Causes of subsidence

Properties which have suffered from subsidence can be hard to find insurance cover for. This guide outlines the causes of subsidence and gives recommendations of how to gain insurance which meets your needs. One in five properties in the UK is considered to be at risk from subsidence.

Here are some of the main causes of subsidence:

Soil shrinkage
Around 75% of subsidence claims occur from soil shrinkage, making it the main cause of subsidence in the UK. There are two classes of soil; cohesive, sometimes known as shrink/swell, and non-cohesive, though both types will invariably contain water.

  • Cohesive soils, such as clay and silt, vary in their ability to change their volume when wet or when dried. This is also referred to as volumetric change. In essence, the soil swells excessively when wet and shrinks when dry.
  • Non-cohesive soils, such as sand and gravel, do not show volumetric change. They can, however, be washed away when subjected to water flow due to their small water particles.

Natural decomposition
Organic soils, such as peat, naturally decompose due to oxidisation, leading to a reduction in its volume. Oxidisation occurs when the organic material comes into contact with the air and this usually then causes the water table to fall.

Man-made subsidence is usually caused by old in-filled sites and vibration. In-filled sites are pits and quarries, which have been filled in with a variety of materials, not excluding refuse in some circumstances, and covered over. The fill can degrade over time, normally through the decomposition of the materials, causing a reduction in the volume of the soil which leads to subsidence.

Subsidence and underpinned property insurance

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