In policy terms subsidence is a loss of support of the foundations of an insured dwelling. This loss of support may cause localised differential damage (trees and drains), widespread damage (mining / tunneling) or complete collapse (swallow holes). Fortunately the most common type of subsidence is of the differential type, which can be rectified by well established techniques. Note that whilst all of these terms are often labelled as foundation settlement, in strictly engineering terms settlement is movement associated with the applied load of the building on the supporting soil. Settlement is therefore most common in the first few years of a buildings life as the new buildings load “settles” into the soil.

Differential subsidence occurs when only a part of the soil support for foundations is affected. This is most commonly the case via two mechanisms:

• Via drainage failure washing away or softening the soil supporting foundations
• Via trees removing moisture from clay soils which in changing the “volume” of soil allows that area of the foundation to subside.

There will also be occasions in which “slope creep” or peat collapse occurs although these are far less common events.

Subsidence damage can occur anywhere across the UK, however tree related subsidence will most commonly occur in two situations:

• In the presence of trees and clay soils
• In the presence of failed drains

For subsidence caused by the presence of clays soils, an understanding of what clay soil is and how it behaves is important.


Clay soils exist throughout the English, Scottish and Welsh lowlands, however the principle clay deposits are to be found to the south of the Mersey and Humber Estuaries. For the south east this high level of clay deposit is compounded by the drier climate and higher population density. Essentially the further south and east you travel, the drier the weather, with more trees, many more people and housing on clay soil.

In the South-East of England, the overall low rainfall, high population density, concentration of clay soils and tree population all create intense pressure on soil water resources and lead to high levels of tree related subsidence.

Subsidence damage can occur anywhere across the UK, however tree related subsidence will most commonly occur in two situations:

• In the presence of trees and clay soils
• In the presence of failed drains

For subsidence caused by the presence of clays soils, an understanding of what clay soil is and how it behaves is important.

CLAY particles are flaky. Their thickness is very small relative to their length & breadth, in some cases as thin as 1/100th of the length. They therefore have high to very high specific surface values. These surfaces carry a small negative electrical charge that will attract the positive end of water molecules. This charge depends on the soil mineral and may be affected by an electrolyte in the pore water. This causes some additional forces between the soil grains which are proportional to the specific surface. Thus a lot of water may be held within a single clay mass.

Plants require water to live, this water is usually supplied from precipitation. Clay soils can intercept this water and bind it to the clay particles. This action increases the volume of the clay soil. When plants remove water to live, this reduces the soils volume and if the soil is supporting foundations then differential movements may occur.

• Trees represent the single most important factor in relation to water loss from clay soils .
• Large shrubs can cause localised soil drying.
• Grass or other herbaceous plants are rarely an issue.

The common approach to tree related subsidence is to classify tree species by risk rating, therefore the large Oak becomes associated with high risk and the small Birch with low risk. This is a simplistic approach and one we do not support.

Effectively all plants need water and hedges, climbers, trees and shrubs have all been implicated in subsidence cases. What is needed is a project team involving the site investigation specialists, engineers and arboriculturists to tailor investigations and mitigation strategies to the specific site.

Drainage difficulties and drain failure, poorly founded extensions and recent extensions can all contribute to subsidence problems

For more information contact CET Group Limited for drainage issues and Cunningham Lindsey for engineering advice.

There are various mitigation options that can be adopted:

• Structural repairs
• Underpinning works
• Root Barriers
• Tree pruning
• Tree felling and replacement

All of these options have advantages and disadvantages however underpinning works, root barriers and tree pruning have all consistently failed to offer lasting or economic advantages and have materially added to the duration of claims in the UK.

Pruning has proven unreliable in maintaining tree water use and recurrence of claim is common. Pruning damages trees and must be continually applied if hazards are not to occur as trees re-grow. Pruning is costly and damaging to amenity.

Underpinning and root barriers are costly, time consuming, disruptive and wasteful of resources. They are often difficult to implement and require a very high level of expertise to complete. Targeted tree felling followed by structural repairs is a proven, fast and effective remedy to tree related subsidence and is the most commonly utilised form of mitigation adopted.