Clay shrinkage is a common form of subsidence, particularly in the South East of the Country. Clays shrink as they are dried and expand as they rehydrate. This shrinkage causes the cracks that are often seen in lawns during the summer months. If structures are supported on clay soils that shrink in this way then the building will move with the clay often resulting in damage to the superstructure.
Normal seasonal variations in the ground moisture content can cause damage to structures built on fill or shallow foundations. Damage tends to be more common and more severe when moisture is extracted from clay by roots from trees or shrubs. Larges trees can extract moisture from many metres depth and this can result in significant movement and damage.