Are the AS2870 standard designs for residential rafts on reactive clay satisfactory?Keynote Address

The standard raft designs set out in AS2870 are adopted for a significant proportion of residential buildings being built around Melbourne (if not in other Australian cities). Of recent years there has been increasing media coverage relating to the poor performance of footings for residential buildings, particularly in areas underlain by Newer Volcanics basaltic clays. There are many aspects which govern the performance of footings founding on or in reactive clays, and questions regarding the appropriateness of the recommendations in AS2870 are being raised. One such question is whether or not the standard raft designs in AS2870 are satisfactory for reactive clay sites. This paper presents the results of three dimensional finite element soil structure interaction analyses of the performance of standard type waffle and stiffened rafts founded on reactive clay. The performance of the rafts analysed is shown to be unsatisfactory with respect to vertical differential movement and structural capacity when subjected to design characteristic surface movements. Whilst significantly more analyses and research is required, the results of these analyses has significant implications for all of the standard footing designs contained in AS2870 for reactive clay sites. This is particularly relevant given the projected levels of population growth in the coming years in the greater Melbourne area which are underlain by highly reactive basaltic clays.