Year

2015

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Due to the recent utilisation of faster and heavier trains, traditional railways consisting of granular media overlying a subgrade or natural formulation have become increasingly overstressed; this may lead to their excessive deformation and degradation and could result in more frequent and increasingly expensive track maintenance. On this basis, an assessment of the stress-strain and degradation of ballast under traffic loading is of paramount significance. Over the past decade, a number of constitutive models that address the behaviour of a wide range of engineering materials under different loading conditions have been developed, and there has also been a rapid expansion in the application of general purpose geotechnical software by practicing engineers, especially for nonlinear analysis. Thus, it is important to develop a reliable and versatile constitutive model for granular soils such as ballast and to incorporate it into finite element analysis.

Large scale triaxial tests have been conducted to evaluate the permanent deformation, stiffness, and degradation of ballast subjected to monotonic and high frequency cyclic loads. Different confining pressures were applied during monotonic testing which revealed that particle breakage has a profound effect on the critical state, the peak state, hardening, and the stress-dilatancy parameters of ballast. In these cyclic loading tests various frequencies, deviator stress magnitudes and confining pressures were utilised to study the combined effect of these load factors on the deformation and degradation of ballast. It was observed that different deformation mechanisms exist for ballast, namely, elastic shakedown, plastic shakedown, ratcheting, and plastic collapse.

An elasto-plastic constitutive model based on the critical state soil mechanics framework was presented to capture the stress-strain behaviour and degradation of ballast and constitutive parameters were determined from large-scale laboratory tests. The model could predict the monotonic shear behaviour of ballast that corroborated with laboratory measurements, after which it was extended to capture important aspects of cyclic loading such as the load frequency and predict plastic deformation under repeated traffic loading.

A numerical integration of the ballast model was executed by using a fully implicit Euler-backward algorithm based on an elastic stress predictor. The tangent operator was derived by consistent linearisation of the updated stress. The finite element model was validated by analyzing the triaxial specimen with a 2-D axisymmetric approach, from which it was found that the stress-strain response from the simulation and experiment matched each other to an acceptable degree. A finite element case study was then carried out where the settlement of a railway track during the passage of a train was simulated.

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