Why do rectangular pools cost more?

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Introduction

This another article that was first published on our blog

This week I am tackling another very topical and also technical issue. My objective is to help the pool buyer to make a choice between a rectangular pool built using traditional technology or a circular pool or pair of linked circles that rely on tension in the pool walls.

Bluepools installs the rectangular type of pool in reinforced concrete.

The Double Bubble pool manufactured by Christal Pools and sold under licence in France by Piscine Plus is probably the biggest selling product of the circular / kidney shaped type and is made from galvanised steel.

There are also rectangular pool kits available on the market and so I have also considered these as well.

All these pools rely on PVC liners - we dealt with pools that incorporate ceramic tiled finishes in the post dated 7th January. Ceramic tiled pools that are built properly are 2 or 3 times as costly as the pools that we are dealing with here.

How the different types work

The Bluepools approach

The pools that Bluepools constructs are based on walls that are supported by a foundation beam around the outside of the pool as shown in the following sketch. The concrete wall is reinforced with 8mm high tensile bars that also connect the wall to the foundation.

When the pool is empty (sketch on the right) the earth pressure behind the wall is resisted by the pink pressure underneath the wall foundation.

When the pool is full the pink pressure changes shape as shown. The wall is stable under all types of ground loading provided that the wall is founded on good ground.

One of the main advantages of this type of pool is that the earth pressure loadings on the wall can vary from zero (ie no soil against the wall) to very high (water-logged running sand) and the walls will still be stable - provided that the foundation below the wall is good. Poor foundation conditions below the walls may require deeper excavation down to good ground and backfilling with concrete.

Roman ends and steps of all shapes and size can be freely installed in pools of this type.

Many French artisans also build pools in a similar way but they often replace the wall foundation with a reinforced concrete slab that covers the whole pool floor. This is not really necessary when the ground conditions are good.

The heavier nature of the construction of reinforced concrete rectangular pools (and the higher capacity filtration equipment required- see below) results in a cost penalty of about 200 euros per square metre of pool area.

The kidney shaped pool approach

The kidney shape approach is illustrated on the following sketch.

When the pool is empty the ground pressures are resisted by compression in the galvanised sheet pool walls and when the pool is full of water the compression changes to tension.

It is very important that the backfill around the kidney shaped pool is a consistent sand that is uniformly compacted. The tension / compression in the pool walls is taken to a heavy beam that spans across underneath the pool at the point where the two circles meet - as shown in the following sketch. The walls are stablised by concrete that is cast between the sheet steel walls and the side of the excavation. The kit relies entirely on the galvanising to stop corrosion.

The heavy main beam must be supported by good ground and it is very important that the sand backfill around the walls is uniformly compacted to ensure that wall loading is even. This means that the backfilling must exist right around the perimeter of a kidney shaped pool.

Small sets of steps can be installed in circular pool wall but they are a complicating factor in a structural type that relies on hoop tension in the walls of a pool.

The Christal Pools design is protected by patent and hundreds have been successfully installed and used in France for over 10 years.

Rectangular pool kits

There are many different types of rectangular pools that can be purchased in a kit format. There are many different structural systems available. Some rely on a prop from the bottom of the pool excavation, some rely on anchors in the soil surrounding the pool and some seem to rely on blind faith!!!

Some are very cheap and some are very expensive.

Many originate from overseas where consistent dry ground conditions - such as in various areas of the US - allow the use of very light systems.

The universal truth that applies to all of them is that they rely on the ground for support but are never bespoke designed for the installation site. Ground conditions and environmental loading on pool walls in SW France are highly variable and anyone that purchases a rectangular pool kit is taking quite a risk.

Water treatment and filtration

The main wall of kidney shaped pools is a thin galvanised sheet that is not wide enough or strong enough to support conventional skimmers, inlets and vaccuum points through the walls of the pool.

As a result all the pool "through the wall" fittings are incorporated in the ladder and this restricts the rate at which the pool water that can be filtered and chlorinated. This results in a reduced capacity in terms of the time it takes for the pool water to be changed. The pumps and filters are sized to suit and it is very difficult to achieve a 6 to 8 hours water change period in larger pools (As is normally adopted for private pools for family use). The skimmer in a rectangular pool also take off the surface water where any harmful pollutants or viruses are concentrated. The intakes through the pool ladder do not do this as efficiently.

It is also true that the circular shape of a kidney shaped pool promotes better mixing of the water than in a rectangular pool with its corners. Most circular type pools are also built without a hopper (deep end) and this also increases the water circulation efficiency.

As a result the water circulation and filtration system in a kidney shaped pool can be installed at a cost that is a good bit less than the equivalent cost for a rectangular pool. However on the other hand it is very difficult - if not impossible - to meet DDAS standards for community pools for instance. (We are preparing a future post to deal with DDAS standards for gite complex pools)

Property Valuation

The shape of a kidney pool makes it impossible to integrate the coping stones and the surrounding paving. At many installations the paving runs under the coping stones - this is both unsightly and a trip hazard.

Many people prefer the classic lines of a rectangular pool with a Roman End and surrounding paving where the joints in the coping stones (margelles) are continued through the paving slabs (dalles). This has the affect of increasing the value of a property where a rectangular pool has been installed.

Conversely many people do not like circular type pools and so the value of the property may even be decreased if people take the removal of an existing pool and replacement by a rectangular pool into account.

Summary

Rectangular pools are a very different product from a circular pool and will normally cost about 10,000 euros more for a pool of about 50 square metres in area.

Rectangular pools in reinforced concrete can be adapted to suit any ground conditions.

A kit pool of any type is not suitable for highly variable or poor ground conditions.

Kidney shaped pools are an excellent product when used as a leisure facility for a single family.

If a pool is required for a gite complex rectangular pools are necessary to meet DDAS requirements

Rectangular pools that are carefully designed and built increase the value of a property - circular or kidney shaped pools may detract from the property value.

I hope this helps in your choice of pool - it may be be a difficult decision but the two options are really quite stark:

• If you want to enhance the long term value of your property buy a rectangular pool with a structure built from reinforced concrete
• If you just want a pool for the family to swim in and enjoy then buy a Christal Pool

I know the people at Christal Pools and I think that they have an excellent product - however the pools that we build are aimed at a different market - I hope this short analysis helps you make a choice on a more informed basis.

Regards

Will Witt