Effects of laboratory compaction schemes on the mechanical response of ballast in large-scale triaxial tests

Railroad ballast, consisting of uniformly graded coarse aggregates with angular and irregular particle shapes, plays a critical role in maintaining track stability. Accurate laboratory characterization of ballast mechanical behavior, such as stiffness, shear strength, and permanent deformation, is essential for track design, maintenance, and performance prediction. Large-scale triaxial testing is widely used to investigate these properties; however, standardized compaction procedures for specimen preparation are lacking. This gap has resulted in inconsistent preparation methods across studies, potentially contributing to variations in test outcomes. In this study, the effects of compaction method (vibratory vs. impact) and number of compaction lifts on ballast mechanical behavior were systematically examined through a series of large-scale monotonic and cyclic triaxial tests. Specimens were compacted to achieve a consistent void ratio using various combinations of compaction methods and lift counts. Results show that ballast stiffness and permanent deformation are significantly influenced by the compaction scheme. Compared with vibratory-compacted specimens, impact-compacted specimens exhibited lower stiffness and greater permanent deformation, likely due to weaker particle interlocking formed during compaction. The stiffness difference between compaction methods became more pronounced under higher confining pressures, indicating a strong confinement dependency. Moreover, increasing the number of compaction lifts enhanced aggregate interlocking, leading to higher stiffness. These findings underscore the importance of accounting for compaction effects in laboratory ballast testing and provide practical guidance for developing standardized specimen preparation procedures in large-scale triaxial experiments. • The effect of compaction lifts on granular materials was investigated using large-scale triaxial tests. • Increasing the number of compaction lifts significantly enhances stiffness but has little influence on shear strength. • Impact-compacted ballast exhibits greater deformation under cyclic loading, especially at low confinement. • Additional compaction lifts improve aggregate interlocking, resulting in higher overall stiffness.