How to Prevent Subsidence with Root Barriers

Subsidence is a significant structural issue that can lead to costly repairs if left unaddressed. Among the primary causes, tree roots are a common subsidence culprit. This is especially the case in clay soils where moisture fluctuations result in ground movement. Having an understanding of how to prevent subsidence with root barriers equips professionals to safeguard foundations from potential damage. This approach is particularly effective in mitigating the risks associated with clay shrinkage and soil movement.

In this guest article for Root Barrier Store, Dr. Graeme Allen—a leading chartered civil engineer and subsidence specialist consultant, with a doctoral thesis on root barriers and clay shrinkage subsidence—shares his expert insights. From pinpointing the causes of subsidence to exploring the solutions, Dr. Allen offers practical guidance for addressing this complex issue. He highlights proven, sustainable methods to fix subsidence caused by trees,providing long-term protection for affected properties.

What is Subsidence?

Subsidence is the downwards movement of one part of a foundation causing cracking and distortions to a building. This process can occur naturally due to geological conditions or be triggered by human activities. Damage is often covered by insurance policies, depending on the definition of subsidence within policy documents.

Data from The Association of British Insurers (ABI) reveals that there were 23,000 domestic subsidence claims in 2022. This resulted in subidence related insurance claims totalled at around £219 million.

Understanding what causes subsidence allows homeowners, builders, and engineers to take measures that can prevent substantial damage and costly repairs.

A gap between layers of bricks caused be separtion along the cement layer due to subsidence

Causes of Subsidence

The list below highlights some common causes of subsidence:

Clay Shrinkage Subsidence occurs when clay soils lose moisture - this can be as a result of tree root activity.
Mining Activities: Excavation and mining can lead to significant ground movement and destabilization of the soil.
Leaking Drains: can saturate the soil, leading to washing out of fine material on gravel soils or softening of clay soils.
Decomposition of Organic Materials: The decay of organic materials in the soil can create voids and lead to ground movement.

a vertical crack running all the way up a brick wall casued by building subsidence

Soil Types

In the first instance, it is important to establish the type of soils that the foundations are resting upon. Different soils respond in distinct ways to changing environmental conditions.

Clay or Cohesive soils have a particular property whereby they change volume with changing moisture content. This is because the particles are exceptionally small and the soil’s behaviour depends on the clay mineralogy and pore water chemistry.

In contrast, sands and gravels consist of relatively larger, discrete particles that hold water within the available void spaces. As a result, these soils do not undergo significant volume changes when moisture content fluctuates.

The soil type considered in this article are clay soils, such as London Clay, as root barriers can resolve clay shrinkage subsidence on clay soils.

Clay Shrinkage Subsidence

In clay or cohesive soils, clay shrinkage subsidence occurs due to volumetric changes caused by moisture content fluctuations.

Trees near a building can affect the soil’s moisture content and volume by drawing large amounts of water from clay soil. In summer, when trees require the most moisture and rainfall is minimal, they extract water from the soil, causing it to shrink. During wetter winter months, when trees are mostly dormant, the soil absorbs moisture and swells. This is usually referred to as seasonal movement.

Most buildings will accommodate normal seasonal movement and this will not lead to structural damage. Structural damage can occur if the moisture content changes significantly. This often happens during very dry summers or when large trees are nearby.

The main factors to consider with clay shrinkage subsidence are tree species, height and distance from the property, foundation type, depth, soil characteristics and weather conditions.

Not all trees are equally likely to cause damage. Broad-leaf trees such as Oak and Willow have roots that spread over long distances and they have a high water demand. Conifers and Pines have lesser affect than broad-leaf trees as their roots generally do not spread out as far.

A vertical crack running up the brickwork with different layers of underground material also exposed, including clay

Solutions for Subsidence Caused by Trees

There are a number of solutions for dealing with the effects of clay shrinkage subsistence:

Underpinning

This traditional method involves stabilizing the foundation by extending it to a depth below the influence of tree roots. Underpinning can be effective in many cases but can also lead to further damage, be costly, disruptive and time-consuming.

Tree Management

Regular pruning can help reduce the moisture a tree extracts from the soil. However, it may not lower the water demand enough to prevent subsidence. Tree removal is a common solution but can negatively impact the environment. Trees offer significant monetary, aesthetic, and ecological benefits, making removal less desirable in many cases.

Root pruning

This involves cutting back the roots of trees that are causing subsidence. However, this is often a temporary solution and will need to be repeated over time.

Root Barriers

This involves installing a tree root barrier to provide a physical block between the tree and the building, leaving the tree in place whilst preventing tree roots from damaging the building.

Root Barrier as a Solution

Given the challenges, there is growing interest in more modern and sustainable solutions, such as the installation of root barriers.

The stated benefits of root barriers are summarised below:

Avoids tree removal: Root barriers allow for the preservation of trees, maintaining their monetary, environmental and aesthetic benefits.
Cost-effective: Usually, installing root barriers is a more cost effective option than underpinning. Furthermore, if the value of trees is properly recognised in the assessment, root barriers are often more economical than tree removal or pruning in cases where there are multiple trees or large trees in urban areas.
Preserves environmental value: Root barriers preserve trees maintaining their numerous beneficial effects whilst dealing with clay shrinkage subsidence.

Half the end of full roll of permeable root barrier

Value of trees

Critical to the assessment of the root barrier solution is appreciating the value of trees.

Trees provide over 40 different benefits contributing positively to urban environments. These benefits include increasing property values, reducing crime, helping to reduce the effects of climate change, improve air quality, improve shading, improve drainage and encourage wildlife.

In pure monetary terms, the value of a tree in UK can be calculated using a variety of tree valuation systems. An individual tree can be valued up to £500,000 depending on its size, species and location.

Root Barrier Materials

Different materials are available to construct root barriers.

Non-Permeable Barriers: These traditional options block both roots and moisture from penetrating the barrier.

Permeable Barriers: These allow for the movement of water whilst providing a physical barrier to root growth. Some specialist barriers provide more than just a physical barrier. For example, CuTex incorporates a micro-perforated copper layer that creates a chemical barrier to inhibit root growth.

Design Considerations

Design needs to be carefully considered when opting for the root barrier solution. You must consider:

Barrier depth: Root barriers need to be installed up to 5m deep, depending on site-specific requirements, to ensure roots do not grow under the barrier.
Barrier length: The length of the barrier should be considered carefully to avoid roots from growing around the barrier.
Proximity to trees and buildings: Careful consideration of the location of barriers is essential to ensure installation of the barrier does not damage the tree, by being too close for example.
Durability and installation constraints: The durability of the materials used and practical installation challenges must be addressed to ensure long-term effectiveness.

Diagram visual of a residential house (left) and an oak tree (right). Tree roots are spreading underground, but stop at a 'vertical root barrier' (text included)

When to Use Root Barriers for Subsidence

Root barriers should be considered as an option in the following situations:

Properties affected by large or multiple trees: Particularly trees with higher values and with diameters greater than 400mm.
Sites where preserving high-value trees is essential: This includes areas where trees provide significant monetary, ecological, aesthetic, or social value.
Situations where underpinning or tree removal is less desirable: In cases where other solutions are not feasible or acceptable.

Root Barriers may not be suitable for:

Trees very close to buildings: There may not be sufficient space to install the barrier in some cases.
Scenarios where alternative solutions are more cost-effective: In some cases, tree removal might be a more economical choice, especially where trees provide minor monetary, ecological, aesthetic, or social value.
Multiple services or obstacles: Root barriers may not be viable in cases where access is particularly difficult or where there are multiple services, or obstacles such as walls, buildings or features.

Challenges

Despite their benefits, there are challenges associated with the implementation of root barriers for subsidence.

Currently, there is not any guidance or standardised design protocols to employ, when root barriers could provide a viable solution. My thesis examines this issue and offers practical solutions:

'A Framework for Subsidence Practitioners to Optimise the Benefits and Practical Use of Root Barriers to Resolve Clay Shrinkage Subsidence and Prevent the Unnecessary Loss of Valuable Trees'

Summary

In summary, understanding the causes of subsidence, including clay shrinkage and tree root activity, is essential for implementing effective solutions.

Traditional methods such as underpinning and tree management still play a vital role, but modern solutions like root barriers offer a promising avenue for addressing the issue while preserving the social economic and wellbeing value of trees.

With proper design and implementation, root barriers can provide a cost effective and sustainable solution to the challenges posed by clay shrinkage subsidence.