Shell flexible pavement design methodology

The method uses the elastic theory in multi-layered systems and distinguishes up to five distinct layers, namely, (a) bituminous layers, which may be subdivided into two layers (b) the unbound layer or two unbound layers with different moduli and (c) subgrade. 

The traffic volume is expressed in equivalent standard (80 kN) axles. The process of converting axial loads into typical axles uses equivalency factors, determined from the equivalence law to the power of 4. The strength of all layers is expressed by their elastic moduli and Poisson ratios. The method also considered environmental temperatures, that is, monthly average air temperatures converted to weighted layer temperature, since they directly affect the stiffness of the asphalt considered in the design. 

For the determination of the critical strains in the asphalt layer and subgrade, the equations given in Sections 13.2 and 7.7.3 have been used. For the strains developed in the pavement structure, the in-house BISAR software was utilised to develop the design charts. 

In 1992, the first release of software SPDM-PC (Valkering and Stapel 1992) was issued, followed the philosophy of the manual. The computer program allowed the use of a wide variety of temperatures, bitumen grades and material properties. 

The last Windows version release of SPDM 3.0 (Shell 1998) contains modules for thickness design, rutting calculations and asphalt overlay design. Additionally, more emphasis was given to material properties incorporating the use of polymer-modified bitumen. The rutting calculations are now based on creep characteristics obtained from dynamic and not static tests, since it appeared not to be applicable to modified bitumen (Lijzenga 1997). 

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With regard to analytical flexible pavement design methodologies, a brief description is given to the Shell method (Shell International 1985) and the ALIZE-LCPC (2011) method. 

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