French pool owners warned over early draining

Pool owners in France are being warned about the risks of draining their pools entirely of water for the winter season, as changes in soil conditions can see structures face increased pressure. 

Draining can lead to an issue called hydrostatic uplift (remontée hydrostatique), lifting the pool by several centimetres and causing it to become misaligned from surface level features.

In some cases, pool walls and shells can begin to crack. 

It can happen across France, but particularly where clay soils, more susceptible to changes from the shifts between dry and wet weather, are prevalent alongside high water tables.

Millions of homes across France are built on clay rich-soils, and pools built near rivers or areas with high water tables are potentially at a greater risk.

What causes the issue? 

In-ground swimming pools face pressure on all sides from surrounding soils which are filled with moisture levels that vary throughout the year. 

However, whilst a pool is filled with water it should be strong enough to resist eternal pressure, bolstered by the volume of water inside. 

If the water is drained from the pool, it can become susceptible to any increases in pressure from rising water tables and increased groundwater in soils.

An increased external pressure can cause the basin of the pool to shift its position, rising higher than the deck of the pool. 

Once this happens, the pool’s lining can deform, and in some severe cases can see the basin detach almost entirely from the base.

There are reports of pool basins moving several centimetres in a single night if they have been emptied too early. 

While conditions may be too cool to be using the pool, draining the water while soils are expected to receive plenty of autumn rainfall increases the risk of pools being affected by hydrostatic uplift. 

What should pool owners do? 

Risks increase significantly if you drain your pool entirely, regardless of whether you do this during the autumn or winter.

The simplest way to avoid hydrostatic uplift therefore is to ensure you keep a certain amount of water in the pool throughout the winter.

Exact amounts will differ between pools, the materials they are made of and surrounding soils, but specialists generally recommend keeping water to a level around 10cm below the jets. You can contact a local pool expert in your area or the company who installed your pool for specific advice.

Reducing water by this much prevents jets from freezing over in winter, while retaining enough water to resist pressure from surrounding soils. 

It also allows for the pool to be cleaned and kept in good condition whilst not in use. More information on ‘wintering’ your pool can be found in our article here, including both passive and active wintering. 

Ideally, you should not begin to close down your pool for the winter season until water temperatures drop to between 12C - 15C, even if you are not using it.

Note that during heightened drought restrictions, pool owners may be prevented from filling pools that have been ‘put to sleep’ for the winter months or from topping up pools that have been opened again. 

In some cases, these restrictions can even cover the emptying or draining (vidange) of a swimming pool, so are worth checking before taking any action.