That was always the thing in FL (which is, as you have likely heard, literally a giant sand bar (the peninsula, anyway)). My dad was a builder. We would set up sprinklers way in advance of any digging in dry weather (like a week if possible) and the sides would collapse anyway--just not as much.The beauty of living on a sand dune. I could easily dig the hole for the greenhouse in one day with my backhoe. Building the retaining walls would be another situation. Of course before building the retaining walls, I would definitely ditch in a water line for a frost free hydrant.
It would go quicker digging the ditches for the ventilation system if I had a helper to lay the pipe as the ditch progressed in order to avoid having to re-dig collapsed ditches.
We had 2 big ones in north FL, that was fun. Our property was on a sand hill and talk about easy digging! You could easily go straight down 6' with just a shovel.
LOVE your little photographic essay. So cool!