A Critical Review of Resilient Modulus Characterisation in Unbound Granular Materials

Abstract The resilient modulus (M R ) is a fundamental parameter used to characterise the resilient response of unbound granular materials (UGMs), which are widely employed in the construction of flexible pavement systems. M R plays a pivotal role in estimating the stress-strain behavior of pavement layers under traffic loading and serves as a key control metric during construction and quality assurance processes. Moreover, it is critical in understanding the mechanisms behind common pavement distresses, including fatigue cracking and rutting. The primary objective of this study is to conduct a comprehensive literature review on the resilient behavior of UGMs, emphasizing the conceptual framework of M R , the key influencing factors, and the evolution of mathematical models developed to estimate and predict this mechanical property. The review concludes with insights into current research gaps and recommendations for future investigation. Despite significant research over the past decades, the resilient behavior of UGMs remains incompletely understood, largely due to the inherent heterogeneity of these materials and their nonlinear, anisotropic responses under varied cyclic loading paths and moisture conditions. The mechanical behavior of UGMs is influenced by both macroscopic and microscopic material characteristics, such as gradation, density, porosity, surface texture, mineralogical composition, particle shape, and orientation. Environmental conditions, particularly temperature fluctuations, also play a critical role. One major limitation of existing M R models is that their parameters are typically difficult to determine through experimentation and are not true intrinsic material constants. Instead, they act as state-dependent variables, influenced by stress level, moisture content, boundary conditions, and loading history. Furthermore, most models are calibrated using data from repeated load triaxial (RLT) tests, which fail to replicate the complex, multiaxial stress states experienced by UGMs in actual pavement structures especially the simultaneous action of vertical, horizontal, and shear cyclic stresses. While these studies have provided valuable insights, further research is needed to establish definitive conclusions and to enhance the predictive reliability of existing M R models.