Concrete base problem in garage shop. Any construction folks?

You need to to find some "red dog dirt" part sand, part clay, and very little dirt in it. I am sure you can find some around where you live, it is often used on secondary roads. I am showing you a link what I used on top of 6C road pack. Rent a compactor and spend all day, this will work for what you need.

Red Dog dirt, sandy gravel pit run material and ponds - $10 (pawnee)