Interim revision of the South African Mechanistic-Empirical pavement design method for flexible pavements

Abstract

Pavement design methods, in combination with network level management systems must enable road authorities to develop reliable long-term financial plans based on the estimated structural capacity of the road network. Inaccurate design models at the core of such a design system could, however, result in significant design risk and inappropriate financial planning. The design model for unbound granular layers contained in the 1996 version of the South African Mechanistic-Empirical Design Method (SAMDM) for flexible pavements was shown to be overly sensitive to minor changes in certain input variables at the Conference on Asphalt Pavements for Southern Africa held in 2004 and the general accuracy of this model was challenged. This paper presents an interim revision of the SAMDM recommending increased contact stress values at the tyre-pavement interface; updated unbound material characterisation parameters; effective stress analysis for unbound material; and revised damage models for estimating the structural capacity of unbound granular layers including the pavement subgrade. The internationally accepted subgrade vertical strain damage model is replaced with a model calibrated for local material and environmental conditions. The Factor of Safety (FoS) model unbound, granular base and subbase layers is replaced with a Stress Ratio (SR) model which is explicitly calibrated for the effect of material density and saturation levels. The revised models are shown to be less sensitive to variation in the resilient input properties of unbound pavement layers and better suited to simulating the permanent deformation of granular pavement layers under repeated traffic loading.