Performance related design and construction of road foundations - review of the recent changes to UK practice

Recently there has been a radical change to incorporate performance related design and com-pliance testing for UK highway foundations. New guidance has been introduced in the Highway Agency's In-terim Advice Note 73/06. The potential rewards of this approach include the wider use of more sustainable marginal materials and savings due to the thinning of the upper structural pavement layers, as well as obtain-ing useful information relating to expected pavement life. This new framework relies heavily on performance-related testing during construction to assess if set performance targets have been met. This in turn requires very clear specifications for construction, to ensure the risks of non-compliance are managed and the potential for dispute is limited. Therefore the use of pre-construction site trials are proposed, which in some cases may be costly or impractical (due to accessibility of the location, or relevance of the subgrade conditions). A more holistic approach to pavement design is provided which whilst providing some benefits has resulted in little guidance on the prediction and management of sub-grade conditions which strongly influence the overall pavement performance. Combining the above with an ‘observational’ method would enhance the performance approach by utilising a greater requirement to understand the site conditions and permit simple and appropri-ate changes to be made during construction to overcome any variability encountered, and ensure small areas of low performance are adequately managed and remediated. This paper describes the development and key elements of the current performance related guidance, and describes case studies for the observational ap-proach used in road construction schemes. It discusses the merits and limitations of both approaches, and pro-poses an appropriate step that could be made to better combine and integrate these procedures into a robust practical method for designing and specifying road foundations for the future.