Earthern floor (or slab) on expansive soil

Are there any strategies for building on-grade floors on top of expansive soils? I'm planning an earthen floor, but strategies for concrete slabs would probably be appropriate. I don't have a soil test but I know others have dealt with expansive soils in the area so I want to play it safe.

The foundation will be a rubble trench 4' deep. It is in a pretty dry climate to begin with, and will have wide overhangs directing water away from the house. I'm hoping that these factors will help maintain a fairly constant moisture level in the soil under the house. What else can I do? The floor itself will probably be 1" finished floor, 4-6" road base, 4-6" perlite, 6" crushed stone. I'm thinking about just making the bottom layer of crushed stone extra deep, maybe digging down an extra foot or even two. How much would that actually help? Would it help by getting down to stabler soil, or by isolating and buffering the floor from movement below? It's obviously a little extra money and work, but if it keeps the floor from moving it'll be worth it.

Your plan sounds pretty good.  The gravel layer is important. Use some landscape fabric on top of the gravel and perlite layers to keep fines from sifting down into the gravel/perlite. If fines gradually sift into the gravel, the floor can become unstable and crack, etc. also the pore space in the gravel is important for moisture control (capillary break).

Instead of gravel and perlite, I used 1/2" screened pumice (14" at the perimeter, 9" in the middle)(no expansive soils).  If pumice is available, you might want to consider it instead of perlite.  I think it probably provides a more stable base.  

My guess is removing another foot of soil probably wouldn't accomplish much.  Normally expansive soils that are problems are fairly thick. Dig a hole and see what the soils look like.  

Sometimes you can get decent information from the USDA NRCS Web Soil Survey too.  

Are you going to build in Vermont, or somewhere with a drier climate?

IN ALASKA. they will dig down to the permafrost (5-7feet) and backfill the whole with gravel from a river ... Then poor concrete or a wood floor over that .