Calculating screwpiles requirement in clay ground

I made a sandbox a couple of decades ago for kids. It had a plywood bottom and was built with legs that held it up about seven inches off the ground. The bottom had drainage holes to let water out. The plywood lasted for a long time. How long? Hard to say I sold the thing in a garage sale after 10 years when the kids grew out of sand boxes.

BTW - I had a lid over the top made out of two pieces of plywood that were hinged together. The lid kept out cats and most of the rain. If the sand got too wet just opened it to the sun and worked the sand around with a small garden rake over the course of a day or so.

When I redid my fence 2 years ago, I went with PostMaster metal posts (http://contractors.masterhalco.com/Contract.nsf/woodpostmaster). They weren't substantially more than a good PT post at Home Depot. You can also easily wrap them with a picket so you don't see metal.

I dug the holes about 30" deep, then just drove the posts into the dirt til they were the right height - much easier than cutting them. You could put a couple inches of pea gravel in the hole, but I just did concrete. My hope is that even if I have to rebuild the fence in 20 years, I'll never have to replace the posts.