Revision of TRH 11 (1999-2000). Recovery of road damage – discussion document on a provisional basis for possible new estimation of mass fees – under review for TRH 11 (2000) – final summary report V1.0

Abstract

This summary report 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 South African mechanistic-empirical 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 practise 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 typical standard pavement structures found in South Africa. This was done for both relatively dry and wet pavement conditions. This study includes examples of eleven selected Mobile Cranes and eight other 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.