Description

This technical publication presents the recalibration of HDM-4 unsealed road deterioration models using long-term field data from Southern Africa. Unpaved roads comprise a vast portion of South Africa's secondary road network, playing a vital role in agricultural logistics and rural mobility. Effective asset management requires reliable models to predict gravel loss, roughness progression, and maintenance intervention timing (such as blading and regraveling). The study evaluates the HDM-4 gravel road relationships against empirical monitoring data collected across diverse wearing course materials (ranging from quartzitic gravels to weathered pedocretes and shales) and climatic zones. The research recalibrated specific parameters controlling gravel loss rates as a function of annual rainfall, material plasticity, grading coefficient, traffic volume, and road geometry. Revised equations for roughness progression between blading cycles were also established, incorporating the dampening effects of routine grading. The results highlight that material selection standards significantly influence lifecycle costs and that localized calibration reduces prediction errors by up to 40%. The report provides practical guidance for provincial road departments to integrate recalibrated HDM-4 models into their asset management systems, facilitating cost-effective maintenance programming.