Current situations,advantages/disadvantages and future developments of resilient deformation characterizations of unbound granular material:A state-of-the-art review

A state-of-the-art review was delivered in the manuscript to analyze the current situations and the advantages/disadvantages of resilient deformation related issues to the unbound granular material(UGM),including testing methods,influencing factors and constitutive models,through which the bottlenecks in UGM resilient deformation characterizations were located and new theories and future research recommendations were proposed.Compared with other testing methods,the hollow cylinder test and true triaxial test are the only two to determine the anisotropic moduli of the UGM.Although significant advantages are observed in more advanced test apparatus such as true triaxial test,further improvements should be made,such as more advanced process identification control algorithm,to more conveniently control the nine independent hydraulic system and accurately measure UGM anisotropic properties.Confusions and conflicts in understanding the influencing factors to UGM resilient modulus were located,such as the stress history,the residual stress,the dry density,the gradation/fine content and the freeze-thaw cycle,based on which future research could be conducted.Through reviewing evolutions of the UGM constitutive models and comparing their representativeness and complexity comprehensively,major drawbacks were discovered and a new theory was proposed afterward to modify the UGM constitutive model,although further validations are still needed.