4-5 feet. Four preferred, but a portion may have to go to five.
I was planning on a soil barrier fabric, and then a 1-2" diameter gravel back fill behind the stones, maybe eight to twelve inches from stones to soil, with a 3-4" pvc drain at the bottom of the gravel, similar to how you'd French drain a foundation.
But I have no idea how to do the base under the stones so they don't walk with water penetration and frost heaving
