Rayleigh backscatter fiber optic sensing permits dynamic strains to be measured along an optical fiber with a gauge spacing and temporal resolution sufficient for rail applications. However, this sensing technology is highly sensitive to vibration. A 7.5 m long section of rail was instrumented with optical fiber and strain measurements were recorded during passage of a freight train slowed to 8–11 km/h. This strategy to minimize rail vibration was successful in permitting distributed dynamic rail strains to be measured under freight car loading. The measured rail strains were used to determine the rail shear forces, which were then used with the static wheel loads to determine the rail seat load for 14 consecutive sleepers as the train passed over the field monitoring site. These data were then combined with measurements of dynamic rail displacement captured using digital image correlation to infer the rail seat load–deflection relationships for individual sleepers. These relationships were observed to provide significantly more detailed information about unsupported voids and the sleeper contact stiffnesses than the traditional consideration of the relationship between applied load and rail deflection and highlights how track behavior at a monitored location can be dependent on the conditions and behavior of neighbouring sleeper.
The measurement of track displacement during the passage of a train is an important parameter for the assessment of track condition. Digital image correlation (DIC) is a non-contact camera-based technology that can be used to measure these displacements. However, ground vibrations induced by the train can result in camera movement, adding error to the measured displacement. This paper presents a two-camera method that can account for the camera movement when measuring track displacements using DIC. The method is validated on a stationary track and then used to measure track displacement during the passage of two trains travelling at different velocities. The results of the two-camera method are then compared to the track displacements found using a low-pass filter. The two-camera method was found successfully to reduce error due to camera movement while removing the subjectivity of choosing a cut-off frequency for filtering.
Abstract:Railway tracks over peat subgrades can experience large ground deformations, increased pore-water pressures, formation of pumping holes, and pumping of fines during the passage of trains, which can lead to accelerated track deterioration and risk of derailment. One approach to mitigate these issues is to improve the subgrade stiffness using mass stabilization, which involves mixing a binding agent, such as cement, into a soil to improve its physical properties. This paper describes the development and use of a method to calculate trackbed modulus to quantify the improvement due to mass stabilization at a site with peat subgrade. Track modulus was calculated using in-service freight trains by measuring track displacements using digital image correlation and wheel loads from a nearby wheel impact load detector. Because of the voids that existed between the rail, sleepers, and ballast it was found that using displacements of the ballast crib to calculate the trackbed modulus, instead of the overall track modulus using rail or sleeper displacements, provided a way to quantify the improvement of the subgrade that was not affected by the presence of voids. The results indicate the post-rehabilitation trackbed modulus was double the original baseline value for the track section, indicating that mass stabilization can be an effective rehabilitation strategy to improve the stiffness of problematic peat subgrades.Key words: railway, track modulus, mass stabilization, peat, digital image correlation.Résumé : Les voies de chemin de fer par-dessus des fondations de tourbe peuvent subir de grandes déformations du sol, l'augmentation de pressions interstitielles, la formation d'orifices de pompage, et le pompage de fines pendant le passage des trains, ce qui peut conduire à l'accélération de la détérioration de la voie et le risque de déraillement. Une façon d'atténuer ces problèmes est d'améliorer la rigidité de la plate-forme en utilisant la stabilisation, qui implique le mélange d'un liant, comme le ciment, dans un sol pour améliorer ses propriétés physiques. Cet article décrit le développement et l'utilisation d'une méthode de calcul du module du tracé afin de quantifier l'amélioration due à la stabilisation de masse à un site avec des fondations de tourbe. Le module de la voie a été calculé à l'aide de trains de marchandises en service par la mesure de la corrélation d'images numériques à l'aide de déplacements et des charges de roues à proximité d'un détecteur de charge d'impact de roues. En raison des vides qui existent entre les rails, les traverses, et les ballasts, il a été constaté que l'utilisation des déplacements de la case de ballast pour calculer le module du tracé, au lieu de l'ensemble des modules des voies en utilisant les déplacements de traverses ou des voies, a fourni un moyen de quantifier l'amélioration de la fondation qui n'est pas affectée par la présence d'espaces vides. Les résultats indiquent que le module de la tracée suite à la réhabilitation était double de la valeur de référence initia...
Rail tracks on peat subgrades can experience significant deflections, some of which have led to derailments. A potential ground stabilization strategy is to use screw piles to reduce rail displacements; however, limited research has been undertaken to investigate the effect of these piles and their performance under cyclic train loading. A field site was instrumented and monitored before and after screw pile installation. The piles were instrumented with strain gauges, piezometers were installed in the peat, and high-speed cameras were used to measure track and subgrade displacements. The load carried by each instrumented pile was approximately 50–60 kN 3 weeks after installation, but reduced to approximately 30 kN after 6 months. Although 1 year’s worth of piezometric data were collected before installation of the piles, it is difficult to conclusively attribute the changes in pore pressures to the piles, particularly because the piezometric data collected after the pile installation were only available for the 7 months that exhibit the period of greatest seasonal fluctuations. The track support system deformations showed no significant difference pre- and post-pile installation. Therefore, based on the monitoring data at this site, it appears that the designed transfer of load from the ties to the piles through arching within the ballast layer was not realized.
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