What is a polder?
A polder is a low-lying area separated from the surrounding water by dykes or embankments, in which the water level is controlled artificially. Without pumping a polder would flood; with pumping, the level can be held at a predetermined height.
The word is Dutch and is used internationally for reclaimed land of this kind. The comparable English landscapes are the Fens and the Somerset Levels, which are drained and pumped on the same principle. A polder created by draining a lake is called a droogmakerij; these lie deepest of all.
Why this matters for building
Polder soils are largely clay and peat: soft, compressible and prone to settlement. That has direct consequences.
- Foundations. On soft ground a shallow foundation is usually not viable. Buildings are piled down to a load-bearing sand layer. Always commission a ground investigation; the depth of that layer varies over short distances.
- Settlement of the ground surface. The ground beside a piled building sinks; the building does not. That produces the familiar problems: a sunken garden and drive, a doorstep left standing proud, and strain on services. For drainage, a flexible settlement compensator is therefore fitted at the building line.
- Groundwater. The water table generally sits high and moves with the managed level. A basement is a water-retaining structure and must also be checked against uplift.
- Timber piles. In older stock founded on timber piles, a falling groundwater table is the main risk: pile heads left above water are attacked by fungi. Lowering the level in an existing area is therefore contentious.
- Peat oxidation. Where peat dries out it shrinks and releases CO₂. That is one reason water authorities are cautious about further lowering.
Before you build
- Obtain the managed water level and groundwater records from the water authority.
- Commission a ground investigation for the founding depth.
- Ask the local authority about made ground and fill and the residual settlement to expect.
- Check whether a consent or licence is needed for dewatering during construction.
Related terms
- Settlement
- Piled foundation
- Groundwater
- Ground investigation
- Water-retaining concrete
- Polder roof
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