Why has the paving around my pool settled and cracked?

Introduction

Last week we looked at the highly technical issue of why tiled pools leak.

This week I want to look at a slightly less technical  issue but one that is of fundamental importance to pool buyers - the failure of pool paving. It is interesting that the answer to the question posed in the title above is exactly the same as the answer to "why do tiled pools leak" ie  "Most paving cracks and settles because of faulty design and installation procedures"

It is worth remembering that a pool is a hole in the ground, filled with water and surrounded by paving. So the paving will always make or break the appearance of a pool.

First of all what do we mean when we say paving?  It is an engineering term that refers to a hard surfacing laid on the ground for use by pedestrian or wheeled traffic.

In the context of pools the hard surfacing is the area around the pool that is needed to:

• Drain rain water away from the pool so that dirty water, sand and earth does not drop into it
• Allow use of the pool in wet weather so that bathers do not carry mud and dirt into the pool
• Allow access to the pool throughout the year for maintenance
• Discourage frogs and other amphibians from using the pool

- and last but not least for sunbathing and entertaining!

We will examine how pool paving is normally built below.

The background

Lets look at what happens when a pool is installed. First of all a hole is dug in the ground. This hole has to be bigger than the pool area to allow space for wall erection and the pool pipework. This  is known as the pool "overdig". Many of the kit pool manufacturers boast about how small the overdig requirement is for their particular pool design. This is misleading because the main benefit that accrues from a small over dig is a small saving in excavation and backfilling costs. Most kit pools also require the top of the pool wall to be stabilised by a concrete "ring beam" that runs right around the perimeter of the pool and so a narrow overdig also reduces the volume of concrete required.

You are probably wondering what all this has to do with the paving, but if the soil is poor - and it nearly always is in the top metre of ground in SW France - the excavation weakens the surrounding ground as shown in the sketch below - and it is this soil that supports the paving.

The excavation will always weaken the volume of soil shown in the sketch unless the ground is solid rock. If the soil is clay or sandy clay the horizontal restraint from the soil that has been excavated is removed and the wedge of soil tends to stretch horizontally and  the vertical face of the excavation moves towards the pool.  In fact in our experience quite large elements of soil often do fall into the excavation thus considerably widening the "overdig". There are other consequences from this method of construction:

• The gravel fill that is supposed to reduce the soil loading on the kit pool wall absorbs water (it is a hole in the ground) that overloads the pool wall and keeps on softening the already weakened soil
• The concrete anchor beam on which the kit pool relies for stability is supported by weak soil that has been badly disturbed at the ground surface by pool construction operations
• The kit pool designs never take the loading from the weakened soil into account - this is always written out in the fine print of the contract
• The paving settles and cracks

I have concentrated on kit pools because they suffer from a major disadvantage when considering how the pool paving can be installed in such a way that it does not settle and crack. This disadvantage is that the pool walls are so light that they cannot be used to support the weight of the pool paving.

The pool paving specification

There are 3 basic options:

1. Paving slabs - 15 to 35 mm thick - laid on a thick sand bed
2. Ditto laid on a sand bed on a consolidated stone base 150 to 200 mm thick
3. Ditto laid on mortar on a 150 mm thick reinforced concrete slab

Type 1 should never be used - the slabs will always need constant lifting and relaying.

Type 2 can be used when the ground is good - see below.

The only form of construction that can be guaranteed to work in most of the weak surface soils of SW France is Type 3 as shown in the following sketch.

Note how the large diameter 16 mm high tensile steel dowel bars transfer the weight of the reinforced concrete slab onto the top of the pool wall. 

The sketch also shows how the paving is supported by the reinforced concrete slab that in turn is supported by the pool wall on one side and by undisturbed ground on the other side. Reinforced concrete is the only type of pool structure that is capable of providing this type of support to the pool paving - so if a kit pool is installed - the paving is likely to settle  and crack unless the pool is built in very good ground such as rock or dense gravel (in these circumstances the paving can be supported as described in 2. above). As noted in previous posts on this blog most pools in SW France are built in poor ground and so the likelihood of having a problem with paving built around a kit pool is quite high.

Paving slabs

There are a few other issues regarding the paving that are worth noting.

Frost

Overnight temperatures as low as -10 degrees Celcius are quite common in SW France. Frosts as hard as this will break up all thin medium to low density paving slabs. High density slabs 35 mm thick are recommended - and even some of these may fail from time to time.

Grout

Paving durability and appearance depends on the grout - and the proprietary grouts that are needed are quite expensive - make sure that any paving is laid with grout that comes in a bag that shows that it is intended for use outside with heavy duty slabs. Use of anything else is a false economy as the grout may need to be relaid after the first winter.

Falls

Both the reinforced concrete support slab and the paving needs to be laid at a minimum gradient of 1 in 60 falling away from the pool. If  the paving boundary is formed by the wall of a building you must install a drain or the wall will get very damp and water may even enter the building when the floor level is lower than the pool paving outside.

Joints

To obtain a stunning appearance the margelles (coping stones in English) and the dalles (paving stones in English) must be manufactured by the same maker so that the joints can be lined up. With care - and if the pool is built to the right size - the dalle and margelle joints can be made to line up  both across and along the length of the pool.

Laying Margelles

The top of the pool wall must be absolutely flat and level or the margelles will look dreadful - this can be very difficult to achieve with some kit pools where manufacturing tolerances are too large.

Type

Bluepools offers 35 mm thick Sahara or Ardoise paving by Carra Terra as a standard on all its pools. These are supported in all instances by a reinforced concrete slab spanning on to the pool wall as described above.

Cost

High quality paving including the reinforced concrete slab and the paving slabs will cost of the order of 100 euros per square metre. The margelles will cost between 300 and 800 euros depending on the pool size and the Roman End / steps configuration.

Remember the pool paving will make or mar your expensive pool - do not economise on it.

I will be writing a post on choosing between paving and decking in the next few weeks - so please look out for that if you are in the process of planning a pool.

Regards

Will Witt

 

Comments

[Mosaic Patterns]

thanks for the information......and also thanks for remembering....

[Charles Lee]

Hi there

My swim pool paving was done two years ago. at that time, one pool wall slight leaned towards inside. Now this leaned wall leaned more inside and make vanyle a hole. i worried that this leaned wall falls more and destroy my pool. How can i do to put this steel wall back straight and my pave around pool need to be redone?

My pool is rectangle and steel wall. The leaned wall is on shadow part and only one wall. my pave is concrete. i can provide viedo to you if you are willing to help.

Thank you in advance

Charles

[Pomor]

Do you have a layout diagram for in stal a concrete slab around a pool...

Please include gravel base, concrete depth, wire mesh / rebar comments...

Also, I plan to install flagstone or pavers on a 1 1/2 - 2 in portland cement base