The big one took a few more bags of sand than I thought, something like 6 or 7, but I took a few more precautions with it as well.
I found several interesting research papers and articles about the strength and behavior of sand with cyclic loading, penetration tests, reinforcement with geotextiles, etc.
Takeaways include, but not limited to
1. Sand has a period over which it "settles" and then all but stops deforming under cyclic loading (needs to be well compacted or my platform will sink over time)
2. Can be reinforced by a grid material that helps transfer shear stress and stop motion
3. Sometimes has an epoxy poured into the top layer to form a stiff crust (keeps sand from spreading and improves strength)
There are many more strategies for increasing the strength of a sand based foundation, but I decided to apply these three with what resources I had on hand. After taping down the frame and applying the sealant, I just stapled together and taped down these scrap strips in a manner I thought would help to prevent any migration of the sand over time.
I poured the sand and screed it, then I took a piece of scrap plywood and placed it on the sand, and then jumped on it as hard as I could. I did this around the entire bed of sand and then added more and screed it again. It made an immediate and considerable difference, I'm glad I didn't skip that step.
I didn't have any epoxy, but I did have this craft adhesive stuff around, so I just sprayed a bunch of it on the surface and laid leftover pieces of landscape fabric on top. The fabric stuck to a thin layer of sand with glue on it. It serves to help lock the top layer in place a bit.
The pieces of PVC pipe were there so I could lay the platform down on top and then move it around easier. That way I could put my glue on with it down instead of trying to carefully set the giant thing down on it. After setting it on the rollers, I laid my construction adhesive and then pulled the pieces one at a time, guiding it into place.
I had plates on top at first, but I realized it wasn't squeezing the adhesive out, so I went around the perimeter installing screws to pull the platform down to my frame and get rid of all the gaps. I trimmed my mats back to fit and now everything is in place.
I did a few clean and jerks and drops from overhead and let me tell you what, it is a joy to use this platform now. Its completely solid, no give, no flex, no rocking around like before. Now that I'm done I'm thinking I might just go ahead and add trim on the edges to make it look good. It would also help keep the mats in one place, and if I leave the top edge proud; keep the bar from rolling off the platform. Other than that I need to add some kind of system on the sides of the squat rack to accept heavy bands so I can do band squats, but I haven't thought of anything very noteworthy yet.