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International audience; Modélisation numérique et caractérisation du comportement HM de la craie de Château Landon (77)

International audience; Cet article présente les résultats d’essais obtenus sur des échantillonssaturés visant à une caractérisation du comportement hydromécanique de la craie deChâteau-Landon (77). Un modèle de comportement élastoplastique de ladite craie, inspiréde la micromécanique est en cours de développement. Les premières simulationsassociées aux résultats expérimentaux vont permettre d’enrichir sa formulation.

International audience

International audience; The Callovo-Oxfordian (COx) claystone is considered as a potential geological host formation for high-level and intermediate-level long-lived radioactive wastes in France. In 2000, the French National Radio-active Waste Management Agency (Andra) began to build an Underground Research Laboratory (URL) at Meuse/Haute-Marne in order to demonstrate the feasibility of a geological repository in the COx formation. The excavation of galleries at the main level (490 m depth) of the URL, essentially following two directions of in-situ minor (σh) and major (σH) horizontal stresses, shows a significant anisotropy of the excavation induced fractured zones (Armand et al 2014). Different factors contribute probably to this anisotropic re-sponse of the COx to the excavation operation, such as inherent anisotropy of the stiffness and strength, anisotropic initial stresses, excavation induced anisotropic pore pressure repartition, excavation induced instability of quasi-brittle rock, etc. The benchmark “Transverse action” consists in developing numerical models to characterize the COx be-havior and using them for modeling the COx response due to the excavation. In the framework of this benchmark, different models, such as anisotropic elasto-visco-plastic models incorporating with non-local or second gradient modeling, anisotropic elastic-damage model, discrete modeling, have been proposed to successful reproduce the excavation induced fractured zones and the convergence measurement for both drifts drilled following σh and σH (Seyedi et al 2017). However, those models are limited in 2D modeling, which cannot show some 3D effect on the claystone response, for instance: the delay of convergence at different sections, the pore pressure distribution at the drift front, etc. This study focuses on developing a nonlinear model including both the elastic anisotropy and the induced anisotropic plasticity. The basic assumption is that the failure of an anisotropic material is due to either fracturing of weakness planes (ubiquitous joints) and the failure of the rock matrix. The ubiquitous joints are introduced based on the orientation of the induced fractures to reproduce the induced anisotropy. Thus, the rock is composed of a matrix and of potential weakness planes as observed from the biaxial tests under plane strain conditions. The proposed model is implemented in FLAC3D (Itasca 2017). Comparisons with in-situ observations on two drifts within the URL, namely GCS and GED drilled following two directions of σH and σh, were made. Analysis is focused on the excavation induced the fractured zones and the gallery convergence.

The shallow underground room-and-pillar mines where chalk rocks were extract-ed are now abandoned and are enduring the effects of time and weathering which increase their risk of collapse. The Parisian Basin in France contains many abandoned underground chalk mines and number of them were already instrumented or subjected to a feedback analysis aimed to aid the development of prevention strategies. Campaigns of laboratory characterization are often conducted to complete the understanding of the hydromechanical behavior of these chalks. For instance, laboratory characterization was conducted to understand the long-term behavior, among other, of Saint-Martin-le-Noeud chalk with respect to the relative humidity for levels ranging between 90% and 100%, the most representative of the in situ conditions of abandoned underground openings. The main conclusions are: (a) the existence of stress and relative hu-midity thresholds from which the creep of this chalk starts, (b) the flooding at the last levels of stress and hygrometry leads to an accelerated creep causing the ruin of samples without transient phase.

In the context of radioactive waste management, one of the options currently being considered is to store it in deep geological formations. Clay formations, in particular, show very favorable confining conditions as repositories for long-term safety of nuclear waste due to their low hydraulic conductivity and significant retention capacity for radionuclides. In France, the National Radioactive Waste Management Agency (Andra) began to build the Meuse/Haute-Marne underground research laboratory (MHM-URL) in the Callovo-Oxfordian (COx) claystone formation, lying between depths of 420 m and 550 m. With regards to the main objectives of feasibility and safety assessment of a potential deep geological repository (Cigéo), a comprehensive scientific research programme of specific in situ experiments, laboratory tests and numerical modelling have been carried out to characterize and understand the behavior of the COx claystone formation in the framework of hydromechanical (HM) processes (Armand et al. 2017). In addition to the HM couplings and the associated parameters, it appears necessary to consider at least the inherent anisotropies (mechanical and hydraulic) of such rock, but also its time-dependent behavior (Armand et al. 2017). In this paper, we consider the transient creep mechanism determined from laboratory tests in order to study its effect on anisotropy and hydromechanical behavior of COx clay. The hydromechanical couplings are then evaluated around the GCS drift of MHM-URL (drift excavated following the major principal stress and without rigid lining) under satured conditions using Comsol Multiphysics code.

International audience; The development of nuclear energy raised the issue of radioactive waste disposal in deep geological formations. Clay formations in their natural state show very favorable confining conditions as repositories for radioactive waste due to their generally low values of hydraulic conductivity and significant retention capacity for radionuclides. Three clay formations in Europe are currently being studied in details as potential host rocks for high level radioactive waste disposal, i.e. the Boom Clay in Belgium, the Opalinus Clay in Switzerland and the Callovo- Oxfordian Claystone (COx) in France. Lastly, the French national radioactive waste management agency (Andra) started building the underground research laboratory (URL) at Bure in the Meuse district. Several specific in situ experiments were carried out with the main objectives in term of underground civil engineering to: (a) evaluate the geological environment of the future Deep Geological Repository (DGR), (b) collect data from the clay formation in order to assess the performances of the future DGR, (c) test and demonstrate the feasibility of the future DGR. The feedback of these experiments and in situ measurements as well as the numerous theoretical analyzes and numerical modeling works undertaken on the COx allowed to identify the key mechanisms (mainly in the framework of THM processes) governing the behavior of the COx [1]. In addition to the THM processes and the associated parameters, it appears necessary to consider the anisotropies (mechanical, hydraulic and thermal) of such rock, but also its time-dependent behavior. This paper deals with these issues.

Characterization and modeling of the mechanical behavior for compressible materials are well-known issues, and have been studied for a long time in the field of : (a) soil mechanics, including the foundations on soft ground or stability of embankments, structures on compressible soils etc.; (b) rock mechanics for highly porous geomaterials; (c) metallurgy for the behavior of metal foam containing voids (one of the first ductile damage model has been proposed by Gurson (1977), and widely used in the literature for geomaterials) or for compaction of metal and pharmaceutical powders, snow or crushed salt compaction (for instance) or in the ceramic industry. The common thread for these materials is that when they are subjected to loadings, significant modifications of their microstructure (compaction, stiffening, increase of their cohesion, etc.) are experimentally observed. Indeed, several models of mechanical behavior for highly porous materials (chalk for the oil industry, metal powders or PVC in metallurgy or pharmaceutical industry, ceramics, etc.) can be found in the literature (Baud et al. 2006, Azami & Khoei 2006, Wu et al. 2005). In this paper, we propose a macroscopic phenomenological model for the mechanical behavior of the compressible materials for use at the interface between support elements and the host rock at the Meuse/Haute-Marne underground research laboratory (URL). Such materials are intended to absorb the strain energy of the host rock in the near field due to its creep and therefore to minimize the transmission of forces in the support elements during the transient phase following their emplacement. A specific compressible material has been designed and uniaxial, triaxial and oedometric tests have been carried out with the aim to characterize the short-term behavior of this material, but also to specify the main strain and failure mechanisms. According to the composition of the compressible material, two types of shear behavior have been observed: (a) a strain-hardening after a small elastic phase or (b) an elastic behavior up to the peak followed by brittle failure and a strain hardening. Based on this characterization, a constitutive model was proposed. Fig. 1 summarizes the proposed modeling of each mechanism which is characterized by (a) an elastic limit with or without a brittle failure based on the Drucker-Prager's criterion. The advantage of this criterion compared to Mohr-Coulomb or Hoek-Brown ones is the dependency on the intermediate principal stress s2; (b) a strain hardening modelled by an exponential function with respect to the internal plastic variable (plastic distortion), (c) a cut-off in tension. For the pore collapse mechanism (volumetric cap yield), the yield function is based on the Gurson (1977) criterion, that is to say, an incorporated elliptical yield surface for both pore collapse and compaction. The main advantage of this micromechanics-based criterion is that the yield function depends explicitly on the porosity of the medium. The hardening due to pore collapse is governed by a local hardening variable which depends on the macroscopic plastic deformation with respect to the pore collapse mechanism (e.g. volumetric plastic strain due to the hydrostatic failure). Finally, the densification mechanism occurs after the pores collapse results in a medium becoming more and more cohesive during its compaction. These two phenomena are determined by the principle of equivalence plastic energy and approached on the basis of the ductile damage model of Tvergaard and Needleman (1984). The model parameters have been identified despite of the small number of the available tests. The proposed multi-mechanism model has been implemented in the 3D computation code, FLAC3D. As a verification of the proposed model and its numerical implementation, two laboratory tests (triaxial compression and oedometric tests) carried out on this compressible material were simulated. The experimental curves have been satisfactorily reproduced by the numerical simulations as illustrated in Fig.2 for oedometric test. The contribution of this mechanical model for compressible materials compared to a classical elastoplastic model was evaluated under simplified conditions (2D) for a GVA2 gallery of the Meuse / Haute-Marne underground research laboratory (URL). This is a gallery of large diameter (f = 6.37 m) oriented in the direction of the minor horizontal stress was simulated in 2D following the convergenceconfinement method. A first support consisted of projected concrete with an average thickness of 0.185 m was set up after the complete passage of the face front. A rigid liner of outer and inner radii of 2.85 and 2.42 m respectively, was also set up and a soft packing material (compressible material) is used to fill the gap between the two different supports (Fig. 3). The thickness of the compressible material is about 0.15 m. Both supports elements have an elastic and perfectly plastic behavior based on the Mohr- Coulomb criterion. Otherwise, in the framework of a scientific collaboration between Andra and INERIS, a macroscopic isotropic visco-elastoplastic model, which accounts for the impact of induced damage (modeled through a plastic hardening function) on the viscoplastic strain rates, has been developed (Souley et al. 2017) and used in several design and predicting studies of the mechanical and hydromechanical behaviors around the Meuse/Haute-Marne URL structures. It is also this model that has been used in this study for the viscoplastic behavior of Callovo-Oxfordian claystone (COx). Finally, for the short-term mechanical behavior of the compressible material, two models of behavior were used: one corresponding to the proposed model and the other perfectly elastoplastic type. In particular, we evaluated the discrepancies between the use of the proposed model and the hypothesis of an elastic or elastoplastic behavior for the compressible material on the distribution and magnitude of stresses in the rigid liner as well as the liner bending. Compared to the use of classical compressible material, the proposed model reproduces a reduction of 30 to 50% of the stresses in the rigid liner, for a duration of the simulation about 100 years. This constitutes a considerable gain for a more accurate design of different URL structures as well as in terms of realization costs.

Clay formations in their natural state show very favourable confining conditions with respect to underground storage due to their generally lo hydraulic conductivity, low molecular diffusion and significant retention capacity for some elements. This research work has been performed on Callovo-Oxfordian Clay (also called “Argillite”) in which Andra has constructed an underground research laboratory. As illustrated in figure 1 several specific in situ experiments were carried out with the main objectives basically to characterize the short and long term response of the rock to different drift construction methods in order to optimize the design of component and to assess the Excavation Damaged Zone (EDZ) and its evolution. The model developed by Souley et al. (2011 [4]) is phenomenological and elastic-visco-plastic and takes into account the short- and long-term responses of COx claystone. However, it only considers mechanical behaviour. More recently, Souley et al. (2017) [5] propose to extend this model in order to reflect: (a) the impact of excavation induced fracturing and damage on the delayed strains as indicated by in situ measurements at the Meuse/Haute-Marne URL; (b) hydro mechanical couplings in saturated conditions, (c) changes in the permeability, firstly in the EDZ. The new model is used to numerically simulate the excavation of the GCS drift of MHM-URL (figure 1). Comparisons with in situ measured data are presented in terms of change in strain and pore pressure fields, and then discussed to contribute to the understanding of the COx claystone. Improvement and discrepancy of the modelling are also discussed., L’Ineris a été sollicité par l’Agence nationale pour la gestion des déchets radioactifs (Andra) pour contribuer aux travaux de recherche en amont sur le comportement géomécanique des roches argileuses. De nombreux travaux ont déjà été réalisés depuis plusieurs années sur le comportement et les propriétés des matériaux argileux et les ouvrages souterrains qui y sont réalisés. Le creusement d’excavations souterraines entraîne généralement la création d’une zone perturbée (EdZ) ou zone endommagée (EDZ) au sens de [1], résultant de la micro et la macro-fracturation induites par une redistribution et un réarrangement des contraintes initiales. Il est important de connaître la géométrie, les extensions des EdZ/EDZ, la densité des fissures, leur connectivité et les variations de perméabilité qui peuvent en résulter. Les formations argileuses dans leur état naturel présentent des propriétés favorables au stockage souterrain en raison notamment de leurs faibles conductivités hydrauliques, de leur faible diffusion moléculaire et d’une capacité de rétention de certains éléments.

The Callovo-Oxfordian claystone is considered as a potential geological host formation for high-level and intermediate-level long-lived radioactive waste in France. Since 2000, the French National Radioactive Waste Management Agency (Andra) has been constructing an Underground Research Laboratory (URL) at Bure with intent to demonstrate the feasibility of a geological repository in the Callovo-Oxfordian claystone (COx) formation. The excavation of galleries at the main level (490 m depth) of the URL showed a significant fracturing induced by the excavation (Armand et al. 2014). Indeed, the authors have reported the existence of two adjacent zones around the laboratory galleries: a "connected" fracturing zone and a more extended "discrete" fracturing zone overhanging the first zone. The "connected" fracturing zone would correspond to the well-known EDZ (Excavation Damaged Zone) and would represent a domain for which the fracturing is particularly developed, whereas the "discrete" fracturing area located between the EDZ and the intact rock mass would represent the EdZ (Excavation Disturbed Zone). The in situ observations show an anisotropic extent of the induced fractured zone even for the drifts excavated following the in situ major horizontal stress for which, the initial total stress is quasi-isotropic in the drift section. Different factors contribute probably to this anisotropic response of the COx to the excavation operation. Based on the experimental results, various failure criteria for anisotropic materials exhibiting a visible inherent anisotropy have been proposed and constitutive models were developed in the literature. The developed models can be split mainly in (a) the empirical models based on the theory of variational cohesion and / or friction (e.g., Wang et Yu 2014); (b) the models built on the concept of ubiquitous joints with several planes of weakness (Sainsbury et al. 2008), (c) the models where damage and/or plasticity are incorporated and formulated in the framework of irreversible thermodynamics (Pietruszczak et al. 2002). For this purpose a phenomenological macroscopic anisotropic model (including both the elastic anisotropy and the induced anisotropic plasticity) has been proposed in this paper. The basic assumption is that the failure of an anisotropic material is due to either fracturing of bedding planes (or weakness planes) or the failure of the rock matrix. The formalism is therefore based on approaches with weakness planes commonly called “Discontinuous weakness plane” or “Ubquituous joints” aimed to account for rock mass strength and its anisotropy within large-scale continuum approaches. In the proposed model, any predefined weakness plane is not considered. The joints are introduced following an estimated orientation of the induced fractures to reproduce the induced anisotropy. In this framework, it is assumed that the rock is composed of a matrix and of potential planes of weakness (reflecting the induced damage approached by the theory of plasticity) as observed on the results of biaxial tests under plane strain conditions with X-ray micro-tomography (Besuelle and Lanata 2014) aiming to characterize the deformation mechanism. Then the matrix is assumed to be linear, transversely isotropic and the plasticity is described by an isotropic non linear yield function derived from the laboratory characterization. A non-associated flow rule is used with a distinction between compression and extensional stress paths, as well as the absence of volumetric strain beyond large plastic distortion. Induced fractures are modelled as the planes of weakness represented by joints. Experiments on brittle failure reveal two fundamental types of fractures: tensile/extension (mode I) and shear (modes II) which individually induces different orientation of the facture failure plane relative to the principal stresses. Mode I, fractures are associated to (a) tensile fractures occurring when the minimum principal stress s3 reaches the tensile stress, or (b) longitudinal splitting when the minimum principal stress is close to zero as in uniaxial compression. The fracture orientation is perpendicular to the tensile stress (tension case) or parallel to the major compression stress (uniaxial compression). Mode II, shear fractures occur under triaxial compression configuration with fracture plane angles θ less than 45° with the maximum compressive stress. In the latter case, we assumed that the rocks fail along conjugate fractures with θ=±(45° - ϕw/2) where ϕw is the friction angle along the weakness plane. This is in accordance with the experimental results of failure plane orientation reported by Zhang (2016). Finally, a perfectly plastic behaviour according to the Mohr-Coulomb criterion is assumed along these weakness planes; while the elastic part is considered as linear and transversely 396 isotropic. The constitutive equations were implemented in the three-dimensional explicit finite-difference code, FLAC3D, where fully coupled hydromechanical modelling can be performed under fully saturated conditions for both isotropic and anisotropic rock masses. The ability of the proposed model to reproduce the plastic zones around a drift excavated following in situ major horizontal stress (GCS drift) at the main level of the Meuse/Haute-Marne (M/H-M) URL, is firstly successfully tested from a purely mechanical (monophasic) approach. Comparison between simulation results and the in situ measurements around the GCS drift provides new insights on the understanding of the deformation mechanisms observed around the structures of the M/H-M URL. More precisely, numerical simulation predicted convergence ratios (horizontal / vertical) at different measurement section positions along the length of the gallery, which remain between 1.5 and 2 as illustrated in Fig. 1. Those obtained with in situ measurements for the instantaneous response are ranged between 1.3 and 2.1 (Guayacan-Carillo et al. 2016). The extension of the predicted plastic zones with the proposed model varies from 0.7xD to 1xD (Fig.2), which well corresponds to the in situ observations. Indeed, according to Armand et al. (2014), the extent of EDZ is 0.15xD and 0.5xD in the roof/floor and drift sides, respectively, against 0.5xD and 1xD for EdZ. Hydromechanical coupling was also investigated with the proposed model on the same GCS drift. For simplicity, the coupled hydromechanical simulation was carried out on a 2D geometry (a cross section of the GCS drift, excavated with a drainage condition along its wall). The initial pore pressure at the main level (-490 m) is about 4.7 MPa. The instantaneous (0+) response leads to interstitial over-pressures with a maximal amplitude of 1.5 MPa in the direction of the initial minor stress (horizontal) and underpressures in the direction of the initial major stress (vertical) with respect to the GCS section (Fig. 3ab), which is in agreement with a poroelastic analysis presented by Guayacan-Carillo et al. (2016). With time and in the direction of the initial minor stress, the peak of pressure moves towards the interior of the rock mass while increasing in intensity at first and then decreasing the value of the peak of pore pressure with time. This remains in agreement with the in situ measurements (Seyedi et al. 2016). The apparition of overpressure is explained by elastic anisotropic behaviour of rock as described by Guayacan-Carillo et al. (2016) or Fig. 3a. However, the authors showed the overpressure reaches the maximum value at the instantaneous (0+) response of excavation in the vicinity of the drift wall and this overpressure disappears between 1 to 6 months. When the mechanical behaviour of the COx is elastoplastic (as the case of the present study), the peak of pore pressure is reached at 15 days after the excavation and the overpressures vanish after 2 years. This is closer to the in situ observations.

In the framework of the design of the French nuclear waste repository called Cigéo, a two years collaborative work including French nuclear waste producers (EDF, CEA and AREVA) and the French National Radioactive Waste Management Agency (Andra) has been conducted in order to improve methods of modelling support for design of drift. Among a lot of identified issues, the main idea was to well take into account the real excavation steps and all elements of the support depending on the excavation/support method (tunnelling machine, road header, delayed in place casted support,…) in the modelling approach and identify the effect on the long term loading of the lining assuming creep of the rock mass. Calculations have been performed with the finite difference code FLAC using two different elasto-viscoplastic models (Kleine 2007, Souley et al 2011). The sensibility to various numerical parameters has been analysed as: mesh of support elements, interfaces between elements,…Supports taking into account compressible material have also been considered in the modelling with models for the Highly Deformable Elements. The paper sums up the main results of this fruitful collaboration and exhibits conclusions which could feed and help the project designer of Cigéo.

International audience; The Callovo-Oxfordian claystone (COx) is considered as a potential geological formation to host an industrial radioactive waste repository in France, thanks to its favorable characteristics for radioactive waste containment, as it has a very low hydraulic conductivity, small molecular diffusion and significant retention capacity for radionuclide. A deep understanding of the thermo-hydro-mechanical (THM) behavior of the COx is a key issue for design of different repository structures and the safety calculations of the project. Several constitutive models have been developed/used in the framework of the R&D and simulation programs of Andra. These models aim to reproduce the THM behavior of COx under different repository situations and for different structures. A model benchmark exercise has been launched since 2012 to provide an overall view of the developed models, their basic assumptions, their mathematical descriptions, variables and parameters, and to investigate the consequences of these basic assumptions on the calculations results regarding to the experimental observations. Three main constitutive model families are considered; namely, visco-elasto-plastic models, damagemechanics based and computational homogenized (CHM) ones. Nine teams using different models have been participated to this benchmark exercise.

An anisotropic constitutive model is proposed in this paper accounting for both structural anisotropy and induced anisotropic plasticity. It is assumed that the rock is composed of a matrix and of potential planes of weakness. The matrix is assumed to be linear, transversely isotropic and the plasticity is described by a non linear yield function where the parameters are deduced from the nonlinear Hoek-Brown envelopes in pre- and post-peak, and derived from the laboratory characterization. A non-associated flow rule is used with a distinction between compression and extensional stress paths, as well as the absence of volumetric strain beyond large plastic distortion. The planes of weakness are considered as known a priori or assumed to be oriented perpendicular to the direction of the current minor principal stress. An elastic-perfectly plastic behavior according to the Mohr-Coulomb criterion is assumed in the planes of weakness; while the elastic part is considered as linear and transversely isotropic. Finally, the proposed model was implemented in FLAC3D and used to simulate triaxial compressions to provide a verification of the implementation. The applicability of the implemented model to reproduce damage (pre-peak) and/or failure developments around a circular opening is checked. The GCS drift, one of the mine-by experiments set up at the main level of the Meuse/Haute-Marne Underground Research Laboratory, is selected for this first application.

Since 2000, the French National Radioactive Waste Management Agency (Andra) has been constructing an Underground Research Laboratory (URL) at Bure to perform experiments in order to obtain the in situ data necessary to demonstrate the feasibility of a geological repository in the Callovo-Oxfordian claystone (COx) formation. The excavation of galleries at the main level of the laboratory showed a significant fracturing induced by the excavation. One can distinguish a connected fractured zone (with extensional and shear fractures) and a zone of discrete fracturing. The extent of the fractured zones depends on the orientation of the drift with respect to the initial stress state (Armand et al. 2014). The short and long terms hydromechanical behavior is closely related to the behavior of the induced fractured zone as indicated by the in situ strain measurements. Similarly, the nature and extent of these fractured zones directly act on the magnitude of loading transferred to the concrete supporting elements. Hence it is necessary to account for the damage induced by the excavation in order to understand the long-term behavior of the couple COx – structural supports. Indeed, the loading-unloading of the plate test carried out at the wall of the GET gallery (parallel to the major horizontal stress) shows a change in the stiffness of the fractured zone as a function of the load (Vu et al., 2015). The increase of the rigidity can be explained by the mechanical closure of the induced fractures under compressive loading. This phenomenon is similar to those observed in the mechanics of rock joints or the re-closure of initial gap observed at the beginning of triaxial tests on soft rocks or soils. The evolution of the stiffness of the fractured zone, i.e., stiffness degradation due to excavation and partial stiffness recovery due to fracture closure affect the loads transferred to structural elements. The main goal of the present paper is to provide insights on the role of this stiffness change and its effect for a more accurate design of different repository structures.

National audience; The paper presents the first results on the mechanical behaviour of two chalks from underground quarries in the Paris Basin depending on the relative humidity of air. The influence of water on the short term was demonstrated from uniaxial compression testings conducted on unsaturated samples for several saturation degrees (corresponding to relative humidity of air from 36.5% to 98%). It has been established that unconfined compressive strength decreased inversely with the degree of saturation. The effect of water on the long term has also been studied. Ageing of chalk seems to be related to water-chalk chemical interactions. Variability of data put aside, It has been stated that physical and mechanical properties evolve considering the range in the pillar: the densities and compressive strength of chalk seems to increase from the pillar centre to the edge.; La communication présente les premiers résultats concernant le comportement mécanique en fonction de l'hygrométrie de deux craies en provenance de carrières souterraines du Bassin Parisien. L’influence de l'eau sur la craie à court terme a été mise en évidence à partir d'essais de compression uniaxiale effectués sur des échantillons à divers degrés de saturation correspondant à des humidités relatives variant de 36.5 % à 98 %. Il a été établi pour les deux craies que la résistance en compression simple diminuait lorsque le degré de saturation augmentait. L'effet de l'eau à plus long terme a été également étudié. Il apparaît un vieillissement de la craie lié aux interactions chimiques entre l'eau et la craie. A la variabilité des données près, il a été observé une diminution des masses volumiques (sèche et humide) et de la résistance en compression simple du bord de pilier vers le centre.

The paper presents the first results on the mechanical behaviour of two chalks from underground quarries in the Paris Basin depending on the relative humidity of air. The influence of water on the short term was demonstrated from uniaxial compression testings conducted on unsaturated samples for several saturation degrees (corresponding to relative humidity of air from 36.5% to 98%). It has been established that unconfined compressive strength decreased inversely with the degree of saturation. The effect of water on the long term has also been studied. Ageing of chalk seems to be related to water-chalk chemical interactions. Variability of data put aside, It has been stated that physical and mechanical properties evolve considering the range in the pillar: the densities and compressive strength of chalk seems to increase from the pillar centre to the edge., La communication présente les premiers résultats concernant le comportement mécanique en fonction de l'hygrométrie de deux craies en provenance de carrières souterraines du Bassin Parisien. L’influence de l'eau sur la craie à court terme a été mise en évidence à partir d'essais de compression uniaxiale effectués sur des échantillons à divers degrés de saturation correspondant à des humidités relatives variant de 36.5 % à 98 %. Il a été établi pour les deux craies que la résistance en compression simple diminuait lorsque le degré de saturation augmentait. L'effet de l'eau à plus long terme a été également étudié. Il apparaît un vieillissement de la craie lié aux interactions chimiques entre l'eau et la craie. A la variabilité des données près, il a été observé une diminution des masses volumiques (sèche et humide) et de la résistance en compression simple du bord de pilier vers le centre.

International audience; In the present work, an attempt has been achieved to describe the effects of ageing and of water on the hydro-mechanical behaviour of chalks from underground quarries of northern Parisian basin. Physical and mechanical properties were obtained as a function of distances from the pillar face (horizontal core drillings, from the wall to the centre) and relative humidity (Hr) of air. It has been stated that physical and mechanical properties change with depth in the pillar: the density and compressive strength of chalk tend to decrease from the pillar face (at least 1m from the wall) to the centre, contrary to the classical observations on the ageing of geomaterials. Uniaxial and triaxial compression tests under controlled relative humidity put into light the sensitivity to water (and then relative humidity in the quarry) on the mechanical strength and deformability of chalks.

Under high in situ stresses and high anisotropic stress ratios, the excavation of underground openings generally causes the creation of a disturbed (EdZ: Excavation disturbed Zone) and/or damaged (EDZ: Excavation Damaged Zone) zone, resulting from the initiation and growth of cracks and fractures and by a redistribution and rearrangement of the initial stresses. Due to the modifications of mechanical and hydromechanical properties in the EdZ/EDZ, the latter constitute a potential risk for the efficiency of the geologic and/or engineered barriers. To estimate the performance of a site of radioactive waste storage, it is essential to know the geometry, the extensions of the EdZ/EDZ, the density of cracks, their connectivity and the variations of permeability which can result from it. In particular, their extension depends on numerous factors among which the nature of the rock, the pre-existent fractures and their reactivation, the initial stress field (magnitudes and/or rate of anisotropy), the time, the geometry and the techniques of excavations etc.. In addition, the evolution of EdZ/EDZ properties in the medium term is little or poorly known, particularly under the influence of environmental conditions such as the reconfining by near field rock creep in contact with a rigid structural support or the evolution of hydric conditions (desaturation and resaturation). Ultrasonic experimentation under the OHZ experiment (one of the numerous in situ experiences carried out in the Meuse/Haute-Marne Underground Research Laboratory) has been implemented to characterize the EDZ extension and its evolution in time according to the structural support type (soft or rigid) and the environmental conditions present in the laboratory. This study consists of two experimental components : (1) the prior auscultation of the sole and face of the gallery by ultrasonic transmission tomography, (2) the monitoring the EDZ and the analysis of measurable changes in the propagation of ultrasonic waves in the medium term. Mechanical modelling of the gallery GCS (gallery with soft support and oriented parallel to the horizontal major stress) has predicted an extension of EDZ between 1.4 and 1.9 m according to the front advancement, the position around the gallery (roof, bottom and gallery side) and the rock characteristics. Moreover, a former experimentation showed that below the bottom of a gallery with this same orientation, ultrasonic velocity falls related to the damage were significant up to 2 m deep. The area of interest of experimentation OHZ encompasses at least this area with in addition one reference measurement in the undisturbed area. To escape heterogeneous variations of clay layers and the main anisotropy effect, two devices were selected with a triplet of horizontal drillings in the siding and a doublet of vertical drillings in the bottom of the gallery. P-wave tomographic data allowed rebuilding the images of the velocity on the 4 plans defined by the drillings. These images (figure 1) have shown a significant perturbation of the velocity field over 1.8 m deep in the gallery side. This area has been associated with the EDZ whose extension is of the same order of magnitude as which had been modeled during the blind predictions. Furthermore, there is an area of 0.4 m in depth on this siding with very low velocities or disappearance of ultrasonic rays in the 3 plans. That can be associated with fracturing of argillites. This very low velocity zone is not visible at the bottom, where velocity perturbations are much lower, with constant velocity over 1.6 m. The containment of the concrete floor, which is particularly thick at this location, might play a role in this observation. Finally, some anomalies of velocity are still visible beyond the end of the zone associated with the EDZ. They are probably caused by isolated fractures that occurred during excavation of the gallery and identified on extracted cores from the drillings. The magnitude of these anomalies would suggest that these fractures are closed. A program of inversion of these continuous in situ measurements of five elastic wave velocities (i.e. VP(0°), VSh(0°), VP(90°), VSv(?) and VP(?), where ? is off-axis) has been developed under Mathematica. Then, the five dynamic elastic cosntants for the assumed transverse isotropic character of the argillites are derived as a function of time and the distance from the gallery wall (Balland and Souley, 2011). From these elastic constants, some indicators of damage rate were proposed with respect to the space and the time (Sayers and Kachanov, 1995). Finally, in addition to the experimental data analyses, synthetic data (generated from the given elastic constants and velocities derived from the theoretical formulation) as well as data from literature (Sarout et al, 2007) (measurements of ultrasonic velocities in the laboratory on samples) allowed us to successfully evaluate the consistency of the results obtained with this program. The next step is the implementation of the same ultrasonic experimentation around the GCR Gallery which presents a rigid structural support, with the same orientation of GCS.

Research presented here is justified by several problematics in relation to the geoenvironmental applications: post-mining with regard to the mechanical stability of the abandoned mines due to a progressive rise of ground water after the end of the dewatering process; the deep underground storage of hydrocarburs or radioactive wastes. The approach adopted in this research consists to jointly undertake an experimental study, a theoretical modeling of physical phenomena (conceptual and numerical developments) and in situ applications. On the basis of the laboratory tests and/or in situ experiments carried out on several geomaterials (iron ore, claystones and granite; rock joints of granite, schist and sandstone), several conceptual models and rheological laws were developed. These constitutive models were also implemented in numerical codes and applied to interpret specific in situ experiments and to perform retro-analyses with confrontations between measurements and predictions. The practical applications involve structures of deep underground radioactive waste repository (such as the underground laboratory of Meuse/Haute-Marne in France) and the mining works. The details of methodology and the applications carried out are widely discussed in Souley (2010). Three examples are illustrated here: (a) a new experimental device allowing to characterize the hydraulic exchanges between fracture and matrix in a double porosity environment, (b) the developments of a macroscopic viscoplastic model of argillites aimed to improve the viscoplastic strain prediction in the EDZ (Extent of Damaged Zone) around the LS/MHM drifts are proposed based on the in situ observations, (c) the modeling of REP experimentations (including the hydromechanical behaviour) at the underground laboratory of Meuse/Haute-Marne (LS/MHM)., Après une période d’exploitation intense des mines, la France et d’autres pays européens sont confrontés aux problèmes posés par la fermeture et l’abandon de ces ouvrages, notamment leur stabilité mécanique sous l’effet d’une remontée progressive des eaux souterraines après l’arrêt des pompages d’exhaure. Par ailleurs, la gestion des ressources naturelles (eau, gaz et hydrocarbures), explique le grand intérêt pour la communauté scientifique d’étudier le comportement hydromécanique des massifs rocheux fracturés, composés d’une matrice (roche poreuse et microfissures) et de fractures souvent disposées en réseaux. Enfin, l’exigence de sécurité et de fiabilité des sites de stockage, notamment de déchets radioactifs a donné lieu, depuis plus d’une vingtaine d’années, à des études approfondies sur le compor tement thermohydromécanique de la roche hôte et des barrières ouvragées. Ce sont quelques exemples de problématiques géoenvironnementales, mettant en jeu les comportements mécanique, hydromécanique et thermohydromécanique (THM) complexes et illustrant la nécessité de mieux comprendre les phénomènes s’y rattachant. L’approche adoptée dans cette recherche consiste à mener de front un travail expérimental, un travail de modélisation théorique des phénomènes (développements conceptuel et numérique) et un travail d’applications aux structures sur site. Plus précisément, à l’issue des essais en laboratoire et/ ou d’observations in situ sur de nombreux matériaux (minerai de fer, argilites de l’Est et granite ; joints rocheux de granite, de schiste et de grès), plusieurs modèles conceptuels et lois de comportement ont été développés. Ces modèles constitutifs ont été implantés dans des codes de calculs, et des applications à des cas réels ont été réalisées aussi bien pour des interprétations d’essais ou d’expériences in situ spécifiques que pour des rétro-analyses et des confrontations avec les mesures. Les applications concernent les structures de stockage de déchets radioactifs (laboratoire souterrain de Meuse/ Haute-Marne et celui de Manitoba au Canada creusés entre 400 et 500 m de profondeur, la mine de Kamashi au Japon située à 1 000 m) et les ouvrages miniers (bassins houillers de Provence, bassins ferrifère et salifère lorrains). Cette méthodologie sera illustrée par trois exemples mettant en oeuvre l’expérience, la modélisation théorique et les applications.

International audience; With the objective to better understand the evolution of overburden damage on top of an underground solution mining, an in-situ experiment is undertaken above a salt cavity in the Lorraine region (NE of France). The overburden overlying the salt cavity is characterized by a competent layer where most brittle damage, with the associated microseismicity, is expected. This work presents a coupled continuum-discrete modelling approach to simulate the mechanics of fracture initiation and propagation in the rock mass, a continuum approach for the marls and salt layers, and a discrete micromechanical approach for the competent layer. A methodology of a large scale coupled continuum-discrete modelling is proposed and applied to the site of Cerville-Buissoncourt. The numerical verification of the hybrid approach response is achieved by comparing the results with those from the fully continuum model at the site scale when the competent layers behave elastically. The first results of a large scale modelling based on the hybrid approach suggest that the fracture mechanisms in the competent layers are predominantly tensile. Finally, these explicit microcracking simulations open interesting perspectives for comparison with the observed microseismicity of the study area (transient pressure experiment or future final collapse).; Dans l'objectif de mieux comprendre la microsismicité induite dans des mines de sel, une expérimentation in situ a été réalisée sur une cavité saline de la région Lorraine (Nord-Est de France). Le recouvrement est caractérisé par la présence d'un banc raide dans lequel l'essentiel de l'activité microsismique est attendue lors de la reprise de l'exploitation. Ce papier présente une modélisation basée sur une approche couplée : continue (pour les marnes et le sel) et discrète (pour les bancs raides) pour évaluer les mécanismes de développement des fractures dans le recouvrement. Une méthodologie de modélisation à grande échelle couplant les approches continue et discrète a d'abord été proposée, puis appliquée au site de Cerville-Buissoncourt. La validation numérique de cette approche hybride est réalisée par comparaison avec la réponse d'un calcul continu à l'échelle du site. Les premiers résultats de cette modélisation hybride à grande échelle montrent que les mécanismes de fracturation prédominants dans les bancs raides restent les tractions. Ceci ouvre des perspectives intéressantes pour des confrontations avec les mesures qui seront enregistrées lors de l'effondrement provoqué (essai de pression, future effondrement après la reprise de l'exploitation).

With the objective to better understand the evolution of overburden damage on top of an underground solution mining, an in-situ experiment is undertaken above a salt cavity in the Lorraine region (NE of France) until 2005. The overburden overlying the salt cavity is characterized by a competent layer where most brittle damage, with the associated microseismicity, is expected. This work is focussed on a coupled continuum-discrete modelling approach to simulate the mechanics of fracture initiation and propagation in the rock mass. A methodology of a large scale coupled continuum-discrete modelling is proposed and applied to the site of Cerville-Buissoncourt. The numerical verification of the hybrid approach response is achieved by comparing the results with those from the fully continuum model at the site scale when the competent layers behave elastically. The results are quite satisfactory. Moreover, the development of microcracking is in accordance with the predicted tensile zones obtained with a fully continuum approach. The sudden rise up of the microcracks cumulative number from a specific brine depth level on, can be interpreted as the fracture development by coalescence of microcracks within the competent bed, which may possible, indicate the beginning of a generalized instability of the overburden. Finally, the coupled continuum-discrete methodology presented herein provides a good opportunity to explicitly simulate the microcraking in the overburden. A conclusive statement about failure mechanisms, with eventually a quantitative comparison, will be achieved when information about microseismic event locations and magnitudes will be available., L’INERIS instrumente depuis 2005, dans le cadre du Groupement d’Intérêt Scientifique GISOS, une cavité saline en exploitation dans le bassin salifère lorrain. Le site d’expérimentation se situe à l’intérieur du périmètre de la concession minière de Cerville-Buissoncourt appartenant à la société SOLVAY. Il s’agit de mettre à profit l’occasion unique, offerte par l’évolution d’une grande cavité souterraine jusqu’à son effondrement provoqué volontairement (partie intégrante de la méthode d’exploitation), pour valider, en vraie grandeur, différentes techniques d’auscultation et de surveillance, employées par l’INERIS et d’autres partenaires du GISOS.

In the framework of the feasibility study of a radioactive-waste repository in a geological formation, Andra (French radioactive waste management agency) has been built an underground research laboratory within a Callovo-Oxfordian argillite formation located in Eastern France. During the sinking of the laboratory's access shaft, the hydromechanical behaviour of the argillites was monitored through an in situ experimental program called REP. The experimental zone is located between 460 and 476m depth. From a drift located at -445 m, 15 instrumentated boreholes were drilled downwards and equipped with 120 mechanical and hydraulic sensors. A predictive modelling of the shaft sinking has been performed in the framework of the European Modex-Rep project, using a poro-elastoplastic model based on a generalized Hoek and Brown criterion. Results of blind prediction emphasize that the model reproduces the in situ phenomena, but is not able to reproduce the amplitude of the drop in pore pressure during the shaft sinking. This article presents the analysis of this discrepancy and discusses new approaches aimed at improving the previous model. Finally, permeability changes around the shaft based on the in situ measurements and/or damage-induced permeability changes implemented in the model allow to better reproduce in situ data.

International audience; This paper presents several large-scale numerical modellings of valley and tray situations with the presence or not of vertical fractures. Through these modellings, we firstly attempt to evaluate an influence zone, in terms of stress variations induced by the creation of the valley. Next we study more particularly the behaviour of a stiff overburden, according to situations, with the aim to estimate if a valley and/or the presence of subvertical fractures influence the overburden massivity.; Ce papier présente plusieurs modélisations à grande échelle d'une situation de vallée et de plateau en présence ou non d'une fracturation verticale. A travers ces modélisations, nous tentons d'évaluer une zone d'influence, en termes de variations de contraintes induites par la création de la vallée. Puis nous étudions plus particulièrement le comportement d'une couverture raide, en fonction des situations, dans l'objectif d'estimer si une vallée et/ou une fracturation subverticale influencent la massivité du recouvrement

International audience; With the objective to better understand the induced microseismicity in a salt mine environment due to an underground solution mining, an in situ experiment is undertaken by GISOS in the Lorraine salt basin. The overburden overlying the salt cavity is characterized by the presence of two competent layers where most microseismic events are expected. This paper presents a coupled continuum-discrete modelling to simulate the mechanics of fracture initiation and propagation in the rock mass overlying the cavity: discrete approach for the competent layers and continuum approach for marls, salt and other rocks and soils. For the competent layers, numerous calibrations of the model microparameters based on the laboratory results are firstly performed. The first coupled modelling results suggest that the mechanism of fracturing in the competent layers is predominantly tensile as it could be expected. The results also show that the microseismic events associated to the progressive damage in the competent layers through microcraks development can be modelled. This opens interesting perspectives to assess the feasibility of seismic monitoring of underground cavities by comparing, in the future, the numerical modelling results with the recorded seismicity of the study area.; Dans l'objectif de mieux comprendre la microsismicité induite dans des mines de sel, une expérimentation in situ a été entreprise par le GISOS sur une cavité saline de la région Lorraine. Le recouvrement est caractérisé par la présence de deux bancs raides dans lesquels l'essentiel de l'activité microsismique est attendue lors de la reprise de l'exploitation. Ce papier présente une modélisation basée sur une approche couplée : continue (pour les marnes et le sel) et discrète (pour les bancs raides) pour évaluer les mécanismes de développement des fractures dans le recouvrement. De nombreuses calibrations des microparamètres du modèle discret à partir des paramètres macroscopiques ont été nécessaires. Les résultats de la modélisation montrent que le mécanisme de fracturation dans les bancs compétents résulte essentiellement d'efforts en traction. Les premiers résultats montrent aussi que les évènements microsismiques peuvent très bien être mesurés à travers la formation des microfissures. Ceci ouvre des perspectives intéressantes pour des confrontations avec les mesures qui seront enregistrées lors de l'effondrement provoqué.