Effect of Particle Size and Shape Characteristics on Ballast Shear Strength: A Numerical Study Using the Direct Shear Test

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.