Railway transitions such as bridge approaches experience differential movements related to differences in track system stiffness, track damping characteristics, foundation type, ballast settlement from fouling or degradation, as well as fill and subgrade settlement. Identification of factors contributing to this differential movement and developing design and maintenance strategies to mitigate the problem are imperative for the safe and economical operation of both freight and passenger rail networks. Findings are presented from an ongoing research study at the University of Illinois that focuses on the instrumentation and performance monitoring of railroad bridge approaches with multidepth deflectometers. Sensors installed at the selected approaches are introduced, and details of the instrumentation activity are explained. Track settlement data acquired over time are presented to compare the contributions of different substructure layers with the permanent deformation accumulation. Similarly, transient track deformation data gathered under dynamic train loading are analyzed to quantify the contribution of individual track substructure layers to the total transient deformations. Finally, a new approach is presented; it quantifies the support conditions under instrumented ties and assesses the percentage of the wheel load carried by the instrumented tie. Instrumentation of track transitions with multidepth deflectometers has been shown to quantify the contributions of substructure layers to track settlement adequately. In the bridge approaches instrumented with multidepth deflectometer technology, the ballast layers appear to be the primary source of accumulation for both permanent and transient deformations.

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.

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This paper presents findings from an ongoing research study at the University of Illinois that aims to develop and calibrate improved models for unbound aggregate rutting through laboratory characterization of aggregate materials used for unbound base and subbase applications in the state of North Carolina. Extensive triaxial laboratory testing was performed to establish a robust link between the number of load applications, stress levels, shear stress and sheer strength ratios, and permanent deformation responses. A framework was established for considering the strong correlation that commonly exists between permanent deformation and shear strength characteristics, as opposed to resilient modulus properties, in the laboratory characterization of the permanent deformation behavior of various types of aggregate materials. Trends of permanent strain accumulations from repeated load triaxial tests were adequately captured in a new rutting model whose development took into account the shear stresses applied at 25%, 50%, and 75% of the shear strength properties of these materials under similar field loading confinement conditions. The research shows that this model is an improvement on the rutting damage model for unbound aggregate currently used in AASHTO's mechanistic–empirical pavement design approach because it offers better material characterization and rutting prediction of the unbound base or subbase layer.

Characterizing railroad ballast behavior under repeated train loading is of significant importance for evaluating field settlement or permanent deformation potentials of unbound aggregate ballast layers. For the proper characterization of ballast behavior under dynamic loading, a new triaxial test setup was recently developed at the University of Illinois at Urbana–Champaign. Capable of accommodating cylindrical specimens with a diameter of 305 mm (12 in.) and a height of 610 mm (24 in.), this closed-loop servohydraulic test setup used a load cell and four displacement transducers mounted on the specimen to quantify deformation behavior under loading. Preliminary test results evaluating effects of different applied stress states as well as geogrid reinforcement on ballast behavior established the consistency and repeatability of this new test equipment. Laboratory findings are presented from an ongoing research study aimed at investigating the effects of different ballast types and field degradation trends on permanent deformation accumulation. The ballast type with the highest mill abrasion value was found to accumulate the highest permanent deformation under repeated load triaxial testing. Permanent deformation trends observed for four other ballast types showed direct correlations to the degrees of particle degradation observed in track sections constructed with these ballast materials and trafficked for approximately 18 months with a total track usage of 320 million gross tons.

Geogrids are well known for improving the performance of unbound aggregate layers in transportation applications by providing confinement and restraining movement through interlock between individual aggregate particles and geogrid apertures. Geogrid reinforcement offers an effective remedial measure when railroad track structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents findings from an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength behavior of railroad ballast. The effects of two geogrid types on ballast shear strength were evaluated through laboratory testing and numerical modeling. An imaging-based discrete element method (DEM) modeling approach was used to identify the optimal position for geogrid reinforcement to achieve the maximum shear strength gain in cylindrical triaxial specimens. Geogrids were installed at five depths within the cylindrical specimen and tested for shear strength properties with a large-scale triaxial test setup to evaluate the effectiveness of both geogrid aperture shape and reinforcement depth. Placing two layers of geogrids in the middle of the specimen was found to result in the maximum shear strength gain. Such placement of the geogrid ensured the intersection of the shear failure plane with the reinforcement layer, ultimately leading to significant shear strength gains. The DEM simulations were observed to capture accurately the ballast shear strength behavior with and without geogrid reinforcement.