Characterization of permanent deformation properties of densely-compacted unbound granular materials subjected to cyclic loading

To investigate the long-term deformation properties of unbound granular materials (UGM) that are ordinarily adopted to construct subgrade for high-speed railway, a series of medium-sized cyclic triaxial tests were performed to obtain the relationship between permanent strain and loading cycles under different cyclic stress levels. Moreover, DEM analysis was conducted for the samples to reveal the deformation mechanism and verify the strain developing tendency. It is found that the UGM samples present different long-term deformation properties under different cyclic stress levels. As cyclic stress increases, the permanent strain of UGM sample transfers from rapid stabilization to tardy stabilization, then to tardy failure and finally to rapid failure. Furthermore, the exponent in a power law function was selected as the critical indicator of deformation developing tendency. With the exponent obtained precisely in accordance with the strain rate, the deformation tendency can be analyzed quantitatively. Finally, the characteristics of interparticle force chains induced by different cyclic stress levels were obtained by DEM analysis, which provided evidences for the classification of long-term deformation properties of UGM samples. The achievements have guiding significance for the design of subgrade of both ballasted and unballasted high-speed railway.