A new modelling approach for compacted clayey soils using specific water volume as a state variable

One of the key challenges of the present geotechnical engineering community is the accurate definition of unsaturated soil behaviour in routine engineering practice. This is because despite the remarkable progression of unsaturated soil mechanics as a branch of geotechnical engineering over the last few decades, the gap between unsaturated soils research and practice has widened significantly as the models to predict the soil behaviour have become more and more complex. Therefore, a simple, reliable and more importantly practical approach of defining the behaviour of unsaturated soils is vital in bridging this existing gap and to advance the modelling of unsaturated soil behaviour as a whole.
   
   By now, it is globally accepted that the definition of mechanical behaviour of unsaturated soils fundamentally requires two constitutive variables due to the inherent nature of complex behavioural patterns as a result of soil being unsaturated. Net stress and matric suction are the universally used constitutive variables in modelling unsaturated soil behaviour and on the other hand the number of attempts reported in adopting alternative constitutive variables is limited. Very recently, motivated by practical needs, Kodikara (2012) introduced a new concept for the unsaturated soil modelling known as Monash-Peradeniya-Kodikara (MPK) framework which utilises the net stress and moisture ratio as the constitutive variables. Based on this original concept, the aim of the present research is to introduce a new modelling approach for compacted unsaturated soils in net stress-specific water volume (moisture ratio) space which can bridge the existing gap in unsaturated soils research and practice.
   
   The research presented in this thesis was conducted under two major components, theoretical developments and experimental investigations. Under theoretical developments a mathematical basis for modelling the volumetric behaviour of compacted unsaturated soils in net stress-moisture ratio space was first formulated based on the qualitative framework presented in Kodikara (2012) and then the proposed volumetric framework was extended to deviatoric stress space devising a generalised elasto-plastic critical state framework linking volume change and shear strength behaviours of compacted unsaturated soils. Under experimental investigations, a comprehensive experimental programme comprised of targeted constant moisture content isotropic and triaxial compression tests was performed for the validation of the proposed critical state framework.
 
   The high level of agreement shown in between the laboratory data and the model predictions gave significant insight for the capability of the proposed model in defining the behaviour of compacted unsaturated soils in the net stress-moisture ratio space. With the promising results shown in the current research study, it is believed that further refinements of this primary version of the proposed framework presented in this thesis would better predict the mechanical behaviour of compacted unsaturated soils.
   
   The most prominent feature of the proposed model is the use of specific water volume as a constitutive variable in modelling unsaturated soil behaviour making it relatively simple, straightforward and feasible in engineering practice as moisture content is an easily measurable, commonly used parameter in field. In addition, with the use of specific water volume as a constitutive variable, hydro-mechanical coupling is readily available in the proposed framework which has been demanding for many other net stress-suction based models. Therefore, it is expected that modelling compacted unsaturated soil behaviour in net stress-specific water volume space could significantly simplify the application of unsaturated soil mechanics in routine engineering practice.