Mechanical behaviour of unsaturated highly expansive clays
This thesis concerns the mechanical behaviour of unsaturated highly expansive clays. Given the high cost of damage to buildings, structures and roads caused by unexpected ground movements associated with unsaturated highly expansive clays and the increasing use of compacted expansive clays for engineered barriers for environmental protection and other purposes, it was considered important to investigate the behaviour of these materials. Previous researchers had reported the occurrence of substantial irreversible components of either wetting-induced swelling or drying-induced shrinkage during cycles of wetting and drying performed on unsaturated highly expansive clays containing active clay mineral such as montmorillonite. This form of irreversible behaviour cannot be represented by existing elasto-plastic models for unsaturated non-expansive clays. It had therefore been suggested that the behaviour of unsaturated highly expansive clays was fundamentally different to that of unsaturated non-expansive clays, and that the constitutive models developed for unsaturated non-expansive clays were inappropriate. The behaviour of an unsaturated highly expansive compacted bentonite/kaolin mix under isotropic stress states has been studied in a programme of controlled-suction tests incorporating isotropic loading and unloading at constant suction and cycles of wetting and drying (variation of suction) performed either under constant load or at constant volume. A smaller series of tests was also performed on unsaturated compacted samples of non-expansive pure kaolin. Either net shrinkage or net swelling occurred over a wetting/drying cycle for samples of the same soil, depending upon the compaction pressure. Irreversible shrinkage or swelling occurred during subsequent wetting/drying cycles, not just during the first cycle. Net shrinkage during a wetting/drying cycle was sometimes accompanied by a distinct change of stiffness (a yield point) during the appropriate drying path, but at other times was not. Yield points were never observed during both drying and wetting stages of a cycle. Net shrinkage or swelling during a wetting/drying cycle was always accompanied by a substantial net change of degree of saturation Sr during the cycle (termed hydraulic hysteresis). Experimental evidence showed that there is no fundamental difference between the mechanical behaviours of unsaturated highly expansive soils and unsaturated non-expansive soils. The experimental results suggested that unified modelling for both unsaturated highly expansive soils and unsaturated non-expansive soils is desirable. A new constitutive modelling framework is therefore proposed. In the modelling framework new forms of stress variables, with conjugate strain increment parameters, are used for the first time. Results from theoretical analysis of the influence of suction on inter-particle forces are used in developing the modelling framework. The influence of hydraulic hysteresis on stress strain behaviour is included in the framework, which has been developed in such a way that the otherwise difficult, transition between saturated and unsaturated conditions can be modelled in a relatively straightforward fashion. In the new modelling framework, both net shrinkage and net swelling during wetting/drying cycles, can be explained.