[Cob] Tires For a Foundation?

[beno]

Hi, everyone;
I'm new ;) Working on building a 3-room school in the Dominican Republic 
where I live. I have read enough to recognize the problems inherent in 
using concrete for a foundation. Where I live we don't have lots of 
large stones (stones larger than a baseball). But we have lots of small 
'caliche' limestone. I was thinking about using a configuration of tires 
kind of like this:
http://web.vi/foundation.jpg
That image isn't perfect nor is it necessarily representative of what 
we'll end up building, but the idea depicted is 3 circular classrooms on 
top of a bed of old tires (of different sizes) that would be stuffed 
with caliche tamped down hard. The tires would be bound together by 
nylon rope, wire or something (suggestions?). Outside the tire system 
we'd also tamp down caliche. What do y'all think? Has anybody done 
anything similar?
Thanks,
beno