Pavement foundation stiffness testing: a new regime

Traditionally the construction of the foundations of paved infrastructure followed a recipe approach where specified materials are laid using specified plant in an approved way following a method specification. This approach is prescriptive and limits material use to those that meet the recipe assuming a given level of performance after completion. To encourage sustainability the UK Highways Agency launched new pavement and foundation design guidance that is moving away from this prescriptive approach (IAN 73/06 revised in 2009, and HA 26/06). The guidance aims to allow a more flexible design and assessment of the required foundation performance parameters of strength and resistance to permanent deformation. This also introduced a stiffness assessment of the constructed foundation to confirm compliance with design. In contrast to the previous regime, the actual performance of the foundation can influence (and provide savings to) the design of the structural pavement layers above. The new guidance permits the use of lightweight deflectometers to assess stiffness compliance. These are becoming increasingly common tools in the checking of foundations of paved infrastructure. This paper presents the background to the use of deflectometers within the new guidance, and elements of a recently completed good practice guide for their use.