A Fully Coupled Hydromechanical Model For Unsaturated Soils And Its Numerical Simulations

Characterization of the coupled stress-strain, water flow and gas transport pro-cesses in gcomaterials is a fundamental issue for performance/safety assessment and optimization design in large-scale hydropowcr engineering, slope engineering and large scale underground space engineering, and in recent decades, there has been an increasing interest in the analysis of coupled hydro-mechanical (HM) phenomena in gcomaterials. In this thesis, a coupled solid-liquid-gas mathematical model for skeleton deformation, water flow and gas transport was developed for unsaturated soils based on the principles of the continuum mechanics and the averaging approach of the mixture theory. On the basis of soil’s microstructural observations, a constitutive model for fully coupled stress-strain and hydraulic hysteresis behaviors was proposed in a thermodynamic consistent framework. The proposed model was implemented in a finite element code, and was applied to modeling of coupled deformation, water flow and gas transport in a soil slope subjected to rain infiltration, and modeling of coupled processes of non-steady seepage flow and non-linear deformation in Shuibuya concrete-faced rockfill dam. The major achievements obtained in this study arc summarized as follows:(1) An elastoplastic constitutive model accounting for the inter-particle bonding effect was proposed for unsaturated soilStarting from the experimental observation that the inter-particle bonding effect induced by water menisci has a significant influence on the stress-strain behavior of unsaturated soils, an inter-particle bonding factor of rigorous physical meaning, defined as the bonding force per unit cross-sectional area that the force acts on, has been used as an independent variable in our model to represent the magnitude of equivalent bonding stress. Based on an empirical relationship between the bonding factor and the void ratio, a new loading-collapse yield function was proposed, and then a constitutive model in triaxial stress states was presented within the framework of Modified Cam-Clay Model. Compared with the classic Barcelona Basic Model (BBM), which contains two yield surfaces and twelve parameters, the proposed model only consists of one yield surface and eight parameters. All of the model parameters have clean physical meanings and can be obtained through standard triaxial and suction-controlled laboratory tests. Comparisons between experimental data and model results show that in most cases, the proposed model is not only able to reproduce the stress-strain behaviors that can be described by BBM, but also able to model the drying-induced elastoplastic deformation.(2) A deformation-dependent hysteretic water retention curve and hy-draulic conductivity model were proposed for unsaturated soilsIt has been well recognized that the deformation of soil skeleton and the change in pore size distribution (PSD) have a significant effect on water retention behavior of unsaturated soils. Although the PSD evolves rather complicatedly during deformation, experimental data showed that the overall shapes and the distribution characteristics of the PSD function are not significantly altered. Based on these findings, the PSD function at a deformed state was obtained by horizontal shifting and vertical scaling of the corresponding PSD function at a reference state. On this basis, a hysteretic water retention curve model was formulated to account for the influences of deformation and hysteresis on the variation of saturation. Since the effect of deformation on water retention curve is represented by the change in PSD, all of the model parameters have clear physical meanings and can be calibrated by standard laboratory tests. Fourteen experimental data sets were used to validate the proposed model, showing that the model can reasonably capture important features of retention properties for deformable soils under different loading paths and different stress states, including main wetting, main drying, drying-wetting cycles and scanning curves.On the other hand, the permeability of soils is also dependent on soil deformation and PSD evolution. Although Mualem statistical model is widely used to determine un-saturated hydraulic conductivity, Mualem model is not adequate for describing the un-saturated hydraulic conductivity in deformable soils. In this thesis, the original Mualem model was modified by incorporating a new variable defined as the mean radius of pores completely filled by water in a given degree of saturation to account for the effects of soil deformation on the unsaturated hydraulic conductivity. An unsaturated hydraulic conductivity model was then proposed by combining our water retention curve model and the modified Mualem model to account for the effect of deformation on permeability evolution. Experimental data of four types of soils were used to evaluate the modified model, which indicated that the modified model is able to describe the permeability for deformable soils with higher accuracy.(3) A therrnodynamic consistent model for fully coupled hydromechani-cal behaviors of unsaturated soils was proposedWithin the framework of therrnodynamic, a general therrnodynamic potential was defined for the three phases in unsaturated soils, and then the expression of dissipation energy for solid deformation, capillary hysteresis and water flow were derived. Based on the proposed models mentioned in (1) and (2), the constitutive model for fully cou-pled stress-strain and hydraulic hysteresis were proposed in a thermodynamie consistent framework, and the coupling mechanism between deformation and capillary hystere-sis was discussed in details. Laboratory tests with complex stress paths were used to evaluate the coupled model, showing that in most cases, the proposed model can rea-sonably capture the important features of coupled stress-strain and hydraulic hysteresis behaviors in unsaturated soils.(4) A numerical model for coupled solid deformation, water flow and gas transport in unsaturated soils was proposedA coupled solid-liquid-gas mathematical model for unsaturated soils was developed based on the principles of the continuum mechanics and the averaging approach of the mixture theory. A three dimensional computer code was developed, in which a fully implicit return-mapping algorithm was adopted to integrate the stress-strain and water retention relations. The computer code was validated by Liakopoulos’ draining test. On this basis, the coupled processes of unsaturated flow, gas transport and soil deformation were simulated with the proposed coupled solid-liquid-gas model, and the numerical results demonstrate the delaying effects and impacts of gas transport and soil deformation on the propagation of the wetting front and the evolution of slope stability.(5) Coupled analysis of non-steady seepage flow and non-linear deforma-tion processes in Shuibuya concrete-faced rockfill dam was performedAs for the non-steady seepage flow problem subjected to variable water head condi-tions, a new parabolic variation inequality (PVI) formulation mathematically equivalent to the PDE formulation was proposed, and the corresponding discretized FEM formu-lation was established and validated by laboratory tests. The proposed method was used to model the coupled processes of non-steady seepage flow and non-linear defor-mation in Shuibuya concrete-faced rockfill dam. Comparison of model predictions with in-situ measurements shows that the presented model has an acceptable performance for capturing the main features of non-steady seepage and non-linear deformation behav-iors, which provides a theoretical basis for dam impounding decision and safe operation management.