I have a 18'x36' standard 42" steel walled pool with a vinyl liner and 8'deep end. The current pool base is sand but needs total reworked due to holes in liner and water erosion. Local pool contractors will only install liner if base is retroweled with vermiculite product which has doubled in price since last year. I see people putting XPS foam board under above ground pool liners. Any thoughts on replacing sand base with 100 psi XPS which has a compressive strength of 690 kPa? My one concern would be hopper walls shifting when vacuum is applied before refilling with water.