YOUR POOL CAN “POP” OUT OF GROUND
An empty swimming pool can pop, or float out of the ground. This is due to possible hydrostatic pressure from unknown underground water. Think of your swimming pool as a “boat” in the ground, and it will float out of ground if there is underground water pressure.
There is no way to tell if you have underground water in your specific area unless you performed core sampling supervised by a soils engineer.
The best way to prevent pool popping is to drill several 1” holes in the floor of the pool, all the way through to the underground soil as soon as it is emptied. Several holes should be drilled if there is suspicion of ground water. The holes should start at the shallow end and continue across the break, with two or three in the deep end. One must be careful not to drill through the main drain line, which typically runs from the main drain to the skimmer.
Holes should remain clear during the time the pool is empty. New plaster can fill holes to reseal.
Do not drain your pool and leave empty without taking measures to prevent such popping.