for the partial basement.

Using a slab under the ground floor would not be recommended because (a) it would need to be below the frost and (b) it would tend to settle in the disturbed soil adjacent to the partial basement, although the latter objection could be remedied with proper design.

BA

 

[molibden]

I will explain how we design structural foundation slabs in our part of EU. I see some difference between EU and US practice.

Thickness of the slab is 250mm or more. Under the slab there is compacted gravel - thickness 500mm or more. Today is common practice to put thermal insulation (around 150mm) and water barrier UNDER the slab. So in effect the thickness of all layers is 250 + 150 + 500 = 900mm. And that is common frost depth. This way most of residential houses today are built.

Probably what OP is asking is how to deal with uneven settlement of ground floor slab and basement slab. One way is to design stepped foundations between both slabs to provide enough rigidity so whole structure will act as one. Other way is to put movement joint between basement wall and ground floor slab but this may not be possible in your case (both parts seem to be connected).

 

[BAretired]

In my experience, the basement usually occurs under the house but not under the attached garage. The house is founded on spread footings. The depth of basement usually (but not always) means that footings are below the depth of frost penetration. The garage is normally founded on friction piles or screw piles with a grade beam around its perimeter. The grade slab for the garage and for the basement is non-structural, usually a four inch thick slab. Some care must be taken at the junction of the house and garage to ensure the slab does not settle unduly.

Methods vary considerably depending on locale and soil conditions.


BA

 

[n3jc]

tnx for answers... its a new building/house...
Im really confused right now and dont know what to do.

 

[BAretired]

Maybe a good first step would be to find out what kind of soil you have on your site. Secondly, what is the depth of frost penetration? What is the usual way of handling the problem of partial basements in your area? What type of equipment is readily available in your area?

BA

 

[oldestguy]

In your area I would go to jurisdiction office with rules over new construction and get their requirements. If none, talk to a local engineering firm for advice.

 

[KootK]

Assuming strip footing and frost wall construction:

1) One option is to incrementally step your footings down adjacent to the basement so that they meet at the same elevation. Another option is to "hang" your house footing foundations from the basement wall in an attempt to reduce excavation and material costs. This is fine so long as the associated loads are dealt with, frost concerns are tended to, and utilities are properly coordinated. This may not work well in your specific situation, however, because your main building front wall is outboard of your basement wall perimeter, creating a cantilever condition.

2) Main building slab on grade settlement adjacent to the basement is always a concern. In high end commercial buildings, they will often install structural, suspended slabs on grade (12-16") spanning from the basement wall back to the limits of foundation wall excavation. A bridge of sorts to span the back fill most likely to settle. On residential projects, I usually see a nominal slab on grade (4") simply either terminated on top of the basement walls or doweled into the side of them. It's cheaper and, frankly, I haven't experienced any difficulty with the detail to date.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[n3jc]

tnx for answers!

@KootK can you post a sketch for the last option you described? English is not my first alnguage so its kind of hard to understand everything?

 

[KootK]

@KootK can you post a sketch for the last option you described? English is not my first alnguage so its kind of hard to understand everything?

It was a mouthful even for folks whose first language is English. By "last option", do mean my #2 or the second option discussed in #1? Could you perhaps post a copy of the text where I mentioned it? It takes a while to sketch and scan a detail. I want to make sure that I'm sketching the right one for you.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

As a note, for high end residences we generally always design a structural slab as noted in option #2 by Kootk. Not uncommon for us to have all at-grade foundations pier supported to minimize differential settlement from the hardspot caused by the basement.

 

[KootK]

@jd:

- "pier" = pile?

- Have you ever encountered any problems where structural slabs on grade were not used?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

"pier" was intended to mean drilled pier. Generally 18"-24" diameter. Depending on your usage of pile it may mean the same thing. Piles to me are driven not drilled.

I'm not aware of any issues with the slab-on-grade option, however most geotechs we work with do not allow it and where we are located we obtain geotechnical reports for all basement projects. The fear as you know is settlement of the backfill. The geotech is generally camped on the site to make sure it hits 95% compaction, but due to the value of the homes (all new homes are at least $1 mil in construction in our area) they generally do not like to take chances.

 

[KootK]

but due to the value of the homes (all new homes are at least $1 mil in construction in our area) they generally do not like to take chances.

I'd consider that a market nicety. Big money homes = ability to do better engineering.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[n3jc]

@KootK
I was thinking about this:

2) Main building slab on grade settlement adjacent to the basement is always a concern. In high end commercial buildings, they will often install structural, suspended slabs on grade (12-16") spanning from the basement wall back to the limits of foundation wall excavation. A bridge of sorts to span the back fill most likely to settle. On residential projects, I usually see a nominal slab on grade (4") simply either terminated on top of the basement walls or doweled into the side of them. It's cheaper and, frankly, I haven't experienced any difficulty with the detail to date.

Thank you very much.

 

[n3jc]

This is what I came up with... what do you think?

 

[BAretired]

This does not appear to be a high end residence.
Do you really need a basement under the garage?
Are you going to place properly drained weeping tile around the basement area?


BA

 

[KootK]

As for the "what to do here" bit, you can step the foundation as you have elsewhere. Just step it around the corner. Remember, nothing's spanning here with regard to the stem walls. All of them are assumed soil supported.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.