Foundation Degree Site Operations The Trouble with Tree Roots

Site Operations Aims – to understand the problems associated with tree roots Objective 1 – explore and understand the effect tree roots can have on a structure. Objective 2 – explore and understand the geological conditions associated with tree root problems. Objective 3 – Identify the main type of trees with the potential to cause structural damage to a property.

Site Operations Whenever we are planning to build either a new structure or an extension to an existing structure, it is always worth having a look at the local vegetation. In particular the type of tress that are growing locally and the type of soil conditions within our site. In this age of climate change, Property developers and House Builders are actively encouraged to plant trees, and if planted correctly, can add a desirable feature to any development. Many developments are also built around existing trees, and old trees growing too close to any structure can pose a serious danger to the drains or even worse the foundations.

Site Operations Any Damage that is caused by ground settlement associated with tree root activity can cause a lot if damage to a property. As seen in the illustration, step work cracks appear in the brickwork, and roofs stop being waterproof as lead flashings start to crack and tiles become displaced.

Site Operations Every building can expect to have a degree of settlement, and this is normal and is caused by the dead load of the building gradually compressing the soil on which it has been constructed. Ground subsidence can be a very real problem especially in long dry conditions, where any trees will ensure they absorb as much moisture as possible from the ground in order to survive. This will result in subsidence and we can expect damage from the ground movement, caused by soil drying out.

Site Operations All trees have the potential to cause problems, however the Poplar species of trees are amongst the most problematic. They grow to a great height and their root structure can exceed the height of the tree by half as much again.

Site Operations The roots are a very important part of any tree, they will seek out water and also offer stability to the tree. In areas where we have groups of trees, all these roots are competing against each other, all of them trying to ensure they seek out enough water for the tree they are supporting. This competition will also influence the direction and spread of the tree roots

Site Operations Tree roots do not spread out in a nice even pattern, and they do not stick to a certain depth. They adapt themselves to the different types of soil and local Geology, in order to help the tree survive.

Site Operations In areas where a thin depth of soil sits on top of the rock, the tree has to spread its roots over a large area in order to compete with other vegetation. So the type of soil in an area will have an effect on how a trees roots develop.

Site Operations Soil shrinkage and expansion is a common problem, especially in areas where there are large seasonal changes in Rainfall. In times of drought, local trees will remove as much of the water as possible from the ground, causing it to shrink, however in flood, once the tree has had its fill, the ground will start to swell. This is particularly true in areas where firm shrinkable clays are present. It is not always easy to spot this type of soil, as in a lot of cases it may be buried underneath a layer of top soil, sitting 1 – 2 metres below ground level.

Site Operations In areas where both houses and trees have lived together for many years, a state of equilibrium is reached, especially when the tree has reached its maturity. However old houses that may have a lot of brickwork within its foundations, and have become saturated and rotten, can be a major attraction for tree roots. As well as the foundations, the drains can also be affected, with trees such as Ash seeking out any cracks that may have developed, causing blockages, and any leaks from drains into the soil will encourage more tree roots. As a rule of thumb no tree should be within a third of its height, or less than 4m from a house. (I would take this to be a third of its height when fully grown.)

Site Operations As the use of land becomes increasingly important, new housing developments are often situated in areas of soil shrinkage and expansion. In areas where a lot of trees and vegetation have been removed, particularly where clay is present, the ground can take a long time to recover, as the clay slowly becomes saturated. In some cases we could find the soil level increasing as much as 100mm over a period of 7 – 10 years. This has an effect of upward heave on the foundations of a house and can cause floors and foundations to crack. This in turn can cause the damp proof course to fail , which again causes rising damp into a property.

Site Operations

Site Operations When investigating any potential site, gaining a thorough soil analysis is important. Throughout this presentation we have highlighted problems associated with Trees and ground heave, If we know what we are dealing with, then it is possible to design our developments to withstand these problems. Foundations could incorporate a layer of Polystyrene to help withstand the effects of up heave pressures. Maybe we could use piled foundations to drive through the ground liable to subsidence.

Site Operations Conclusion. Trees can cause potential hazards for all construction developments. Trying to kill off trees can be both expensive and time consuming. Do not forget tree preservation orders. Most areas will have large scale geological maps showing the drift and solid rock formations. The Local Authority will also be able to advise. A geotechnical survey will also be of value, depending on the type of construction required.