Imagine a small boat in a calm lake next to a dock. If someone steps into the boat, it sinks down a bit. If they step out of the boat, it pops back up. If someone pushes the boat down as far as they can without filling it with water and then let go, it will jump back out with some force. This is pressure in action. Hydrostatic pressure is the force exerted on other objects when not in motion. When you dive to the bottom of the pool, you can feel it.
A swimming pool is much the same. When full, the water exerts pressure on the walls and floor. It pushes it down. When empty though, it is subjected to all the pressures around it pushing up. When precautions are not taken, this can lead to disastrous results, including the practical destruction of the entire pool, deck and surrounding areas. Online searches of “popped pools” will show nightmare images of these pools popped out of the ground. Fiberglass pools are at a much higher risk of this. In other cases, the damage may not be as noticeable. It can crack concrete basins, warp things ever so slightly and then the pool will leak in the future.
Groundwater is the main cause for hydrostatic pressure on a drained pool and is regional. Pools installed in low lying areas, next to natural bodies of water or anywhere with a naturally high groundwater can cause significant hydrostatic pressure on the pool.
In order to prevent this damage from occurring, hydro relief caps/valves are usually present. Commonly installed under a main drain cover in the deepest part of the pool. When a pool is drained, these must be opened or removed if a cap. They may need to be opened to drain the last few inches out of the pool, or in some cases opening them may let the ground water into the pool, which is the point. This gives the groundwater a relief point, so instead of pushing on the pool it enters the pool, equalizing pressures.
Some pools may not be equipped with hydrostatic relief valves. In those cases, holes are sometimes drilled into the bottom of the pool to act as a relief. Then they are repaired when the pool is ready to be filled.
Location and type of relief valve vary. It may be hidden under the main drain cover or may be in the plaster of the pool. It might be able to be unscrewed, or it may require chipping off plaster around the cap to remove it. There may even be more than one valve. Consult your original engineering drawings and other documents for details specific to your pool.
If unsure on how to relieve pressure, do not drain your pool. Consult a professional before draining a pool. If hydro relief valves/caps are present, they must always be opened when draining the pool. ♦
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