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GL0143- -
2004, A. Grammatikopoulou, PhD thesis, Imperial College London

Abstract:

The pre-failure behaviour of overconsolidated clays is now well known to be highly non-linear and inelastic. Conventional constitutive soil models fail to predict this observed behaviour and recently a family of elasto-plastic soil models, the kinematic hardening models, has been developed to overcome these limitations. The kinematic hardening models allow for plasticity and non-linearity to be invoked within the conventionally defined yield surface, through the introduction of kinematic surfaces. The aim of this thesis is to improve the predictions of the behaviour of overconsolidated clays, particularly the pre-failure behaviour, through the study of elasto-plastic constitutive models based on this concept. Two existing kinematic hardening models, formulated within the framework of critical state soil mechanics, were chosen to be implemented into the Imperial College Finite Element Program (ICFEP), the two-surface model developed by Al-Tabbaa (1987), and the three-surface model developed by Stallebrass (1990). Both models were generalised in order to make their implementation into a finite element code possible. Although these models form a substantial improvement in modelling the behaviour of overconsolidated clays they cannot predict a smooth transition from elastic to elasto-plastic behaviour. This feature of behaviour, which proves to be an important drawback when simulating pre-failure non-linearity, was improved by changing the hardening modulus of each model and in this way two new generalised models were formulated. The implementation of the four models into ICFEP was validated through a number of single finite element analyses. The performance of the models was then evaluated through simulation of a series of laboratory tests on overconsolidated clays and comparison of the predictions with experimental data, where possible. The models were further used in the finite element analyses of two boundary value problems. The first of these modelled an embankment founded on a soft clay deposit. The second boundary value problem involved the analyses of tunnels excavated within the heavily overconsolidated London Clay. An analysis of the twin tunnels beneath St. James?s Park, London, constructed as part of the Jubilee Line Extension of the London Underground network, is also presented. The predictions of the models were compared with field data, where available.

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