UK flexible pavement design methodology

The methodology has introduced the concept of the foundation and of the pavement structure being either flexible or rigid. The design criteria used to develop the design charts are as follows for the foundation design, the deflection of the foundation surface and the minimum thickness of the upper foundation layer (Chaddock and Roberts 2006), and for the pavement design, the strain of the asphalt layer and the stress of the hydraulically bound layer (Nunn 2004). 

The foundation, constructed over the subgrade, consists of the sub-base layer and the capping layer, if used. The foundation is distinguished into four different classes of materials, known as foundation classes. The four foundation classes are defined by their surface modulus, known as foundation surface modulus. The materials for the foundation of the pavement may be unbound or hydraulically bound material. The latter are distinguished into fast-setting and low-setting material. 

The pavement structure consists of upper and lower layers. The upper layers are asphalt layers and the lower (base) layers are either asphalt layers or hydraulic bound layers. 

The thickness of the foundation depends on the subgrade CBR or subgrade stiffness modulus, while the thickness of the pavement structure depends on the foundation classes and the traffic volume. 

This revised/modified methodology gives a wider choice of material and design configurations. It also enables a stronger foundation, incorporating hydraulically bound materials to be used, which results in reduction of thickness of the more expensive asphalt layers. 

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Figure 13.12 Flow chart of UK flexible pavement design methodology.

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