Multi-field Coupled Model For Soils Based On Porous Media Theory And Its Application In Modelling The Constitutive Behaviour Of Unsaturated Soils

With the development of modern geotechnical engineering such as the construction of high level radioactive waste repositories, exploitation of natural gas hy-drate and geothermal resources, heat-supply pipelines and high-voltage cables burial in city constructions, research into the multi-field coupled problems in soils including stress, seepage, temperature and chemical effect has become an important issue interna-tionally. Under the effect of multi-field coupling, soil behaviors are different from those under a single field, which including the deformation of soil skeleton, heat transfer, pore-water seepage, advection and diffusion of pollutants in soil pores, the chemical reaction and phase change, etc. And how to determine the soil response under these coupled fields has become an urgent but difficult problem in geotechnical engineering. The study of soil behaviors under multi-fields is of great demand in many applications, e.g., civil engineering construction, geo-disaster prevention, diffusion and migration of pollutants, and high-efficient exploitation of new energies. After several decades're-search on multi-field coupling theory, great achievements have been made. Most of them are based on intuition, experience, or macroscopic recognition, and are lack of a united and consistent scientific theoretical basis. These empirical theories normally have their own limitations and often fail to describe the behaviors of soils under multi-field coupling conditions strictly and effectively. Therefore, the solutions cannot meet the needs of engineering practice. Especially in some modern geotechnical and geoenvi-ronmental engineerings such as high level radioactive waste repositories, the influences of temperature on the deformation-strength and seepage characteristics of unsaturated soils are very significant. However, the related studies are still in its infancy, so valuable research achievement is fewer and the theoretical system has not been established.In light of the thermodynamics-based porous media theory, a nonlinear multi-field coupled model is proposed in this paper. Following the balance equations, entropy in-equalities of the system and reasonable constitutive assumptions, rigorous theoretical derivation for the new model is presented. Based on the theoretical framework, a new mathematical model that describes the coupled skeleton deformation-fluid flow-heat transfer behavior in unsaturated soils is proposed via proper choices of free energy and dissipation function. In addition, a constitutive model that deals with the ther-mal-mechanical modeling of unsaturated soils is proposed based on simplification of the new coupled theoretical model and available experimental results. Temperature effects on volume change and seepage behavior of unsaturated soils is modeled and simulated by self-developed computer programs. Finally, a new temperature controlled triaxial test systerm for unsaturated soils has been developed to verify the proposed model. A series of SWCC tests are carried out on silt samples using the developed equipment at differ-ent temperatures, which show valuable results. The main achievements of this thesis are listed as follows:(1) The balance equations with three levels (constituents, phases and the whole mixture soil) are set up under the assumption that soil is composed of multi-constituent elastic-plastic solid skeleton, viscous liquid and ideal gas. With reasonable constitutive assumptions in such restrictive conditions as the principles of determinism, equipre-sence, material frame-indifference and the compatible principle in continuum mechanics, a theoretical framework of constitutive relations modeling three-phase soil in both non-equilibrium and equilibrium states is established, thus the closed field equations are formed. In the theoretical framework, the concept of effective generalized thermody-namic forces is introduced, and the nonlinear coupling constitutive relations between generalized dissipation forces and generalized flows within the system at nonequili-brium state are also presented. On such a basis, four special coupling relations, i.e., sol-id thermal elastic-plastic constitutive relation, liquid visco-elastic-plastic constitutive relation, the generalized Fourier's law, and the generalized Darcy's law are put forward. Based on a specific dissipation function, the concrete form of generalized Darcy's law is deduced; without considering temperature and other coupling effects, the nonlinear coupled model in this thesis can degenerate into a soil elastic-plastic constitutive model.(2) The coupled skeleton deformation-fluid flow-heat transfer behavior in unsatu-rated soils is very common physical phenomena and engineering problem. Based on the established theoretical framework, constitutive models and a new mathematical model that describe the coupled skeleton deformation-fluid flow-heat transfer behavior in un-saturated soils are proposed via proper choices of free energy and dissipation function. The difference between the proposed model and the existed models in literatures is the nonlinear constitutive equations that considering the effects from every physical field.(3) Based on the nonlinear multi-field coupled model for soils and by using exist-ing experimental results, an elastic-plastic constitutive model of unsaturated soils under non-isothermal conditions is developed. The model is used to predict and analyze the influence of suction and temperature on the deformation properties of unsaturated soils under isotropic conditions. The proposed model is successfully verified by the compar-isons between the model predictions using Fortran-based computer programs and ex-isted experimental results.(4) The thermodynamic based expression of suction for unsaturated soils is pre-sented. This expression can consider the temperature effects on suction for both surface tension and wetting coefficients. On contrast of expressions by experimental data fitting, its theoretical basis is more strict, and hence it has broader applicability. By adopting the van Genuchten expression of soil-water characteristic curve, a new model considering the temperature effects is established. On this basis, an indirect method for predicting permeability coefficients of unsaturated soils at different temperatures is presented. The new expressions are made for the entire suction range of SWCC; and hence it has a broader applicability. Based on the experiments of MX-80bentonite and loess, model predictions and the relative permeability coefficients at different temperatures with suc-tions are made.(5) A new temperature controlled triaxial test systerm for unsaturated soils is de-veloped, which can be used to study the temperature effect on seepage and strength-deformation characteristics of unsaturated soils. With the organic combination of the existing static GDS triaxial test systerm and a new temperature controlled pres-sure chamber, the new test systerm can realize better control of temperature. Using the new temperature controlled triaxial test systerm, the soil-water characteristic curves of silty clay obtained from a foundation ditch of Beijing under different temperatures are tested, which reveal the variation law of the SWCC due to temperatures.