[Saturn488]

I am looking at putting a pier in my backyard but the only area I can do it is on the elevated tiled area.

There is about 400-500mm of packing sand which is the foundation for the tiled area then under that is the 'clay' based ground.

What would be the recommended depth I would need to dig into the clay ground so nothing moves or shifts? Should I dig enough into the clay and fill it up with concrete up to the packing sand and bolt the pier in? Or should I dig deep enough to put a 200mm PVC pipe in, fill it with concrete and place my adapter at the top?