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How Trees Cause Subsidence and What Can Be Done About It

One of the most common causes of property loss and damages in the UK is subsidence caused by trees.  Subsidence is the gradual caving in or shrinking of an area of land due to various factors. According to reports, trees create 70% of all subsidence. Though planting trees seem like a positive move in a race of global warming, they can be hazardous to a certain extent. So trees, in some soil, in some climate are problematic. The cost of repair works due to subsidence depends on the size of the property and the extent of damages, but it is always an expensive and lengthy process costing anywhere between UK £500 to £50,000.

Causes and effects

Subsidence of soil is caused by external as well as natural factors. The main reasons are drainage of the organic soil, tree roots, climate or weather, leaking drains and pipeline and mining.

The types of soil determine whether it will shrink or swell and cause subsidence. The first one is called non-cohesive soil. Since sand and gravel do not absorb water or change in volume when they are wet or dry, they do not pose much threat.

The second type of soil is called cohesive kind of soil. The most common example is clay and silt. Clay is most prone to shrink or swell due to change in water contained. Since they are made up of particles that bind together when mixed with water and can give up moisture which makes them expand or swell when wet and shrinks or contract when dry. Hertfordshire, Sussex and Kent are some of the places where the soil is made up mostly of clay, along with limestone and chalk.

Generally, most trees do not pose a threat of subsidence. Those trees planted on clay soil are the ones that gradually cause problems. This is because trees absorb water and nutrients from the soil through their roots and disperse the water through the leaves by the process of photosynthesis.  Each tree has a circle of influence and the quantity of water absorbed varies with each tree species. For example, Poplars, Elms, Oaks and Willows absorb the largest amount of water, causing their surroundings to dry out faster especially in spring and early summer when tree growth is at its peak.

An extended period of dry season or no rainfall leads to vertical and horizontal movement of soil which may result in subsidence of buildings and other structures with weak foundations. As a consequence of this seasonal process, a large number of trees have been removed from London Borough to save properties and buildings.

Subsidence also occurs when there is a leakage in pipes and drainage systems. Condensation around the drain attracts tree roots that can dislodge the underground pipe-work. The root of the trees can get into the tube through faulty cracks or joints, resulting in blockage and disrupting the sewers.

Preventive measures

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Since there is no way of predicting which trees are going to cause damage, the first and foremost way to prevent them is to choose a safe distance from trees, shrubs and bushes while building any structures.

Careful consideration as to what kind of trees are to be planted should be taken into account. Tree species that absorb a significant amount of water can be avoided in areas with extreme climate.

Remove trees and bushes planted near a building with utmost care as they can cause the soil to heave if not done correctly. For this, specialists can be consulted for professional guidance.

If the tree cannot be removed, regular pruning to curb the growth of the tree should be maintained so that absorption of water is lesser.

Selecting trees species with slow-growing roots that are less aggravating also prevent subsidence in later years.

Sensible planting distance should also be maintained to provide enough growing space for the trees. This can be done by checking the distance table as planting distance on other soil types often varies depending on the species of the trees and the climate of the location.

Considering the long-term effects of climate change on the soil by the tree is now possible through charts that show a time frame of the weather in 10 or 100 years. Looking at this kind of structure also provide useful insight of the impending tree growth. The expected increase in temperature in the future should also be considered as the hotter it is, the drier the soil is going to be. This process will make the land more prone to shrinkage and subsidence. Central London is already considered 9 degrees hotter than its surrounding countryside in the summer, so it is likely that the Victorian and Edwardian buildings and the local buildings constructed in the 30s and 50s are likely to be destructed by this effect.

However, not all trees cause subsidence, and it is not possible to predict which ones will cause problems, so it is usually advised to consult geologists and other services that specialise in different types of subsidence.  For example, Blue Sky is a remote sensing company that maps the location and height of every tree over 3 meters high all over England, Wales and across Scotland.

Cracks on the ground and building structures are the warning signs of subsidence. Quick measures should be taken if they are found on the interior or exterior walls, close to windows and doors and if they spread diagonally across the wall or they are 3mm wide and thicker. Surveyors can be consulted to assess the extent of damage and movement, risk of further change and loss, which remedial measures need to be taken by sampling the soil and how to claim insurance.

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When trees cause subsidence, the quickest way to provide a solution is to remove the tree, but sometimes that can lead to the opposite of subsidence known as heave. At such times, it is necessary to consult tree surgeons, tree specialists or arborists to treat them without further damage and minimum expense.

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