Discussion of “Discrete-Element Analysis for Compaction-Induced Stiffness Variation of Ballast Aggregate in Large-Scale Triaxial Testing”

Railway ballast, an unbound coarse-grained aggregate derived generally from crushed rock, gravel, and slag aggregates, is typically used as the uppermost layer of a ballasted railway track structure on which the railroad sleepers and rails rest.Adequate compaction of the ballast is essential to dissipate the static and dynamic stresses transmitted from the rail cars to the underlying layers of the roadbed to minimize aggregate degradation and permanent deformation, among other deformation features of the various roadbed structural components.The objective is to ensure the overall stability of the track system and, hence, the safety of the rail traffic.The authors have presented the findings of a comprehensive research study using the discrete-element model (DEM) to evaluate the results of large-scale triaxial tests to demonstrate the differences and assess the reasons for strength and stiffness characteristics of ballast comprised of a fixed gradation when subjected to compaction by three typical compaction methods.According to the authors, the findings of their study demonstrate the need to consider the influence of compaction methods when conducting large-scale triaxial testing or evaluating the test results.The original study also reflects on which type of laboratory compaction would provide better results to represent the behavior of in-track ballast.Some thoughts on this aspect are presented in addition to comments on the study.The original study involved developing DEM simulations for three compaction tools: an electric jackhammer (JH), a modified Proctor Hammer (MPH), and a Portable Tamper (PT) typically used to prepare large-scale triaxial test samples.These simulations were calibrated by conducting large-scale static triaxial tests on a prescribed ballast gradation prepared at a specific void ratio using each of the three compaction tools.Two tests were performed using each compaction tool.The results of these tests have been used to calibrate the DEM simulations and to understand the development and evaluation of the characteristic features of the tests, such as peak strength, stiffness derived from initial tangent modulus and secant modulus, contact shear force, contact normal force, and contact moment.The authors presented the results in a series of graphs and explanations demonstrating the efficacy of each compaction tool.Overwhelmingly, the best results were achieved by using the JH compaction tool.This finding reinforced the general understanding that preventing the ballast particles' upward movement beyond the base of the JH tool during compaction would allow for more intimate contact with the ballast particles.This