The top three causes of subsidence that require repair in the UK in 2023 are:
- Root induced clay shrinkage (60%)
- Running water from leaking drains or mains water supply pipes (18%)
- Poor ground, infill, and consolidation issues (18%)
These are based on the most up-to-date insurance data available from valid subsidence claims collated by the Association of British Insurers (ABI). The remaining 4% of claims cover ground heave, landslips, the appearance of sinkholes or subsidence related to mining issues.
The causes of subsidence are linked to the quality of the ground beneath the foundations. Subsidence happens when the support provided by that ground is no longer sufficient to hold the weight of the building, and part or all the foundations, and the building above sinks into the ground. If subsidence affects only a section of the building this can, depending on the scale of the effect, cause shearing forces to be set up within the structure and this can lead to large cracks appearing in the walls.
The top cause of subsidence, soil shrinkage, is due to the cohesive nature of clay soil that means it will expand and contract as its moisture content rises or falls. If it is dehydrated, by tree roots seeking out new sources of water for example, then it will shrink and, if it is underneath foundations, they, and the building above, will sink into the ground. Unusually hot summers will also present more instances of subsidence linked to the shrinkage of clay soils that are not directly related to the influence of tree roots.
Running water underneath foundations can wash away the soil, especially if it is non-cohesive, such as sand and gravel, causing subsidence as the foundations around the leak sink into the gap that has been created. The last of the three causes, where, for a variety of reasons, the ground beneath the foundations is not sufficient to bear the load of the foundations, reinforces the link between subsidence risk and ground stability.