[Lynn]

You can see from this shot that when the PO had the upper floor poured, they trucked in loads of sand for a base. The “solution” to keeping the sand in place was to place a lot of boards between the upper floor and the old wooden floor. MUCH of the sand had migrated out, going down under the wood floor.
We needed a retaining wall if I wanted a concrete floor on that lower level. Also, those two brick supports under the two wooden support posts were crumbling. If you look closely, you can see that we used some of the old floor joists to shore up the tilted wooden posts. The floor joists were probably overkill for this, but they were free. They are about 2 to 2.5 times as dense and heavy as modern wood, and there are no knots.
Look at the pic below. The floor joists are the darker wood. They are dead plumb. You can see how tilted the old posts are. Fortunately, the concrete pilasters at the base of each pillar (original to the building) are large enough to accommodate the extra wood.
In the second pic, I have removed the crumbling brick bases that went between the pillar and the wooden posts. I built temporary supports for that part of the building during the process. I sawed off the bottom of each post. I did not take pics, but I build new supports out of 4 inch steel posts (1/4 inch thick walls) welded to steel plates on each end. When we poured the retaining wall, we encased all the steel in concrete.