Most fiberglass pools and some concrete pools will have a hydrostatic valve fitted  the deepest part.  

Is designed to let ground water INTO the pool, especially when empty or nearly empty. They made to stabilize the water pressure under the pool with pool itself to avoid any damage made by high water table that can be due to a geology or heavy contentious rain.  Be aware that such pressure if not managed correctly can damage the shell of swimming pool when it is empty or near empty.  More so for fiberglass pools which don’t have the mass of a concrete pools. Usually a concrete pool is engineered to withstand considerable water pressure when empty so the hydro-static valve is omitted and when the pool was built it would have remained empty for a month or so while the concrete cured. So you can usually empty concrete pools without issues.

Fibreglass pools have much less mass and thus  may buckle or even lift out if the ground but that water beneath. It does not happen often, as a matter of fact only twice in our long history , pools were lifted. However you never know, you could be that third one. 

Some pools will have an inspection pipe near the pool which can be used to determine the ground water level. It’s usually located in the surrounding tiled area and will have a shower waste grill on top, check water depth with broom handle. For fibreglass pools we advise not emptying if the water table is high, say less than 1 M deep and if between 1 – 2 M, proceed with caution. Also don’t leave a fibreglass pool empty if heavy rain is expected. 

Monitor the situation by emptying the pool about 1/3 and leave overnight so as to check on any pressures, bulges or movement of the shell. If  shell stays stable,  empty a further 1/3, so you can release the hydrostatic valve.  To release valve lift off cover plate and then pull up the mushroom shaped cap, in the sump, which should let dirty water into the pool.  If seized, then try to force up with screw driver under lip.  ALWAYS a good idea to replace when pool empty, they have not changed much through years so any pool shop would have replacements. Leave overnight and slowly continue empty pool. The water may will keep on coming through the valve for some time but that is a nature and one has to work around of it. 

You may notice some springiness in the floor, though hopefully no bulges. Try not to walk on any springy areas and while preparing the pool use some plywood sheets to spread the load. Fibreglass is quite resilient, but continuous flexing is not good. Also older fibreglass pools may be somewhat brittle with age or if of poor quality and may crack if flexed.

Bracing should be considered if any doubt about the water pressure and use 2 – 3 braces across the pool. (Place as soon as you can get into the pool to work) These can be Acrow props (Hire Company) or timber posts, both with a plywood panel about 600mm (2ft) square to spread the load. They will require moving as surface preparation and resurfacing are undertaken.

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