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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.

Models predicting the fate of water and dissolved chemicals in vegetated soils are required for a wide range of applications. Substantial uncertainty is present due to measurement errors, parametric uncertainty, and structural issues related to model con- ceptualisation. Due to the costs and intrusiveness of subsurface measurements there are limited datasets available to interrogate models against. Furthermore, the models are typically computationally intensive, making it di±cult to fully explore parametric and other uncertainty spaces. Hence there are two pressing needs which must be met to improve the utility of models: more data and constraints are required to quantify the impacts of uncertainty, and e±cient methodologies to explore sensitivities and uncer- tainties are also needed. This dissertation presents and applies a stochastic framework addressing the above concerns. Approaches and underlying assumptions to modelling water °ow and solute transport within soils and plants are examined, and two ex- isting models extended. The problem of uncertainty is investigated, and appropriate approaches suggested. Monte-Carlo techniques, including Markov chain Monte Carlo methods, are developed for application to the models, and tested using a comprehen- sive hydrological and radiological dataset from a plot-scale lysimeter experiment. The integrity of the experimental data is examined. Sensitivity analysis and calibration of the hydrological and radiological data sets is performed, with the ability of the model and framework to recover parameters interrogated. Structural uncertainty and e®ects of erroneous inputs are discussed. Results demonstrate the power of the methods to generate insights into process response and quantify uncertainties. The e±ciency of Markov Chain Monte Carlo techniques is demonstrated, but the advantages of retain- ing simple set search methodologies are also clear. Consideration of model structure also signi¯cantly reduces the uncertain parametric space. However, despite the unusu- ally comprehensive experimental dataset, major issues of uncertainty remain, of which data issues are a dominant component.

The stability of over consolidated fissured clays and clay shales is a central problem in Soil Mechanics. In this thesis the problem has been approached using field evidence; the actual failure surfaces of slips being analysed by the generalised stability method rather than the most critical circle or wedge for a failed slope being determined. development has resulted. A different emphasis on slip Case records of over 50 slips have been analysed in a spectrum of argillaceous sediments ranging from the Oligocene to the Carboniferous. The London Clay, Oxford Clay and Lias Clays provided the bulk of the data, with the support of several Cretaceous Clays, and mention is made of the Coal Measure shales, bentonitic shales and, shales weakened by flexural slip. Laboratory tests were undertaken. These included classification tests on nearly all slips, except those in London Clay, and several sets of direct shear tests on Oxford Clay, two of which illustrated the importance of directional (sedimentary) properties on the path to the residual strength. From the analysis the time effects on the stability of soils were obtained. Decay in strength with time was found to be predictable in the London Clay and, to a lesser extent, the Oxford Clay, although the Lias Clays were more erratic. In general, it was found that the initial failure of a slip was explainable with the average angle of friction equal to, or greater, than the laboratory peak value, and that the decay in strength - at least over periods of the order of 100 years - was a drop in the effective cohesion term only, ie. a softening process. This general trend may not apply where any form of progressive failure occurs. The modes of progressive failure do, however, appear to be very limited and deformation, due either to lateral expansion towards a cutting or simple swelling of susceptible layers, needs to be severely constrained in order to develop strains of sufficient magnitude to produce residual conditions. Some information on the average drop in strength due to movement of slips is given, and it has been found that, a slip will generally need to be reactivated once or twice to give average conditions near to the residual. An attempt has also been made to show the time development of progressive failure mechanisms in some London Clay slides. Generally good agreements were obtained between the residual angle of friction of reactivated slides and that found by conventional laboratory tests, e.g. direct shear or tests on cut planes, However, most slides take place at very much smaller values of the average effective normal stress than are used in laboratory tests and the implications of these two facts, combined with the possibility of curvature of the failure envelope, are mentioned. Occasional wide discrepancies between the field and laboratory values of the residual angle of friction were also encountered; these perhaps being due to bands of different clay minerals in the slope or excessive movements. In Section I of the thesis the generalised stability analysis is described and the effect on the results of several variables and assumptions is investigated. Comparisons are also made between this method and the other conventional (hand) methods of stability analysis.

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Previous research has shown coupling of mechanical behaviour and water retention behaviour in unsaturated soils at a constitutive level, with degree of saturation (in addition to suction) influencing mechanical behaviour and volumetric strains influencing water retention behaviour. An innovative elasto-plastic modelling framework incorporating coupling of mechanical behaviour and water retention behaviour has been proposed by Wheeler, Sharma and Buisson (2003) for isotropic stress states. These authors presented a single constitutive model for both mechanical behaviour and water retention behaviour. They did not, however, fully validate the model against experimental results. The objectives of the current research included undertaking an experimental programme specifically designed to investigate the coupling between mechanical behaviour and water retention behaviour, and using experimental results to investigate the validity of the Wheeler et al. (2003) model. Developments and refinement of the model were also to be explored An experimental programme of suction-controlled testing was carried out on one-dimensionally compacted samples of speswhite kaolin in a single steel-walled triaxial cell and an isotropic cell. In addition to standard stress paths, such as isotropic loading, unloading, wetting, drying and shearing, many non-standard stress path tests were also performed. These produced a unique data set, providing evidence of aspects of behaviour never previously studied. In terms of constitutive model development, the Wheeler et al. (2003) model was extended to triaxial stress states, to include the role of deviator stress. In addition, bounding surface plasticity concepts were used to develop simple realistic water retention models for rigid or deformable unsaturated soils, and a new bounding surface plasticity version of the Wheeler et al. (2003) model was developed. However, a problem of theoretical inconsistency in the Wheeler et al. (2003) model was identified, which occurs if plastic volumetric strains are predicted while the soil is fully saturated. A simple, but rather unsatisfactory, solution to this inconsistency was identified, and further research is required to identify a more satisfactory solution. Codes were developed for stress point simulations with the original version of the Wheeler et al. (2003) model, the new bounding surface plasticity version of the model and the conventional Barcelona Basic Model. Simulations were performed of all the experimental tests performed in the current research, to explore the performance of the different models. Comparison of model simulations with experimental results showed that the Wheeler et al. (2003) model was able to represent basic concepts of the mechanical behaviour of unsaturated soils, but sometimes not with the same level of accuracy or flexibility as the Barcelona Basic Model. The Wheeler et al. (2003) model was however able to capture features of mechanical and water retention behaviour that could not be represented by the Barcelona Basic Model or by other conventional models for mechanical or water retention behaviour. The bounding surface plasticity version of the Wheeler et al. (2003) model sometimes produced improved predictions. There remained, however, specific aspects of behaviour that were not well matched by either versions of the Wheeler et al. (2003) model. Some of these may be solved in the future by refinement of specific constitutive equations within the Wheeler et al. (2003) model, but others appear more likely to be insoluble without a major change to the proposed modelling framework.

Includes bibliographies / 1 v. (various pagings) : / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (D.E.)--University of Adelaide, 1991

Relic periglacial solifluction features cover most of Britain, particularly the Midlands and the South, causing serious and continuing earthwork and construction problems. Although the engineering significance of the presence of periglacial solifluction deposits is widely appreciated, the mechanics of emplacement of these deposits has received surprisingly little attention. The principal objective of the research has been to investigate the freeze-thaw behaviour of some clay soils in Britain which have been exposed to periglacial conditions. The behaviour was examined using a Permafrost Oedometer or Permode, based on the apparatus used by Morgenstern and Smith, (1973). Tests have been carried out on Lias Clay, Weald Clay and Oxford Clay. In each test, a specimen was placed under an applied stress and subjected to 24 hour cycles of freezing and thawing. Freezing was mainly imposed from the top of the specimen downwards. The tests were carried out undrained. The pore water pressures were found to increase as a result of cyclic freeze-thaw conditions. Greater pore water pressures were generally recorded at the top than at the base of the specimen. This was considered to be due to moisture migration towards the freezing front during freezing, and impeded filtration, (due to underlying frozen material), during thaw. The significance of the additional pore water pressures in terms of slope stability was considered. A semi-infinite slope analysis model was used primarily, but reference was made to other, similar research. The analysis found that the predicted slope angle of failure is reduced significantly by relatively low increases in pore water pressure. Greater increases in pore water pressure are gained for soils of lower residual shear strength/higher plasticity. It was determined that the generation of excess pore water pressure and corresponding decrease in residual strength have considerable implications for the stability of slopes previously exposed to periglacial conditions.