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.

Ballast fouling, often associated with deteriorating railroad track performance, refers to the condition in which the ballast layer changes its composition and develops a much finer grain size distribution. Fouling is commonly caused by degradation or breakage of ballast aggregates under traffic loading, although other fine materials including but not limited to coal dust, fine-grained subgrade soils, and sand can also contaminate a clean and uniformly graded ballast layer. An experimental approach is described to characterize stages of railroad ballast degradation studied through Los Angeles abrasion testing in the laboratory. An aggregate image analysis approach is used to investigate ballast particle abrasion and breakage trends at every stage through detailed quantifications of individual ballast particle size and shape properties. The experimental study indicated that the fouling index (FI) commonly used by practitioners was indeed a good indicator of fouling conditions, especially when all voids created by larger particles were filled by fine materials as FI values approached 40. Image analysis results of ballast particles larger than 9.5 mm (3/8 in.) scanned after a number of turns of the Los Angeles abrasion drum showed good correlations between percentage changes in aggregate shape properties, that is, imaging-based fatness and elongation, angularity and surface texture indexes, and the FI. The establishment of such relationships between in-service track fouling levels and ballast size and shape properties with similar field imaging techniques would help to understand field degradation trends better and as a result improve ballast serviceability and life-cycle performance.

Railroad track transitions such as bridge approaches may experience differential movements due to variations in track stiffness; impact loads due to train speed and excessive vibration; ballast settlement from fouling, degradation, or both; tie–ballast contact condition and gap; and settlement of fill, subgrade, and foundation layers. A research study completed recently at the University of Illinois focused on identifying the major causes of this differential movement and implementing suitable rehabilitation measures to mitigate recurrent problems with settlement and geometry. Transient and permanent deformation trends were observed in track substructure layers at two instrumented bridge approaches along the Amtrak Northeast Corridor. Multidepth deflectometer systems installed through crossties successfully recorded both permanent (plastic) and transient deformations of individual track substructure layers. Strain gauges mounted on the rail effectively measured vertical wheel loads applied during train passage and monitored the support conditions under the instrumented crossties. Track settlement (or permanent deformation) data revealed that the ballast layer was the primary source of differential movement contributing to recurrent settlement and geometry problems. Transient layer deformations recorded under train passage were higher in the ballast than in any other substructure layer. Transient displacement and wheel load data were consistently higher at near-bridge locations than at open-track locations. Rail-mounted strain gauges indicated that load amplification levels were significantly higher at near-bridge locations than at open-track locations.

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Numerous studies have targeted using numerical modeling, field instrumentation, or combinations of both to gain insight into track substructure behavior under loading. In-depth understanding of track substructure behavior serving both passenger and freight trains is critical to developing suitable design and maintenance/rehabilitation methods to ensure adequate performance under loading. This manuscript presents findings from a recently completed study involving advanced instrumentation and numerical modeling to investigate track substructure-related issues at several problematic railroad bridge approaches in the United States. Multi-Depth Deflectometers (MDDs) were installed to measure transient as well as plastic deformations experienced by track substructure layers under loading. Strain gauges were installed on the rail web to measure the vertical wheel loads applied during train passage. Data from the field instrumentation was used to make inferences regarding the relative contributions of different substructure layers towards the differential movement problem. A 3-D Finite Element (FE) model was developed to further understand the behavior of the instrumented locations, and was calibrated using the field instrumentation data. An elastic layered track analysis program, GEOTRACK, was first used to iteratively backcalculate individual track substructure layer moduli from the field measurements; these backcalculated modulus values were subsequently used in the FE model to predict track response under transient loading conditions. Modulus values estimated for the ballast layer were found to be significantly affected by the presence of gaps at the tie-ballast interface at track transitions. Once validated, the model was further modified to match transient displacement results acquired in the field using a quasi-static moving load approach. Good agreement was found between the model predictions and field instrumentation results. Development of advanced numerical models augmented by field instrumentation data can facilitate the design and maintenance of well-performing track structures.