Earth Oven - Foundation Question in USDA Zone 6b

Hi permies!

I'm starting on my earth oven build.  I'm on a slightly sloped area on the edge of a little forested area in Western PA, USDA zone 6b.  Our subsoil at the homestead is just...extremely difficult to get through (solid clay and tons of stone, making each shovelful a real battle).   Just wondering if anyone has experimented with alternative foundations to getting below the frost line.

While I'm obviously concerned about frost heaving, it may take me all summer to dig down to get below the frost line  Would it be possible to do a slightly thinner foundation (rubble trench, perhaps) and then raise up the cob oven base on about 6" more gravel for drainage?  Would I still deal with frost heaving or would that be enough?

Thanks!

If the ground is that hard would frost heave be occurring anyway?