The ballast layer in a railroad track helps distribute loads from the superstructure to the formation; a well-designed ballast layer is also meant to prevent excessive vertical, lateral and longitudinal movement of the track under loading. When subjected to repeated loading, the granular ballast particles often undergo breakage leading to significant changes in the shear strength as well as drainage characteristics of the ballast layer. Excessive ballast degradation leads to increased vertical settlements, and is often associated with speed restrictions and increased passenger discomfort. 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 paper presents findings from an ongoing research effort at Boise State University aimed at studying the phenomenon of ballast breakage under repeated loading using a commercially available Discrete Element Package (PFC3D®). Ballast particles were simulated as clusters of balls bonded together, and were allowed to undergo breakage when either the maximum tensile stress or the maximum shear stress exceeded the corresponding bond strength value. Different factors studied during the parametric analysis were: (1) load amplitude; (2) loading frequency; (3) number of cycles of loading; (4) bond strength; and (5) particle size distribution. The objective was to identify the relative importance of different factors that govern the permanent deformation behavior of railroad tracks under loading.
The ballast layer serves as a major structural component in typical ballasted railroad track systems. When subjected to an external load, ballast particles present a complex mechanical response which is strongly dependent on particle to particle interactions within this discrete medium. One common test used to study the shear strength characteristics of railroad ballast is the Direct Shear Test (DST). However, it is often not feasible in standard geotechnical engineering laboratories to conduct direct shear tests on ballast particles due to significantly large specimen and test setup requirements. Even for the limited number of laboratories equipped to accommodate the testing of such large specimens, conducting repeated tests for parametric analysis of different test and specimen parameters on shear strength properties is often not feasible. Numerical modeling efforts are therefore commonly used for such parametric analyses. An ongoing research study at Boise State University is using the Discrete Element Method (DEM) to evaluate the effects of varying particle size and shape characteristics (i.e., flakiness, elongation, roundness, angularity) on direct shear strength behavior of railroad ballast. A commercially available three-dimensional DEM package (PFC3D®) is being used for this purpose. In numerical modeling, railroad ballasts can be simulated using spheres (simple approach) and non-breakable clumps (complex approach). This paper utilizes both approaches to compare the ballast stress-strain response as obtained from DST. Laboratory test results available in published literature are being used to calibrate the developed numerical models. This paper presents findings from this numerical modeling effort, and draws inferences concerning the implications of these findings on the design and construction of railroad ballast layers
Geogrid reinforcement of railroad ballast improves its structural response under loading, limits lateral movement of ballast particles, and reduces vertical settlement through effective geogrid-ballast interlocking. This improved performance can be linked to improved shear strength and resilient modulus properties. An ongoing research study at Boise State University is focusing on investigating the effects of different specimen and test parameters on the mechanism of geogrid-ballast interaction. A commercially available Discrete Element Modeling (DEM) program (PFC3D®) is being used for this purpose, and the effect of geogrid inclusion is being quantified through calculation of the “Geogrid Gain Factor”, defined as the ratio between resilient-modulus of a geogrid-reinforced ballast specimen and that of an unreinforced specimen. Typical load-unload cycles in triaxial shear strength tests are being simulated, and parametric studies are being conducted to determine the effects of particle-size distribution, geogrid aperture size, and geogrid location on railroad-ballast modulus. This paper presents findings from the research study, and presents inferences concerning implications of the study findings on design and construction of better-performing ballast layers.
The Unified Soil Classification System (USCS) uses the 4.75 mm sieve opening size (#4 sieve) as the boundary between ‘coarse’ and ‘fine’ particles. Particles larger than 4.75 mm are classified as ‘coarse’, whereas particles smaller than 4.75 mm are classified as ‘fine’. However, applying these definitions to railroad ballast can be erroneous, as most particles in a ballast material are larger than 4.75 mm (often as large as 63 mm in size), therefore indicating the absence of any ‘fine’ particles. However, depending on relative distribution of particle sizes within a granular matrix, certain particles serve to create voids (coarse fraction), and certain particles serve to fill the voids (fine fraction). Accordingly, rather than using the standard definitions of ‘coarse’ and ‘fine’ particles, as has been done in the literature, the analysis of packing conditions in a ballast matrix may be better served by studying the relative packing between different size fractions. This paper focuses on the development of a new gradation parameter, termed as the “Coarse-to-Fine (C/F) Ratio”, which can shed some light on the importance of different size fractions in a ballast matrix. Changing the ‘coarse’ and ‘fine’ fractions within a particular gradation specification, the resulting effect on ballast shear strength was studied through simulated Direct Shear Strength Tests. A commercially available threedimensional Discrete Element Modeling (DEM) package (PFC3D®) was used for this purpose. Details of the numerical modeling effort are be presented, and inferences are drawn concerning the implications of simulation results on the design and construction of railroad ballast layers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.