Nonlinear unsaturated shear strength behaviour of compacted crushed rock class IV material: implications for corrugation in unsealed roads

Unsealed roads make up over 65% of Australia’s transport network and are prone to surface deterioration, such as corrugation, especially under dry climatic conditions. Corrugation formation is closely associated with progressive shear failure in the surface layer, often exacerbated by moisture loss. Crushed rock class IV material is widely used for unsealed road construction in Australia. The unsaturated shear strength behaviour of this material has not been characterised properly. This study presents a systematic experimental investigation into the unsaturated properties of compacted crushed rock class IV, focusing on the role of degree of saturation and fines content. Laboratory indirect tensile and direct shear tests were conducted on specimens compacted at optimum moisture content and subsequently dried back to a wide range of saturation levels. Results demonstrate a strongly nonlinear relationship between shear strength and degree of saturation, with cohesion peaking at intermediate saturation (around 40 %–50 %) and diminishing under both very dry and near-saturated states. The apparent friction angle increased with drying, reaching a peak at fully dry conditions. Comparison of fines-included and fines-removed specimens highlights the critical role of fines in generating suction-related cohesion and maintaining structural stability, as the specimens containing fines exhibited higher unsaturated cohesion. Nonlinear failure envelopes and an empirical Gaussian-based cohesion model are proposed to capture saturation-dependent behaviour, addressing limitations of conventional linear Mohr–Coulomb analyses. Practically, the results suggest that maintaining moisture within intermediate ranges and preserving fines during grading are essential strategies for improving the durability and performance of unsealed roads.