Keywords: box testing, field loading, lateral confinement, ballast settlement, ballast breakage, ballast settlement models, track monitoring, track maintenance.

The simulation of field conditions on ballast in the laboratory aids the identification of the overall response and performance of the ballast layer and the resulting impact on track structure deterioration. The permanent deformation of ballast under cyclic loading often accounts for the largest portion of track settlement of all layers that constitute the track structure. Hence, utilizing best-fit ballast settlement models aids in identifying the need for track maintenance interventions and adequate planning. Furthermore, the use of high-quality ballast is essential to maintain geometric stability and to enhance the durability of ballast particles on heavy haul tracks. In addition, it is important to ensure adequate compaction and confinement of ballast on railway lines. This will limit the occurrence of various types of ballast breakage which affect the integrity of the track structure. The objective of this paper is to describe best-fit ballast models that can predict the gradual deformation of ballast as well as different types of ballast breakage under field loading and boundary conditions. Ballast deformation is accurately predicted by existing best-fit ballast settlement models defined by different levels of lateral confinement. Ballast breakage results reveal that attrition of asperities and corner breakage are the foremost types of breakage, where an increase in lateral confinement of the ballast layer could limit excessive ballast breakage to acceptable levels. From the findings of this research, ballast settlement can be accurately predicted for regular track geometry monitoring and maintenance using best-fit settlement models for any level of lateral confinement. Furthermore, high quality ballast material utilized on railway lines could extend track maintenance intervals.

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