A Critical Review of the Modelling and Factors Considered for Permanent Deformation of the Unbound Granular Materials

Abstract The elastoplastic behavior of Unbound Granular Materials (UGMs) under repeated loading plays a critical role in the performance and longevity of flexible pavements. This paper presents a comprehensive review of the permanent deformation modelling and factors considered in UGMs. The factors influencing resistance to permanent deformation such as stress magnitude, moisture content, gradation, density, porosity, particle morphology, mineralogical composition, are discussed, emphasizing their complex interdependence and impact on material behaviour. The study also examines the development and evolution of mathematical and empirical models used to predict permanent deformation under cyclic loading, highlighting that most existing models, primarily derived from repeated load triaxial (RLT) tests, remain empirical in nature, lack physical interpretation, and do not capture the true three-dimensional stress state experienced in the field. Rutting, the predominant distress mode in flexible pavements, arises from the accumulation of both elastic and plastic deformations across all pavement layers. Although numerous predictive models express accumulated strain as a function of load repetitions and deviator stress, they often neglect critical influences such as environmental conditions and shear strength of the subgrade. This review identifies limitations in current design practices and modeling approaches, provides insights into modern concepts in rutting analysis, and outlines future research needs for developing more mechanistic and physically meaningful models to predict the permanent deformation behavior of UGMs in pavements.