As part of the foundation for the posts. You can see here in the picture:

"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