Most of us love the sight of trees and countryside, especially if we are fortunate to enjoy these wonders of creation from our own living room window. But whilst trees do wonders for our mood, trees that are located close to buildings can lead to ground conditions that can have a damaging effect on the structure.
We all know that plants and trees take moisture from the ground. But in some cases, the amount of water drawn up through the root system of a tree can be surprisingly high. One source quotes that a mature oak will draw up 190 litres of water each day. Granted, trees typically have a large root system to draw upon, but the continuous need for copious amounts of water has an effect on the conditions of the soils on which they stand, especially during dry weather.
This effect is most pronounced in clay soils. Why? Clay soils are known to be shrinkable, depending on the amount of moisture that they contain. When clay soils are hydrated or moist, they have more volume (expand) than when they are dry or desiccated (shrink).
In the UK we would expect to find our winter months to be cold and wet and our summer somewhat warmer and hopefully drier. This then leads to a seasonable volume change in clay soils, they expand during winter and shrink during hot, dry summers. In the South East, where the summer climate is often warmer and drier, these seasonal changes can be more pronounced than in many other areas of the country where clay soil deposits are found.
What does this seasonal ground movement mean for foundations? Clearly, the main point of having a foundation in the ground is to provide stability to the structure built above. It makes sense then that excessive ground movement could upset that important relationship, rigid masonry doesn’t react too well to movement.
When the ground does shrink and affects the foundation enough to cause movement (usually a rotation of the foundation), we call that ‘subsidence’. Interestingly, the opposite can also happen. When clay re-hydrates and lifts a structural element (foundation or floor), we call that ‘heave’.
However, the good news is this seasonal volume change on its own rarely creates serious structural issues. Often the damage is superficial, cracks (usually up to 1mm wide) will appear in the summer as the ground shrinks and the same cracks close back up in the winter months as the ground returns to a hydrated state and lifts back up. These types of crack will end up only needing cosmetic repair, but this is usually needs to be an ongoing process.
We’ve noted above that trees remove large amounts of moisture from the soil. When this is coupled in to the natural seasonal variations of moisture level and volume of clay soils, we can start to see how problems may arise.
But again, we have to acknowledge that rigid masonry properties and trees have cohabited in close proximity relatively well for centuries. So what tends to be the problem?
Issues can start to occur when variations in soil volume become more extreme. If changes from the norm are only minor and the soil moisture content stays in relative equilibrium, the effects of the subsequent movement will be minor. However, if we experience hot and dry conditions for a prolonged period, such as in the famous summer of 1976 where there was no rainfall for 45 days – or indeed the summer we have just had in 2022 - the shrinkage of the soil is far more pronounced than would normally be expected and movement is far more likely to occur.
Trees close to buildings, coupled with these hot and dry periods of weather can combine to create conditions that can cause clay soils to shrink to the extent that subsidence of foundations can occur, leading to potential structural damage.
Yet it’s not just shrinkage of clay soils that can be an issue. If you are considering having an extension and there is a tree in the vicinity of your new kitchen, you may want to have this taken down as part of the works right? However, you need to consider another potential cause for tree related structural issues – heave.
What effect could taking this tree down have on the soil, particularly clay soils? Over the years of the life of the tree it will have absorbed vast quantities of moisture from the soil that it sits upon. This area, usually extending to the edge of the canopy of the tree, will be less moist, less hydrated, than the surrounding ground. If the tree is cut down, you have removed the draw on the moisture on the area that the tree stood. This ground will now hydrate back to normal level and will potentially expand as it does.
For this reason, you need to carefully consider when you actually remove the tree. Doing this too close in time to when you intend to excavate for foundations and floors, could mean these structural elements are forced to move with the ground as it hydrates and expands over time. The resultant expansion can lead to the lifting effects of heave, often a far worse scenario than subsidence, especially for floors. Taking out a tree well in advance of you starting the works and allowing the ground to naturally settle, could help alleviate the risks.
As a rule of thumb, think carefully before you consider adding or removing trees close to your property, especially if there are already any signs of seasonal cracking. In all cases, speak to a structural engineer before you make a decision, so as to help avoid issues later on.
So how do we avoid these issues from the start in new construction? Our next article will look at foundations and the role their location plays in avoiding the worst of the effects caused by these natural processes.