Engineers Australia

Seasonal variations of soil suction profiles in the Perth Metropolitan Area

James Harris, Fred Davenport and Barry Lehane


For the purpose of residential slab and footing design, the Thornthwaite Moisture Index (TMI) is used by AS2870 – 2011 to estimate the depth to which the in situ moisture content changes on a seasonal basis in order to classify residential sites based on the soil foundations reactivity. It does so by assigning a TMI value to a given climatic zone, which can then be used to predict the depth of design suction change, Hs, within the soil profile and, hence, calculate the characteristic surface movement. However, the most effective method for determining the potential ground surface movement is to establish seasonal soil suction profiles at the site. Unfortunately, little research has been undertaken in Western Australia to assess these profiles. This paper attempts to address this by comparing soil suction profiles for both wet and dry periods at two locations within the Perth metropolitan area, from which the depth of design suction change was better established. This allowed for a direct comparison of the experimental values to the existing values given in AS2870 – 2011. It was shown that the currently accepted value of 1.8 m given in the standard is possibly too low and that, given the drying climate, a higher value may be necessary. However, more research into soil suction profiles around Perth would help to generate more reliable data that could be used to better understand the seasonal moisture changes occurring in clayey soils of the Perth metropolitan area.