Performance based specification for road foundation materials

UK Pavement foundations are currently designed using a method specification whereby tightly specified materials are constructed using specific compaction methods and layer thickness. This process does not necessarily guarantee the performance of the materials, but it is assumed to be adequate based on past experience. However, it can be inefficient, leading to unnecessary restrictions when using stabilised, recycled or marginal materials and/or the inappropriate use of good quality aggregates. The UK Highways Agency (HA) funded a recently completed three-year research project to produce a draft performance-based specification for road foundations. The performance-based specification aims to enable more appropriate and efficient use of a wider range of materials, both natural and recycled. The performance parameters required of the materials were established as the stiffness and the resistance to permanent deformation, with both measured, ideally, in the laboratory for design purposes and during construction to ensure their performance on site. Pre-construction trials to demonstrate adequate material performance (both as individual layers and as a composite structure) are expected to feature prominently when the new approach is adopted. A further HA-funded project started in January 2000 to evaluate the implementation of this new specification. This paper outlines the philosophy of the draft performance-based specification produced, including what needs to be measured and how and when it should be measured. Its impact on the highways industry is then discussed.