Low rise buildings situated on shrinkable clay soil and with significant trees or other woody vegetation growing nearby may under certain circumstances be subject to movement caused by a moisture deficit within the soil. Trees use water and can have a desiccant effect upon the surrounding soil. However the degree of desiccation is the result of many factors, of which trees are only one.
Shrinkable soils undergo volumetric changes depending on soil moisture content. When desiccated, the soil shrinks. If this soil is adjacent to or beneath a foundation, the structure upon that foundation may experience movement, or subsidence. Trees are sometimes implicated as a cause of the soil desiccation.
When the desiccation is reversed, the soil swells. This may in turn cause building heave. For this reason, even if the tree is implicated, its removal may not be the obvious answer.
Tree Related Subsidence
Investigation of Tree Related Subsidence
When tree related subsidence is suspected then insurance companies, or other organisations, may request an investigation by a qualified arboriculturist to determine the likelihood that the trees are to blame, and to assess possible solutions. Additionally recommendations can be made to minimise or eliminate future risk. Various pieces of information are collected. These include an accurate identification of the implicated trees, the age class and dimensions of those trees, and the distance of those trees from the affected property. This enables the investigator to determine whether the affected property is within the tree's zone of influence. Additional information such as the underlying geology, soil type, its plasticity index, age of the affected property in relation to the age of the implicated trees, foundation type and depth etc. The history of the affected, and other nearby, property can be important i.e. have any extensions, patios, garages been added? What evidence exists of previous problems, especially within event years?
All these, and other, factors are combined to assess the likelihood that the implicated trees, or other woody vegetation, are in fact to blame.