[Cob] foundation question

[Bob Smolen]

Time and again I read that a foundation must extend below the frost line to avoid frost heave. I also read that a floating slab is suitable for building a structure. These slabs dont always extend past the frost line. The posts in the old barn on my place was supported by piles of rocks placed on the surface of the earth . The barn  failed after 100 years because the roof was not maintained. 

Being cautious and conservative, I lean toward following the common advice that a foundation should extend below the frost line. That is at least 4 ft. here in Wisconsin and maybe more. But, I also understand that rubble foundations are acceptable for buildings.Do rubble foundations have to extend below the frost line? Can someone explain how extending past the frost line really prevents heave compared to say a 2ft deep foundation. How is heave a problem in a rubble foundation when the gravel and rubble would seem to give to the movement caused by frost? Have studies actually been done to prove that extending below the frost line prevents heave more than a foundation above the frost line. Why was my barn able to survive 100 years sitting on loosely piled field rocks? Why are earth bags suitable for foundations when placed at very shallow depths? Is it all dependent on location? It is all a bit confusing. Is the advice based on science ?
Thanks,
Bob