Study On The Principle Of Generalized Effective Stress And Constitutive Modeling Of Unsaturated Soils

So far the theory basis of soil mechanics is still far from consummate and still in a semi-theoretical, semi-empirical stage. It is necessary to develop a unified theoretical basis for soil mechanics. The multiphase porous medium theory, developed from 1960s, can describe the complex interaction in each phase and their response under external pressure or in complex environment, which provides a theoretical basis for soil mechanics. An unsaturated soil is a three phase porous media, consisting of soil skeleton and pores, with water and gas filled in the pores. The existence of gas in the soil makes its properties much more complex than saturated soils. Owing to the complexity of unsaturated soils, it is difficult to predict the deformation and strength behaviour of unsaturated soil uniquely. In this thesis, based on the multiphase porous media theory, the principle of generalized effective stress is derived, and the constitutive models of unsaturated soils can be further developed. The following conclusions can be drawn:(1) Based on the multiphase porous media theory and thermodynamics, the work and energy balance equations for unsaturated soils by using the geotechnical terminologies are derived. Then a principle of generalized effective stress for unsaturated soils is proposed, which makes preparation for constructing the constitutive models.(2) The free energy and the dissipation equation can be derived from the work expression. Based on the principle of generalized effective stress, the constitutive equations for unsaturated soils can be established using the thermomechanical method. The choice of internal state variables is important to build the relationship among the three phases. A solid-liquid-gas coupled constitutive modeling framework is established, which can consider many complex situations, for example the influence of gas dissipation on soil behaviour.(3) Based on the constitutive framework of unsaturated soils, a three phase coupled constitutive equations in triaxial stress state is presented with some reasonable assumption and simplification. The constitutive equations can be easy to apply in practice. The model is then used to predict the exiting experimental data, results showing the model agrees well with the test points in both low degree of saturation and high degree of saturation situation.(4) Studying on SFG model, it is found that the model could not consider the influence of degree of saturation on the soil behaviour. Using the principle of generalized effective stress deduced from the work expression, a modified model is proposed which can take the degree of saturation into account without increasing any more parameters. The modified model is then compared with SFG model, and model predictions are also given to simulate the existing experimental results.