previous user of Intex pools (the 18 foot one...)
ground doesn't have to be level- just the location that you put the metal feet! nothing sharp underneath, but small hollows in ground are ok...
if your land is flat to start with, sand will not have anywhere to wash away to (it flows down hill!)
If not flat, I found lime to be a good base, with landscape rock holding it in place

I had about 5 foot drop across 20 foot of space, used 20ton lime (1 dump truck, cost $100 roughly) with a pallet of landscape rock from lowes/home depot to build retaining wall- pool was in place for 4 years without problems!