Engineers Australia

Elements of expansive clay roadway specifications

Burt G. Look


Technical specifications provide guidance on quality levels and documents the procedures to be followed to achieve an acceptable standard. Road specifications sets out the precise requirements of the road authority and varies between Australian States and Territories, and internationally. A road construction specification cannot be overly prescriptive as it must integrate standard procedures, with varying climatic and material conditions as well as the significant testing variability. Elements used in developing road specifications using expansive clay material is presented.

A key quality control parameter is density. Soil compaction control requires a lower characteristic value to obtain the required density (and associated implied increase in strength and modulus with increased density). However, increased density can result in higher suction and swell for highly expansive clay soils. An upper characteristic value needs to be considered for such materials. The Optimum Moisture Content (OMC) is associated with the Maximum Dry Density (MDD). For expansive clays, the Equilibrium Moisture Content (EMC) is more important than an OMC placement target. Thus, the standard compaction approach which applies to non-expansive material cannot be directly used to expansive soils because such soils are dependent on climate, which influence its soil suction and movement potential. Movement rather than strength often governs for expansive soils. Zonal strategies are appropriate for embankment constructed of expansive clays.

Additionally, residual soils are very common in Australia, with a high granular content in “clayey” soils. The commonly used Plasticity Index (PI) used as an initial indicator of likely expansive behaviour, is not representative of the whole sample, with a significant “error” in this most basic of classification tests when used as a screening measurement to identify expansive clays in Australia. This screening is the first step in establishing appropriate design and construction procedures. The weighted plasticity index (WPI) accounts for the portion used in the PI test and this is more relevant for classification of residual soils. A simple ± 2% of Optimum Moisture Content (OMC) which may be relevant to nonexpansive materials cannot be applied to expansive clays. Additional testing variabilities also apply, which influences the interpretation of the reference maximum dry density (MDD) and even the CBR values used.