Today I am working on a continuing project; a deck on the East side of my house. I didn't have it built when the house was constructed because I wanted to move in. I was living in a motorhome on the edge of town commuting out to the house each day and that was getting old, so I had the house inspected and moved in without several patio areas. This is the largest and will take the most time and money to build. Enough background, I had to dig a hole to put in a support to keep the deck from bouncing and sagging. This is one of those pre-made ten inch concrete things that you put under things to hold it down in the wind and support it when walked on. Normal stuff.
So I dug a hole roughly 18 inches square and 12 or so inches deep to hold the support; here's a picture of it.