Pool manufacturers recommend setting above ground swimming pools on level ground to prevent an unnatural shallow side to the pool, potential collapsing of the walls and possible injury to bystanders or swimmers. According to BackYardPools.com, the walls of the pool distribute the force of the water pressing against them to the ground at a 45-degree angle. With an unleveled pool, these forces cause a collapse.
Each gallon of water weighs over 8 pounds. Since pools hold several thousand gallons, the force is tremendous. When the pool is level, the forces work evenly along the pool walls, keeping it upright. If the pool is not level, the force of the water presses against one side of the wall more than it does on the other side. These unbalanced forces become more than the walls can withstand, which causes the pool to collapse and dumps the contents of the pool, including swimmers, into the yard.
After removing the sod from the ground, installers lower the soil to match the lowest spot in the installation area to level the pool. Many owners make the mistake of filling in any low spots using soil removed from high spots. Under the weight of the pool, this soil compacts, leaving the area unleveled..