| Ballast is an essential layer of the railroad track structure, and provides primarily drainage and load distribution. In general, ballast aggregates are considered as uniformly graded, angular shaped with crushed faces. However, various ballast aggregate gradations and particle shapes are in use yet their effects on ballast performances remain unknown. In previous designs and modeling practices, railroad ballast has usually been treated as a homogeneous and continuous layer. This approach is not suitable to model the deformation behavior of the particulate nature railroad ballast aggregates under dynamic moving loads. Further, continuum solutions do not take into account realistically the morphological characteristics of aggregates such as particle size distribution and shapes. A combined digital image and Discrete Element Modeling (DEM) methodology has been developed in this PhD thesis to study effects of aggregate particle size and morphological characteristics on ballast performances. The approach has been calibrated using actual ballast aggregates through laboratory shear box texts and validated by further laboratory as well as field experiments. Using the DEM ballast model, individual effects of aggregate particle size distributions and shape properties on railroad ballast strength, lateral stability, and settlement potential were studied. From the DEM simulation results, it was found that aggregate particle size distribution and shape have significant impact on ballast performances. Ballast with broader size distribution was shown to yield less settlement potential than ballast with more uniformly graded aggregates. Also, ballast with angular aggregate particles were found from the DEM simulations to have higher strength as well as better lateral stability than ballast with rounded aggregate particles due to better stone on stone contact and aggregate interlock. In summary, the developed DEM ballast model has been proven in this PhD research to be a promising tool for studying railroad ballast load and deformation characteristics and could lead to the ultimate goal of designing better "engineered ballast."... |
