Numerical analysis of a near-to-real scale in-situ experiment of a deep geological repository

Deep geological repository involving a multi-barrier system constitutes one of the most promising options to isolate high-level radioactive waste from the human environment. In order to certify the efficiency of waste isolation, it is essential to understand the behaviour of the confining geomaterials under a variety of environmental conditions. The efficiency of Engineered Barrier Systems (EBS) is highly based on the complex behaviour of bentonite. To improve the understanding of the processes involved in the EBS, results from a near-to-real scale experiment, the FEBEX experiment, are studied by means of a thermohydro- mechanical (THM) finite element approach including a consistent thermo-plastic constitutive model for unsaturated soils. The model also features a coupled THM approach of the water retention curve. The extended literature available on the behaviour of the FEBEX bentonite is used to calibrate the parameters. The results of the numerical simulations are compared with sensor measurements and show the ability of the model to reproduce the main features of the mechanical behaviour of the system. The hydraulic and thermal response is also realistically described by the model. The link between the confined swelling behaviour as tested in laboratory and the results of the real-scale experiment is clearly established by this simulation

Heat-exchanger piles for the de-icing of bridges

Of the various types of road structures, bridges are the most exposed to icing; the problem of icing is widely addressed through salting, which reduces the lifespan of the bridge. One promising solution to avoid the use of salt is the seasonal storage of solar heat energy captured directly through the asphalt layer; however, this solution can only be achieved cost effectively if a necessary geostructure is used as a heat exchanger. In this study, such an approach is studied for a bridge crossing a canal, and the geotechnical and energy-related challenges of such a solution are discussed. Bridge piers and abutments are located on piles, which are used as heat exchangers. Depending on local conditions, seasonal storage and natural thermal reload are two possible solutions for the operation of such a system. In particular, the presence of underground water flow is thought to be a significant factor in such a design and is considered here. This study aims to determine the geotechnical and energy design parameters through thermo-hydro-mechanical simulations. A three-dimensional finite-element model analysis is necessary given the distance between bridge piles. Various underground water flow scenarios are studied. The capture of energy and de-icing requirements is based on the few existing structures that use other means of energy exchange with the ground. The results indicate that the use of heat-exchanger piles for de-icing bridges can only be considered at specific sites; however, the efficiency of the solution at those sites is high. Possible foundation and structure stability problems are also considered, such as vertical displacements due to the dual use of the foundation piles

Significance of rock THM parameters in geological nuclear waste storage simulation

A deep geological repository involving a multi-barrier system constitutes one of the most promising options to isolate high-level radioactive waste from the human environment. In order to certify the efficiency of waste isolation, it is essential to understand the behavior of the confining geomaterials under a variety of environmental conditions. The efficiency of an Engineered Barrier System (EBS) is largely based on a combination of bentonite and host rock characteristics. To contribute to a better understanding of the processes involved in the EBS, a case study for sensitivity analysis has been defined and is studied using a thermo-hydro-mechanical (THM) finite element approach including a consistent thermo-plastic constitutive model for unsaturated soils. The model also features a coupled THM approach of the water retention curve for bentonite, using the ACMEG-TS constitutive model. Regarding rock parameters, intrinsic permeability and relative permeability effects are evaluated. Two regimes are found regarding the importance of the estimation of rock permeability: in the first one, precise assessment is unnecessary due to water inflow control by bentonite, while in the second one, a precise assessment is necessary to correctly estimate resaturation time. This study highlights the effects that need to be taken into consideration for a correct assessment of EBS behavior, from bentonite characteristics to the correct quantification of the thermo-hydro-mechanical couplings in host rock

Heat Exchanger Anchors for Thermo-active Tunnels

Shallow geothermal power represents an important energy resource for the heating and cooling of the buildings. Due to relatively low temperature levels encountered at shallow depths in the soil, between 10°C and 20°C, heat pumps are required to process the extracted heat, forming the so called ground source heat pump system. Different types of heat exchangers with the ground were developed in order to optimize the heat exchanges, from simple geothermal loops grouted in boreholes reaching depths up to a couple of hundreds of meters to complex energy geostructures. Indeed, embedding geothermal loops within concrete foundation structures increases the heat exchange efficiency as well as it saves the cost of additional drillings. Recent developments suggested that applying the concept of energy geostructures to tunnel structures that are in contact with the ground should also be efficient. The present study investigates the potential of using tunnel anchors and nails as heat exchangers with the surrounding soil. Two main structures of urban tunnels were investigated. A cut and cover tunnel, whose diaphragm walls are maintained with long anchors, was modelled first. Thermal influence of the soil surface and unsaturated conditions were taken into account because of the shallow depth of the tunnel body. Nevertheless, mechanical implications of the heat extraction on the cut and cover tunnel were neglected because of the low mechanical confinement observed on the structure. Then, an urban bored tunnel was investigated. Soil conditions encountered at this depth were assumed always saturated and the thermal influence of the surface was neglected. Mechanical implications of the heat exploitation were assessed because of the high confinement of the bored tunnel body induced by the soil weight. Different types of heat exploitation cycles were tested for the different configurations. The heat extraction is based on the external air temperature in order to meet a simplified building heat demand. Cycles with and without heat injection were also investigated. All the exploitation cycles were optimized in order to reach a temperature threshold in the ground to prevent freezing it. Next, comparisons between extracted and injected heat of the different cycles allow drawing an optimum exploitation method. It is found that injecting heat during the hot period is necessary for the cut and cover tunnel as the natural heat reload isn’t high enough to ensure the sustainability of the heat storage. Conversely, the bored tunnel beneficiates from an increased natural heat reload, turning the heat injection into a more expensive solution. Furthermore, mechanical implications of the heat exploitation on the bored tunnel are found to be more significant when injecting heat. This shows the importance of a thermo-mechanical design of such a system. Finally, considering heat injection or not, it is estimated that heat extraction ranges from 0.6 to 1.2 MWh per year and per meter of cut and cover tunnel, and from 2.8 to 4.0 MWh per year and per meter of bored tunnel

Utilisation des structures géotechniques pour l’extraction d’énergie dans les routes

Les géostructures énergétiques ont vu un important développement depuis leurs premières réalisations au début des années 1980. Elles permettent par la circulation d’un fluide caloporteur dans des parois enterrées ou des pieux une forte récupération d’énergie géothermique de surface pour un coût modique (pompe à chaleur). Une des pistes de développement les plus récentes consiste à étendre leur usage à de nouveaux ouvrages enterrés, que l’on rencontre dans le cas des tunnels, ou à de nouveaux usages tels le contrôle thermique des chaussées. Le LMS travaille sur ces deux aspects ; Une étude concerne le réchauffement de chaussées verglacées par les géostructures énergétiques, par exemple des pieux de culée. Une revue d’ouvrages significatifs tels le pont de Saiwae (Japon) est présentée. Une autre étude concerne le comportement d’ancrages énergétiques dans les cas d’une tranchée couverte ou d’un tunnel urbain a été effectuée. L’objectif d’un tel ouvrage est le chauffage d’un bâtiment proche. L’efficacité et le potentiel thermique est évalué, ainsi que les effets mécaniques. Complétée par une autre étude économique et technologique, ces travaux permettent d’identifier les pistes les plus prometteuses et les risques techniques pour le développement de l’extraction d’énergie dans les routes

Comportement du béton sous fort confinement : Étude en compression et en extension triaxiales à l'échelle mésoscopique

This Ph.D. thesis aims at characterising and modeling the mechanical behaviour of concrete under high confinement at the mesoscopic scale. This scale corresponds to that of the large aggregates and the cementitious matrix. The more general scope of this study is the understanding of concrete behaviour under dynamic loading. A dynamic impact can generate mean pressures around 1GPa. But the characterisation of a material response, in an homogeneous state of stress, can only be achieved through quasi-static tests. The experimentations led in 3S-R Laboratory have underlined the importance of the aggregates in the triaxial response of concrete. Modeling concrete at the mesoscopic level, as a composite of an aggregates phase and a mortar phase, permits a representation of the aggregates effect. An experimental study of the behaviour of mortar phase is performed. Usual tests and hydrostatic and triaxial high confinement tests are realised. The parameters of a constitutive model that couples plasticity with a damage law are identified from these tests. This model is able to reproduce the nonlinear compaction of mortar, the damage behaviour under uniaxial tension or compression, and plasticity under high confinement. The biphasic model uses the finite element method with a cubic and regular mesh. A Monte-Carlo method is used to place quasi-spherical aggregates that respect the given granulometry of a reference concrete. Each element is identified by belonging either to the mortar or to the aggregate phase. Numerical simulations are compared with the experimental tests on this concrete. The parameters for these simulations are only identified on the mortar. The simulations reproduce the different phases observed in hydrostatic compression. The evolution of axial moduli under growing confinement is shown, as is the good reproduction of the limit-states experimentally observed under high confinement. The fracture aspect of numerical simulations is comparable with that of experimental tests. The triaxial extension loading shows the limits of this numerical model.Ce mémoire de thèse a pour objectif de caractériser et de modéliser le comportement mécanique du béton sous fort confinement, à l'échelle mésoscopique, celle des granulats et de la matrice cimentaire. Le cadre plus général de cette étude est la compréhension du comportement du béton sous chargement dynamique de type impact, pouvant générer des pressions moyennes de l'ordre du GPa. Mais la caractérisation de la réponse d'un matériau, dans un état de contraintes homogène, ne peut se faire que par des essais quasi-statiques. Les essais déjà réalisés au laboratoire 3S-R ont mis en évidence l'importance des granulats dans la réponse en compression triaxiale du béton. La modélisation du béton à l'échelle mésoscopique, sous la forme d'une phase mortier et d'une phase granulats, permet une représentation de l'effet des granulats. Une étude expérimentale du comportement de la phase mortier est réalisée. Des essais usuels et des essais hydrostatiques et triaxiaux entre 60 et 650MPa de confinement permettent d'identifier les paramètres d'un modèle de comportement de type plasticité couplée à l'endommagement. Celui-ci reproduit la compaction non-linéaire du mortier, l'endommagement des essais de compression ou de traction simple et la plasticité sous fort confinement. Le modèle biphasique utilise la méthode des éléments finis, en utilisant un maillage cubique régulier. Une méthode de Monte-Carlo est utilisée pour placer sur cette grille des granulats quasi-sphériques selon la granulométrie mesurée sur le béton de référence. Les simulations numériques sont comparées aux essais expérimentaux sur ce béton. Ces simulations, dont les paramètres sont identifiés par les essais expérimentaux sur mortier, reproduisent les différentes phases observées lors de la compaction hydrostatique. L'évolution des raideurs axiales avec le confinement est soulignée, ainsi que la bonne reproduction des états-limites des essais triaxiaux sous fort confinement. Les faciès de rupture des essais numériques sont comparables à ceux des essais expérimentaux. Les chargements d'extension triaxiale mettent en évidence les limites du modèle biphasiqu

Numerical analysis of canister movements in an engineered barrier system

The mid-term safety of deep geological nuclear waste repositories is based in part on the presence of a buffer, the main role of which is to isolate the environment from radionuclides. A design evaluation of such a repository is necessary to assess the potential vertical canister movement inside the drift that could reduce the buffer efficiency. A thermo-hydro-mechanical (THM) simulation is performed in a vertical cross-section of the drift. The THM couplings are described, and their influences on the mid-term (300 years) response of the engineered barrier system (EBS) are revealed. This study uses an advanced constitutive model to simulate the THM processes that occur in a specific EBS design case. The near-field simulations of the nuclear waste canister are performed in a two-dimensional finite element configuration that considers the effect of gravity. The focus of this study is on the mechanical behaviour of the buffer, which consists of two different forms of bentonite. Such an approach allows realistic consideration of the effect of the wetting and drying of the buffer material in non-isothermal conditions. Due to a specific design that includes bentonite blocks and pellets, the canister is observed to heave slightly during the re-saturation period, which extends up to 100 years

THM COUPLING SENSITIVITY ANALYSIS IN GEOLOGICAL NUCLEAR WASTE STORAGE

Abstract. A deep geological repository involving a multi-barrier system constitutes one of the most promising options to isolate high-level radioactive waste from the human environment. In order to certify the efficiency of waste isolation, it is essential to understand the behaviour of the confining geomaterials under a variety of environmental conditions. The efficiency of an Engineered Barrier System (EBS) is largely based on the complex behaviour of bentonite. To contribute to a better understanding of the processes involved in the EBS, a case study for sensitivity analysis has been defined and is studied using a thermo-hydro-mechanical (THM) finite element approach including a consistent thermo-plastic constitutive model for unsaturated soils. The model also features a coupled THM approach of the water retention curve. Various couplings were studied separately and in combination in order to determine the significance of each. The same principle is applied to physical phenomena such as vapour diffusion. This study clearly highlights the effects that need to be taken into consideration for a correct assessment of EBS behaviour. Fabrice DUPRAY, LI Chao, Lyesse LALOUI