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Consulting Engineers

I love trees…..but buildings don’t!

The problems that trees can cause for buildings. A lot of buildings have trees near them. The problem with trees is that they need water to survive, which they draw from the soil via their root systems. The bigger the trees, the bigger their root systems are.

A rule of thumb is that the root system is approximately the same size as the crown of the tree, which for very large trees is large and they draw a lot of moisture from the soil.

Trees can be classified into low, medium and high-water demand trees, and draw corresponding amounts of water from the soil. This becomes particularly significant if the soil on the site is a highly shrinkable clay, as it will be affected significantly by changes in moisture.

If the building’s foundations are not deeper than the zone of influence of the trees on the site, the tree will remove moisture within the zone of the foundation and there will be settlement due to the soil shrinking due to loss of moisture. The zone of influence can be as deep as 2.5m, depending on the type and size of the tree and the distance of the tree from the foundations, and the type of soil on the site.

The structural engineer can calculate what the depth of the foundation would need to be to limit the effect of trees on the site, or if the tree was removed.

Removing the tree does not necessarily solve the problem, as if the tree is removed, it will no longer be absorbing the water that it did before. This means that the soil will now absorb the water again after a long period of drying out, which if the soil is a highly shrinkable clay, it will absorb the water over time and will start to swell and increase in volume, which could cause cracking to nearby buildings.

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