I have torn down my cottage & had someone come in to sink sona down 5' to bedrock to sit a 24x38 cottage on. Well it turns out that it is 5' of clay & the job was halted after the tractor with a post hole digger buried its front wheels up to the axle. It was suggested that 2' of clay be removed & replaced with crushed stone tamped down, a 6" concrete floor be poured with reinforcing rods etc. This can't be done until the spring & I am looking for alternatives if there are any.
Answer:
Unless this is going to be a 4-season, insulated, heated and fully occupied structure, I'd float the foundation on top of the existing grade. If, on the other hand, you plan to live there, I'd put the slab on gravel but make sure there is a crawl space that can be heated or comfort during the heating season will be hard come by. The slab idea works very well but I would not dare use it as a floor - that's what cold feet are made of.
