Effect of Particle Breakage on Ballast Permanent Deformation — A Study Using the Discrete Element Method

The ballast layer comprises relatively large (often as large as 63 mm) angular particles that mainly function to dissipate train-induced stresses from crossties to the underlying subgrade soils and to provide rapid drainage for surface water. Moreover, the ballast layer is also critical towards ensuring a smooth riding track profile, dampening dynamic loads, as well as providing lateral, longitudinal and vertical resistance against excessive track deformations. Under train loading and during track maintenance processes like tamping, individual ballast particles can undergo significant breakage leading to fouling of the ballast layer. The fouling mechanism leads to gradual deterioration in ballast shear strength as well as drainage properties. Inadequate drainage of surface water can lead to further reduction in the ballast layer’s ability to resist excessive track deformations, ultimately leading to geometric defects such as severely ill-conditioned geometry parameters like profile, alignment, gauge, cant and twist. About 76% of the fouling in a ballast layer can be attributed to ballast degradation and breakdown under repeated train loading. In extreme cases, ballast breakage may potentially lead to derailment as a major portion of track vertical settlement or permanent deformation occurs within the ballast layer. Several researchers in the past have studied the phenomenon of ballast breakage in a laboratory setting. However, due to complexities associated with these large-scale laboratory tests, detailed parametric studies are often not feasible. In such cases, numerical modeling tools such as the Discrete Element Method (DEM) become particularly useful. This master’s thesis presents findings from a research study aimed at understanding the significance of ballast breakage considerations in the associated vertical permanent deformation of the ballast layer under repeated loading. A commercially available Discrete Element Package (PFC3D®) was used to simulate the response of the ballast layer under loading. Various factors that affect ballast breakage and eventual permanent deformation accumulation within the ballast layer were studied, and analyzed. First, the ballast particles were modeled using simple ellipsoid shapes. Parametric studies were conducted to quantify the effects of different parameters such as cyclic load amplitude, loading frequency, number of loading cycles, particle strength, and particle size distribution. As a subsequent enhancement to the research approach, laboratory tests were conducted to quantify the crushing strengths of individual ballast particles. A DEM model was prepared to simulate the Single Particle Crushing Test (SPCT), and calibrated against the laboratory test results. The calibrated parameters were then used to study the response of polyhedral ballast particles (simulated as clumps in PFC3D®) under repeated loading. Comparisons of permanent deformation and ballast breakage were made for polyhedral ballast particles and ellipsoid ballast particles. From the results, it was observed that there was a significant deviation in permanent deformation (polyhedral ballast layer underwent 80% more permanent deformation than the ellipsoid ballast layer). When the relative shift in particle distribution curves (the area between the particle size distribution curves before and after loading), were compared for the ellipsoid and polyhedral ballast layers it was seen that polyhedral ballasts underwent about 53% more breakage compared to ellipsoid ballasts. This showed the importance of polyhedral shape simulation to accurately study ballast layer response under loading. As a potential approach to reduce ballast breakage and permanent deformation, the model was subsequently modified to incorporate geogrid reinforcement. Two different geogrid types, with square and triangular aperture, were modeled, and the response of geogrid-reinforced ballast layers were compared against unreinforced configurations. From the results, it was seen that the unreinforced ballast layer showed the highest permanent axial strain (approximately 15%), and the same for all the geogrid-embedded ballast layers were found to be approximately 10%. This showed that geogrid reinforcement reduces permanent deformation accumulation within the ballast layer. However, no significant effect of geogrid reinforcement was observed when the extent of ballast breakage was compared. Detailed understanding of different factors governing ballast breakage and permanent deformation accumulation can help facilitate the design and construction of better performing railroad tracks