An integrated approach to dynamic analysis of railroad track transitions behavior

Railway transitions like bridge approaches experience differential vertical movements due to variations in track stiffness, track damping characteristics, ballast settlement from fouling and/or degradation, as well as fill and subgrade settlement. Proper understanding of this phenomenon requires the integration of field instrumentation with analytical and numerical modeling. This paper introduces an integrated approach to dynamic analysis of the railway track transitions behavior using field instrumentation, analytical modeling, as well as numerical simulations using the Discrete Element Method (DEM). Several bridge approaches have been instrumented to monitor the track response on a problematic portion of the US North East Corridor (NEC), which is primarily a high-speed railway line with occasional freight traffic, carrying high-speed passenger trains operating up to a maximum speed of 241 km/h. Previous publications by the authors have focused on findings from geotechnical instrumentation of railroad track transitions, as well as the validity of a fully coupled 3-dimensional track dynamic model and image-aided discrete element models. The primary contribution of the current manuscript involves the combination of these three components to propose an integrated approach for studying the behavior of railroad track transitions. Track response data from instrumented bridge approaches were used to determine track substructure layer properties and calibrate a fully coupled 3-dimensional track dynamic model. Loading profiles generated from this model were then used as input for a discrete element based program to predict individual particle accelerations within the ballast layer. The importance of modeling the ballast layer as a particulate medium has been highlighted, and the particle to particle nature of load transfer within the ballast layer has been demonstrated. 2014 Elsevier Ltd. All rights reserved. Introduction the sudden change in track stiffness, the ‘‘stiff’’ side of a Railway track transitions present a significant challenge as far as maintenance of track profile is concerned. Due to track transition undergoes lower deformations under loading, compared to the ‘‘less stiff’’ side. This differentialmovement often results in the formation of a ‘‘bump’’ in the track profile. Bridge approaches qualify as an ideal example of track transitions, with the approach track on either side of the bridge abutment being much less stiff compared to the bridge deck often supported by deep foundations. Differences in track system stiffness and/or damping characteristics, settlement of the ballast layer due to degradation and/or fouling, and settlement of the subgrade and/or fill D. Mishra et al. / Transportation Geotechnics 1 (2014) 188–200 189 layers are some of the factors commonly reported as mechanisms contributing to the differential movement at track transitions. Proper understanding of different mechanisms contributing to this phenomenon requires the combined application of field instrumentation with analytical and numerical track modeling. Several research studies have focused on investigating the bump development at both highway and railway bridge approaches (Zaman et al., 1991; Stark et al., 1995; Briaud et al., 1997; White et al., 2005; Briaud et al., 2006; Nicks, 2009). A detailed review of these past research efforts, different factors contributing to the bump development at railway track transitions, as well as the effects of possible remedial measures has been presented by Mishra et al. (2012). Due to the sudden change in track profile, railway track transitions are often exposed to magnified dynamic loads as a train passes over them. Such magnified load levels ultimately result in rapid degradation in track geometry and profile, requiring frequent maintenance and resurfacing. The annual expenditure to maintain track transitions in the US has been reported to exceed USD 200 million (Sasaoka et al., 2005; Hyslip et al., 2009). Nicks (2009) reported that approximately 50% of railroad bridge approaches in North America experienced differential movement problems, characterized by the development of a low approach, usually 6–102 mm in depth. Read and Li (2006) reported that the bump problem is more significant at the ‘‘exit’’ side of a transition, as the train moves from a high-stiffness track to a low-stiffness track. The ‘‘bump’’ formation at railway bridge approaches is usually within 15 m from the abutment (Plotkin and Davis, 2008). The differential movement at track transitions is particularly problematic for high-speed rail infrastructure as the ‘‘bump’’ is accentuated at high speeds. The issue is even more critical for shared corridors carrying both freight and high-speed passenger trains. Transitions along shared corridors need to be maintained to satisfy the high ride quality requirements associated with high-speed trains. Additionally, these transitions also need to withstand the heavy loads imposed by slow-moving freight trains without undergoing excessive deformations. With the current impetus for development of high speed lines in the US and the challenges associated with shared corridors for operation of passenger trains at increased speeds, preventing andmitigating the problem of differential movement at bridge approaches and other track transitions has become more significant. This paper introduces an integrated approach to dynamic modeling of railway track transitions through field instrumentation and analytical and numerical modeling. Field instrumentation data collected from Multidepth Deflectometers (MDDs) and strain gauges are used to determine individual track substructure layer deformations and dynamic wheel loads, respectively. Track response data from instrumented bridge approaches are then used to calibrate a fully coupled 3-dimensional dynamic track model. Loading profiles generated from this model are used as input for a numerical simulation program based on the Discrete Element Method (DEM) to predict individual particle accelerations within the ballast layer. Shortcomings associated with other track analysis and numerical modeling approaches based on the principles of finite element, or finite difference methods to characterize the ballast layer as one continuum are highlighted. Accordingly, through the integrated approach, the importance of modeling the ballast layer as a particulate medium is mainly emphasized, and the particle to particle contact for load transfer within the ballast layer is demonstrated. Therefore, the primary objective of this paper is to emphasize the importance of adopting an integrated approach for realistic analyses of ballasted railroad track systems. This has been accomplished with the help of instrumentation and numerical modeling results from different track transition sections. Proposing solutions to mitigate the differential movement problem at track transitions is beyond the scope of this paper. Field instrumentation of selected track transitions A research study sponsored by the US Federal Railroad Administration (FRA) is currently being carried out at the University of Illinois with the overall objective of identifying and mitigating different factors contributing to the differential movement at railway transitions. Several problematic track transitions have been instrumented under the scope of the current study to monitor the track response under loading near Chester, Pennsylvania on Amtrak’s North East Corridor (NEC). There are 8–10 closely-spaced undergrade bridges with recurring differential movement problems at the bridge/embankment interfaces. The NEC is primarily a high-speed railway with occasional freight traffic, carrying high-speed passenger trains operating up to a maximum speed of 241 km/h. This segment of the NEC near Chester comprises four tracks, with Tracks 2 and 3 maintained for high-speed Acela Express passenger trains operating at 177 km/h. The predominant direction of traffic along Track 2 is Northbound whereas Track 3 primarily carries Southbound traffic. Data from the instrumented track transitions are being used to calibrate different analytical and numerical models to predict the dynamic track behavior under train loading. A brief description of the instrumentation used in the current study is first presented in the sections below. Instrumentation details The instrumentation used in the current study comprised Multidepth Deflectometers (MDDs) for measuring track substructure layer deformations, and strain gauges mounted on the rail for measuring the vertical wheel loads and tie support reactions. The MDD technology was first developed in South Africa in the early 1980s to measure individual layer deformations in highway pavements (Scullion et al., 1989).MDDs typically consist of up to six linear variable differential transformers (LVDTs) installed vertically at preselected depths in a small diameter (45 mm in the current study) hole to measure the deformation of individual track layers with respect to a fixed anchor buried deep in the ground (DeBeer et al., 1989). More details on the operation principle of MDDs can be found elsewhere (Mishra et al., 2012; DeBeer et al., 1989). It is noteworthy 190 D. Mishra et al. / Transportation Geotechnics 1 (2014) 188–200 that the use of MDDs to monitor railway track performance has been extensively pursued in South Africa (Gräbe and Clayton, 2005; Gräbe and Shaw, 2010; Priest et al., 2010; Vorster and Gräbe, 2013). Any boundary effects and associated stress concentrations introduced in the track substructure layers due to drilling of theMDD hole (hole diameter is 45 mm;mean particle size or D50 of ballast is approximately 35 mm) are assumed to be of negligible significance, and therefore have not been considered during analyses of the instrumentation data. Fig. 1 shows the schematic of an MDD system with five LVDT modules each installed at track substructure layer interfaces. Individual LVDTs are placed at different depths inside the borehole to measure the deflections at (1) top of ballast layer, (2) top of fouled ballast layer, (3) top of embankment fill layer 1, (4) top of embankment fill layer 2, and (5) top of the subgrade layer, respectively. The approach fill layer or the subgrade as indicated in Fig. 1 can conceptually be divided into two separate segments: ‘‘deformable’’ and ‘‘non-deformable,’’ respectively. The MDD anchor is fixed at a depth that is assumed to be in a ‘‘non-deformable’’ subgrade or approach fill. Note that the deformations of substructure layers measured using the MDD technology is dependent on the assumption that the anchor is located in a non-deformable layer, and does not undergo any elastic or permanent deformation. Accordingly, for an accuratemeasurement of layer deformations usingMDDs installed on a bridge approach, it is important to ensure that the anchor is located sufficiently below the track level. Track construction reports and/or reports from previous geotechnical explorations (if available) can be used to make educated inferences regarding the potentials of individual track substructure layers to accumulate transient (elastic) and permanent (plastic) deformations under loading. This information can also be used to identify suitable locations for placement of the MDD anchor. The MDD system involves the installation of a flexible lining tube in the drilled hole; individual LVDT modules are subsequently mounted inside the hole by gripping Fig. 1. Schematic representation of multidepth deflecto against the flexible lining tube. Additionally, a polyurethane grout is used to ensure proper contact between the flexible lining tube and the adjacent soil layers. Presence of the polyurethane grout and the flexible lining tube provides sufficient flexibility to the LVDT mounting points to ensure adequate insulation against vibrations transmitted from the upper layers. Moreover, no physical connection exists between individual LVDT modules in an MDD system. Accordingly, transmission of vibrations from the upper layers can be adequately avoided. Note that due to the presence of overhead catenary cables, the boreholes for MDD installation in the current study could not be drilled deeper than 3.0 m below the top of the tie. Accordingly, the MDD anchor was placed at a depth of 3.0 m below the top of the crosstie. However, this is likely to be sufficient to ensure the ‘‘non-deformability’’ of the anchor, as the instrumented sites have been in service for more than 100 years. Accordingly, the embankment fill and subgrade layers have likely been fully consolidated, resulting in no significant elastic or plastic deformations. Track substructure layer configuration The track substructure layer profile was determined during the drilling by visual classification of the soil coming out of the drilled hole. The drilling was carried out in small (25–50 mm) increments to ensure that the depths of substructure layer interfaces could be identified with reasonable accuracy. The positions of individual MDD modules correspond to changes in the track substructure layer interfaces as determined during the drilling process. Fig. 2 shows the layer profile established during the instrumentation of one of the bridge approaches. Note that the term ‘‘fouled ballast’’ has been used to indicate the section of ballast layer that was contaminated with fine materials from underlying layers. Accordingly, this terminology is not intended to make references to the quality of ballast material used in the instrumented approaches. meter (MDD) system installed in railroad track. D. Mishra et al. / Transportation Geotechnics 1 (2014) 188–200 191 It is important to highlight that the extent of fouling in the ballast layer is a transient phenomenon, and hence the position of the ballast-fouled ballast layer interface is likely to shift over time. For the purpose of this study, the position of this interface has been assumed to be stationary at the position identified during the drilling process. Subsequently, the ballast and fouled ballast layers have been treated as two separate layers for analytical and numerical modeling purposes, and different layer moduli have been assigned to the two layers. Characterization of the ballast-fouled ballast layer interface transient behavior requires extensive investigations through periodic subsurface explorations potentially utilizing ground penetrating radar (GPR) scanning and is beyond the scope of the current paper. In addition to the MDDs, strain gauges were also installed on the rail to measure the vertical wheel load and tie reaction forces. A total of eight (8) strain gauges (two sets of four, constituting two different Wheatstone bridges) were installed next to each MDD hole. Periodic monitoring and data acquisition Two types of data are currently being collected from the instrumented bridge approaches to monitor and evaluate their performances. Tomonitor the permanent deformation accumulations in individual track substructure layers, ‘‘offset measurements’’ are being collected from the instruments at one to two week intervals. Additionally, transient (recoverable) deformations are also being collected for each approach under train loading at two to Fig. 2. Substructure layer profile for instrumented track section analyzed (the numbers 1 through 5 indicate the positions of the LVDTs installed along the hole for measuring individual substructure layer deformations). threemonth intervals. Periodic data acquisitionwas carried out using a laptop computer and a signal conditioner connected to the installed sensors. All LVDTs used in the MDD system were manufactured using an inductive half-loop configuration, and were excited using an AC power of 1 V at 4.8 kHz. The strain gauge circuits were powered by an excitation of 5 Vmagnitude. The data acquisition frequency for monitoring the transient track response under train loading was 2000 Hz. Fig. 3 presents the transient data collected fromone of the instrumented bridge approaches during the passage of an Amtrak Acela Express passenger train. Note that both transient and permanent track deformations recorded in this study have been analyzed in view of the contributions of individual layers only. In other words, only the percentages of total track deformations contributed by individual layers have been determined. Analyses of individual mechanisms contributing to the individual layer deformations are beyond the scope of the current paper. Accordingly, different mechanisms such as ballast degradation, ballast migration, particle rearrangement, that can contribute significantly towards deformations within the ballast layer have not been separately investigated. A typical ACELA Express train comprises two power cars (one at either end) separated by six passenger cars. This configuration of the train is clearly reflected by the vertical wheel load values registered by the strain gauges installed on the rail (see Fig. 3-a). The heavier power cars apply higher loads on the rail compared to the passenger cars. Moreover, the peaks corresponding to individual axles (32 in total) passing over the instrumentation location is clearly evident from the graph. Fig. 3-b shows the transient deformations recorded by the top LVDT under the passage Fig. 3. Field measured values for (a) vertical wheel load applied on top of rail, (b) ballast layer deflection, and (c) ballast layer acceleration. 192 D. Mishra et al. / Transportation Geotechnics 1 (2014) 188–200 of the same train. Note that the top LVDT was mounted within the concrete crosstie, just above the top of the ballast layer. As the concrete crosstie is rigid, and does not undergo any deformation under loading, the deformations recorded by the top LVDT can be assumed to represent the deformation within the ballast layer. However, it is important to note that the deformations registered by the top LVDT may include excessive deformations of the crosstie resulting from inadequate support conditions underneath, also referred to as ‘‘hanging tie’’ conditions. Fig. 3-c shows the layer acceleration values calculated for the ballast layer from the LVDT-recorded transient deformations. Estimation of track substructure layer moduli using