Theodore R. Sussmann

Fundamental Nonlinear Track Load-Deflection Behavior for Condition Evaluation

Locations of rapid track condition deterioration are typically well known to railway track engineers, although the cause and methods of improvement may be unclear. The rapid deterioration of these locations can be due to many factors such as poor conditions of the components (rail, tie, and ballast) or failure of any of the components or subgrade. The exact cause of the problem is generally unknown as attempts to maintain the track in acceptable condition (by surfacing or undercutting) are implemented. In conjunction with track geometry measurements, which help to identify poorly performing track locations, track load-deflection behavior can contribute to the knowledge of the cause of the problem, with an ultimate goal being identification of a cost-effective, long-term solution to improving the performance of the track. Fundamentals of track load-deflection behavior, how track behavior relates to performance, indices that can be used to determine suitable maintenance strategies, and the relation of these indices to actual track performance all contribute to the evaluation. As indications of the track load-deflection behavior, several track stiffness measurement techniques that have been proposed and tested are described. The goal is to clarify the need to measure track stiffness and to identify rational means of relating the data to track condition and, ultimately, to ensure maintenance, renewal, and safety.

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.

Source of Ballast Fouling and Influence Considerations for Condition Assessment Criteria

Railway ballast is a critical element in the railway track support structure. The ballast is often overlooked when inspection tools are developed for track. When ballast is not functioning correctly, the strength of the track structure may be inadequate and thus compromise track stability. Track stability–related failures vary from rapid deterioration with little warning to slow and progressive deterioration with often predictable required maintenance. Ballast-related deterioration is progressive and usually provides visual evidence to warn maintenance personnel of needed rehabilitation. However, the blocked drainage that develops with fouled ballast can result in a saturated roadbed that is not stable and could rapidly deteriorate to an unsafe condition with little warning. Although massive failures are rare, if a side hill fill or embankment deteriorates to the point of becoming susceptible to massive failure, then the challenge becomes evaluation. More detailed knowledge of the track support condition will be needed for a thorough evaluation than can be provided by current track inspections, except for costly detailed visual inspections. The current standard of practice for ballast inspection and maintenance can be improved to reduce the risk of sudden failure. Much of the required technology, knowledge, and resources is already available and being utilized under the current system. A more precise evaluation of ballast condition is essential to identify thresholds related to unsafe track support conditions and to support effective maintenance plans.

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.

Development of material properties for railway application of ground-penetrating radar

A research project is being conducted to identify methods of using ground penetrating radar (GPR) to improve railway track condition assessment and enhance track inspections and safety. The safety of passing traffic can be improved if better indicators of problematic track conditions can be developed and utilized to better inspect the track for safety and to guide maintenance. The research effort has included evaluation of data collection techniques including testing a variety of GPR systems, identification of data interpretation techniques, and comparison of GPR data to track condition information. One limitation that has been identified is a lack of information on the electrical properties of track materials. Although data from geologic materials is well documented, the specific characteristics of railway track materials are different. For example, granite is documented widely as an intact rock mass and is used as ballast for track. However, in the railway ballast application, granite is used as crushed stone. During this research, tests were conducted to measure the dielectric permittivity of a variety of track materials to verify and supplement field measurements and to provide reference data for data interpretation. This paper describes the research project and the results of the testing and analysis.

Evaluation of Tie Support at Transition Zones

This paper discusses two instrumentation techniques, linear variable differential transformers (LVDTs) and accelerometers, used to monitor and evaluate track structure behavior with the goal of nondestructively and quickly identifying track structural problems that eventually cause track geometry problems. LVDT results at a poorly performing bridge approach and corresponding open track site are used to show a relationship between poor tie support and the observed permanent vertical displacements. The existence of a gap between the bottom of the tie and the top of the ballast is expected to increase permanent ballast vertical displacements because of increased loads and vibration applied to the underlying ballast. Similarly, accelerometers show larger peak tie accelerations at ties with tie–ballast gaps and suggest that poor tie support increases applied loads to underlying ballast. Collected field data show that the tie–ballast gap can increase with time, which results in progressive loss of tie support at that tie and an increasing load on adjacent ties because of redistribution of wheel loads. The results show the need for a nondestructive monitoring system to be used with existing track geometry detection systems to improve identification of poorly supported ties. This system will guide maintenance to reduce the gap, because even a small gap can decrease tie and ballast performance and thus require remediation of a track section rather than a single tie.

Examination of the effect of concrete crosstie rail seat deterioration on rail seat load distribution

One of the more critical failure modes of concrete crossties in North America is the degradation of the concrete surface at the crosstie rail seat, also known as rail seat deterioration (RSD). Loss of material beneath the rail can lead to wide gage, cant deficiency, reduced clamping force of the fastening system, and an increased risk of rail rollover. Previous research conducted at the University of Illinois at Urbana–Champaign (UIUC) identified five primary failure mechanisms associated with RSD: abrasion, crushing, freeze–thaw damage, hydroabrasive erosion, and hydraulic pressure cracking. Because the magnitude and distribution of load applied to the rail seat affects four of these five failure mechanisms, effectively addressing RSD requires an understanding of the factors affecting rail seat load distribution. As part of a larger study aimed at improving concrete crossties and fastening systems, UIUC researchers are attempting to characterize the loading environment at the rail seat by using matrix-based tactile surface sensors (MBTSS). This instrumentation technology has been implemented in both laboratory and field environments and has provided valuable insight into the distribution of a single load over consecutive crossties. This paper focuses on the analysis of data gathered from MBTSS experiments designed to explore the effect of manufactured RSD on the load distribution and pressure magnitude at the rail seat. The knowledge gained from these experiments will be integrated with associated research conducted at UIUC to form the framework for a mechanistic design approach for concrete crossties and fastening systems.

Estimation of rail bending stress from real-time vertical track deflection measurement

High traffic volume, heavy axle loads, and high train speed can produce large rail bending stresses which contribute to increased track deterioration. Rail stress problems are further exacerbated by poor support conditions such as abrupt changes in vertical track modulus and poor track geometry. This paper summarizes the development of a measurement technique, based on a system being developed over the past few years at the University of Nebraska and sponsored by the Federal Railroad Administration, to determine the actual bending stress in the rail in real-time from a car moving at revenue speeds. The UNL system measures the rail height relative to the line created by the wheel/rail contact points. The system functions continuously over long distances and in revenue service. The system establishes three points of the rail shape beneath the loaded wheels and over a distance of ten feet. These points include the location of high bending stress below the loaded wheels. This direct measurement of the rail shape can then be mapped into rail stress through the curvature of the rail and beam theory. As verification of the UNL measurement system, results from tests conducted on the Union Pacific Railroad’s Yoder Subdivision are discussed. In these tests, bondable resistance strain gages were mounted to the lower flange of the rail at several locations. The track was then loaded by spotting the measurement car over the strain gages and by moving the car over the gages at various speeds. The loaded and unloaded rail profiles were measured using surveying equipment and the relationship between the UNL deflection measurement and the measured rail stress was explored. These early results suggest the UNL system is capable of measuring real-time bending stress in the rail.Copyright

INFLUENCE OF TRACK MAINTENANCE ON LATERAL RESISTANCE OF CONCRETE-TIE TRACK

Adequate lateral resistance is required to provide the stable track structure necessary for safe rail operations on passenger and freight railroad track. Insufficient lateral resistance, coupled with a large thermal compression force in the rail from high rail temperature, can buckle the track structure. Railroads typically use mechanical stabilization, slow orders, or both, following maintenance operations that disturb the ballast section, such as track surfacing and alignment. Tests were conducted to improve the understanding of lateral resistance variations on concretetie track caused by surfacing and subsequent stabilization or compaction. Factors influencing track stability are summarized, maintenance procedures are described, the single-tie push test is described, and test results are presented. Tests were conducted to evaluate the changes in lateral resistance, from the trafficked, well-consolidated track structure before surfacing and alignment through the laterally weak track structure after surfacing. The influence of stabilization on the lateral resistance of the track structure was evaluated. The tests results indicate that surfacing significantly reduces the lateral stability of the track to a potentially critical level. Mechanical stabilization following surfacing significantly increased the lateral stability of all sections tested.

Resilient Modulus Backcalculation Techniques for Track

The layer moduli of track substructure components are important factors in the evaluation of track condition and estimation of maintenance costs. Layer moduli can be estimated from in-situ tests such as the cone penetration test (CPT); however, for layers of open graded aggregate, such as railway ballast with large individual particles, the results of the CPT are significantly influenced by individual particles and may not reflect the behavior of the mass. A method of evaluating layer properties using downhole measurement of layer deformation is presented in this paper. Tests were conducted on track using downhole measurements of layer deformation under both quasi-static test loads and passing traffic. The applied loads were measured by instrumenting the track superstructure. The top of rail load and substructure deformation data were used with a geotechnical model of track to backcalculate the moduli of individual layers. The process is similar to the procedure for processing data from the falling weight deflectometer; however, data from actual traffic can be evaluated. The results are compared to the CPT results from the same site and also to expected results for similar materials.