I am trying to dig holes that are 18" square, and 12"+ deep.
The site is mostly undisturbed soil, although there is a thin layer of topsoil or fill in some areas. The soil is glacial till, so a mix of sizes. Plenty of rocks.
I use a mattock to loosen the ground, then a shovel to, err, shovel it out and clean up the sides. I have trouble when I get close to the 12" mark, because I can no longer swing the mattock into the hole. If I need to go deeper (because I hit soft fill dirt) then I'm kinda stuck.
It's also taking a while, and I have 30 holes to dig, so I'd like to think about ways to speed things up.
For context: I'm going to fill with gravel and set pre-cast pier blocks on top.