Laboratory Evaluation of Naturally Abraded Ballast Behavior and Properties

Laboratory tests are commonly used to investigate the performance and behavior of ballast in various conditions. In large part, these investigations use freshly quarried rock prepared to meet gradation requirements of AREMA or similar organizations as the ballast material. When worn ballast is desired, some studies run the fresh ballast through an apparatus, such as the one used in LA Abrasion, to create an artificially worn ballast. Past investigations of the effects of angularity on the strength properties of granular materials provide mixed results. There is a clear need for further understanding of the behavior of abraded ballast. In this investigation, a naturally abraded ballast, sourced from an active rail, is used in laboratory testing to assess its mechanical properties and behavior. Based on gradation testing, this material was most likely an AREMA #4 graded material when it was first placed into the track bed. However, particle breakage that occurred during the time that it was used in track led to a broader gradation that included smaller ballast pieces. Triaxial testing was performed to determine the strength properties, stress-strain behavior, and volumetric strain behavior of clean ballast in varying moisture conditions. Box testing was used to investigate the settlement of the ballast under dynamic loading. The ballast in the box test was prepared to the same density as the triaxial tests so that the results are analogous. The results of the triaxial tests exhibited typical behavior, with the samples undergoing initial contraction followed by dilation. However, the results showed that the deformations were larger than might be expected from a similar angular ballast. Additionally, strengths do not appear to be significantly reduced compared to angular ballast. The box tests also showed typical results, though less total settlement occurred than might be expected. The unexpected results from these tests could be explained by the broader gradation of the abraded ballast. It has been shown that a wider range of particle sizes in a granular material increases the strength. It is likely that this holds true for this particular ballast, despite the increased level of abrasion. However, the more rounded particles are still less likely to interlock, resulting in the high deformations exhibited in the triaxial tests. The introduction of water to the tests does not have hugely negative effects on the strength, likely because the lack of fouling prevents large amounts of water from being held by the ballast. Abraded ballast appears to achieve the desired strengths needed for support railroad loads, however the increased deformability of the material makes it less than ideal for use in track.