Emptying a Swimming Pool
Every swimming pool built or manufactured must, at one time or another, be emptied for repair or resurfacing. There is no such thing as a pool that will never need to be drained, regardless of what you may have been led to believe.
Draining a pool can cause serious damage to its structure.
If your concrete or gunite pool has hydrostatic relief valves (usually located near the bottom of the steep slope), remove them as soon as you can get to them while emptying the pool. If you have an operating bottom drain, remove the cover and see if it has a hydrostatic relief valve (almost all do), unscrew and remove the valve as soon as the pool is drained.
The last 30cm or so of water probably won't drain out, so be ready with a sump pump to immediately place at the drain to remove the remaining water. This is extremely important, and must be done quickly, so the bottom drain can be accessed for removal of the cover.
After opening the valve, place a pipe into the valve (about 3’ to 6’ long) and put a hose through the pipe, Bring the hose up over the top of the coping and attach a pump to draw the water from under the pool. This is just ground water, so there is no chemical content to be concerned about. Point the water flow to a drainage area away from the pool
Type of damage that can occur:
After draining, the liner can shrink which may then tear when refilling. The older the liner, the less it will stretch when refilling. Generally if the water has been out of an above ground pool for any length of time, a new liner will be required.
Because fibreglass pools weigh the least, as compared to concrete or gunite, the danger of lifting is greater. Never empty a fibreglass pool during the rainy season unless the pool has a built-in system for relieving the underground water pressure (which most do). If you have standing ground water hours after it rains anytime during the year, there is a strong possibility that any pool (gunite or fibreglass) will lift.
A fibreglass pool is the most dangerous to drain and should only be done by a professional. Older pools may not have been built structurally to hold back the weight of the dirt against it when the pool is drained. This can cause the walls to collapse. These pools were backfilled with dirt as the water level came up, equalizing the pressure as it fills. A fibreglass pool must be braced if emptied.
In ground Concrete Pools
Here, you are dealing with the same groundwater problems as for a fibreglass pool. Most in- ground concrete pools are built structurally to withstand the weight of the dirt against them when drained. However, if the ground water is high enough, it can push the entire pool out of the ground. The pool shell acts like a ship and floats up in the ground water.
Bottom Line: - Do not attempt this if you are unfamiliar with the process and the risks.
When telling pool owners about the above concerns, we are often asked about the hydrostatic relief valve and why this wouldn't protect the pool. A hydrostatic relief valve only allows as much water to flow through as the force of gravity permits. You are draining the pool much faster than water can flow through the hydrostatic valve. It is designed to equalize the water level in the pool to the ground water to compensate for a small leak or water loss.