When I installed my 12' x 24' oval above ground pool seven years ago, I was daunted by the thought of hauling in and leveling out enough sand to cover the base two inches deep. Instead, I purchased 4' x 8' x 1" tongue-and-groove foam insulation boards (the type used under cement floors). I then cut and laid this over the leveled dirt area, then duct-taped the seams twice so that the weight of the water would not create a groove. After that, I only needed enough sand to slope around the interior edge before installing the liner. The end result is a long-lasting, puncture-resistant pool bottom with a bit of give.