A quantitative evaluation of the impact of soft subgrades on railway track structure

  • Author / Creator
    Roghani, Alireza
  • The railway networks in Canada traverse from coast to coast, and pass over diverse terrain with large stretches of very soft soils, including clay deposits and peat formations. The majority of these lines were constructed approximately 100 years ago. Railway loads have since increased significantly, resulting in the imposing of higher loads on older infrastructure, particularly those constructed on soft soil foundations. This combination has resulted in the need to upgrade these lines to handle the increased loads and the expected increase of volume of traffic. The main challenge in upgrading these lines is the limited knowledge about the location of poor subgrades as the extent and relative stiffness of the foundation have not been mapped and documented over long distances. The lack of this information has also limited the studying of the influence of subgrade on track performance as well as quantifying the value obtained from the investment in improving track and substructure. An extensive trial with a newly developed rolling deflection technology, over 12,000 km of track, was conducted to assess its potential to map the variability in subgrade conditions over long distances. It was evident from the collected data that unprocessed deflection measurements are heavily affected by the track surface condition such as joints and geometry irregularities so as to obscure the deflections because of poor subgrade support. A methodology was developed to minimize the influence of the surface condition that occur at short wavelengths and show the variations in track deflections because of changes in subgrade conditions which occur at longer wavelengths. The comparison of the processed data at different subgrade and geology condition confirmed that they are consistent with field conditions and are representative of the subgrade conditions. Mapping the subgrade condition over extensive lengths of track presented the opportunity to investigate the impact of subgrade stiffness on the prevalence of track geometry defects and degradation of track quality indices (TQI). This investigation was consisted of the analysis of 800 km of subgrade data and track geometry measurement from two subdivisions from different physiographic region of Canada. The analysis showed that the geometry defects have a strong correlation with both subgrade condition and its variability whereas the TQI are only related to the variability of subgrade condition. These results showed that the locations that have a large deflection and a high variability in deflection are those that are difficult to maintain, and at which maintenance is not always able to keep up with the degradation of the track geometry. It also suggested the processed data from rolling deflection measurement systems provides an evaluation of the underlying causes that result in the degradation of track conditions and allow for the identification of sections where it most likely that maintenance will not always be able to keep up with degradation; even if maintenance has done so recently. A methodology was also developed for quantifying the effectiveness of different methods used to improve the railway track performance on soft subgrades. This methodology is comprised of quantifying the changes in track stiffness from before and after vertical track deflection (VTD) measurements, and the evaluation of the roughness of the track that has developed since the track upgrades. A project was discussed as a case study to explain the steps of this methodology. The result showed that replacement of joints with heavier continuously welded rail (CWR) can reduce the track deflection up to 60%. The results of replacing the suggested 600 mm of subballast with 300 mm of subballast and a geogrid showed no change in the performance of the track under the CWR.

  • Subjects / Keywords
  • Graduation date
    Spring 2017
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.