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11 results
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ItemSimulation of the Unsaturated Excavation Damage Zone Around a Tunnel Using A Fully Coupled Damage-Plasticity Model(Georgia Institute of Technology, 2013-07) Le Pense, Solenn ; Arson, Chloé ; Gatmiri, Behrouz ; Pouya, AhmadDuring tunnel excavation, stress redistribution produces plastic deformation and damage around the opening. Moreover, the surrounding soil can be either saturated or unsaturated. Suction has a significant influence on the mechanical behaviour of geomaterials. Depending on their stress state and on their moisture content, clay-based materials can exhibit either a ductile or a brittle behaviour. Plasticity leads to permanent strains and damage causes the deterioration of the soil elastic and hydraulic properties. The damage-plasticity model proposed in this work is formulated in terms of a damaged constitutive stress, defined from the principle of Bishop's hydro-mechanical stress (for unsaturated conditions), and from the principle of damaged effective stress used in Continuum Damage Mechanics. The evolution laws are obtained by using the principle of strain equivalence. This hydro-mechanical damage-plasticity model was implemented in a Finite Element code. The excavation of a tunnel is simulated at different constant suctions. The results obtained illustrate the influence of suction on the development of plastic and damaged zones.
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ItemNumerical Study of Damage in Unsaturated Geological and Engineered Barriers(Georgia Institute of Technology, 2011-04) Arson, Chloé ; Gatmiri, BehrouzThe theoretical framework of a damage model dedicated to non-isothermal unsaturated porous media is presented. The damage variable is a second-order tensor, and the model is formulated in independent state variables. The behavior laws are derived from a postulated expression of Helmholtz free energy. The damaged rigidities are computed by applying the Principle of Equivalent Elastic Energy (PEEE). Internal length parameters are introduced in the expressions of liquid water and vapor conductivities, to account for cracking effects on fluid flows. The damage model has been implemented in Θ-Stock Finite Element program. The mechanical aspect of the damage model is validated by simulating a triaxial compression test on a dry isothermal brittle material. Then, a sophisticated model of nuclear waste disposal, involving two non-isothermal unsaturated porous media, is reproduced. The results obtained in elasticity are in good agreement with the results presented in the corresponding reference article. A parametric study on initial damage is then performed to assess the influence of the Excavated Damaged Zone (EDZ) on the response of the nuclear waste repository during the heating phase. The trends meet the theoretical expectations.
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ItemThermo-Hydro-Mechanical Modeling of Damage in Unsaturated Porous Media: Theoretical Framework and Numerical Study of the EDZ(Georgia Institute of Technology, 2011-01) Arson, Chloé ; Gatmiri, BehrouzThe damage model presented in this article (named ‘THHMD’ model) is dedicated to non-isothermal unsaturated porous media. It is formulated by means of three independent strain state variables, which are the thermodynamic conjugates of net stress, suction and thermal stress. The damage variable is a second-order tensor. Stress/strain relationships are derived from Helmholtz free energy, which is assumed to be the sum of damaged elastic potentials and ‘crack-closure energies’. Damage is assumed to grow with tensile strains due to net stress, with pore shrinkage due to suction and with thermal dilatation. Specific conductivities are introduced to account for the effects of cracking on the intensification and on the orientation of liquid water and vapor flows. These conductivities depend on damage and internal length parameters. The mechanical aspects of the THHMD model are validated by comparing the results of a triaxial compression test with experimental measurements found in the literature. Parametric studies of damage are performed on three different heating problems related to nuclear waste disposals. Several types of loading and boundary conditions are investigated. The thermal damage potential is thoroughly studied. The THHMD model is expected to be a useful tool in the assessment of the Excavation Damaged Zone, especially in the vicinity of nuclear waste repositories.
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ItemNumerical study of a thermo-hydro-mechanical damage model for unsaturated porous media(Georgia Institute of Technology, 2010-05) Arson, Chloé ; Gatmiri, BehrouzThe “THHMD” damage model presented in this article is dedicated to non-isothermal unsaturated porous media. The proposed frame is based on the use of independent state variables (net stress, suction and thermal stress). Stress/strain relations are derived from a postulated expression of the free energy, accounting for the existence of residual strains. The damaged mechanical rigidities are computed by applying the Principle of Equivalent Elastic Energy for each stress state variable. The influence of damage on liquid water and vapor transfers is accounted for by introducing internal length parameters, related to specific damage-induced intrinsic conductivities. The “THHMD” model has been implemented in Θ -Stock Finite Element code. The mechanical aspects of the model have been validated by comparing the numerical results with experimental reference data. A nuclear waste repository model has been reproduced. The elastic predictions are in satisfactory agreement with the reference results. The parametric studies performed on damage parameters meet the theoretical expectations. Damage gets higher with higher damage rigidities. Water permeability grows with damage and with the internal length parameter.
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ItemA Multiphase Analysis for Environmental Impact Assessment with Theta-Stock Finite Element Program(Georgia Institute of Technology, 2010-03) Gatmiri, Behrouz ; Hemmati, Sahar ; Arson, Chloé ; Amirzehni, E.In the THM modeling of multiphase medium, the coupling effects of skeleton, suction, and temperature have been integrated via the concept of state surfaces of void ratio and degree of saturation. Based on proposed formulation, a fully coupled numerical model for the behavior of soil deformation, water flow, air flow, heat flow in unsaturated soil has been developed and integrated in a finite element code θ-Stock by the first author. This program is conceived with this idea that it will be able to analyze the response of a soil in different states of humidity to mechanical, thermal loading, and also damage phenomena. Damage model is dedicated to unsaturated brittle rocks. It mixes phenomenological and micromechanical concepts and is formulated based on the use of independent state variables. The expression of the liquid permeability is modified in order to represent the influence of fracturing on interstitial fluid flows. The final matrix form of established field equations of the proposed model for unsaturated case has been encoded for this particular purpose, in a finite element program which had been developed for dry and saturated soils previously.
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ItemA mixed damage model for unsaturated porous media(Georgia Institute of Technology, 2009-04) Arson, Chloé ; Gatmiri, BehrouzThe aim of this study is to present a framework for the modeling of damage in continuous unsaturated porous geomaterials. The damage variable is a second-order tensor. The model is formulated in net stress and suction independent state variables. Correspondingly, the strain tensor is split into two independent thermodynamic strain components. The proposed framework mixes micro-mechanical and phenomenological approaches. On the one hand, the effective stress concept of Continuum Damage Mechanics is used in order to compute the damaged rigidities. On the other hand, the concept of equivalent mechanical state is introduced in order to get a simple phenomenological formulation of the behavior laws. Cracking effects are also taken into account in the fluid transfer laws.
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ItemOn damage modelling in unsaturated clay rocks(Georgia Institute of Technology, 2008-10) Arson, Chloé ; Gatmiri, BehrouzThe aim of this paper is to present the main problems encountered in the modelling of damage in an unsaturated quasi-brittle rock mass. Micromechanical damage models are based on a physical definition of damage, related to fracturing. Phenomenological formulations are less straightforward, but offer huge modelling possibilities by means of economical computation processes. Due to the dissipative aspect of damage, the Inequality of Clausius–Duhem (ICD) has to be satisfied. Strain softening and crack localization are regularized by means of a non-local formulation, founded on microstructure concepts, homogenisation and space averaging or gradient-enhancement. In an unsaturated damaged porous medium, suction effects combine with mechanical loading and fracturing, which induces complex couplings. On the one hand, Continuum Damage Mechanics well represents stiffness degradation for dry materials. On the other hand, fracture network models give a good estimation of complex flows. It is difficult to reconcile both theories. A new mixed model, HHMD, is proposed. It is a fully coupled formulation, involving independent state variables.
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Itemθ-STOCK, a powerful tool of thermohydromechanical behaviour and damage modelling of unsaturated porous media(Georgia Institute of Technology, 2008-09) Gatmiri, Behrouz ; Arson, ChloéA brief review of the basic points of a suction-based heat, moisture transfer and skeleton deformation equations for an unsaturated medium is presented. The main issues such as: two temperature-dependent state surfaces of void ratio and degree of saturation which are used to present the coupling effects of temperature, moisture content and deformation of skeleton; the new thermoelastoplastic constitutive law, etc. are briefly mentioned. The Bubnov–Galerkin integral form of field equations has been developed as the basis of spatial and temporal discretized matrix form. The single-step integration in time is described. The numerical solution algorithm of the finite element package, θ-STOCK, is presented. Some application cases are presented and discussed to show the strong ability of presented model and the prepared numerical package.
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ItemSite-specific Spectral Response of Seismic Movement due to Geometrical and Geotechnical Characteristics of Sites(Georgia Institute of Technology, 2008-04) Gatmiri, Behrouz ; Maghoul, P. ; Arson, ChloéIt is well-known that the response of a site to a seismic solicitation depends on local topographical and geotechnical characteristics. Many aspects of seismic site effect still need to be studied in more detail and they can be incorporated in the seismic norms after quantification. The purpose of this paper is to contribute to establishment of a simple method to include complex site effects in a building code. Horizontal ground movements in various points of two-dimensional (2D) irregular configurations subjected to synthetic SV waves of vertical incidence are calculated. The parametric studies are achieved by means of HYBRID program combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The results are shown in the form of pseudo-acceleration response spectra. For the empty valleys, we can classify the spectral response according to a unique geometric criterion: the “surface/angle” ratio, where surface is the area of the valley opening, and angle denotes the angle between the slope and horizontal line in the above corner. To assess the influence of the 2D effect on the spectral response of filled valleys, the response of alluvial basins are compared with the response of one-dimensional columns of soil. Finally, an offset criterion is proposed to choose a relevant computation method for the spectral acceleration at the surface of alluvial basins.
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ItemSeismic site effects by an optimized 2D BE/FE method. II. Quantification of site effects in two-dimensional sedimentary valleys(Georgia Institute of Technology, 2007-10) Gatmiri, Behrouz ; Arson, ChloéThis paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed.