cracks in the slab

We're having a house built in a rural mountain area where the predominant composition of the soil is decomposed granite and a little clay. Our house is a passive solar design and uses a rock floor (about 3 feet high, 2" rocks or gravel) over a water barrier. The floor of the house is 4" thick reinforced concrete on top of the rock floor. Because of concerns about ground water, a French drain was installed around the perimeter of the house. In one room of the house, the concrete floor shows signs of cracking (about 1/16" cracks with about 1/8" uplifting). The design of the rock and floor and been approved by engineers and been used many times by this builder with no structural problems. At certain times of the year, a hole at one side of the house that is deeper than the rock floor seems to have water in it which makes me think that the ground water can rise and fall.
The contractor speculates that areas with a little more clay may have swollen a bit and caused the uplifting. Does this seem likely? Is there a way to control this type of uplifting? I realize it is hard to make an assessment without seeing the site. What sort of professional should we look for for advice?
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This is exactly what happened to the slab in my p/s house. The soil had a lot of clay in it so there was a lot of movement between winter (wet & clay expanded) and summer (dry & clay shrunk). The slab was laid according to council regulations with extra cement pourings needed (which meant doubling the cost) because the clay went deeper than was first thought (of course!!).
There was no long term effect on the p/s performance but tiles laid on the cement in the living areas had hairline cracks in them from the seasonal movement. The tiles were laid within 2 months of the slab being poured and I was told (much later) that in order to avoid the hairline cracks I should have waited 12 months to put them down. The bedroom floors were not tiled but were painted with (outdoor) cement paint and the cracks were much less apparent there. Cement paint on the slab or epoxy sealers will be the way for me next time. No cracked tiles and much less expense to finish off the floor.
Judanne

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Are you referring to footings? Is the concern the frost depth, rather than the depth of clay?
Banty
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Cracks in concrete are normal but what would concern me is the uplift. You are probably looking for some type of reassurance, get that from your builder for he is the one responsible for the finished product.
Banty wrote:

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No, because there is no frost. It was a floating slab.
Judanne
Judanne says...

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Judanne wrote:

Clay is the best foundation for concrete slabs because the superfine particles compact into a solid mass. In fact clay is hauled in and used as a stabilizer over looser soils. *Any* soil will expand & contract with moisture variations, but clay will do it less. That is why the old saying is: "the only sure things in this life are death, taxes, and cracks in concrete".

Concrete takes a while to shed it's water and cure to size & strength. In fact the cureing curve of concrete extends upwards to 30 years. So your waiting for 12 months would have had little effect on hairline cracks. Those are characteristic of concrete and *cannot* be avoided. Larger cracks are avoided by wire and rebar in the concrete - it cracks but doesn't separate into large cracks.
Bob
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Thanks, Bob
Judanne

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Judanne wrote:

I waited 10 years to install ceramic tiles on a floating slab (several inches of SAND under it to isolate it from the gumbo [ultra heavy clay] soil underneath it), and I still have several tiles with hairline cracks in them.
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You should hire yourself the best structural engineer you can find and make sure he/she represents your interests. Don't rely on the builder's or his own expert's explanations, which may be designed to lull you into a false sense of security until all warranty deadlines run out.
I don't know about the state you're in but well over half the states have one version or another of a "right to repair law" now, and you/your lawyer will need to be familiar with it. It's a builder protection law and will waste more of your time, but if you have it, it's not an option, you will have to follow its steps.
Good luck. Both vertical and horizontal displacement are, shall we say, not good.
snipped-for-privacy@bigfoot.com wrote:

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