A design method and performance based specification for UKroad foundations

摘要

Research has been conducted since the 1980s by the Transport Research Laboratory on behalfof the Highways Agency to develop, for a wide range of materials, designs for road foundations and proceduresfor their in-situ assessment. In 2006, guidance was issued in Highways Design document HD 26 on the design ofpavements for four classes of foundations that provide different degrees of support for the overlying pavement.Also issued as Interim Advice Note IAN 73 was design guidance for the foundation and a performance basedspecification, which aims to provide some assurance to the client of both the short and long term performance ofthe foundation. The main features of the design guidance and the performance based specification are outlinedin this paper. A practical application of IAN 73 in the form of a two stage design method is then proposed forfoundations whose subbases are slow setting, hydraulically bound materials that can act substantially as unboundgranular materials in their early life but later cure to form hardened materials. This approach is illustrated bythe retrospective application of aspects of the performance based specification to a foundation built recently ona UK trunk road. In addition, the potential range in structural performance of unbound granular foundationsis illustrated by several case histories. The potential benefits for client and contractor of a performance basedspecification for road foundations are highlighted.