The effects of vegetation on the swelling and shrinking of soils in Australia

Soil movement is of great importance in Australia because about 20% of the total area consists of expansive soils, most of which occur in the semi-arid climatic zone. The first measurements of soil movement were made in Adelaide under natural grassland. From this limited information, it was deduced that there was little soil movement below 1·6m. Recently, movements have been measured in clay layers at much greater depths. These movements have been attributed to shrinkage due to desiccation by deep rooted vegetation, and swelling due to ponding of water. Several case histories are discussed where the effects of vegetation have caused movements of buildings of up to 150mm settlement and 100mm heave. To calculate potential soil movement caused by vegetation, the extremes of total soil suction and a quantitative value of the volume change properties of the soil are required. The instability index, Ip has been suggested as a measure of this soil property and was measured on two of the most expansive and widesp...