The Design Standard HD 26 (2006) provides pavement designs that are based on four foundation stiffness classes FC1, FC2, FC3 and FC4, whose support for the pavement increases with class number. Interim Advice Note IAN 73 (2006) provides two design approaches for these foundation classes. Firstly, ‘Performance Related Designs’ are given to provide flexibility to the designer but only when used in conjunction with a ‘Performance Related Specification’. Secondly, a limited number of conservative ‘Restricted Designs’ are given primarily for schemes of limited extent. This report describes the evolution of these designs. In the design method adopted, the foundation was modelled as a layered, linear elastic structure and the thicknesses of the subbase and/or capping were chosen so that critical structural responses caused by simulative traffic loading were controlled to avoid excessive foundation deformation and deflection. The model was calibrated to replicate the previous designs given in HD 25 (1994) to provide continuity of experience. Examples of Performance Related Designs are given for all four classes of capping/subbase only foundations as well as designs for classes FC2 and FC3 of subbase on capping foundations. However, the method can be used to produce alternative designs. Restricted Designs are provided for a limited number of materials and are confined to foundation classes FC2 and FC3 of subbase only foundations and to foundation class FC2 of subbase on capping foundations. The conservatism of these designs is attained by use of increased layer thicknesses and/or superior materials over that required for equivalent Performance Related Designs.
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