Keynote paper. Granular drainage layers in pavement foundations

Factors influencing the design thickness and selection of materials for sub-bases or capping with respect to their drainage function are examined critically. Although it appears that a permeability of 10 -3 m/s may be needed to prevent saturation, a lower value (often 5 × 10 -4 m/s) is common in UK specifications. This gives good to fair drainage depending on the layer thickness and carriageway width. Developments in laboratory testing and specifications for permeability are discussed.