Indirect damage to buildings : Part 2
Much ignorance surrounds the subject of tree damage to buildings. The usual fear is that if tree roots grow close to the foundations of a house they will undermine them and cause damage. This is rarely the case. Tree roots do not have the capacity to break up the large concrete foundation block of a modern building. However, they can be an indirect cause of damage through their absorption of water from a shrinkable clay soil.
Buildings located on sand and gravel based soils (i.e non shrinkable soils) do not generally experience subsidence caused by changes in the water content of the soil. They can still experience subsidence caused by other reasons such as the movement of a poor subsoil structure or frost heave.
Clay Soils change their volume in relation to their water content. They shrink as they become drier and expand as they become wetter. They act like a sponge. Some types of clay shrink and expand more than other types. It is possible to determine the exact shrinkability of a clay soil by taking a sample and having it analysed in a laboratory. The laboratory will carry out a number of tests and calculate the soils plasticity index (PI); the plasticity index relates proportionally to the shrinkage potential of the soil. Builders of new homes use a number of factors including the plasticity index of a soil to determine the design and depth of new house foundations.
A clay soil shrinks and expands in reply to climatic changes. In Summer, where rainfall is reduced and evaporation, through the increase in temperature, is higher, a clay soil will shrink. In Winter, rainfall increases and evaporation decreases, the soil increases its volume. This is a natural process called Seasonal Fluctuations. Other factors such as vegetation, drainage and wind can modify the level of these fluctuations.
A clay soil which has been saturated with water but allowed to drain naturally would be termed as being at its Field Capacity.
A clay soil which has lost additional water and is below its field capacity is termed as being desiccated and having a Soil Moisture Deficit (SMD).
A clay soil which fails to return to its field capacity after the wet season and enters the next dry season with a soil moisture deficit already in place, is termed as having a Persistent Moisture Deficit sometimes known as a Permanent Moisture Deficit.
All plants use water to carry out necessary growth processes. The production of food within the leaves uses carbon dioxide, sunlight and water to produce a sugar which is then stored. The amount of water required for this process is very small (1%); the remaining water is lost through the leaves during the process of transpiration. Transpiration occurs during the process of carbon dioxide absorption; where CO2 is taken into the leaf and water is evaporated and diffused out into the atmosphere. On a hot day, a large mature tree, may process hundreds of gallons of water through its leaves.
The presence of plants (trees) and their need for water has an effect upon the clay soil moisture content. Water is already lost through drainage and evaporation; the actions of plants increase the rate of water loss which in turn reduces the volume of the clay soil further. The presence of trees can increase the seasonal soil movements; sometimes by a considerable margin.
In some situations a tree can cause significant soil drying to the point where it causes an on going Persistent Moisture Deficit.
Trees and Buildings
The majority of houses in Britain were built before the actions of clay soils and their shrinkage potential were considered. Subsequently most of these houses are built on inadequate foundations. Additionally, the problems of trees on clay soils were not considered either, so many older houses have large, mature trees located very close to them.
However, a modern house, located upon an area of clay soil, should be able to cope with the natural soil movements. This is because most modern buildings should have been designed with adequate foundations to deal with the natural shrinkage rate of the soil. Things, however, do not always go to plan and building damage can occur on modern buildings through soil movements produced by natural seasonal fluctuations alone.
If the presence of trees are not considered during a house design stage it may be that the foundations of a building are able to cope with the natural seasonal soil movement but not with the increased soil movement that an adjacent tree may cause.
Although the presence of trees can increase the seasonal clay soil movements; it is not conclusive that the mere presence of a tree close to a building will cause building damage. There are many factors which must be considered before implicating trees as a source of damage; with the most important being the determination of the soil type.
Geological maps are available for all areas of the UK and are fairly cheap, 15 pounds or so, they show the soil geology for a given area and are useful as general guidance. However, its possible that small pockets of a different soil type are located within the larger indicated type and these may not be shown on the map. Determining the actual soil type therefore requires analysis of a sample.
Types of Damage
Damage occurs to a building through the distortion of its structure; these are usually caused by foundation movement. There are 2 main actions of soil movement which can cause building damage. These are subsidence and heave. In simple terms, subsidence describes a contraction of the soil and would normally be associated with a downward movement of the foundations. Heave describes an expansion of the soil and would normally be associated with an upward or lateral movement of the foundations.
If the whole building structure moves together then damage is unlikely to occur. Damage is normally caused when only a section of foundation moves and the remaining section stays static. This results in a distortion of the structure which then causes damage such as cracks in walls. Trees can be an attributable cause of such damage.
This crude diagram shows how a drop in soil volume has produced a drop in the foundation level from A to B. The brick structure is unable to cope with the resulting distortion so has failed and produced a crack in the wall.
Serious problems can occur when an area of land has been cleared of trees for development purposes. The soil may have a persistent moisture deficit due to the action of the trees, and be in a 'state of shrinkage'. The removal of the trees will reduce the soil moisture loss, so the soil will begin to recover as rainfall increases. It may take several years for such a desiccated soil to recover back to its saturated volume. If houses were constructed before such a soil recovery is complete, problems with heave damage may occur as the soil slowly expands.
The same situation can apply to a house which was constructed close to existing trees. The trees may already be mature and have caused a significant moisture deficit and shrinkage in the soil. If, in the future, the tree is removed, the soil will begin to recover and increase in volume; and unless provision was made in the house foundation design for such an eventuality, building damage may occur.
If a tree was planted after the construction of a building and then at a later time is a cause of soil shrinkage, removal of the tree would normally restore the soil back to its previous state and is unlikely to cause a heave problem. However, as with all operations involving trees and buildings, specialist advice should be sought prior to taking action.
A house which suffers from subsidence may have visible cracks which open and close in response to the seasons. Sometimes these can be quite large. In summer, as the soil shrinks, the crack opens, in winter the reverse occurs. Usually, dust and grit locate themselves within the crack which prevents full closure during the winter. It is important not to 'fill' a seasonal crack while it is open during the summer, filling the gap could cause serious damage elsewhere in the house as the structure responds to the soils swell in the winter. Contact a structural engineer for advice.
It is now common place for a mortgage lender or insurance company to demand a survey and arboricultural report detailing the relationship between a house and its adjacent trees. They require assurance that the trees are not likely to be a cause of damage to the buildings in the future. This is understandable, since over the last few years, insurance companies have had to pay out many millions of pounds against claims for subsidence damage. Unfortunately, due to the multitude of variables at play, the absolute prediction of building damage from trees is almost impossible.
The Arboricultural Association have developed a system called the Subsidence Risk Assessment (SRA) which uses a mathematical model to calculate the risk that a tree poses to a house. The system is still fairly new and its accuracy and usefulness is questioned by many arboriculturists including a leading consultant in the field of trees & buildings. The UKTC Forum archives contain many letters between arboriculturists which discuss the pros and cons of the Subsidence Risk Assessment system.
The ISA (through SORP) have also developed a method of assessing the risk of tree damage. Their method acts like a check list, guiding the arboriculturist through a data collection procedure which ensures that all relevant factors have been considered. The arboriculturist must then use his/her own judgement to form the end assessment.
Useful Contacts and Information Sources
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