Year

2013

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

The ballast layer is responsible for distributing the applied wheel load to the subgrade soil and maintaining the track alignment. However, upon repeated load applications, the ballast due to its unbound nature deforms and degrades thereby significantly affecting the performance of the railway track. In this view, it is necessary to stabilise the ballasted rail tracks so that they can carry high-speed trains without creating any major track problem. In recent times, the use of geogrids to stabilise the ballast is on the rise. However, the effectiveness of reinforcement depends on the degree of ballast-geogrid interaction. Therefore, it is necessary to identify a suitable geogrid to stabilise the ballast. Also, it is imperative to investigate in detail the effect of geogrid on the deformation behaviour and degradation characteristics of ballast for typical loading and boundary conditions specific to railway tracks.

In this research, an experimental investigation using the large-scale direct shear apparatus was carried out to study the ballast-geogrid interface behavior and establish the effect of geogrid aperture size on the interface shear strength. A process simulation test (PST) apparatus to simulate the realistic behaviour of ballast was designed in this study. Following the design of apparatus, the influence of geogrid on the permanent deformation and degradation of ballast was assessed by conducting the model track tests. In addition, the study investigated the possible use of optical fiber Bragg grating (FBG) sensors in monitoring the railroad ballast deformations.

The large-scale direct shear tests reveal that the normalized geogrid aperture size (A/D50) has a profound influence on the shear strength of the ballast-geogrid interfaces. In this respect, the ratio A/D50 based on the variation of interface shear strength is categorized into three key zones: (a) Feeble Interlock Zone, with A/D50 < 0.95 (b) Optimum Interlock Zone, with 0.95 < A/D50 A/D50 < 2.50. The best geogrid aperture size to optimize the interface shear strength is determined to be 1.20D50 and (c) Diminishing Interlock Zone, with 1.20 < A/D50 < 2.50. The best geogrid aperture size to optimize the interface shear strength is determined to be 1.20D50. The minimum and maximum aperture sizes desired to attain the beneficial effects via geogrids are established as 0.95D50 and 2.50D50, respectively. The minimum and maximum aperture sizes desired to attain the beneficial effects via geogrids are established as 0.95D50 and 2.50D50, respectively.

While the ballast due to its discrete nature is known to undergo non-uniform lateral spreading, the existing state-of-the-art for laboratory testing does not permit such non-uniform lateral spread. Therefore, to realistically simulate the ballast behaviour under cyclic loading, the process simulation test (PST) apparatus available at the University of Wollongong was modified. The modification involved the replacement of the central portion of the side wall of the existing prismoidal chamber with a setup of five independent movable plates assembled along the depth. The free lateral movement of each individual plate under the applied loading is representative of the non-uniform lateral spreading of ballast under track operating conditions.

The model track tests reveal that the geogrid effectively arrests the lateral strains in ballast, thus reducing the extent of ballast settlement and minimizing the particle breakage. However, the effect of geogrid decreases with vertical distance from its placement position. Two new parameters, namely, the lateral spread reduction index (LSRI) and geogrid influence zone (GIZ) are proposed in the current study to assess the performance of geogrid-reinforced ballast. The GIZ is found to vary from 160 mm (4.60D50) to 225 mm (6.45D50) based on the geogrid placement position. The study reveals that the LSRI has a profound influence on the settlement and breakage of ballast with both ballast settlement and particle breakage exhibiting a significant reduction with the increase in average LSRI. The ideal geogrid placement location is determined to be a function of A/D50 ratio. The study further highlights the ability of FBG sensors to capture the deformations in ballast thereby encouraging their use in the monitoring of track stability under operating conditions.

The current study offers comprehensive understanding of the behavior of ballastgeogrid interfaces and their subsequent effect on the ballast behaviour with potential applications to rail track design. Moreover, it also benefits the rail industry in the form of reduced maintenance costs through enhanced track longevity.

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