Sog with drilled piers

[Gellis3068]

Hey there
I am in Colorado and we have highly expansive soils and into most case we drill piers or over excavated for foundations. I have a home builder who agrees that piers are best but wants a sog instead of structural floor. And instead of native clay soil under the slab use a base or sand. My foundation over ex for footing and sog was over ex 7’ below footing and replace with A-2-4 or better. And to over ex and remove the native and import is to expensive. Other than mixing like 80% import with 20% is the only thing I can think of to cut cost.

 

[phamENG]

Which is more expensive? Doing it right now, or doing it wrong now and fixing it later? (It's the second one every time.) That is a lot of excavation and fill...but with a proper pier layout, void forms, and rebar, a structural slab won't be too bad.

I've only been in business for myself for a short time, but I've already learned that the clients that try to cut costs on the really important stuff are the kind to avoid...partly because they tend to try to cut my costs, too, but mostly because their risky behavior isn't worth it. If you force the contractor to do it right (or, at least, don't give him permission to do it wrong), you may lose the client...or you may not. If you do, probably a good thing, if not...then they know where you stand on dealing with expansive soils.

 

[MTNClimber]

Can you clarify what you're proposing. Are you using piers or footings with the overexcavation? Will there be a basement level. How expansive are the soils in the upper 30' from finished grade? Can you give a general subsurface profile?

 

[geotechguy1]

How thick would the sand be, and what are it's gradation and permeability characteristics relative to the surrounding soil?

Careful that you don't build a shallow bathtub under the house to draw water into that will sit on top of the underlying expansive clay and ruin the house.

 

[theCorkster]

If you use piers to prevent uplift, expect the sog to lift. That's likely why industry practice is to use structural floors to avoid the movement to come. If you over-excavate and rebuild with non to low expansive fill, then use shallow spread foundations with the sog.