What is peat?

Peat is a soil made largely of undecomposed or partly decomposed plant remains. It forms in waterlogged conditions where oxygen is scarce, so dead plants do not rot away but accumulate. Peat contains over 75 per cent water, has very low bearing capacity and is highly compressible, making it one of the most difficult soils to build on. In Dutch it is called ‘veen’.

Fen peat and bog peat

• Fen peat (Dutch laagveen) is fed by nutrient-rich groundwater, for example where a lake fills in with reeds and sedges. According to the Dutch planning agency PBL, about 9 per cent of the Netherlands is fen peat.
• Bog peat (Dutch hoogveen) grows above the water table and is fed by nutrient-poor rainwater, mostly sphagnum moss.

Subsidence: shrinkage and oxidation

When peat is drained, the ground surface drops in two ways:

• Consolidation and shrinkage: water leaves the soil and it compresses, more so under fill, roads and buildings. Settlement in peat can continue for a long time after construction.
• Oxidation: peat above the water table reacts with oxygen and literally disappears, releasing CO2.

The loss is irreversible. In parts of the Dutch peat and clay areas the ground sinks by up to about 2 cm a year.

Effects on foundations

• Shallow foundations: a house on strip or raft foundations sinks with the peat, often unevenly, leading to cracks and tilting.
• Piled buildings: where piles reach firm sand below, the surrounding ground sinks relative to the house. Paths, patios and service connections get damaged.
• Timber piles: if groundwater drops, pile heads dry out and rot. Dutch foundation experts (KCAF) report that 10 to 20 years of cumulative dry exposure can seriously weaken a foundation.

Tips and points of attention

• Get a ground investigation before building, extending or raising ground levels on peat.
• Limit added load: every layer of fill compresses the peat further. Lightweight fill reduces this.
• Watch the groundwater: drainage or dewatering next to a building on timber piles can cause damage.
• Check local hazard maps: in Great Britain, the British Geological Survey maps compressible ground, including peat.

Related terms

• Soil type
• Clay
• Silt
• Groundwater level
• Foundation problems with timber piles
• Soil investigation

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