Subgrade treatment design for expansive soils in Adelaide using Australian Standard AS2870 — is this the right approach?

The Darlington Upgrade Project is an important part of the development of Adelaide’s north-south corridor and will deliver an upgrade of 3.3 km of the existing Main South Road between the Southern Expressway and Tonsley Boulevard. The project alignment is located within the area defined as the ‘upper outwash plain’. The natural soils encountered in this area are typical of the Tertiary/Quaternary sediments found across much of the Adelaide Plains and comprise expansive soils of the Pooraka and Hindmarsh Formations.

In Adelaide, insufficient subgrade treatment for construction of pavements over expansive soils has resulted in poor pavement performance and as a result, in 2014 the South Australia Department of Planning Transport and Infrastructure (DPTI) implemented the use of the Australian Standard for Residential Slabs and Footings (AS 2870-2011) for subgrade treatment design on many of its major road infrastructure projects. The use of AS 2870 is not widespread in subgrade treatment design for pavements in Australia, as many States adopt subgrade treatments based on soaked CBR swell values. This paper discusses the expansive nature of the soils of the site, the effect of wetting and drying on expansive soils and the implementation of AS 2870 to meet stringent maximum movement criteria of 20 mm from seasonal and long-term moisture change. Comparisons with subgrade treatment using other States’ approaches are also presented.