As part of the foundation for the posts. You can see here in the picture: https://image.nostr.build/82e111af3abb535f15e16efbb668d3265ae16994e9f15b27f86ab62f879b92cd.jpg

"The tire Foundation book" I found online back then.
It solved a problem for me because I have rising humidity from the ground due to being below the level of a water channel going by 50 meters away from home. So water is almost at ground level because my land is a couple of meters below.

So the tire is filled with compacted gravel. And the post is just on top. That way I think (as I studied back then) no rising of water should happen.
It could have been just rock masonry stem wall or just a large boulder as foundation.
But there aren't angled square stones or large boulders around. So it was cheaper easier and practical to use tires.

For the house I failed to build I used 25  truck tires 1meter in diameter filled with gravel. But as I said. It was demolished after all. I still have some tires and I use them for exercise  

 Thats really cool! Great that you had the chance to build your own home.  Have you considered building a rocket mass heater too?