Engineers Australia

Cover requirements over expansive soils in flexible pavement design in Western Australia

Ester Tseng, Geoffrey Cocks and Fred Verheyde


The design of flexible pavements in areas of expansive soils requires careful consideration as shrink-swell movements can result in increased pavement roughness and cracking. One of the common strategies to minimise the detrimental effects of expansive soils in pavements is the use of a non-reactive material cover over the expansive subgrades. The Austroads pavement design guide does not provide guidance regarding how to calculate the thickness of this cover. Some Australian state road agencies provide tables and charts to select minimum cover requirements over reactive material. However, the requirements do not typically take into consideration the magnitude of the swell, climate, thickness of the expansive material layer or soil suction variations.

This paper focuses on utilising the Australian Standard on Residential Slabs and Footings (AS 2870) methodology to select the required thickness of cover over expansive soils taking into consideration CBR swell, climate and the thickness of expansive material, as well as other potentially varying parameters such as changes in suction at the soil surface. A sensitivity analysis using the AS 2870 methodology has been conducted in addition to investigating possible correlations between shrink-swell index and CBR swell. Required cover thicknesses obtained following the proposed methodology are compared to state road agencies requirements. Calculated vertical movements are compared to the vertical movements measured for the Karratha-Tom Price Road (in Western Australia) from 2008 to 2015. Finally, a new suggested design chart to calculate required cover over reactive soils as a function of climate zone and subgrade CBR swell is presented.

The analysis is presented considering flexible pavements and Western Australian (WA) conditions; however, similar methodology could be used for different conditions.