The prediction of moisture induced foundation movements using the Instability Index

It is now well established that reactive clays are a major source of failures in Australia, particularly for lightly loaded structures such as houses and pavements. Large structures on deep clays, can also be affected. The weight of a structure is no defence against shrinkage movement resulting from tree drying as shown by the settlements experiences by the Art Gallery in Adelaide.

Fortunately in Melbourne, often the reactive soil consists of a relatively thin residual clay and the design moisture regim is dominated by seasonal drying. This has allowed the simple classification of soil profiles to be related to seasonal ground movements and the geological origin of the soil (Walsh, Holand and Kouzmin 1976). Even so, there are still many other parameters involved, for example drainage and clay depth can substantially influence the movement of basically similar clays.

It was originally intended that the Melbourne soil classification system was no more than a first attempt at the problem. Further revision as a result of experience of footing performance is essential to the success of this system, and an opportunity may exist in the SAA code ‘Residential Footings and Slabs’ which is currently under development.

Specific recommendations are available for most of the clay profiles in Melbourne but only limited guidance is given for the rest of Victoria. An extensive body of local knowledge has been collected in Adelaide and some studies have been commenced in Sydney. CSIRO is currently establishing test sites in a variety of locations in an attempt to extend the knowledge of individual clays, but until a comprehensive data base is established for the particular soil type, the prediction of movements must be based on laboratory tests. The most promising experimental technique uses suction profiles and the instability index.