Damage may occur as tree roots take up water from the soil, causing the ground to dry out and shrink. Shrinkage can cause uneven settlement leading to subsidence at the surface. This occurs predominantly during spring and summer. The drying results in vertical and horizontal movement of the soil, which may lead to the subsidence of buildings with shallow foundations.
Tree roots grow in the direction of least resistance and where they have the best access to water, air and nutrients. In towns and cities root growth can cause physical damage to structures by simply pushing the ground apart.
All clays are susceptible to some shrinkage and swelling due to changes in moisture content. Those with a higher proportion of expansive clay minerals, such as smectite, are more prone. The depth of shrinkage and swelling is contained by the zone where moisture changes are most likely to occur. This is usually the upper 1.5 m–2 m, but exceptionally 5 m, of the subsurface. The depth affected can be greatly influenced by the presence of tree roots and surface cracking.
Areas with many older houses and old style shallow foundations can be seriously affected. A number of London Boroughs have seen large numbers of street trees removed due to subsidence related insurance claims. Recommendations on the safe planting distance of a tree from a building are published. However, existing trees also affect the foundations beneath houses.
Paving of previously open areas of land, such as the building of patios and driveways, can cause major disruption to the soil moisture system.
Paving an area with low permeability materials such as concrete increases water run-off and reduces the amount of rainwater that can soak into the ground. If the paving cuts off infiltration, many trees will send their roots deeper into the ground or further from the trunk in order to source water. The movement of these tree roots will cause disturbance of the ground and will lead to the removal of water from a larger area around the tree.
This situation may be made worse as trees continue to extract water during the growing months, when rainfall is low. If a more permeable type of surface, such as block paving, is used, more rain water can enter the ground and supply nearby tree roots.
|Concrete||Clay Soil||Asphalt||Block Paving||Road Surface||Sandy Soil|
|Low||>---------------------- Increasing Permeability ------------------------>||High|
If paving becomes cracked and open, due to the action of tree roots, large amounts of water can enter the ground causing clay-rich soils to expand and swell, possibly causing heave damage. The impact of paving will depend upon a number of factors including soil type, slope of the ground, type and size of tree and type of paving used.
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