We're working on a project that required a trench to be cut. A kind neighbor brought over his backhoe and did the job for us.
Looking down into where the machinery bit into the earth, we could see the shiny clay sides.
This clay layer is so slick and so impermeable that growing anything has been a constant and unending challenge.
Seeing the interior of the trench reinforced that our decision to garden in tires was the right one. Tires (essentially, raised beds) allow us to bypass the problems associated with clay: weeds, lack of drainage, compaction, and everything else.
So tires it is.