If I use the foam or gorilla padding underneath the pool do I still need to use sand?
Hi. It is not necessary to use sand. The ground should be as smooth as possible however. The Gorilla pad is very thin and will allow every single bump or pebble to show through to the liner so I would use something thicker. Foam is a good choice and just requires level ground, not sand.
Here are a few more pages related to foam and pool bases.