What is a pingo?
A pingo is a hill with a core of ice that forms in permanently frozen ground. Groundwater beneath the frozen layer is under pressure, forces its way upward and freezes there into an ever-growing ice lens, pushing the overlying soil up into a mound that can reach tens of metres. There are no living pingos left in north-western Europe: these formed during the Weichselian, the last ice age, when the region had a tundra climate.
The pingo scar
As the climate warmed, the ice core melted. The heaved soil sank back, leaving a round hollow, often ringed by a low rampart of slumped material. That remnant is called a pingo scar or pingo remnant. In the landscape they show up as circular ponds, wet hollows or strikingly round field parcels; in the Dutch province of Drenthe alone, several thousand are thought to exist.
Over the following millennia the hollow filled with water, organic mud (gyttja) and peat. It is precisely that fill which makes a pingo scar matter in construction.
Why it matters for building
- Very soft layers in an unexpected place. In the middle of otherwise firm sandy ground, a local pocket of peat and mud several metres — sometimes more than ten metres — thick can sit. Its bearing capacity is effectively nil.
- Sharply varying layer thickness over a short distance. That is the real risk: with one corner of a building on sand and another on the peat fill, differential settlement and cracking follow.
- Settlement that runs on for years. Peat compresses for a long time, particularly if the water table drops or fill is placed on top.
- Ecological and archaeological value. Pingo scars hold an unbroken pollen archive and are often protected as geological heritage; digging or infilling may simply not be permitted.
Practical tips
- Read the landscape before you calculate a foundation. A round wet hollow, or the one parcel that floods every winter, is a signal.
- Commission several soundings, spread across the site rather than one in the middle. Only then does the varying layer thickness appear.
- Found through the soft layer onto the bearing sand, or use a single foundation principle for the whole building. The worst damage comes from mixed foundations.
- Be careful with made ground. Sand fill placed over a peat pocket is a load that produces settlement for decades.
- Check the local authority mapping for geological and archaeological designations before applying for permission.
Related terms
- Bearing capacity
- Cone penetration test
- Settlement
- Boulder clay
- Groundwater level
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