I'm looking for input on a foundation technique that I've heard of being used only once (engineer signed off on it and it works well).

Soil in the area is unstable and most houses are driving telephone pole pilings down to 45' for a pier and beam foundation.  A few are using helical soil screws.  Evidently an alternative is simply a grade level layer of foam covered by a thick slab with rebar.  The foam extends beyond the slab perimeter and prevents frost under the foundation.    The structural slab acts as a huge footing and reduces psf loads to something the soil can handle.  I couldn't tell if the foundation slab was larger than the house (to gain even more bearing area). This sounds so simple with only the additional  slab thickness and steel as extra costs.  But no excavation needed.  

Any thoughts on this?  Location is Michigan.

jonr,

Have you looked at Geofoam?  I know it is used instead of rock over unstable ground for roads and such.

Geofoam does look like the right keyword.

This may be called a mat slab or floating raft foundation.   Evidently sometimes a filler (geofoam or floats) is used in a waffle pattern to create a ribbed mat slab for more efficient use of concrete.

Great suggestion, they even GeoFoam under Bridge Towers when crossing Swamp wet land etc, and EPS GeoFoam will remain stable for 1000's of years.

 A few questions.......... Are there any other structures in the area that have used this method and how long have they been in place?  Have you discussed this with a geotechnical engineer?  When you say unstable, do you mean that the soil is uncompacted / undocumented fill or the soil profile itself is not stable?  Do you know what the bearing value of the soil is, i.e. 1,000 p.s.f., 500 p.s.f.?

I mean the skins start delaminating.

One house I know of used it about 10 years ago and it is working well. But I was told that it took some looking to find an engineer willing to design it - which is why I am hoping to learn about it some. The soil is about 5' of sand fill (put in 20 years ago) on top of 30' of marl. I don't know the psf, although I understand this figure is needed.

My understanding is that foam below grade subtracts from the house weight - with enough of it, the house will put no net load on the soil and float. Or even with no foam, loads of 100 psf are possible. I've never had an issue with driving a truck across the property, so it can support a lot more than that.

Very interesting idea. I live in Michigan myself, whereabouts are you? I had talked to one guy that did soil testing, it was kind of expensive, but he was a pro, and might be able to tell you about the psf of the soil. I found him by doing a google for soil bore testing or something similar with the words flint mi after that. I could probably find it in my notes but I don't have but 30 seconds right now, lol.

I'm north of Ann Arbor. Looks like choices are:

post tensioned or rebar - less steel with post tensioned
flat slab or waffle/ribs/beams in the slab - more complex to form and insulate but less concrete

Sounds like 10" thick is common for a structural flat slab. Around $8000 of concrete.

Why not just use EPS Foam with a GFRC skin coating such as this

If I understand the picture correctly, it shows a foundation with footings that would not work with the very low PSF loads that unstable soil needs. Ie, the idea is to spread the weight of the house over the entire area of the house.

Foam can be used to create the waffle pattern without trenching - foam is a cheaper fill than concrete.

I expect the problem to be in finding people who are familiar with this type of foundation in MI.

Yes foam is way less expensive and the savings in temp loss will also add up in the long run. Here is another idea that might work better for you.

OR, or you could do it the HARD way, this works only if you get cheaper labor.

great drawings, thanks.

So here is how to do it right, a save BIG Bucks.

Here is what I am thinking that is easy and has the thickened edge for strength (most of the weight is at the edges of the slab).  Plus post tensioned cables through the concrete.

I see you have the concept, you don't need a very thick concrete slab on the EPS Foam 1.5" reinforced would work fine.

I believe that 8" is about as thin as they go for structural foundation slabs.    5" for post tensioned tennis courts.

That's why Concrete Producers don't like the Super Insulated Panel Systems, but LOVE ICF's that continue the Wast of Concrete, It's the Profits you know.
Things must change, because where won't be any earth left if we keep on building disposable wasteful structures.