John wrote:

Surely a bit dangerous to disregard the forces on the sides. Could lead to a
massive failure.

The presence of the insulating blocks (or kingspan!) will have little
effect on the strength of the pool. You'll still need to handle a
lateral force of one tonne (ish) for every running metre of wall length.

I should add I'm not a pool designer, but I do know a _little_ _basic_
physics.

Andy