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Feasible? Self-Supporting Slab with monolithic slab/footing addition

Guest (Visitor)
21 Feb 01 18:27
Circumstances lead me to explore the following question.

For a small (12' x 16') 2-story addition (workshop on 1st floor) with monolithic slab (4 inch) on grade with integral footing (16 inch wide footings), is there a reasonable alternative for mitigating settling issues by making the slab self-supporting?  I'm prepared to accept an answer of "No" but am compelled to explore for alternatives.  Piers, reinf, thicker slab, etc... are along the lines of what I'm thinking but not sure of the soundness of such directions.

Thanks,
Andrew
JAE (Structural)
22 Feb 01 14:31
One concept is to overdesign your new footings to reduce soil pressure and thus, reduce new settling under the addition.  This is not fail-safe and there will always be some settling.
Guest (Visitor)
22 Feb 01 16:35
JAE

I hesitate to clarify cause you probably understand what I was asking and I'm misinterpreting your response.

But I'm taking your response to be how to mitigate settling of the entire addition.

What I'm really worried about is settling under the slab.  I've had to build up a pad for the slab which now I'm worried about it settling.  I can get footings designed and located such that they are stable within the reasonable controllable limits.  However, I'm thinking about this pad settling and ways to reduce the impact.

What I'd really like to do is auger a couple holes down through the pad and create a couple of piers to support the interior of the slab.  At about 1.5 yds of slab, I'm figuring that the slab could be insulated from settling concerns with a couple of piers.

As I said, I'm prepared for the eventuality that I'll have to redo the pad.  But before I do, I'd like to know if there is an alternative that isn't too imprudent.
JAE (Structural)
22 Feb 01 19:36
Sorry...I see now what your asking.  

Adding a couple of piers under your slab, as a concept, can work.  What you're creating is, essentially, a suspended structure that will try to span from footing wall, to pier, to footing wall.  When your fill settles, there will be small voids directly beneath your slab.

Is this what you're thinking?

If so, you will have to reinforce this slab to span the distances involved and be capable of supporting the self weight of the slab and the intended live loads...a work shop may require quite a heavy load, perhaps 100 psf, compared to a typical residential area that requires 40 psf.

Also, the piers would have to be designed to transmit the reaction from the slab to the underlying soil below.  Thus, you are concentrating floor load onto two small points, which adds to settlement, unless the piers extend deep enough into more competent material.

breaks (Structural)
27 Feb 01 16:34
I would consider making the slab a simple slab-on-grade design if you have the means to provide suitable backfill preparation and material.

Depending on your loads, a 6" slab with minimal reinforcement on a suitable backfill (including 6" crushed aggregate directly beneath the slab and proper compaction) would be more than acceptable for your project, and is more than likely overkill. There is ALWAYS going to be some sort of settlement with slabs-on-grade, but this set-up will make this issue minimal.

Keep in mind (and you may already have) that if you're looking at poor soil conditions, this concept might not give you the results you desire.
emmgjld (Geotechnical)
4 Mar 01 23:28
The problen of deficient subgrade soils are not usually solved by adding more concrete or reinforcement, unless the design is as a 'raft' foundation. I have been involved in a few situations where some additional supports beneath a slab did some good but, I have seen many more where nothing was accomplished and, in some cases, greater damage was done. My advice is to improve the soils.
Jim6758 (Civil/Environmental)
6 Mar 01 18:47
I agree with Emmgjld.  Since the area in question is fairly small and possibly hard to compact, you might want to consider using Controlled Density Fill which is mostly fly ash, a little cement, and a lot of sand and water.

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