Assessment of performance specification approach for pavement foundations

Road foundations are currently designed using established empirical relationships and a recipe specification, according to which specified materials are laid and compacted using specified methods. Such an empirically based specification is unlikely to result in an efficient use of construction equipment or materials and does not allow the use of analytical design procedures. If a move to a performance specification can be adopted, functional subgrade and foundation material parameters can be used in design and compliance testing. This allows for the use of previously untried materials and provides assurance of the “as-constructed” performance of the pavement foundations. The philosophy adopted for a performance specification currently being researched in the United Kingdom is explained. The performance parameters to measure for both design and compliance testing, and when they should be measured, are detailed. The requirements of tests to measure the parameters are specified, and the techniques currently available are reviewed. Finally, selected results from field trials performed to evaluate the proposed specification are presented. Research has shown that techniques are available to allow for a move to a performance specification. However, the assessment of granular materials with large particle sizes requires further research. The trials revealed that adequate compaction of subbase can be achieved on a wider range of supporting materials than might be expected. Significant variability in test data was found for small areas of the same site containing similar subgrades; therefore, any target values set for compliance testing should be ideally both material and site specific.