Discrete element modelling of two-layered ballast in a box test
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ORIGINAL PAPER
Discrete element modelling of two‑layered ballast in a box test Huiqi Li1 · Glenn McDowell1 Received: 5 December 2019 © The Author(s) 2020
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. Keywords Discrete element modelling · Micro mechanics · Ballast mechanics · Railtrack engineering
1 Introduction Railway ballast generally comprises large, angular particles, typically in the size range 20–50 mm. The main functions of railway ballast are to transmit and distribute the load from the sleepers to the formation, to facilitate maintenance operations to ensure ride quality, and to provide rapid drainage [1]. As granular material, the plastic deformation of ballast caused by the cyclic traffic load results in permanent settlement and thus the deterioration of track geometry [2–4]. The reduction of ballast settlement could significantly reduce the maintenance cost for the railway industry. A number of modifications to the conventional ballast system have been proved to be effective in reducing the permanent settlement, such as adding geogrid reinforcement [5–8], attaching resilient pads (under sleeper pads) to the underneath of sleepers [9–12], and adding fibre reinforcement to the ballast [13, 14]. This paper aims to explore the possibility of reducing the ballast settlement by simply using layers of ballast with different grading without any additional reinforcement. * Huiqi Li [email protected] 1
Nottingham Centre for Geomechanics, University of Nottingham, Nottingham, UK
Anderson and Key [15] carried out a ‘box test’ using a two-layered ballast system primarily in relation to railway maintenance by stone blowing and found that raising the level of a sleeper by the addition of smaller stones can result in reduced settlements under repeated load. 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 invest
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