A rigid retaining wall retaining a sandy soil is subject to a load of about 1/2 the weight of the soil it retains ( this is the "At Rest Pressure"). If the wall is allowed to move slightly, the 1/2 drops to about 1/3 (this is the "active pressure). Note that the actual value depends on the actual soil conditions, that clays are analyized differently than sands and that if the water table is retained by the wall, the full weight of the water acts laterally on the wall. In order to acheive this reduction a diplacement of at least .001H for dense sands to .004H for loose sands is required. H is the height of the wall. Thus for a 6 foot wall the required rotation is .0072 to .228 inches. So you are probably loooking at something like 1/8 to 1/4 inch rotation. Although this does not sound like much, it will require some deflection of your pool walls. The walls may be able to accomidate the movment strucurally, but will require some cracking of the concrete which may make it more difficult to waterproof. I would recomend trying to design the walls as rigid unless it is significantly more work. As you may have garnered from above this is not a precise science and being conservative is often the better choice.
Good Luck
