Zakaria, Ideris Yielding of unsaturated soil. PhD thesis, University of Sheffield. Considerable efforts have been made in recent years to develop a better understanding of the mechanical behaviour of unsaturated soils in terms of elastoplastic critical state constitutive models. These models are defined in terms of four independent stress state variables : mean net stress, deviator stress, suction and specific volume. An important feature of the models is the suggestion of the existence of a yield surface in mean net stress : deviator stress : suction space. Suction-controlled triaxial tests were performed to investigate the shape of the yield surface for unsaturated compacted speswhite kaolin with a particular stress history.
Sivakumar, Vinayagamoorthy A critical state framework for unsaturated soil. PhD thesis, University of Sheffield. A critical state framework for unsaturated compacted clay is proposed. Five state variables are included: mean net stress, deviator stress, suction, specific volume and water content.