Investigation of expansive soil for design of light residential footings in Melbourne

Expansive soils are found in most Australian states. In fact, it is estimated that 20% of surface soils in Australia are categorized as moderate to highly expansive clays. Quaternary basalt clays generally exhibit expansive characteristics and are commonly found in the Western part of Victoria. These expansive soils undergo heave and settlement due to moisture changes. Such ground movements are capable of creating differential movement in footings which results in cracks and damage to light structures. Recently, it has been reported that more than 5000 houses experienced damage in Victoria due to changes in soil moisture caused by extreme climate events. Since the climate condition is the natural cause of soil moisture changes, it must be considered in the design stage of relevant structures. This study is part of a comprehensive research programme aimed at enhancing knowledge about expansive soil behaviour and mitigating damages to residential structures due to ground movement caused by climate influences. This particular dissertation focused on estimating soil moisture changes in response to climate conditions and its consequences. The Australian standard (AS2870) considers the effect of climate on footing design in terms of Thornthwaite Moisture Index (TMI). It was found in this study that there are several methods to calculate TMI, which produce different values for a particular climate condition and hence result in different footing designs. Furthermore, TMI depends mostly on rainfall and may not be adequate to consider the variations in soil moisture condition particularly when the TMI values are based on averaging of long periods. Soil moisture changes and subsequent ground movement were monitored in a field site established in Braybrook in Melbourne, which has typical basalt clay soils. The collected data shows that soil moisture contents follow the rainfall pattern and a subsequent ground movement, with seasonal variation. Field monitoring over a two year period revealed that the changes in soil moisture were recorded mainly in top soils which contributed to the ground movement. A comprehensive laboratory investigation was performed to characterise the basic properties of Braybrook soils. These properties suggested that the site has a consistent profile of highly expansive clay. Further investigations were performed to obtain specific expansive properties such as mineral composition, soil suction changes and hydraulic conductivity. These results confirmed the presence of expansive clay soils in Braybrook. Moreover, a series of shrink swell tests were performed using undisturbed soils at various in situ moisture contents. These test results indicated the dependency of shrink swell index on in situ moisture content, which is in contrast to the specifications outlined in the Australian standard AS1289. The outcome of the laboratory investigation led to publication of a comprehensive data set that benefits both practitioners and researchers. The soil properties collected from the Braybrook site were then used to develop a finite element model to predict the soil moisture changes in response to climate conditions. The climate data were obtained from a nearby weather station and the model was validated against the monitoring data over a two-year period. The validated model was used to investigate the soil moisture changes due to long-term climate conditions, including extreme events such as the millennium drought and the subsequent above average rainfall period in Melbourne. In addition, the soil moisture predictions taken from this model were fed into another model, developed as part of this comprehensive research programme, to obtain the ground movement. The results of ground movements obtained from these models were then compared with the outcomes from the current Australian standard (AS2870). The results from the model suggest that considerable additional ground movement has occurred due to the millennium drought, which was not captured by the AS2870. Furthermore, the model predictions were used to consider the mound shapes underneath flexible cover slabs placed at different times of the recent years. Finally, available climate predictions were used to examine the possible future changes in soil moisture and ground movement. This research provides a versatile prediction tool for soil moisture changes due to climate conditions and, therefore, will greatly assist the footing design procedure given in AS2870. The model can be used to observe future changes of soil moisture within the design life of a structure using various climate prediction scenarios. Hence, it is an invaluable tool for designing residential structures that can withstand different severities of climate conditions as well as uphold the homeowner’s expectations.