You will be modeling a water tank, not a pool. Therefore, in order to increase the rigidity of the system and reduce the dynamic effect of the water, you should think with 2 or 3 chambers. lower elevations of the curtains in the middle axles from the foundation level by 1 m. so that the composite container system is formed. (It can be a panel space of 2 * 1 m.). You should give the horizontal thrust to the outer panels, which will be created by the water from inside, and the horizontal thrust to be created by the ground from the outside. In addition, when the tank is empty, you should only analyze under this load, since there will only be an external soil effect. You should leave an entrance hole for each compartment on the floors and consider a suitable cover system for cleaning and maintenance of the tank. Finally, it is beneficial to add a waterproofing gasket along the outer axles to the foundation, outer curtain joint. Greetings G.KÖSE