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Pavements constructed on volcanic soils behave differently to non-volcanic soils and have higher deflections when dynamically loaded. Past relationships between CBR (California bearing ratio) and modulus may therefore not be applicable.

Fast non-destructive methods currently being used rely on these relationships and therefore relationship(s) between the CBR and modulus need to be obtained for volcanic soils.

The project investigated the various volcanic soils within the North Island and attempted to classify the various types into three groups, initially based on the geological description and past work.

In situ testing of the volcanic subgrades was conducted using the falling weight deflectonmeter (FWD) test and in situ CBR  together with other standard tests.

From the in situ testing three correlations were identified for volcanic soils, however the third was not well defined.

The volcanic soil types represented by the correlations were, clayey, pumiceous and silty/brown ash. To enable the structural number of pavements on volcanic soils to be determined, factors were presented, based on the identified relationships and the procedure for determining the structural number of pavement from the FWD modulus suggested.

A modification of the Tonkin & Taylor method of determining the structural number was included which allowed for the use on volcanic subgrades

Also included were factors that allowed the volcanic relationships to be used in Austroads pavement design.

Keywords: volcanic subgrades, CBR, FWD Modules, in situ testing, structural number