Load equivalency factors (LEFs) for abnormal vehicles (AVs) and mobile cranes in South Africa based on the mechanistic-empirical (M-E) design methodology

Abstract

Paper presented at the 28th Annual Southern African Transport Conference 6 - 9 July 2009 "Sustainable Transport", CSIR International Convention Centre, Pretoria, South Africa.

This paper describes the proposed new methodology for the determination of the Permit Mass Fees for Abnormal road vehicles (AVs) based on the estimation of road damage. The existing South African mechanistic-empirical (M-E) pavement design methodology is used to estimate the Load Equivalency Factors (LEFs), based on critical pavement layer life, under static loading conditions. The proposed methodology is not based on the traditional Equivalent Single Wheel Load (or Mass) ESWL (or ESWM), nor on the well known 4th power law for relative pavement damage but on the latest South African Mechanistic-Empirical Design Method (SAMDM) which has been used in practice for pavement design and analysis since 1996. The LEFs were calculated from estimated ratios of critical pavement layer life for each individual AV relative to the Standard Axle (80 kN, 520 kPa) bearing capacities of a range of nine (9) typical standard pavement structures found in South Africa. This was done for both relatively dry and wet pavement conditions. This paper includes examples of eleven (11) selected Mobile Cranes and eight (8)typical selected AVs. The new methodology also includes the effect of tyre inflation (or contact pressure) (Tip), including a sensitivity analysis over a range of 520 kPa to 1200 kPa for all the above vehicles and pavements. It is clear that there appears to be a wide range in the new LEFs for the different vehicles based on the new and what is considered a more rational and fully mechanistic approach (i.e. the SAMDM). Although the new LEFs (hence the associated Mass Fees) are found to be different compared to those calculated according to the existing ESWL method, they are in principle, considered to be based on a more rational (mechanistic)methodology than before and it is suggested that they be refined and applied with draft TRH 11 as soon as possible, but phased in over time.