greybeard
Herd Master
This, /\ is the age old standard way of doing it before sackrete and it's how I set every cornerpost in '64-'65.dig hole, drop in post, then fill 1/3 with water. Push sand in hole, tamp, push in more sand, tamp, tamp some more, fill hole with sand. If it is too dry, add a little water and tamp a LOT. the water helps set the post.
If you've ever watched anyone instal a septic tank or any other underground tank, while and after it's being backfilled with dirt, the installer will run water into the loose soil being dumped in...the backfill settles making a level firm area around the tank so you don't have to go in later and add more dirt to fill in a hollow or low spot around the tank.
Think of a pure sand beach right after high tide goes out and the sand (on Texas beaches anyway) is dark colored..you can drive anywhere on it. Let the water seep down into the sand a few feet a few hrs later and the top couple feet of sand is white and dry and you're apt to get stuck in perfectly dry sand. Wet sand will compact itself. Dry sand never will.