The results of a Federal Railroad Administration research project into the factors that contribute to differential displacements at railroad track transitions are presented in this paper. Data from instrumented high-speed passenger (Amtrak) sites suggest that poorly supported ties increase the loads applied on the underlying ballast and can accelerate differential displacements. Poorly supported ties amplify the tie-ballast interaction, which eventually results in large permanent vertical displacements at those locations. This paper presents the location and depth at which permanent vertical displacements are occurring, the ''root cause'' of these permanent differential vertical displacements, and design and remedial measures that focus on reducing poorly supported ties in 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.
Ballast fouling is a problematic track condition that can lead to inadequate ballast performance. Prioritizing remediation of fouled ballast sites is difficult because no relationship between ballast fouling and track performance exists and fouled ballast performance depends on the amount, grain-size, type, plasticity, and moisture content of the fouling material. This paper provides results of an international industry survey on fouled ballast definitions, parameters, limits/standards, and laboratory test results to aid development of a procedure for quantitatively assessing ballast fouling and assessing the ability to: transmit applied train loads to the subgrade, allow drainage, and maintain proper track geometry as required under §213.103.
Ballast characterization is important for predicting ballast performance and serviceability in track. This paper investigates ballast sampling methods and sizes, reconstitution, splitting, and gradation testing for appropriate characterization of in-track ballast. This paper will review various ASTM test methods and determine how these procedures should be applied to railroad ballast sampling and testing. For example, ASTM D75/75M, Standard Practice for Sampling Aggregates, and ASTM C136/136M, Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates, require sampling ballast with a maximum particle diameter of 75 mm (3 in.) and about 16 to 17 three-quarters full 5-gal buckets to meet the requirements. These methods are important because of the difficulty of obtaining representative in-field ballast samples. Undisturbed ballast samples are not feasible because of the nature of unbonded granular particles, so sampling emphasis must be placed on best methods to collect and reconstitute field ballast conditions in the laboratory. Another aspect that will be discussed is how to deal with local variations in ballast fouling and degradation that can result in significant differences in ballast characteristics. For example, the ballast underneath the tie (i.e., the ballast of interest) will likely be more degraded and fouled than shoulder or crib ballast, but this is also the most difficult location to obtain without track disturbance.
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