There is also the issue of floatation!
As a civil engineer, floatation is most commonly overlooked. You dig a big hole and make it waterproof, then when the surrounding ground water level rises, if a floatation design has not been undertaken the whole thing could effectively pop out of the ground. Seen it happen with empty swimming pools!!!
Make sure you get a chartered civil engineer to do a design for you!
(not pimping out my services here as i'm not yet chartered, 1 year to go for me )