If the pool remains filled with water, the downward force/pressure (holding pool in) will equal the worse case upward force from the ground water. The pool will not pop out. The problem comes in if/when you drain water form the pool. That is when they can pop out. Same is true of concrete pools. In areas of high ground water, builders will install a drain field/line under the pool (a well point) so the ground water can be pumped out under the pool whenever you need to drain it (i.e. repairs/refinishing).
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