Understanding Tree Influence
When planning a development, trees might not seem like a major concern until they begin to affect your foundations. Tree influence zones are one of the most overlooked causes of ground movement, structural damage, and unexpected costs on construction projects.
Understanding how trees impact soil and foundations is essential for anyone working on sites with existing vegetation or nearby mature trees. In this article, we break down why tree influence matters, what to look out for, and how to design the right foundation to protect your build.
What is a Tree Influence Zone?
The tree influence zone refers to the area around a tree where its roots and water uptake can affect the soil. This zone can extend as far as 1.5x the height of the tree (or more), depending on the species and soil type. In particular, clay soils are highly susceptible to shrinking or swelling as moisture levels change.
Why are trees a problem during construction?
During its life, a tree draws moisture from the soil, causing it to shrink (shrinkage), once the tree dies or is cut down, previously desiccated soil re-hydrates and swells (heave). This cycle causes ground movement that places stress on foundations, leading to cracking, distortion, or settlement over time.
Tree Influence and Foundation Design
If your project is located near mature trees, you’ll need to account for this risk in your foundation solution. Ignoring tree-related ground movement can result in costly structural issues post-build and even warranty disputes.
Best practices include:
- Identifying tree species and calculating their potential influence radius
- Conducting soil classification and testing for plasticity index (PI)
- Assessing the age and proximity of trees relative to new foundations
A tree survey is often required for new developments. This combined with a good quality ground investigation should provide sufficient information for an appropriately detailed assessment.
Foundations need to be taken to depths below the zone of tree influence. This can lead to deep and costly excavations over 2m deep if shallow foundation solutions such as strips are to be adopted.
At SPEEDECK, we often recommend tailored foundation systems, such as piled raft foundations, for sites within known tree influence zones. These systems minimise excavation while providing structural flexibility and resilience against shrink-swell cycles.
Do Tree Roots Affect Summer Ground Conditions?
Yes, trees can be put under additional stress during hot weather leading to greater water intake. However it is the long term effect over the lifetime of a tree that is most significant. It can take a very long time, likely a number of years, for water conditions to return to normal after the effects of a tree’s influence are removed.
Common Tree Species That Affect Foundations
Some trees are far more aggressive in their root systems and water demand. Trees commonly found in the UK that exhibit high water demand and generate large zones of influence include:
- Willow
- Oak
- Poplar
- Cypress
- Elm
Foundations near these trees often require deeper or specialist solutions, such as piles or heave protection measures. However the issue should be considered for all trees.
Signs Your Site May Be at Risk from Tree Influence
- Proximity to large or mature trees (even outside the site boundary)
- Presence of clay-rich soils (such as London Clay, often found in large areas of southeast England)
- Cracking in nearby existing structures
- Tree preservation orders (TPOs) likely indicating large and established trees
If any of the above apply to your site, it’s time to consult a geotechnical expert early in the planning process.
How SPEEDECK Can Help with Tree Influence
We’ve worked on hundreds of sites affected by tree influence zones, designing and delivering bespoke foundations that resist movement and protect structural integrity. Our in-house team of engineers can:
- Carry out a full tree impact assessment
- Advise on foundation depth, type, and reinforcement
- Incorporate heave protection
- Provide warranty-compliant solutions for NHBC, LABC, and more
Planning a site with trees nearby? Let’s talk about the right foundation strategy for you.
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