What is subsidence?

Subsidence is a lowering or collapse of the ground. It can be triggered by man-made disturbance, a change in drainage patterns, heavy rain or by water abstraction. Subsidence has the potential to cause engineering problems such as damage to foundations, buildings and infrastructure. Events are associated with Karst in soluble rocks, and coal mining areas. Collapse can also occur in Loess and compressible soils such as peat. However, in these pages the focus is on subsidence due to shrink–swell action.

What is shrink–swell?

Shrink–swell occurs as a result of changes in the moisture content of clay-rich soils. This is reflected in a change in volume of the ground through shrinking or swelling. Swelling pressures can cause heaving, or lifting, of structures whilst shrinkage can cause differential settlement. This shrink–swell behaviour is the most damaging geohazard in Britain today, costing the economy an estimated £3 billion over the past 10 years.

Houses in many parts of Britain are affected by subsidence. In some places subsidence can be caused by the movement of the ground with changing weather conditions, for example dry summers or wet winters.

The amount by which the ground can shrink and/or swell is determined by the water content in the near-surface. Fine-grained clay-rich soils can absorb large quantities of water after rainfall, becoming sticky and heavy. Conversely, they can also become very hard when dry, resulting in shrinking and cracking of the ground. This hardening and softening is known as ‘shrink-swell’ behaviour.

This can be a natural seasonal occurrence or can be enhanced by various means including:

  • Normal seasonal movements associated with changes in rainfall and vegetation growth
  • Enhanced seasonal movement associated with the planting, severe pruning or removal of trees or hedges
  • changes to surface drainage and landscaping (including paving)
  • short term un-seasonal movements as a result of leaks from water supply pipes or drains
  • long-term subsidence, as a persistent water deficit develops
  • long-term heave as a persistent water deficit dissipates

Damage to buildings may occur when the volume change of the soil, due to shrinking or swelling, is unevenly distributed beneath the foundations. For example, if there is a difference in water content in the ground beneath a building, swelling pressures can cause the wall to lift; often called heave. This can happen at the corners or towards the centre of a building.

Where does this happen?

Formations

The rock formations most susceptible to shrink-swell behaviour are found mainly in the south-east of Britain. Here many of the 'clay' formations are too young to have been changed into stronger 'mudrocks', leaving them still able to absorb and lose moisture. Clay rocks elsewhere in the country are older and have been hardened by burial deep in the earth and are less able to absorb water.

Some areas (e.g. around The Wash and under the Lancashire Plain) are deeply buried beneath other soils that are not susceptible to shrink–swell behaviour. Other superficial deposits such as alluvium, peat and laminated clays can also be susceptible to soil subsidence and heave (e.g. in the Vale of York and the Cheshire Basin).

BSG research and shrink–swell data

The BGS has created a dataset which is based on the properties of both bedrock and superficial deposits and also takes into account areas where bedrock is covered by superficial deposits. This generates a map of shrink–swell susceptible areas with a rating for a soils potential for volume change.

Shrink–swell hazard potential mapping at BGS

Currently, BGS are concentrating their hazard potential mapping research on those rock types that demonstrate the greatest susceptibility to shrink-swell behaviour and therefore are most likely to pose problems in the towns and cities of the UK. Research into the clays of the Lias Group, Reading and Gault formations has been successfully completed. Currently research is concentrated on the London Clay Formation and the clays of the Weald.

Example References

Jones, L. D. and Hobbs, P R N. 2004. The shrinkage and swelling behaviour of UK soils: Clays of the Lambeth Group. British Geological Survey Research Report Series RR/04/01

Vane, C H and Jones, L D. 2003. A DRIFT spectroscopy study of clays. Ground movement shrink/swell project. British Geological Survey Report IR/02/033

Hobbs, P, Hallam, J R, Forster, A, Entwisle, D, Jones, L D, Cripps, A C, Northmore, K J, Self, S and Meakin, J L. 2002. Engineering geology of British geology or British rocks and soils: mudstones of the Mercia Mudstone Group. Keyworth, Nottingham, British Geological Survey, 106pp (RR/01/002)

Freeborough, K. 2005. Determination of the shrinking and swelling properties of the Lias Clay: Laboratory report. Nottingham, UK, British Geological Survey. 123pp (IR/07/035)

Jones, L D and Hobbs, P R N. 1998. The shrinkage and swelling behaviour of UK soils: Mercia Mudstone. British Geological Survey Technical Report Series WN/98/14

Jones, L D and Hobbs, P R N. 1998. The shrinkage and swelling behaviour of UK soils: Gault Clay. British Geological Survey Technical Report Series WN/98/13

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