Tree-related subsidence: causes and solutions

Tree-related subsidence is a complex interaction between vegetation, soil, and built structures. It occurs when the ground beneath a building sinks due to changes in soil volume and primarily affects soils with a high clay content.

Clay-based soils act in a similar manner to a natural sponge expanding in volume when moisture levels are high but shrinking significantly when moisture levels drop. This change in soil volume occurs whether vegetation is present or not; it is directly related to seasonality, weather and climate.

While the cause of subsidence is directly related to the soil type, the amount a soil can shrink is exacerbated by the presence of trees. Understanding the causes and implementing effective solutions is essential for mitigating risk and managing urban tree populations responsibly.

As reported previously by Insurance Times, the cost of subsidence to UK insurance companies in 2022 amplified by the heatwave that summer was in the region of £220 million, driven by more than 20,000 claims.

Causes of tree-related subsidence

Subsidence is directly related to the soil type and relative water content which is impacted further by factors including the presence of trees and vegetation. Trees remove water through transpiration (the movement of water from the soil through roots and out of the trees leaves).

Different tree species have varying water demands and root architectures. High water-demand species like oak, willow, and poplar are more likely to contribute to subsidence.

The closer a tree is to a building, the greater the potential for its roots to influence soil moisture beneath foundations. Root systems can extend laterally beyond the canopy, sometimes reaching twice the tree’s height.

Older, larger trees have more extensive root systems and higher transpiration rates. As trees mature, their influence on soil moisture increases, especially during dry seasons.

Tree roots in general are typically found within the top 60cm of soil and the exacerbating effect these have on soil shrinkage in proximity to buildings is also generally related to the foundation depth.

It is fair to say that many subsidence events affect structures with shallow foundations including single storey extensions, garages and outbuildings. In a domestic setting these are often built under permitted development rights and may not properly consider the presence and proximity of trees.

Building on shrinkable clay without adequate foundation depth or planting high water-demand species near structures increases vulnerability.

Solutions to tree-related subsidence

Prevention is always better than the cure when considering tree planting or construction. A thorough site investigation should be conducted to assess soil type, moisture content and shrinkage potential.

Arboricultural consultants play a key role in assessing tree-related subsidence risk. This includes evaluating species, age, size, and proximity to structures. Choosing the right species and planting them at safe distances from buildings is crucial, whilst low water-demand species are preferred and NHBC guidelines for planting distances should always be adhered to.

However, even with the best planning, subsidence can still occur due to the response of the soil to the influences of the seasons, weather and climate. This may in some circumstances result in soil shrinkage in areas that are less prone to subsidence in normal conditions.

When a subsidence situation arises there are a number of possible solutions. This could include a simple repair in circumstances where damage is minor or unlikely to re-occur. However, in situations where damage is more extreme or has occurred on more than one occasion then engineering and arboricultural solutions can help to solve the problem.

Arboricultural solutions may be implemented to manage root systems including root pruning or tree removal. Engineering solutions may include installation of root barriers, while structural solutions like underpinning may be used to improve the depth of a foundation and reduce the effect that soil moisture may have on a building.

In many situations, however, there is no single solution to the problem of subsidence, in which case a holistic approach combining arboriculture, engineering, and planning is the most effective way to address tree-related subsidence. Collaboration between stakeholders is key to balancing risk with the ecological benefits that trees create while also providing affected parties with lasting and cost-effective solutions.

What to expect if subsidence occurs

Tree-related subsidence often involves legal and insurance implications. Tree Preservation Orders may restrict removal or pruning, and insurance claims require evidence of causation.

It is likely that a number of specialists will be required to assist you with solving the problem and this may take a number of months.

These specialists will investigate the cause of the damage and provide evidence back to the insurance companies, legal representatives and local authorities (in the case of Tree Preservation Orders).

Conclusion

Tree-related subsidence is a multifaceted issue requiring careful consideration of soil conditions, tree biology, and structural engineering. Through informed species selection, proactive management, and collaborative planning, it is possible to mitigate risks while preserving the urban forest.

Further information

Contact Tim Williams