Technical Guidance Notes - Clay-related Subsidence
Clay-related subsidence is a major cause of building damage and insurance claims in the UK. BRE estimates that there are around 35,000 claims for clay-related subsidence each year. Clay-related subsidence occurs when clay soils suffer a change in moisture content causes them to shrink (as moisture content reduces) or swell (due to an increase in moisture content). The amount of any movement depends on the type of soil (its susceptibility to volume change) and the amount of any change in moisture content.
Shrinkable clays are found particularly in the southern part of the UK, which is also where summers can be driest and the majority of clay-related subsidence claims originate. Where there is an enhanced risk from clay-related subsidence it is important to make sure that appropriate insurance is in place to cover this.
To help professionals and homeowners make informed decisions on properties, the Terrafirma Ground Report provides information on the susceptibility of properties to clay-related subsidence and practical guidance on next steps before purchase.
Some types of clay soil contain a high portion of clay minerals which can expand or shrink with changes in moisture content (‘shrinkable clays’). During dry summers the amount of water in the upper layers of the ground will reduce significantly as the ground dries out. Movements in ‘shrinkable clays’ of 50mm to 100mm can occur, depending on how ‘shrinkable’ the soils are and how dry the ground becomes. Where soils are exposed this shrinkage is visible as cracking in the ground surface. During the winter, as the ground becomes wetter due to rainfall, this process is reversed and the ground can swell. This shrink-swell effect is particularly pronounced in very dry summers such as 2018.
The growth of vegetation has a significant effect on the moisture content of the ground. As plants grow during the summer water is drawn out of the ground by their roots, increasing the drying effects due to lower rainfall and so the amount of shrinkage. During the winter as plants are dormant this water demand decreases, exaggerating the effects of swelling as soils are wetted by rainfall. The amount of these effects depends on the size and type of plant. Low growing plants will have a very limited effects because their roots are small and shallow and their water demand for growth is low. Larger plants like trees have very extensive and deeper roots with a much higher water demand, so the effects on shrink-swell movements will be greater. Normally the higher the tree the greater the extent of the roots, but particular types of trees (such as willows and oaks) have a very high water demand.
The combination of the susceptibility of soils to shrink-swell movements and the presence and type of tree can have a significant effect on the amount of movement. For soils with low shrinkability, in the absence of trees, the amount of any shrink-swell movement will normally be minimal even after very dry summers. However, for soils with a high shrinkability, movements after dry summers can be sufficient to affect building foundations. In most cases tree roots growing close to shallow foundations have the potential to cause ground movements in clay soils which could affect buildings, and in medium and high shrinkability soils this could result in significant damage.
Shrink-swell movements normal only extend to around the top 1.5m of the soil. Modern foundations should be designed to take account of the expected shrink-swell movements of the soils beneath the property, but this is often not the case for older properties. Even where foundations have been designed to modern standards, if trees have grown or been planted to close to buildings then the increased level ground movements may not have been included in the foundation design. Care is always required when planting new trees around properties because of the effects on foundations. Trees normally need to be planted at last their full mature height way from any buried structures like foundations which could be affected by them and advice from a tree specialist (aboriculturalist) is required if there are any concerns over the possible effects of tree planting.
When shrink-swell movements occur beneath or adjacent to foundations they transfer a downward force to them was they shrink (subsidence) and an upward force to them as they swell (heave). Modern foundations are designed to extend below this zone of potential movement and to include compressible material along their sides which will prevent this force being transferred to them from the adjacent ground. Where this is not the case, as the foundations move they affect the walls supported by them causing cracking. This cracking increases with the amount of movement but is particularly severe where there is differential movement (i.e. some parts of the foundation are moving more then others). Trees in particular can cause this differential movement in foundations, where movements are generated close to roots and increase away from them. Whilst in most case cracking developed as soils shrink during the summer will reduce during the winter as soils heave, in many cases there is a ‘ratcheting’ effect as movements increase during the summer and do not fully recover during the winter.
In many cases the cracking visible will be cosmetic, but in some cases severe structural defects can result which may affect the stability of walls or prevent doors and windows from opening properly. Where damage is significant, action needs to be taken to prevent it getting worse.
In addition to building damage, movement of the ground can affect paved areas like roads or paths where uneven movement of the ground can cause cracking and depressions. Buried pipes under the ground can also become damaged by ground movements, especially those like ceramic drainage pipes which are not flexible. Leakage from pipework may increase the amount of movement by causing local heave or encouraging the development of roots into and around pipes causing further damage and ground movement. Buried soakaways and outfall pipes can have similar effects.
It is important to remember that there may be many other cause of damage occurring to a property, from problems with the building structure (such as due to alterations) or collapse beneath the ground, such as from mining. Specialist advice is therefore always required when damage to a property occurs, including where clay-related subsidence may be occurring. The best source of this advice will be a building surveyor who will be able to check that there are no other causes for the problem and will be able to recommend other specialists, such as structural engineers, if required. Insurers should be advised of property damage and they may be able to arrange for suitable advice and repairs. For new houses an NHBC guarantee may cover any damage.
Normally monitoring of any cracks and investigation of the foundations and the type of soils around them is needed. These investigations may also be able to show whether tree roots are growing near to the foundations and see whether pipes have become damaged. Further work to solve the problem and complete any repairs will depend on the causes and severity of the movements.
In some cases, where trees are the cause of movements, their pruning or removal may help, but a tree specialist (aboricultralist) will be needed to agree what should be done. Some trees have legal protection (Tree Preservation Orders or TPOs) and will need agreement from the local council before any work to them can be completed so checks for TPOs on trees will need to be made. Sometimes, if trees are older than the property, removing them could make problems worse.
Once the cause of the problems has been identified, repairs to properties could range from simply infilling cracks and monitoring, through to underpinning foundations and structural repairs.
Underpinning in involves extending the foundations below the level of movement of the soil. Underpinning works should only be undertaken by specialist contractor and Building Control approval from your local council will be required. The underpinning should provide a warranty or guarantee for the work completed – for example members of the ASUC trade organisation offer an insurance-backed guarantee.
This guidance note is for information only. You should always seek specialist advice on your property if you have any concerns about growth of trees, ground moment or damage. Additional guidance on clay-related subsidence can be found in the following publications:
BRE FB13 Subsidence Damage to Domestic Buildings - a guide to good technical practice 2007
BRE 298 Low rise building foundations - influence of trees in clay soils 1999
BRE CI/SfB(B2d) Good Repair Guide 2 – damage to buildings caused by trees 1996