it would probably be fine, especially if its just a few tyres, and they are not too soft. If you are building on the something bermed in the dirt, the wall even leans back a bit into the dirt so it is well supported and would never fall down. You can get a way with a lot actually, but to be safe I do full..
When you do retaining walls you can miss out the packing totally and can have way more soft spots.. retaining walls lean back right into the slope so they just never move!
Thank you. Of course it's better to be on the safe side. I was wondering if it would slowly come down if these soft spots would turn into cavities or something. I'm learning as much as possible from your posts for the future. I'm very interested in alternative building styles. The sloth centre here is build using sea containers. I find that very cool.