Material Instability Research And Coupled Chemo-Hydro-Mechanical Manifold Element Analysis For Porous Media

In the recent years, material instabilities and coupled physico-chemical nonlinear analysis of multiphase porous media has been one of research focuses in solid mechanics and engineering mechanics. Unlike simple mechanical analysis for single phase materials, this subject is a coupled problem and has comprehensive engineering applications, such as landslide related to the failure of saturated soils, fracture of the reservoir and land subsidence in petroleum extraction and design of engineering clay barrier etc.The emphases of this thesis focus on the following three aspects. Firstly, an anisotropic constitutive relation is introduced to the theoretical research of material instabilities for multiphase porous media under quasi-static and dynamic loading conditions. Secondly, numerical manifold element is presented for dynamic nonlinear analysis of saturated porous media. Thirdly, a chemo-hydro-mechanical constitutive model and the corresponding implicit integration algorithm are presented for numerical simulation of partially saturated porous media.Modelled as the solid-liquid or solid-liquid-gas mixture, saturated or partially saturated geomaterials have non-homogeneous structures in space. A fundamental assumption is to model the non-homogeneous multiphase system as a porous-continuum in the macroscopic level, in which each phase is assumed to fill up the whole domain. Based on this assumption, the theory frame of continuum can be applied to porous media as two-phase or three-phase mixtures.A great number of experiment investigations on geomaterials indicate that the failure mode of geomaterials is related to the anisotropic behavior. As a matter of fact, the anisotropic behavior is caused by the special structures of geomaterials, such as layering etc, or by stress growth. It is necessary to introduce anisotropic constitutive relations to material instability analysis of porous media. An anisotropic constitutive model developed for geomaterials is used to model the anisotropic mechanical behavior of the solid skeleton of saturated porous media. Conditions for static instability and dynamic instability (stationary discontinuity and flutter instability) of saturated porous media are derived based on strain rate discontinuous bifurcation analysis method. The effects of material parameters on material instability are investigated in detail by numerical computations. There is a difficulty in finite element analysis for u-p mixed formulation of saturated porous media. Lots of previous works showed that the interpolation function for the displacement and the pore pressure must fulfill the so-called Babuska-Brezzi stability criteria or the Zienkiewicz-Taylor patch test. Otherwise, oscillations will appear in pore pressure field. These criteria preclude the use of finite elements with the same order of interpolation, such as T3p3 triangles (3-noded linear triangle for displacement and 3-noded linear triangle for pressure). Based on the idea of the numerical manifold method, the manifold elements for dynamic nonlinear analysis of saturated porous media are developed. Numerical examples indicate that the proposed manifold elements achieve the goal of avoiding oscillations in pressure field and can be applied in nonlinear failure analysis of saturated porous media.It has been recognized from experiments that chemical concentration in pore fluid may have negative effect on the hydro-mechanical qualities of clayed soils. Chemical loading may cause elastic and elastoplastic deformation. Understanding of the chemical effect is essential for the design and analysis of civil engineering such as clay barriers and tunnels etc. A coupled chemo-hydro-mechanical constitutive model of partially porous media is developed on the basis of the existing hydro-mechanical constitutive model and chemo-plastic constitutive model. Based on the definition of implicit Euler backward integration scheme for standard plasticity, an implicit integration algorithm and the consistent tangent moduli are presented for the chemo-hydro-mechanical constitutive model. The chemical softening effect and variation of suction are taken into account in the present algorithm. The nonlinear terms arose by suction are also taken into account at global level of manifold element analysis. Numerical examples are presented to demonstrate accuracy and convergence property of the algorithm proposed at local level and the application to geotechnical engineering with chemical loading.