question about slabs and clay soil
I’m currently designing a walk-out basement for the back of my house. The house s on clay soil, and has some minor settling problems (drywall cracks above the corners of the doors, etc.). A lot of the houses in this community have this kind of problem, especially the older houses. Anyway, I was considered as part of the slab to drill some piers down into bedrock if it’s not too expensive. My father-in-law, who will be the contractor for the project, thinks that a slab won’t budge and that piers would be overkill. I’m not so sure, I think a slab might move in the heavy clay soil we have. Who is right?
I'd consult a soils engineer. It depends on what kind of clay it is. If it is an expansive clay like Bentonite you need professional advice anyway. If it isn't one of the most important things to do is to create a drainage plane under the slab with at least 6" of 3/4" clean sharp stone and perf pipe to a sump pit. I'd also use both fibermesh and wire mesh, or possibly rebar in the slab.
if the clay stays wet, you will be fine, If the clay stays dry you will be fine, but if wet clay dries or dry clay wets, then you have major headaches.
I have busted out many slabs that has a big gap between the dirt and slab, maybe they did not compact the dirt too well. I have seen many garage slabs poured with the footings, they seem to have the biggest crack in the middle. The slabs with the least amount of cracks are poured after the stem walls are up, with perimeter felt joints, fully compacted dirt and deeply scored joints. So, I would recommend a floating slab.