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Title: Advanced finite element modelling of coupled train-track systems : a geotechnical perspective
Author: Banimahd, Meysam
ISNI:       0000 0001 3442 5689
Awarding Body: Heriot-Watt University
Current Institution: Heriot-Watt University
Date of Award: 2008
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In this thesis, a series of models have been developed based on the finite element method to study coupled train-railway track systems. In particular a three dimensional dynamic model, which incorporates all track components such as multi-layered ground, sleepers and rails, has been developed. In this model, absorbing boundaries are implemented to prevent artificial wave reflection at the domain boundaries. Nonlinearity of the soil is taken into account by implementing several nonlinear constitutive models into the numerical code. The train is modelled as a combination of rigid masses, dashpots and springs. The train and track are coupled at the wheel-rail contact points, employing a nonlinear-contact theory that incorporates the rail surface irregularities and the possibility of wheel-rail separation. The dynamic problem of the coupled track-train system is solved in the time domain using a modified explicit integration technique. Using the developed three-dimensional model, the effects of train speed and soil nonlinearity are investigated in terms of the track displacement and the stress & vibration level in the sub-soil. Based on these studies, a modified design algorithm is suggested for high speed tracks to minimize railway track maintenance levels. The problem of stiffness transition near bridges and tunnels is also investigated, using the three-dimensional finite element model, in terms of applied load/stress on the track and passenger comfort. A yaiwtrack irregularity model, which is developed to compute the response of a train to ~ giv~~ track irregularity, is also employed to perform parametric studies on transition problems. Relationships between dynamic load amplification in the transition zone ~nd geometrical properties of the transitions, for different train speeds,· are accordingly given. The design principles for transition zones are critically reviewed against the outcome of transition studies conducted in this research, and modifications are suggested toward the sustainable design of transitions. With ever-increasing train speeds, heavier axle loads and mixed passenger-freight traffic, the track sub-soil is experiencing complicated stress paths at relatively high stress levels. Such a stress regime would generate considerable track settlement and even cause foundation failure in a worst-case scenario. The soil behaviour under such stress regimes can be represented only by complicated constitutive models which simulate cyclic, as well as monotonic, soil elastic-plastic behaviour appropriately. An example of such constitutive soil models, called ALTERNAT, is given. Unlike simple elastic-plastic models, ALTERNAT realistically accounts for stress dependency of the friction angle, strain softening-hardening and non-associativity. In order to develop a good understanding of railway track response affected by complex soil behaviour, the monotonic response of shallow foundations, which can represent a railway track, is extensively studied using a finite element code incorporating ALTERNAT. To determine the parameters in ALTERNAT, a calibration procedure is also developed based on an optimization technique, namely a micro genetic algorithm. The effect of footing size, shape, relative density and roughness on the ultimate bearing capacity and settlement characteristics are studied, and the computed results compare very favourably with the experimental trends. It is also shown that ALTERNAT can simulate the basic features of the cyclic response of granular soils, which occurs under train loading. It is therefore suggested that the ALTERNAT cyclic model and the three dimensional finite element model can be· used together towards the prediction ofthe long-term performance of railway tracks.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available