Identification of Railroad Ballast Fouling through Particle Movements

Railroad ballast fouling is a source of major distress that causes track instabilities and high maintenance and replacement costs. When ballast is fouled, the voids among the large aggregate become filled with fine particles that can block drainage and decrease the shear strength of the ballast layer. There have been numerous studies on ballast fouling since the early 1980s. However, to the authors’ knowledge, the effect of ballast fouling on particle movement and the associated impact on the dynamic train-track interaction, which provides better indication of track instability than ballast strength, have not been shown in the literature.

A recent railroad field test was conducted to investigate and visualize the movement patterns of ballast particles under different trackbed conditions. A control section with clean ballast and a section with mud pumping were chosen. These two sections are in close proximity on the same track so that traffic conditions are identical. SmartRocks (Liu et al. 2017), which are wireless sensor devices, were embedded in the crib, 10 cm beneath the ballast surface in each section to record particle movements under train passage. Fig. 1 shows the uncovered SmartRocks. SmartRocks can realistically sense, record, and transmit translation and rotation movements in real-time via Bluetooth. Three-dimensional printing technology is used to generate arbitrary shapes of a SmartRock that resemble realistic ballast particles. An appropriate printing material is used so that the contact stiffness, equivalent specific gravity, and strength of each SmartRock are close to those of other ballast particles.

An example of data recorded by SmartRocks from both sections under an AMTRAK train with an axle load of approximately 160 kN and speed of $115\mathrm{km}/\mathrm{h}$ is presented. Fig. 2(a) shows a comparison of the recorded vertical acceleration time history of the SmartRock in the clean section under dry conditions and for the mud-spot section under wet conditions. The wet mud-spot section experienced more intense vertical vibration than the dry clean section, which is because the lower shear strength in the wet mud-spot section is likely caused by a lack of particle-particle interlocking and buildup of excess pore pressure as manifested through mud pumping. Fig. 2(b) shows a comparison of SmartRock peak acceleration distribution over a one-week period for the clean and mud-spot sections under dry and wet conditions. The clean section under dry conditions has the highest peak and narrowest acceleration distribution corresponding to the best ballast performance, whereas the mud-spot section under wet conditions has the lowest peak and widest acceleration distribution corresponding to the worst performance. Fig. 2(b) also shows that the clean section under wet conditions and the mud-spot section under dry conditions have a similar performance.