James P. Hyslip

Railway track condition indicators from ground penetrating radar

Two embankments with track performance problems related to embankment instability were investigated. Both sites demonstrated potential for GPR to identify substructure instability resulting in track settlement. The capability to non-destructively evaluate track condition and diagnose the problem cause will ensure that ensuing maintenance addresses the root problem cause, thereby enhancing safety and maintenance efficiency. Two characteristics of the GPR data from these two track performance problem locations were identified as potential track condition indicators. These characteristics were applied to GPR data from a third location with observed performance problems. The application of the condition indicators demonstrates potential for simplifying data interpretation.

FRACTAL ANALYSIS OF TRACK GEOMETRY DATA

An ongoing FRA-sponsored research project at the University of Massachusetts has been exploring the use of fractal analysis of track geometry data for indicating track geometry condition, planning maintenance, and evaluating the cause of substructure-related problems. Results indicate that fractal analysis is able to provide unique numerical values (fractal dimensions) that characterize railway track geometry patterns, can discern different orders of roughness within track geometry data, and is effective for maintenance management by providing independent fractal parameters for trend analysis and geometry deterioration assessment. Fractal theory is discussed, and the usefulness of fractal analysis for quantifying railroad geometry data is demonstrated by highlighting key aspects of the research results. The relationship between track structure conditions and fractal dimensions for use in maintenance planning and condition evaluation is also discussed.

Railroad Track Transitions with Multidepth Deflectometers and Strain Gauges

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.

Use of Recycled Tire Rubber to Modify Track-Substructure Interaction

Resiliently bound ballast (RBB) is a new engineered material being developed as an alternative or supplement to conventional ballast for use in transit, passenger, and freight railways to improve mechanical behavior and control the modulus and damping when recycled tire waste material is used. RBB is a stable mixture of standard ballast stone and tire-derived aggregate (TDA) bound together with a purpose-designed resilient epoxy binder. Initial laboratory tests were conducted on specimens 6 in. (151 mm) and 10 in. (254 mm) in diameter of two mixes of RBB. Tests were conducted on fully bound (cemented) samples of ballast, TDA, and epoxy as well as individual particles with TDA bound to the individual ballast particles with the resilient epoxy binder. The tests included static triaxial compression tests and dynamic cyclic triaxial tests. Static tests indicated that the addition of the TDA and epoxy resulted in an increase in cohesive strength. The dynamic tests indicated changes in modulus and damping depending on the mixture of rock, TDA, and epoxy. One proposed use of RBB is to affix the RBB to the bottom of concrete ties to modify the interaction between the tie and the ballast material and improve ballast durability and modify resilience and damping. Box tests were conducted on a section of concrete tie with and without RBB attached. The tie with a section of rail attached was vertically loaded with a sinusoidal load to model repetitive axle loading. Observations indicated that the box test that used a concrete tie without RBB produced more ballast breakage compared with the test that used a concrete tie with RBB; however, more abrasion between particles occurred with the RBB-bound tie.

INVESTIGATION AND MITIGATION OF DIFFERENTIAL MOVEMENT AT RAILWAY TRANSITIONS FOR US HIGH SPEED PASSENGER RAIL AND JOINT PASSENGER/FREIGHT CORRIDORS

As with most highway bridges, railway transitions experience differential movements due to differences in track system stiffness, track damping characteristics, foundation type, ballast settlement from fouling and/or degradation, as well as fill and subgrade settlement. This differential movement is especially problematic for high speed rail infrastructure as the “bump” at the transition is accentuated at high speeds. Identification of different factors contributing towards this differential movement, as well as development of design and maintenance strategies to mitigate the problem is imperative for the safe and economical operation of both freight and passenger rail networks. This paper presents the research framework and preliminary findings from a recently initiated research effort at the University of Illinois at Urbana-Champaign. Aimed at developing design and repair techniques to mitigate differential movement at railway transitions, this research project involves instrumentation, performance monitoring and numerical modeling of new and existing track transitions.

Ground Acceleration Based Characterization of High-Speed Rail Track Bed

A new ground acceleration based method of evaluating High Speed Rail (HSR) track bed integrity has been developed. The purpose of the implementation of the system is to provide an efficient means for monitoring HSR track to predict settlement and deformation conditions that would require maintenance. The method relies upon the use of an accelerometer array that can be installed in the substructure of the track, i.e., within the ballast, subballast and subgrade layers. The array can be installed on a permanent basis and monitored periodically to identify changes in the small strain elastic properties of the substructure layers. The methodology utilizes conventional downhole seismic shear wave testing. The accelerometers are used in concert to simultaneously evaluate the arrival of seismic shear waves produced by surface initiated impact methods. The methodology was successful in measuring initial shear wave velocities. The shear wave velocities were used to calculate shear moduli and elastic moduli for the ballast, subballast and subgrade layers. Shear moduli were in the range of 14×10 3 kPa for the fouled ballast to 118×10 3 kPa for the subgrade. Elastic moduli were in the range of 39×10 3 kPa for the fouled ballast to 320×10 3 kPa for the subgrade. These values were compared with values that were measured separately at the site on similar soils and were found to be consistent. The monitoring can be done on a regular schedule requiring no interruption of track operations. The methodology has other applications including highway pavements, airport pavements and tunnels/underground facilities.

Spectral Analysis of Ground Acceleration-Based Testing

Spectral analysis of downhole acceleration data is used to identify changes in elastic properties of track substructure. The spectral analysis is based on the power spectral density (PSD) as an indicator of frequency content. This method was developed and tested at the Transportation Technology Center, Inc., in Pueblo, Colorado. The system provides an efficient means for monitoring track bed to predict settlement and deformation conditions that would require maintenance. The method relies on a triaxial piezoelectric accelerometer inserted into a permanently installed inclinometer casing installed into the subgrade. Periodic measurement of the dynamic ground response allows for the identification of changes in the small strain elastic properties of the substructure layers. The methodology uses conventional downhole seismic shear wave velocity testing. For the purpose of comparison, test sections were constructed with clean and fouled ballast. Shear wave velocities indicated the differences in material properties. PSDs of the signals were analyzed to determine variation resulting from differences in soil conditions. PSDs were calculated at different intervals of loading. With increased loading, settlement of the subtrack soils occurs. The densifi-cation can be seen as an increase in the predominant frequency of the PSD. The PSD from the section with the clean ballast had a higher predominant frequency than the PSD from the section with the fouled ballast. A decrease in predominant frequency over time would indicate the decrease in stiffness and the potential of increased fouling of the ballast with fines. Changes in the frequency content would indicate changes in elastic response.

Deformation and dynamic load amplification trends at railroad bridge approaches: Effects caused by high-speed passenger trains

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.

Time and Spatial Dependence of the Fouling of Railroad Track Ballast

With the increased use of heavy axle load (HAL) trains, the improvement of railways is more important than ever. Because maintenance costs are so high, any way to reduce the amount of fouling that occurs—or to better predict when maintenance is necessary—and thus reduce the amount of maintenance required, would be financially beneficial. The problem however is that because there are so many variables that exist between railroads, it is difficult to control these variables in the field in a way that would allow experiments to be run that will further our knowledge of ballast fouling and degradation. In the early 1990s, samples were taken from a section of rail in the hopes of understanding the nature of how fouling occurs. The samples of the ballast were taken over the course of three years, and sieve analyses were run on these samples. Sampling was done with location as a major consideration, retrieving samples from specific locations in the ballast cross-section. Using the grain size distribution data, the fouling was quantified using the conventional Fouling Index (percent of particles, by mass, passing the 3/8” sieve). Single-factor analysis of variance tests were designed and run using these data to determine how certain variables related to the fouling of ballast. Parameters of interest were time after placement and location of sample. It was statistically determined that ballast fouling increased over time, and that the fouling was more prevalent in the lower depths of the ballast layer, as well as in the tamping zones.

Stone blowing as a remedial measure to mitigate differential movement problems at railroad bridge approaches

Railroad track transitions such as bridge approaches often experience recurrent track geometry problems due to differential settlement between the bridge and the adjacent track. The resulting “bump at the end of the bridge” leads to significant passenger discomfort and causes rapid deterioration of the track as well as vehicular components. In general, railroad managers address recurrent track geometry defects through track resurfacing methods, such as tamping that involve raising the track through mechanically induced vibration and rearrangement of particles within the ballast layer. Although widely used for track resurfacing, the tamping process tends to destabilize the ballast layer, and the track may rapidly return to its former deteriorated state based on the traffic flow. The method of “stone blowing,” on the other hand, which was developed as an alternative to tamping, relies on the principle of injecting fresh ballast particles into gaps underneath ties and raising the track level rather than disturbing the packing condition of the existing ballast. In a recently completed research study in the United States, stone blowing was successfully implemented as a remedial measure to mitigate the problem of differential movement at a problematic bridge approach along Amtrak’s Northeast Corridor. Advanced geotechnical instrumentation was used to monitor transient deformations within individual track substructure layers before and after stone blowing. Moreover, tie support conditions and track geometry data were also analyzed to quantify the effectiveness of stone blowing on the improvement of track performance.