Research On The Constitutive Model For Unsaturated Expansive Soils

ABSTRACT:In expansive soil layer, projects of underground high-level radioactive waste disposal and geotechnical engineering, such as slope protection, channel excavation, uneven settlement of building foundation and etc., involve the deformation, strength, seepage and retention water behaviors of expansive unsaturated soil. As the object of study in this thesis, unsaturated expansive soils are other than low and moderate plasticity unsaturated soils. Many unsaturated expansive soils, such as natural and compacted clays and bentonite pellet mixtures, have a pore size distribution with at least two dominant values of porosity. Furthermore, the physiochemical effect between the solid matrix and adjacent water are obvious. Thus the property of pore water changes with pore size and location. The above two reasons lead to more complex mechanical and hydraulic behaviors of unsaturated expansive soil than that of the unsaturated soil. Thus, in terms of its specific characteristics of structure and deformation, there is a need to develop and improve the existing theory of unsaturated soil, which could also lay a theoretical foundation for engineering application.The paper mainly contains the following parts:(1) Based on the porous media theory and the essential laws of physics, the input work of unsaturated expansive soils are proposed. In the process of derivation, the pores are classified into macro-and micropores, meanwhile, the pore water is classified into capillary water and adsorbed water. Then several pairs of energy-conjugate variables could be determined, e.g. the macro-and microstructural strains, the macrostructural degree of saturations of capillary water and adsorbed water, the microstructural degree of saturation and their energy-conjugate stress variables.(2) According to the derived input work expression of unsaturated expansive soils and the work-energy-dissipation relations for an open multiphase thermodynamic system, a thermodynamics-based modelling framework for unsaturated expansive soils with double porosity is established. Based on the thermodynamics-based modelling framework, a double-scale constitutive model for isotropic stress state is proposed, involving coupling of hydraulic hysteresis and mechanical behaviors. The input work expression is simplified based on the some common hypothesis. There are three terms in the simplified expression, i.e. the work caused by the macrostructural deformation, the work caused by the change of macrostructural degree of saturation, the work caused by the change of microstructure. In the derivation process of the thermodynamics-based modelling framework, the internal variable is used to couple the three terms. The proposed model takes two factors into consideration:the interaction between the micro-and macro-structure of expansive soils and the fully coupled capillary hysteresis with mechanical behaviors.(3) A new binary-medium model for unsaturated expansive soils is proposed, involving a consideration of the effects of the capillary component and the adsorptive component of matric suction on the behaviors of unsaturated expansive soils. Most constitutive models for unsaturated soils are proposed on the basis of the capillarity mechanism, ignoring the contributions of the adsorption effect to mechanical and hydraulic behaviors. For expansive clays, however, the adsorption effect which leads to more complex behavioral characteristics than that in low plasticity clays cannot be ignored. The expansive clay is assumed as a mixture of two parts, i.e. an ideal capillarity part and an ideal adsorption part, and two ideal constitutive models are established for the ideal parts, respectively. Then a participation function is used to reflect the proportions of the ideal capillarity part and the ideal adsorption part in the actual situation. According to the thought of establishing the disturbed models, a method dealing with the physicochemical effect, i.e. the adsorptive component of matric suction, is proposed, which is extended by the existing unsaturated soil model based on the capillarity mechanism.(4) On the basis of the thermodynamics concept of stored energy, the mechanisms of the deformation behaviors of unsaturated expansive soils under suction cycles paths are discussed. Moreover, a new constitutive model for unsaturated expansive soil is proposed. The generation of the stored energy for soils is attributed to the deformability of the individual aggregate and mutual restrictions among the adjacent aggregates. By means of analysis, the plastic deformation can be decomposed into two parts, which are attributed to the structural rearrangements and the change of stored plastic work, respectively. And then the combined isotropic-kinematic hardening rule is used to consider the mechanism of plastic deformation. The proposed model can quantitatively predict the behaviour of unsaturated expansive soils under suction cycles.