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.

This paper presents findings from a recently completed research study at the University of Illinois aimed at investigating the effects of aggregate type and quality on mechanistic pavement response and rutting performance with implications to over 2.6 million km (1.6 million miles) of unsurfaced roads in the United States. Three aggregate types, crushed limestone, crushed dolomite, and uncrushed gravel, were tested in the laboratory for resilient modulus (MR) and permanent deformation characteristics at different factorial combinations of selected aggregate physical properties. Aggregate properties studied included particle shape and surface texture, type and amount of fines, and moisture and density in relation to required compaction conditions. Stress-dependent material characterization models determined from the laboratory MR tests were used in a nonlinear axisymmetric finite-element analysis program to compute the vertical compressive stress on top of the subgrade as a critical pavement respon...

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.

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.