@GBBUDD: Do you see that azek sheet in stock where you are? Definitely not around here.
I will look at digging deeper for better compression strength. My thought was that static load will be quite low and not need much cover at all. 3” of gravel is decorative only with the sheet gods spreading most of the weight.
The sheet goods are, in theory, taking the place of additional soil cover. Probably doesn’t hurt to have a little more cover, though.
Paging Dr.
@Lisak1: How deep are your aqua blocks buried?