[Cob] Cob On Top of Tires

[beno]

Okay, so it sounds like a test of the tire foundation is in order. But 
what of setting cob on top of the tires? After all, they meet in a 
point. Won't that point be unstable and suseptible to cracking? Should I 
build the foundation one tire higher and then downfill cob to the next 
lowest level, or will that lowest portion of cob not be structurally 
significant? Or, perhaps I should search around for some larger size 
rocks to stack on top of the gaps between the two highest layers of tires?
beno