Quantifying the Impact of Subgrade Stiffness on Track Quality and the Development of Geometry Defects

Abstract

This paper presents the quantification of the impact of subgrade stiffness on the prevalence of track geometry defects and degradation of track quality indices (TQIs). The data included in this study come from two high-traffic subdivisions [>50 million gross tonnes (MGT)/year] in Canada with a total length of 800 km and consist of vertical track deflection (VTD) measurements and 3 years of track geometry measurements. The VTD measurements were used to derive two indices that represent the magnitude and variability of the subgrade stiffness. An analysis of the data shows that the locations at which defects occur correspond to locations with low modulus (higher VTD) and high variability of track modulus. A similar correlation is shown with track roughness represented by a TQI. However, the correlation with the spectrum of TQI calculated was found to be poor. This was attributed to maintenance activities carried out to improve track conditions. The correlation with the TQI greatly improved when arbitrary thresholds were applied and TQI values above this were treated as geometry defects. These results show that the locations that have a low modulus and high variability in the modulus are those that are the most difficult to maintain and at which maintenance is not always able to keep up with the degradation of track geometry. Thus, VTD measurements evaluate the underlying causes that result in the degradation of track conditions and allow for the identification of sections where poor track conditions are most likely to develop.