The railroad ballast layer consists of discrete aggregate particles, and the discrete element method (DEM) is the most widely adopted numerical method to simulate the particulate nature of ballast materials and their particle interactions. Large-scale triaxial tests performed in the laboratory under controlled monotonic and repeated loading conditions are commonly considered the best means to measure macroscopic mechanical properties of ballast materials, such as strength, modulus, and deformation characteristics, directly related to load-carrying and drainage functions of the ballast layer in the field. A DEM modeling approach is described for railroad ballast with realistic particle shapes developed from image analysis to simulate large-scale triaxial compression tests on a limestone ballast material. The ballast DEM model captures the strength behavior from both the traditional slow and the rapid shear loading rate types of monotonic triaxial compression tests. The results of the experimental study indicated that the shearing rate had insignificant influence on the results of the triaxial compression tests. The results also showed that the incremental displacement approach captured the measured shearing response, yet could save significant computational resources and time. This study shows that the DEM simulation approach combined with image analysis has the potential to be a quantitative tool to predict ballast performance.

Ballast containing large aggregate particles with uniform gradation is an essential layer in a railway substructure to facilitate load distribution and drainage. Although it is constructed or maintained in accordance with specifications, ballast is characterized by inherent randomness, not only in particle properties, such as mineralogy, size, and shape, but also in particle arrangement. Because of this randomness, the initial configurations of ballast particle assemblies vary from place to place in the field as well as from specimen to specimen in the laboratory. This paper presents a study on the influence of the aggregate scale randomness, which is the initial particle arrangement, on the geomechanical behavior of ballast. A series of large-scale, laboratory, triaxial shear-strength and repeated-load permanent-deformation tests was performed at the University of Illinois at Urbana-Champaign for the study. Numerical simulations using the discrete-element method (DEM) were also performed to understand better the effect of different initial particle arrangement on the basis of the identical particle scale properties of shape and size distribution. The ballast aggregate particles were modeled as three-dimensional (3D) polyhedral elements according to the properties of particles used in the laboratory tests, and then, the same set of particles was used to simulate laboratory tests with different initial particle arrangements for each specimen. The results from both the laboratory experiments and numerical simulations confirm that the initial particle arrangement may significantly affect the strength and deformation behavior of ballast. Hence, multiple tests or simulations with different particle-packing arrangements are needed to study ballast mechanical behavior. However, a stable average response can be obtained with a minimum of three laboratory experiments or numerical simulations.