Engineering for Texas gulf coast and clay soil

[Texasranchpizza]

Hey folks! Been a lurker fan of this forum for years. I finished my first oven this spring in the Davis Mountains of Texas. A friend has enlisted me to build an oven for him on the gulf coast. Our soil is rock and his is pure clay like play dough. I was planning a 6” slab for the base, 1/2 inch rebar, crushed road base they have on the ranch. Do I need footers around the perimeter or piers? Do I need a plastic layer? This area gets 42” of rainfall and I don’t want this thing to move.

Pics are the oven I just finished. Still have to do some finish work. This is a really remote location.