Paul,
Hmmmm, I think I'm reading your intent correctly, but please correct me if I stray. You are considering use of a concrete filled 4" dia PVC pipe with a cap at the bottom as a vertical support member...? This being the case, an number of engineering issues come to mind: prevention of the the base being driven into the clay bottom, how to stabilize horizontal movement and shear, the compressive strength of PVC to support the vertical load. I s'pose the first issue could be resolved by borrowing the engineering that Tommy Docks uses as a "standard footpad":
Tommy Docks Composition . You may be able to use a flange at the bottom of the pipe instead of a cap, fabricate your own design footpad and attach it to the flange. This may include a short spike welded to the bottom of the footpad to drive into the clay to stabilize horizontal movement at the bottom. Or, you could just use Tommy Dock hardware, or similar.
If I'm way off base, please tune me up.
