We are having an inground gunite pool installed with an inground acrylic 88" spill over spa that will be surrounded in flagstone and coping. Should the spa be placed on a layer of sand or gravel instead of dirt? I am now also worried about the flagstone and coping (that will be placed next week) sticking to the slick surface of the acrylic spa. Any suggestions for how the flagstone and coping should be placed?