Engineers Australia

Assessment of the AS2870 standard designs for residential rafts on reactive clay

Andrew L.E. Lochaden and Chris M. Haberfield

Abstract

The standard raft designs set out in AS2870 are adopted for a significant proportion of residential buildings being built across Australia. Of recent years there has been increasing media coverage relating to the poor performance of footings for residential buildings, particularly in areas of Victoria 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 the reactive clay sites they were formulated for. This paper presents the results of preliminary three dimensional finite element soil structure interaction analyses of the performance of standard type waffle and stiffened rafts founded on reactive clay with H2 classification. The analyses, based on simplified but reasonable assumptions, show that the waffle raft is substantially less robust than the stiffened raft. They also indicate that, for the assumptions made, the calculated performances of both types of raft do not meet the requirements for design of rafts by engineering principles, and that the rafts are unsatisfactory with respect to vertical differential movement and structural capacity when subjected to design characteristic surface movements. Whilst significantly more refined analyses and research are required to increase the absolute accuracy of these results, the results of these analyses cast doubt on the use of AS2870 standard raft designs for H2 sites and potentially for other reactive clay sites. This is particularly relevant in the greater Melbourne area given the projected levels of population growth in the coming years in areas which are underlain by highly reactive basaltic clays.