I was told from a concrete guy that to do concrete or pavers correctly you should go down to clay. Even if that means digging down two feet to remove topsoil. Then backfill with clay, then 6-8" of crush , then sand, then pavers.
Not sure how accurate this is - but requires a heck of a lot of labour if true. I have definitely seen projects with only 6" of crush and sand on top of soil that frost heaves in the winter.