Imaging the ROOT Cause of Subsidence Using Electrical Resistivity Tomography

Throughout the world, ground movement caused by moisture content variation in expansive clay soils is a major problem. In southern areas of the UK particularly, where major urban areas coincide with outcropping clay, subsidence and the resulting structural damage caused by clay shrinkage is at the forefront of both the public and insurers concerns. The principal cause of such damage however, has not only been attributed to expansive clays but more increasingly to the influence urban trees have in promoting clay subsidence through water uptake, especially during times of drought. At present the diagnosis of tree-induced subsidence (TIS) involves verifying clay desiccation via testing soil index properties (i.e. Atterberg Limits) and comparing soil water and strength profiles at points proximal and distal from the tree. These methods however, are costly, difficult, and sometimes unreliable and ultimately provide a very limited and often incomprehensive understanding of tree induced subsidence, especially in the long term. As part of an interdisciplinary study of this system, a geophysical approach is taken to bridge the knowledge gap by using Electrical Resistivity Tomography (ERT) to provide a more comprehensive understanding and visualisation of the interactions between trees, moisture movement and volume change in clays.