This is one for the Soil Mechanics Engineers.
I have a fence post that is 4” diameter and 6 feet long. I want to put this into my garden vertically to have 3 feet above ground. I have a 6” post hole borer to dig the hole. OK up to now?
Hole dug, post put in and the soil that came out of the hole is returned and compacted. BUT there is still more hole than soil to fill it. Have to import another barrow load of soil to fill the hole completely.
This has always puzzled me.
Den.

This is because you live in Australia. If you lived in the Northern hemisphere you would be taking more soil out of the hole once you'd put your post in.

Hugh