Year

2020

Degree Name

Master of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Railways play an essential role in transportation and economy in Australia; however, due to the

increasing demand for rail transport in recent years, railway infrastructure inevitably faces extensive degradation. One of the severe issues causing the degradation of rail tracks is mud pumping, where the fines intrude into the ballast layer to form slurry state under wet condition. Mud pumping is a complex process involving different mechanisms, including subgrade fluidisation, internal erosion, filtration and upward migrations of fines. This thesis focuses on the fluidisation of subgrade soil under increasing excess pore water pressure, which results in fines penetrating overlying ballast.

Traditional methods such as experimental and analytical approaches can capture the

macro-behaviours of soil such as soil settlement and hydraulic conductivity under increasing excess pore pressure; however, they have many limitations when microscopic and localised behaviour must be addressed. Therefore, this study proposed a numerical method that couples the Lattice Boltzmann Method (LBM) with the Discrete Element Method (DEM) to capture soil behaviours under increasing hydraulic gradient at both macro and micro scales. While particle behaviour is modelled using the DEM, the fluid properties can be depicted in greater detail based on the LBM. The numerical results are validated with experiments on a selected subgrade soil. The results show that the numerical method can reasonably predict the hydraulic and soil fluidisation aspects concerning the experimental data. Microscopic properties such as the localised fluid velocity through the porous spaces of the soil are also captured well by the proposed fluid-particle coupling approach. Also, the gas fluidisation is carried out in this study using LBM-DEM coupling to further validate the numerical method. The results are compared with the conventional CFD (Computational Fluid Dynamics) - DEM coupling and show a good agreement between LBM-DEM and CFD-DEM coupling.

FoR codes (2008)

090501 Civil Geotechnical Engineering

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.