[Half-fast eddie]

Put the dirt back in, tamping it down, bring it up to the bottom of the existing slab. For a 2x2x6” hole you will need 4 bags of 80# sakrete. Mix according to the directions … easy on the water…. pour in the hole and smooth it. Done.

 

[Half-fast eddie]

If you are not sure you can compact it good enough, cheat.
Drive 4 2x4 stakes down into the mud and cut them off at the height that will be the bottom of the concrete.

Bad idea. That assumes you want the 2x4 to provide some support for the concrete patch. Pretty good chance the wood will rot away. And unless you pound the wood down to a proper depth, it could also sink under load.

 

[Half-fast eddie]

Ok. I just remember doing new bank branches, and the civil engineer required all organic and deleterious material to be removed from under the building.