There are many drainage aspects to this project.:
- Long drain behind the upper retaining wall to prevent water placing pressure behind the wall.
- Trench drain on top of that wall for surface water.
- Trench drain on the pool deck for splash water and rain.
- Subsurface drain for edge landscaping.
- Drainage for flower bed.
- Drain beneath pool to prevent hydrostatic pressure.
We're addressing this last one today.
If the level of the water under and around the pool ever exceeds the water level in the pool, the pool will become buoyant, putting tremendous upward pressure on it. In many cases, the pool will actually rise out of the ground. This is true of both concrete and fiberglass pools* and most often occurs when emptying the pool or when the pool water level is low and heavy rains raise the level of water under the pool.
The results can be hilarious or catastrophic, depending on whether you're the owner:
Image credit: Daily Mail
On a level site, a standpipe dewatering system is the usual defense against this situation. A standpipe consists of a straight pipe with a cap on top that runs from the pool deck vertically down through the ground to the level of the bottom of the pool (but beside it). If the pool water level ever needs to be lowered significantly, the cap is removed and a hose can be dropped down the pipe to pump out any built up water first.
Our steeply sloped site allows us to install a drain that drains to the atmosphere. That is, a drain from under the pool to an above ground area at a lower elevation than the bottom of the pool.
The entire property slopes downwards to the rear and the right (when facing the rear). For drainage, there is a small rock stream running down the right side adjacent to the boundary fence which drains to the stormwater system. All of our drains will be draining to this location via a 100mm agpipe.
The pipe feeds back uphill and terminate in a a gravel pit adjacent to the deepest point of the pool. The other drainage sources will connect to this pipe at various points.
The pit will be enlarged and protected with a geotextile barrier once the pool base is prepared. Between the pool and the rock stream, the agpipe snakes through the garden:
* Since concrete is denser than water, the exterior water level needs to exceed the interior level by some amount to achieve buoyancy.