Publication Date


Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Civil Engineering


Civil Engineering

Major Advisor

Debakanta Mishra, Ph.D.


Arvin Farid, Ph.D.


Bhaskar Chittoori, Ph.D.


Railroad infrastructure plays a significant role in sustaining the economy of a country, and facilitates fast, safe and reliable transportation of passengers as well as commodities. Significant capital investments are required for the construction and maintenance of a railroad network that is structurally and functionally adequate. The ballast layer is one of the main structural components of a conventional rail track system, and comprises coarse-grained unbound particles, often as large as in size. The ballast as a load-bearing layer resists train-induced stresses through particle-particle interaction. Accordingly, particle-size distribution and packing characteristics are important factors that govern the mechanical behavior of the ballast layer under loading. A well-performing ballast layer should ideally possess optimum drainage characteristics to ensure rapid removal of surface water and adequate shear strength to restrain the track against excessive movement under loading. In-depth understanding of different factors affecting ballast behavior can help reduce recurrent costs associated with ballast maintenance.

Conducting common shear strength tests on coarse-grained geomaterials such as railroad ballast, and performing parametric studies to quantify the effects of different material, specimen, and test parameters on shear strength properties is often not feasible in standard geotechnical engineering laboratories due to the significantly large specimen and test setup requirements. In such situations, the Discrete Element Method (DEM) that facilitates micromechanical analysis of particulate matter becomes a logical alternative. The primary objective of this research effort is to study the effects of particle-size distribution and packing characteristics on the shear strength behavior of railroad ballast. This was accomplished by simulating commonly used laboratory shear strength tests such as Direct Shear Test and Triaxial Monotonic Shear Strength Test using DEM. A commercially available three-dimensional DEM package (Particle Flow Code - PFC3D®) was used for this purpose. Published laboratory-test data were used to calibrate the numerical model. A series of parametric analyses were subsequently carried out to quantify the individual effects of different variables being studied on ballast shear strength behavior. In an effort to increase ballast shear strength through better packing within the granular matrix, a new gradation parameter, termed as the “Coarse-to-Fine (C/F) Ratio” was proposed. Changing the ‘coarse’ and ‘fine’ fractions within a particular gradation specification, the resulting effect on ballast shear strength was studied. In addition to studying the particle-to-particle interaction within the ballast matrix, this study also focused on studying the phenomenon of geogrid-ballast interaction under different packing conditions. A recently developed parameter known as the “Geogrid Gain Factor” was used to quantify the benefits of geogrid reinforcement of ballast. The ultimate objective was to further the understanding of ballast behavior under loading, which will ultimately lead to the design and construction of better-performing railroad tracks.