Many homeowners often live in an area of the country with a high water table. These areas of the country are particularly susceptible to groundwater. Hydrostatic forces from rising ground water could cause an inground swimming pool to pop out of the ground, causing severe harm. Inground swimming pools are prone to floating out of the ground after being completely or partially drained when soils are filled with water. This is because the hydrostatic force of water pushing up from below has the ability to overpower the weight of the entire pool as well as other limiting forces including friction between the pool walls and the ground. The pool will move upward if the hydrostatic uplift force exceeds the restraining forces, causing incredible damage and making the pool a complete write-off. This is known as “pool popping,” and it occurs every year in Florida and some parts of Texas, primarily during the rainy season.
When it comes to in-ground swimming pools, the first thing a pool designer must understand is how to deal with a high water table. A high water table does not compromise the finished height of a pool in the yard. During building, the contractor must gain control of the ground water before the pool is finally filled. Under the rough design of the pool, an under drain must be built. A 12-inch pvc line will be buried in 34-inch stone at least 18 inches below the pool’s final desired depth. When draining the pool, a trustworthy contractor would leave a way for the homeowner to access an under drain for future use.
To help avoid a hydrostatic failure, hydrostatic valves and drain pipes are also used. In the main pool drain line, a hydrostatic relief or check valve is often installed. This valve’s job is to balance the pressure between the water under the pool and the water at the pool’s bottom. The valve is designed to open if the water pressure beneath the pool significantly exceeds the water pressure at the pool’s bottom, enabling water beneath the pool to flow into the pool’s bottom. A muddy or dark cloud near the pool drain may mean that the relief valve has been used recently. The valve being stuck open or failing to open to alleviate high water pressure under the pool are two issues with these valves. In the above case, the pool can pop due to the high water pressure.
Groundwater management may sometimes be accomplished by the use of well points. A plumbing pipe is placed under or beside the pool shell in cohesionless soil (sand) or gravel. Before the pool is drained, the well point is used to draw ground water out from under it, reducing the possible hydrostatic uplift pressure and stopping the pool from popping.
The upward or buoyancy force on an object immersed in a liquid is equal to the weight of the liquid moved by the object. The total weight of the item opposes the buoyancy force. A swimming pool constructed in water-soaked soil follows the same idea. The pool is filled with water under normal circumstances. The buoyancy force on the pool is countered by the weight of the water, the weight of the pool, and the restraining impact, such as friction of the soil on the pool walls, and the pool remains in place. When the surrounding soil is saturated and the pool is empty or partially empty, the hydrostatic uplift force may cause the pool to float or “pop” out of the ground. The pool and its surrounding elements, such as concrete walkways, wood decks, supply and drain lines, lighting, and other finishes, typically suffer major cracks and other damage as a result of this. At the deep end of the pool, pool pop failures are more common.
Because of their light weight, fiberglass shells are vulnerable to popping from hydrostatic uplift. When the pool is complete, the lateral forces of the soil and water around the pool shell are counteracted by the water in the pool. The weight of soil and water outside the shell can cause the side walls of a fiberglass or vinyl pool liner to deflect inward when the pool is drained, as there is no opposing hydrostatic force.
Gunite and shotcrete pool walls are usually 4 to 6 inches thick and steel reinforced. Obviously, this is a much more substantial structure than a fiberglass pool shell. Concrete pools, on the other hand, are also vulnerable to major upward hydrostatic thrust and can also pop from the ground. Many concrete pool hydrostatic failures have been investigated by our service. Because of the strength and stiffness of the reinforced concrete walls, lateral deflection of the side walls of concrete pools is uncommon.
In-ground pools can float or pop out of the ground due to hydrostatic forces, causing damage to the shell and surrounding components. Water should not be refilled into a swimming pool that has “popped” out of the ground due to hydrostatic failure. If you do this, the pool will most likely break even more and/or crack around the bottom. If this happens, the pool’s chances of being rescued are likely to be absolute zero. Cutting the top walls, pressure grouting under the pool to fill voids, and re-backfilling around the pool could be enough to save a pool that is just slightly popped and not otherwise seriously harmed.
Until emptying an in-ground swimming pool, consult an experienced pool maintenance contractor. If a problem arises and a pool “pops” out of the ground, a pool contractor with a structural engineering background should be consulted before any further action is taken to fix major structural problems with the pool. It is not recommended that the pool be refilled with water before this is finished.
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