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A physics based framework to predict ballast degradation

Published: 14 August 2017

Proposing a physics based framework to predict ballast degradation.

Sponsor: Trafikverket/JVTC

Researchers: Elahe Talebiahooie (PhD candidate), Matti Rantatalo (PL)

Duration: 2017-2022

Goal: A physics based framework to predict ballast degradation.

Project status and results:

 Ballast layer has an important role in transmission of stress from train passage on the rail to the formation and its rate of degradation affects the derailment risk which is controlled with maintenance of the track. This degradation is a function of parameters such as properties of the ballast particles, weather condition, sub ballast, subgrade and sub soil condition, loading condition which is a function of speed of the train and the axle load and some other parameters. Both the data driven methods and physics based methods used for prediction of ballast degradation. In this project physics based method selected to simulate ballast degradation.

Finite element method has been widely used to model the ballast domain, but with improves in the computational power, discrete element method is becoming more popular. Ballast is a granular domain and with DEM we can monitor micro-dynamics of the particles and non-homogeneous domain properties such as stiffness, strain and damping ratio.

In this project, PFC software (Itasca Consulting Group, n.d.) used to perform the simulations. In the first step the simulation results from previously published literature improved by changing the contact model from Linear Parallel Bond to Hysteretic model and its parameters. A sensitivity analyses showed us the applicability of 3 different damping formulations that can be found in the PFC software in the simulation. The results from this study soon to be published.

A physics based
Figure 1 Comparison between the vertical settlement result from Hysteretic contact model and results from simulations performed in (Chen et al. 2015) and modified experimental data from (Indraratna, Hussaini, and Vinod 2013).

In order to improve the knowledge about the physical phenomena of ballast degradation in terms of sleeper settlement and particle breakage, 3 tests ran in the Construction Lab of LTU with the same material, particle size distribution and loading profile in order to be used for the second set of simulations with PFC. In the experimental test the loading frequency set on 8 HZ based on the apparatus characteristics and in order to simulate the degradation during the 100,000 cycle of loading, the 2D simulation chose to perform the parameter study and improve the breakage simulation.

a physics 2
Figure 2 2D model of the experiments performed in LTU after preloading state with having zero bond breakage in the model.
a physics 3
Figure 3 2D model of the experiments performed in LTU after 800 cycles of loading which led to 7 broken bonds.

Chen, Cheng, Buddhima Indraratna, Glenn McDowell, and Cholachat Rujikiatkamjorn. 2015. “Discrete Element Modelling of Lateral Displacement of a Granular Assembly under Cyclic Loading.” Computers and Geotechnics 69: 474–84.

Indraratna, Buddhima, Syed Khaja Karimullah Hussaini, and J. S. Vinod. 2013. “The Lateral Displacement Response of Geogrid-Reinforced Ballast under Cyclic Loading.” Geotextiles and Geomembranes.

Itasca Consulting Group, Inc. n.d. “PFC — Particle Flow Code,.” Minneapolis: Itasca.