Coupling of mechanical behaviour and water retention behaviour in unsaturated soils.
PhD thesis, University of Glasgow.
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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.
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