Pool Construction: Excavation, Base Slab, Backfill and Drainage
The earthworks decide the service life of the pool. If you plan the subsoil, excavation, base slab, drainage and backfill carefully, you avoid later settlement, deformation and water damage – mistakes that can hardly be corrected once everything has been backfilled.
Assessing the subsoil: the basis of all planning
Before the first spade goes into the ground, you need to know exactly what the pool will sit on. The decisive factors are the load-bearing capacity of the ground, the soil type and the groundwater level. Undisturbed, load-bearing soil is ideal; cohesive, made-up or variable soils call for special attention.
Check the following points:
- Soil type: sandy-gravelly (drains well, load-bearing) or cohesive-loamy (holds water, prone to settlement)
- Groundwater: the highest level to be expected over the year – not just the current level
- Contamination and made-up ground: old foundations, rubble or uncompacted fill
Where the ground is clearly difficult, on a slope or with high groundwater, you should obtain a geotechnical soil survey and have the foundation sized by a structural engineer or specialist contractor. Structural design and buoyancy safety are not areas for guesswork.
Setting the excavation dimensions with working space correctly
A common mistake is to excavate only to the outer dimensions of the pool. All around, you need working space to erect the walls, fit the built-in parts and later backfill in a controlled way. Around 50–60 cm per side is usual; the recognised minimum working-space width of about 0.50 m to DIN 4124 should not be undercut.
The excavation depth does not result from the water depth alone, but from the sum of the build-up heights:
- the desired pool depth
- the thickness of the base slab
- the blinding and frost-protection layer beneath it
Factor in these layers from the outset so that the pool rim ends up at the planned height. With deeper pits, the stability of the slope must be considered – depending on soil type and depth, battering the slope or shoring is required. The rules on occupational safety in the trench are mandatory; if in doubt, the earthworks contractor carrying out the work can advise.
Base slab and frost protection
The base slab transfers the loads into the ground and must be level and exactly horizontal. The classic build-up from bottom to top:
- Frost-protection/base course: compacted crushed stone or gravel as a capillary-breaking layer
- Blinding layer: a thin layer of lean concrete as a clean working surface
- Reinforced concrete slab: thickness and reinforcement to the structural specification
The frost depth varies from region to region; the foundation should be built frost-free or with a sufficient frost-protection layer. For the concrete thickness, reinforcement and the specific frost depth at your location, the structural design is decisive – have it drawn up by a specialist contractor if in doubt. An uneven or under-dimensioned slab later leads to cracks and settlement that carry over to the entire pool.
Drainage and groundwater
A ring drainage around the excavation pit carries away seepage and perched water and prevents water pressure from building up behind the walls. A drainage pipe OD d110 (DN 100) in a gravel bed, with a slight fall towards an inspection or collection chamber, has proven its worth.
- Pipe position: below the top edge of the base slab, embedded in filter gravel and protected against silting with a geotextile
- Fall: continuous towards the collection chamber, with no counter-fall
- Discharge: into a soakaway or – where permitted – into the sewer
Whether infiltration or discharge is allowed is regulated by the municipality; clarify this in advance with the responsible authority. With high groundwater there is a risk of flotation: an empty pool can float up under groundwater pressure. Anti-flotation measures, groundwater control during construction and permanent dewatering belong in the hands of a specialist planner.
Backfill material and compaction
The backfill provides permanent support to the pool walls and is therefore not simply a way of disposing of the excavated soil. Use load-bearing, easily compactable material – depending on the pool type, compacted gravel/crushed stone or lean concrete.
- Work in layers: place in layers of about 30 cm and compact each layer
- Evenly all around: never fill up high on one side only – the one-sided pressure deforms the wall
- With liner pools: backfill in step with the water level so that internal and external pressure stay balanced
- Unsuitable: cohesive soil, rubble, organic material or large stones against the wall
With polystyrene-block and formwork-block pools, the cavity is often filled with lean concrete; here you must pay particular attention to even, low-pressure placement so that the walls, which have not yet stiffened, do not bulge.
Types of damage from incorrect backfilling
Mistakes in excavation and backfilling often only show up months later – and are then expensive. Typical types of damage:
| Type of damage | Common cause |
|---|---|
| Bulged or displaced wall | Backfilled on one side or too quickly; liner pool filled without a matching water level |
| Settlement, sagging pool rim | Not compacted in layers; unsuitable, settling fill material |
| Wrinkles in the liner | Pressure from settling backfill, uneven slab |
| Cracks in slab or wall | Uneven load transfer, missing frost protection, ponding water |
| Floated-up pool | Groundwater pressure with an empty pool and no anti-flotation measures |
Common denominator: too little or the wrong material, uneven pressure and inadequate water management. If you compact in layers, work evenly all around and install the drainage properly, you avoid most of this damage from the outset.
Creating the excavation pit for the pool – from assessment to backfilling
- Check the subsoil and groundwater Assess the soil type, load-bearing capacity and the highest groundwater level. With difficult ground, obtain a geotechnical soil survey and have the foundation structurally sized.
- Set the excavation dimensions Determine the excavation from the outer pool dimensions plus around 50–60 cm of working space per side (minimum working-space width of about 0.50 m to DIN 4124). Add up the excavation depth from the pool depth, base slab and the frost-protection and blinding layers.
- Excavate and secure the pit Excavate the pit and, depending on depth and soil type, batter the slope or shore it. Comply with the rules on trench safety and do not loosen the base.
- Install the frost-protection and blinding layers Place a compacted, capillary-breaking crushed-stone/gravel layer and, on top of it, produce a thin blinding layer of lean concrete, level and horizontal.
- Lay the ring drainage Embed a drainage pipe OD d110 (DN 100) in filter gravel and geotextile with a fall towards the collection chamber. Clarify discharge into a soakaway or sewer with the authority in advance.
- Build the base slab Cast a reinforced concrete slab to the structural specification, screed it exactly horizontal and let it cure sufficiently before the pool is placed on it.
- Erect the pool and fit the built-in parts Build up the pool walls or set the pool in place and position built-in parts such as the skimmer and inlet nozzles. Provide wall and floor penetrations for the pipework.
- Backfill and compact in layers Backfill evenly all around in layers of about 30 cm and compact each layer. With liner pools, work in step with the rising water level so as not to deform the walls.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| Drainage pipe d110 (DN 100) | – | – | – | Ring drainage around the excavation, laid in filter gravel and fleece, with a fall toward the collection shaft |
| Filter gravel / drainage gravel | – | – | – | Bedding for the drainage and a capillary-breaking layer under the base slab |
| Lean concrete (blinding layer) | – | – | – | Level working surface under the base slab; also for backfill with formwork-block pools |
| Reinforcement / reinforcing steel mesh | – | – | – | For the reinforced-concrete base slab, sizing per structural specification |
| Inspection / collection shaft | – | – | – | Low point of the drainage; allows inspection and discharge of the seepage water |
FAQ
How much working space should I plan for during excavation?
Around 50 to 60 cm per side beyond the outer dimensions of the pool is usual; the recognised minimum working-space width of about 0.50 m (DIN 4124) should not be undercut. This space is needed for erecting the walls, fitting the built-in parts and the controlled, layer-by-layer backfilling.
Do I always need a drainage around the pool?
With well-draining soil and no groundwater problem it is not always essential, but it is recommended in almost every case. With cohesive soil or on a slope, a ring drainage is practically indispensable to carry away seepage and perched water. Against permanently high groundwater, however, it does not help: groundwater may usually not be discharged permanently, so here a watertight construction with anti-flotation measures is needed instead of a drainage – its design belongs in the hands of a specialist planner.
Can I reuse the excavated soil as backfill material?
Only if it is load-bearing and easily compactable. Cohesive, loamy soil, rubble or organic material is unsuitable and leads to settlement; use compacted gravel, crushed stone or lean concrete instead.
Why are liner pools backfilled in step with the water level?
This keeps the internal pressure of the water and the external pressure of the backfill in balance. If you fill only on the outside, the material pushes the still-unloaded wall inwards and deforms it.
What happens to an empty pool with high groundwater?
It can float up due to buoyancy. That is why anti-flotation measures and a planned dewatering scheme are necessary – this design belongs in the hands of a specialist planner.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
