Modelling Of Coupled Thermo-hydro-mechanical Behaviour With Mass Transfer In Unsaturated Soils: Environmental Geomechanics

In recent years, people pay more attention to the prevention of environmental contamination caused by industrial and household wastes. For the purpose of simplicity and economy, a kind of important geotechnical structure, the municipal barrier system is devised for disposing and landfilling the wastes. Clay barrier system is usually situated in the multi physical-chemical processes due to coupled thermo-hydro-mechanical effects including pollutant transport. It has been well known that the hydro-mechanical characteristics of barrier material are much sensitive to temperature. The variation of temperature also causes change of the stress and strain states in the clay barrier. The irreversible inelastic strains will occur as the stresses reach to critical yield surface, even result in the failure of the clay barrier. The chemical reactions among pollutant may not only generate the heat production but also the leaching liquid corroding the barrier material. Consequently, it is necessary and important to perform the long-term prediction to the coupled processes occurring in the whole clay barrier system. The main objective of the present thesis is to simulate the coupled thermo-hydro-mechanical behavior with pollutant transport in the clay barrier system in order to ensure the long-term effectiveness of the clay barrier in preventing the migration of pollutants with underground water/gas flows. The scientific and computational problems presented in numerical modelling of the coupled processes occurring in the barrier system is studied in detail.Modelled as the solid-liquid or the solid-liquid-gas mixture, saturated and unsaturated soils have non-homogeneous structures in space. A fundamental assumption is to model the real non-homogeneous multiphase system as a poro-continuum in the macroscopic level, in which each phase is assumed to fill up the entire domain. This means that at every point all phases are supposed present at the same time (overlapping continuum). Meanwhile, at this level we usually deal with homogeneous media. Thanks to the assumption, the theoretical frame of the continuum can be applied to porous media as two or three phase mixtures.A coupled thermo-hydro-mechanical model including possible pollutant transport in saturated or unsaturated porous media is proposed in Chapter 3 of the thesis on the basis of the related experimental results. The influences of temperature on hydrological and mechanical properties of the media are considered in the present model. The finite element solution procedure for the model is developed. Comparisons of the results obtained from the numerical simulations with existing experimental results validate the applicability and the reliability of the present model in modelling the coupled behaviour in the barrier system.It is important to fulfill the elasto-plastic analysis for the clay barrier to prevent it from damage and fracture. Based on the isothermal and non-isothermal experimental results obtained for unsaturated soils and the existing CAP model, a coupled constitutive model to describe the thermo-hycro-mechanical behaviour and its evolution is presented in this thesis. As far as author's knowledge, the present coupled constitutive model is the first one for describing thermo-hydro-mechanical behaviour in unsaturated soils.Six phenomena governing the miscible contamination transport in unsaturated soils, i.e. advection, molecular diffusion, mechanical dispersion, adsorption, degradation and immobileinwater effect are integrated into the present coupled numerical model as one component of the model. Because of the nature of non-self-adjoint operator due to the convection term in advection-difrusion equations governing the contaminant transport, if to use the standard Galerkin discretization procedure will cause the difficulties in the numerical solution of advection-difrusion equations. In addition, the long term numerical prediction to the contaminant transport in unsaturated soils presents the requirement to use the large time step size in th...