2008 - NZ - Adaptation of the AUSTROADS Pavement Design Guide for New Zealand Conditions

The AUSTROADS document Pavement Design – A Guide to the Structural Design of Road Pavements does not specifically design for plastic deformation in the basecourse; however, both experiments and field observations demonstrate that, with sufficient traffic loading, plastic deformation accumulates in the basecourse, sub-base, and the subgrade. Furthermore, pavement design in New Zealand is critically dependent on subgrade strength, apparently neglecting the accumulation of plastic strain in the granular layers of the pavement.

This study, initiated in 2004, examines the design methodologies presented in AUSTROADS and evaluates them against available New Zealand research. Various subgrade strain criteria are examined for New Zealand conditions. The roughness model from HDM III has been used to generate a pavement design figure similar to ‘Figure 8.4’ of AUSTROADS. The figure indicates that for lower design traffic levels AUSTROADS is highly conservative; for high design traffic levels AUSTROADS is not conservative enough. The results for design traffic between 10^5 and 10^6 ESA might help explain the observation that lives greater than 50 years are being achieved in New Zealand since, assuming the modelling is correct, in effect, these roads have been over designed.