Abstract
It has been recently reported that ballast comprising differently graded layers helps to reduce track settlement. The main goal of this paper is to provide micro mechanical insight about how the differently layered ballasts reduce the settlement by employing DEM and thus propose an optimum design for two-layered ballast. The DEM simulations provide sufficient evidence that the two-layered ballast works by preventing particles from moving laterally through interlocking of the particles at the interface of the different layers in a similar way to geogrid. By plotting the lateral force acting on the side boundary as a function of the distance to the base, it is found that the walls in the region of 60–180 mm above the base always support the largest lateral forces and therefore this region might be the best location for an interface layer. However, considering the weak improvement in performance by increasing the thickness of the layer of scaled (small) ballast from 100 to 200 mm, it is suggested that it is best to use the sample comprising 100 mm scaled ballast on top of 200 mm standard ballast as the most cost effective solution.
Highlights
Railway ballast generally comprises large, angular particles, typically in the size range 20–50 mm
To find out what is occurring at a micro level for the two-layered ballast system and potentially propose an optimum design, discrete element modelling (DEM) is employed in this study to simulate the reported experiments [20]
It is found that the two-layered ballast decreases the average lateral force acting on the side boundary especially for the configurations in which the full size ballast is located underneath the sleeper, which is further evidence in addition to the coordination numbers that the two-layered ballast works by reducing particle lateral movement through interlocking of the particles at the interface between different layers
Summary
Railway ballast generally comprises large, angular particles, typically in the size range 20–50 mm. Key [16] designed a group of triaxial tests and box tests on a twolayered sample with various materials of size 14–20 mm on top of standard ballast to investigate how the track maintenance was affected by stone blowing. He found that the weaker layer of material controls the properties of the two-layered triaxial specimen but the (larger) ballast rather than the smaller material controls the deformation in a box test. To find out what is occurring at a micro level for the two-layered ballast system and potentially propose an optimum design, DEM is employed in this study to simulate the reported experiments [20]
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