Sundowner said:
you can build on any soil if you assume it can take 3000lb/square foot. lot of soils can take more, there are few, if any, that will take less.
Assuming that soil has a bearing capacity of 3,000 psf is a good way to get into a lot of trouble in very little time. 3,000 is considered the average starting point that engineers use to design a foundation. Most will state this somewhere on their drawings and that there should be soil borings taken if there is evidence of bad soils on site.
Common soil bearing capacities that I found in a book on my desk:
Bedrock: 12000 psf +
Rock: 4,000 psf
Gravel: 3,000 psf
Sand and Sandy Clays: 2,000 psf
Silt and Silty Clays: 1,500 psf max.
Organics: 1,000 psf max.
All of these numbers are relative to the local area and need to be checked before designing a foundation system. For instance, most clay soils in my area will support at least 32-3400 psf or more. However, if happen to have a vein of organic topsoil buried a few feet below the surface, the bearing capacity drops to less than 2,000. Don't ask how I know. It's from personal experience.