Evaluating the impact of ballast undercutting on the roughness of track geometry over different subgrade conditions

Abstract

The progressive degradation of railway ballast is often cited as a primary factor that contributes to the development of track roughness, while ballast renewal (undercutting) attempts to manage its long-term development. Soft subgrades have been shown to strongly influence track geometry and are a contributing factor that has not been considered during conventional track maintenance. This study evaluated the impact of undercutting on long-term trends in track geometry roughness, and what impact softer subgrades had on the effectiveness of undercutting. A combined 6.90 km of Class II–IV heavy-haul track in Western Canada (undercut in 2010 and 2011) formed the basis for this analysis. Annual traffic on these sections typically totals 50 million gross tonnes. Long-term trends in the track crosslevel, alignment, and surface roughness after ballast renewal were derived from 50 track geometry surveys carried out over a five-year period (2010–2015). The results showed that undercutting significantly reduced track roughness over sand, silt, clay, or till subgrades; however, it was often ineffective when used over soft organic subgrades. Thus, while ballast degradation is the primary cause of track roughness in segments constructed on mineral subgrades, it is not a mechanism that results in track geometry roughness over soft organic soils.

The spatial correlation between track roughness and ground-penetrating radar inferred ballast degradation

“…Crosslevel roughness is not explicitly investigated here as previous studies have shown it to behave similarly to the track surface in response to changes in ballast degradation levels. 11 Track surface quantifies the vertical offset between a rail projected onto a vertical plane and the coplanar projection of a reference straight-line chord (with a specific length) at the mid-chord position (defined as a mid-chord offset or MCO). 3 Similarly, track alignment is an MCO measurement, only with the rail and chord projected onto a horizontal plane.…”

“…The standard deviation of individual track geometry variables has also been used to relate track roughness to conditions in the ballast. 7,11 However, both of these studies investigated track roughness within fixed sections of track. As this analysis requires a continuous representation of track roughness, the RR is considered the more appropriate metric.…”

“…Finally, the RR measurements presented in Figures 6 and 7 are the result of a single track geometry survey performed approximately two weeks before GPR data collection. Scanlan et al 11 demonstrated that longterm trends in track roughness are influenced by the amount of fines within the ballast. The individual track geometry survey measurements incorporated in Figures 6 and 7 provide no information on the long-term trends in track roughness and are strongly affected by recent track maintenance.…”

“…While it may be possible that significant spatial associations between ballast degradation and track alignment and surface roughness are rare, such a result would be significantly contrary to expectations. [1][2][3]7,11 As such, mitigating factors must be influencing the correlation results. Scanlan et al 11 observed that the relationship between relatively degraded ballast and increased track geometry variability is less prominent in areas of soft, organic subgrades.…”

“…[1][2][3]7,11 As such, mitigating factors must be influencing the correlation results. Scanlan et al 11 observed that the relationship between relatively degraded ballast and increased track geometry variability is less prominent in areas of soft, organic subgrades. While organic subgrades underlie portions of the subdivision analysed, they are not distributed widely enough to account for the approximately 92% of alignment sections of interest and 86% of surface sections of interest that yield no significant correlation coefficients.…”