All pools must be drained at some point in their life. Sometimes it is for the purpose of resurfacing the interior finish, sometimes it is simply because the calcium hardness or total dissolved solids have reached 1000ppm or 2000ppm, respectively. Sometimes because of harsh winters the pools are emptied for winterization. Regardless of why the pool needs to be emptied it is important to remember that in some areas the only thing preventing the hydrostatic pressure of a high water table from literally popping the entire pool out of the ground is the equal, reciprocal pressure of the water in the pool itself. In other words, when the pool is empty there is often a risk of groundwater pushing the entire pool shell out of the ground.
There is a simple remedy and it’s an inexpensive part ($50-$80) appropriately called a hydrostatic relief valve placed in the sump of the main drain. Frustratingly such a simple part is often overlooked in the design and construction of many pools. The part allows for the water table to flood into the pool and relieve pressure, as opposed to forcing the pool out of the ground. Granted, groundwater may have some minerals unfriendly to the interior surface of a pool, but is far preferable to the total destruction of the pool itself. Many years ago we at Aqua Design International saw the 75 meter (246 foot) municipal pool in Ardmore, Oklahoma popped 3 feet out of the ground because of a failure to include a simple, inexpensive hydrostatic relief valve in the design of the pool. While not widespread this phenomenon is depressingly familiar and unnecessarily recurring. Check back with us for brief, related read on the importance of structural engineering and geotechnical evaluation as part of responsible aquatic consulting.