Quantifying moisture effects on the dynamic friction coefficient of compacted crushed rock class IV material and its implications for corrugation of unsealed roads

Unsealed or unpaved roads are prone to surface deterioration such as corrugation, a form of distress closely linked to the interaction between tyre-induced shear stresses and the road surface. A key parameter governing this interaction is the dynamic friction coefficient, which varies with the moisture content of the road surface. However, the dynamic friction coefficient is rarely measured directly for unsealed road materials under unsaturated conditions. This study investigates the moisture-dependent dynamic friction behaviour of compacted crushed rock class IV, a material widely used in Australian unsealed road construction. Laboratory tests were conducted using the British Pendulum Tester on specimens conditioned to degrees of saturation between 0% and 86% to replicate field-relevant drying and wetting cycles. Results show that reduced moisture content increases the dynamic friction coefficient and its variability, due to suction-induced strengthening of particle contacts and increased resistance to surface shear deformation. At higher degrees of saturation, lower and more uniform values were observed, reflecting a smoother surface response. An exponential decay model was fitted to describe the relationship between dynamic friction coefficient and degree of saturation, providing a practical tool for modelling tyre–road interaction under variable climatic conditions. The findings demonstrate the critical influence of moisture fluctuation on road surface frictional behaviour and offer insights for improving the design, maintenance, and safety of unsealed road networks exposed to seasonal drying and wetting cycles.