MODELISATION DU COMPORTEMENT THERMO-HYDRO-MECANIQUE DES MILIEUX POREUX ANELASTIQUES

Les effets de la température sur le sol se ressentent de façon importante sur les eaux libre et adsorbée, par l'expansion thermique de la première et la perte des liaisons de cohésion pour la seconde. Nous observons, par l'intermédiaire des déformations thermo-mécaniques des grains solides, son influence sur le gradient hydraulique et l'écoulement transitoire de l'eau interstitielle. En plus de la dilatation thermo-élastique, ces déformations thermo-mécaniques peuvent être dues à une déformation thermique irréversible. Ne considérant que la gamme de température variant de la température ambiante à celle précédant les changements de phase, nous présentons un modèle mathématique qui décrit le comportement thermo-hydro-mécanique des milieux poreux saturés. En homogénéisant les relations microscopiques et leur affectant leurs fractions volumiques, nous aboutissons à la formulation continue des équations de conservation du milieu équivalent d'un milieu formé, à une échelle microscopique, d'une matrice solide saturée par un fluide. Ce milieu est décrit à l'aide de l'énergie libre comme fonction d'état ayant pour paramètres la déformation et la température. La phase solide a un comportement complexe, au niveau macroscopique, caractérisé par des non-linéarités et de la thermo-plasticité. Ce phénomène dissipatif est modélisé par une extension d'une loi de comportement cyclique multimécanisme viscoplastique à une loi thermo-viscoplastique qui prend en compte l'écrouissage thermique et l'évolution des surfaces de charge avec la température. La loi thermo-viscoplastique, testée à travers un certain nombre de chemins de sollicitation, donne des résultats satisfaisants. Après la formulation variationnelle du problème thermo-hydro-mécanique et les discrétisations spatiales et temporelles des relations du comportement, une présentation de schémas de résolution de la formulation couplée avec leurs critères de stabilité est faite. Nous vérifions sur un cas test de consolidation thermo-élastique la validité de l'un de ces critères. Des applications numériques terminent ce travail. Elles abordent particulièrement l'analyse du comportement, dans différentes configurations, du sol adjacent à un puits où sont enfuits des sources thermiques

Modelling the density homogenisation of a block and granular bentonite buffer upon non-isothermal saturation

This paper presents a numerical analysis of the mechanical performance of a bentonite clay buffer for the containment of nuclear waste in the context of deep geological disposal. The design of the buffer is based on the Swiss concept where the waste canisters are emplaced on pedestals of compacted bentonite blocks and the remaining space between the tunnel and the canister is backfilled with grains of highly compacted bentonite. A complete analysis of the long-term performance of the repository requires a good understanding of the mechanical evolution of the bentonite upon heating from the radioactive waste and hydration from the host rock. Despite its importance, the implications of the initial heterogeneous bentonite layout, characterised by blocks and grains, on the final dry density at the repository scale in the steady state have not been previously studied. The present study aims to shed light into these processes by means of finite element modelling using an advanced constitutive model for the bentonite behaviour that considers several thermo-hydro-mechanical couplings. The constitutive model is shown to be able to reproduce several laboratory tests involving saturation of block and pellets at different dry densities. The model predictions, extended up to 100,000 years, indicate that the bentonite blocks and grains tend to homogenise in terms of dry density as the buffer reaches full saturation. Due to the different swelling potential of the block pedestal and the granular backfill, the canister is subjected to movements, although these remain relatively small. The impact of initial segregation of the granular bentonite is also studied and it is seen to not to affect substantially the mechanical evolution of the buffer, although it might reduce canister displacements

Thermo-hydro-mechanical analysis of the complete lifetime of the bentonite barrier in the FEBEX in-situ test

The FEBEX test was a large-scale demonstration project for the deep geological disposal concept of nuclear waste involving bentonite seals that lasted 18 years. One of the objectives of the test was to evaluate the capabilities of numerical methods to provide reliable predictions of the physical processes in a geological repository. Although previous studies have demonstrated the performance of current models of water, vapour and heat flow to capture the evolution of temperature and relative humidity, some uncertainties remain in the capabilities of constitutive models to predict and interpret the stress–strain behaviour of the bentonite. In this paper a recently developed thermo-hydro-mechanical (THM) elastoplastic constitutive model is used to analyse the bentonite barrier of the FEBEX test by means of the Finite Element method. The model features a two-way hydro-mechanical coupling and includes thermo-plasticity. The associated water retention formulation distinguishes the behaviour of adsorbed water and free water. The predictive capabilities of the model are tested by calibrating the material parameters on the sole basis of laboratory tests. Good predictions of total stress, dry density and water content are obtained and the analysis of the computed THM stress paths provides new insights on the causes of the final heterogeneous state of the bentonite barrier

Non-isothermal plasticity model for cyclic behaviour of soils

On the one hand, it has been observed that liquefaction- induced shear deformation of soils accumulates in a cycle- by-cycle pattern. On the other hand, it is known that heating could induce plastic hardening. This study deals with the constitutive modelling of the effect that heat may have on the cyclic mechanical properties of cohesive soils, a relatively new area of interest in soil mechanics. In this paper, after a presentation of the thermo-mechanical framework, a non-isothermal plasticity cyclic model formulation is presented and discussed. The model calibration is described based on data from laboratory sample tests. It includes numerical simulations of triaxial shear tests at various constant temperatures. Then, the model predictions are compared with experimental results and discussed in the final section. Both drained and undrained loading conditions are considered. The proposed constitutive model shows good ability to capture the characteristic features of behaviour

ACMEG-T: A soil thermo-plasticity model

peer reviewe

Experimental analysis of a thermoactive underground railway station

Little is known about the real energy potential of thermoactive underground infrastructures, such as railway stations, that can act as a heating/cooling provider for the built environment. This study presents the results of thermomechanical full-scale in situ testing and numerical analysis of a thermoactive underground train station. The thermal performance and related geostructural impact of a portion of the new underground energy infrastructure (UEI) installed at the Lancy-Bachet train station in Geneva (Switzerland) are analyzed. Heating and cooling tests simulating real operative geothermal conditions are considered. Particular attention is given to (i) the monitored wall–tunnel hydrothermal interactions, (ii) the thermal response of the UEI to heating/cooling thermal inputs and (iii) the thermomechanical behavior of the energy geostructure. Among the main results of this study, it is shown how the hydrothermal tunnel behavior considerably varies on a seasonal basis, while the train circulation completely drives the airflow in the tunnel. The UEI shows a strong heat storage potential due to the main conductive heat transfers between the geostructure and soil, while lower heat fluxes are detected at the wall–tunnel interface. The extraction potential is of lower magnitude with respect to storage because of the limited range of operative fluid temperatures and of the concurrent action of temperature variations at the tunnel boundaries affecting the materials within the UEI. Preliminary guidelines for the thermal response test execution on underground thermoactive infrastructures are also reported. The monitored thermomechanical behavior suggests different wall behaviors in the vertical and longitudinal directions. Low-magnitude strains are recorded, while the mechanical capacity of the existing geostructure can satisfactorily sustain concurrent thermomechanical actions

Numerical modeling of unsaturated porous media as a two and three phase medium : a comparison.

A numerical simulation of the hydro-mechanical behaviour of an unsaturated porous medium is presented. The coupled hydro-mechanical model used is based on the continuum theory of mixtures and treats the unsaturated soil as a three phase porous medium (solid, liquid and gas). Principal variables are the solid deformation, the liquid pressure and the gas pressure. The two fluid phases are in motion and a non-linear pore pressure - saturation relation is used. The resulting system of equations is discretized in space using the finite element technique and in time by the - method. The comparison of numerical results and experimental test data shows that this hydro-mechanical model is capable to reproduce the principal phenomenon in the case of a hydric solicitation. The three phase formulation is compared to a simplified version considering only a two phase medium with static air phase. Merits and shortcomings of the two approaches are shown

Temperature-dependent internal friction of clay in a cylindrical heat source problem

peer reviewedThe effect of the temperature dependence of the internal friction angle is studied in a boundary value problem simulating the impact of a cylindrical heat source on the soil mass in which it is embedded. This follows a previous study which shows that such temperature dependence may substantially affect the interpretation of thermal failure in laboratory experiments. Even if the thermal increase of the internal friction is quite modest (less than 20% in terms of the critical state parameter, M), it affects quite significantly the effective stress path near the heat source. The effective stress path approaches the yield locus and the critical state at significantly higher principal stress difference values for the variable internal friction than for the M = const case. The 'mean effective stress distance from the critical state' is substantially reduced during heating, which in the case of small perturbations of any parameter may lead to potentially unstable or statically inadmissible behaviour. The solutions obtained allow one to identify zones of influence around the heat source of several variables of interest. The two fields most affected by the thermal sensitivity of M are that of the axial stress, dropping significantly near the heat source, and that of the appearance of the thermoplastic strain. Both zones of influence are reduced in size by almost half when the friction angle is increased by 20% over 70°. The presented results may be of relevance to the design of prototype in situ installations and their monitoring, and eventually of actual facilities for nuclear waste disposal