Residential footing design on expansive soils: a review of Australian practice

The vast majority of Australian homes were made of one- or two-storey brick veneer, consisting of an internal timber structure covered with a single layer of brick on the outside. Such lightweight structures are vulnerable to ground movements as a result of cyclic soil shrinkage and swelling arising from soil moisture (suction) variations. The Australian Residential Slabs and Footings Standard AS2870 recommends the use of Thornthwaite Moisture Index (TMI) to quantify the effect of climate changes on the residential footing design. The TMI based isopleth map for the State of Victoria is endorsed by the standard, which assists geotechnical engineers to estimate the depth of design soil suction change (Hs) needed for the calculation of the characteristic ground movement (ys), which forms the basis for site classification for potential ground movements. This paper presents a review of the current footing design and construction in Australia, including site classification process and two commonly used footing types. The related design parameters in the Australian Standards AS2870 are also introduced and discussed, which include the characteristic design surface movement (ys), Thornthwaite Moisture Index (TMI), the depth of design suction change (Hs) and the instability index (Ipt). A worked example for site classification is presented as well.