Callovo-Oxfordian Argillite Research Articles

A micromechanical model for the elasto-viscoplastic and damage behavior of a cohesive geomaterial

A Hill type incremental homogenization method was recently proposed by Abou-Chakra Guéry et al. [Abou-Chakra Guéry, et al., 2008. A micromechanical model of elasto-plastic and damage behavior of a cohesive geomaterial. Int. J. Solid. Struct., 45(5), 1406–1429] for the elasto-plastic damage behavior of a cohesive geomaterial, the Callovo–Oxfordian argillite. The aim of this paper is then to extend the homogenization method to its time-dependent behavior. The argillite is now seen as a three phase composite with an elasto-viscoplasticity matrix and elastic damaged calcite grains and elastic quartz grains. Considering that the incremental formulation of Hill cannot rigorously be used, we proposed a new modified incremental method. A validation of its predictions against experimental data is then conducted.

Influence of initial iodide concentration on the iodide uptake by the argillite of Tournemire

Among the radionuclides that can migrate from radioactive wastes buried within consolidated argillaceous rocks, 129I poses a particular problem due to its high mobility in geomedia and its very long half-life of 1.6 10 7 years. Previous studies carried out on such rocks showed the existence of a low iodide retardation regarding the other anion species. The purpose of the present study was two-fold. It was hoped (1) to confirm this retardation by means of radial diffusion experiments and (2) to estimate the influence of the initial iodide concentrations towards the iodide uptake. The results obtained from in-diffusion stage show that (1) no iodide uptake occurs at high initial concentrations and (2) an uptake at low initial concentrations cannot be clearly evidenced. Finally, the most singular behaviour of iodide was highlighted for all the initial iodide concentrations, from the out-diffusion stage, during which some amount of iodide, available for out-diffusion, were not involved in the diffusion process. Although the origin of such phenomenon remains unclear, a kinetic effect for the iodide uptake was proposed, assuming that the period of 55days (corresponding to the in-diffusion stage duration) would be necessary for inducing the iodide uptake. This kinetic effect, already shown elsewhere by (Descostes, M., Blin, V., Bazer-Bachi, F., Meier, P., Grenut, B., Radwan, J., Schlegel, M.L., Buschaert, S., Coelho, D, Tevissen, E. Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones. In press for Appl. Geochem.) on similar materials, needs to be deepened.

Damage and sealing of clay rocks detected by measurements of gas permeability

Various damage and sealing experiments have been performed on samples of the Callovo-Oxfordian argillite and the Opalinus clay in order to enhance the knowledge about the development of the excavation damaged zone around nuclear waste repositories in clay formations. Stress-induced damage and permeability changes were examined in triaxial compression tests. Preliminary criteria were derived for the detection of dilatancy, percolation, and failure boundaries. By measuring the gas permeability changes due to re-compaction and re-saturation, the self-sealing behaviour of strongly damaged clay samples was investigated. Short-term re-compaction by increasing the confining stress up to 15 MPa reduced the permeability by several orders of magnitude to 10 −19–10 −20 m 2. After water re-saturation, the permeability of the damaged samples decreased significantly to 10 −21 m 2. All laboratory observations suggest a high self-sealing capacity of the studied clay rocks.

Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones (Meuse/Haute–Marne, France)

Abstract Diffusive parameters of tritiated water (HTO) and several anionic tracers (36Cl−, 125I−, 35 SO 4 2 - and 75 SeO 3 2 - ) were determined in the Callovo-Oxfordian argillite formation (Meuse/Haute–Marne, France), and also in the Oxfordian limestone formation, located just above. Twenty two drillcore samples, from depths of 150–480 m and from almost each lithofacies along the same borehole EST205, were tested. Three distinct and complementary experimental set-ups were used (batch, column filled with crushed argillite and through-diffusion experiments) associated with a thorough characterisation of the rock, care being taken to maintain redox conditions. Tritiated water behaviour is similar to water which diffuses in the total porosity of the rock. Tritiated water and anionic tracers displayed analogous diffusive behaviour in calcareous samples with ea(anion)/ea(HTO) = 1 and De(anion)/De(HTO) close to 0.85, i.e. the ratio of their diffusion coefficients in free water. Lower and scattered values were observed in the lower part of the Oxfordian limestones which is characterized by the presence of mineralogical heterogeneities and dolomitic diagenesis. In the Callovo-Oxfordian argillite formation, diffusive fluxes of 36Cl−, 125I− and 35 SO 4 2 - are similar but lower than that of HTO. The diffusive behaviour analogy between I− and Cl− was verified, as De(halide)/De(HTO) and ea(halide)/ea(HTO) ratios were always lower than 1. This reduction in diffusivity is a consequence of the anionic exclusion, which limits the diffusion-accessible porosity to a value lower than the total porosity for anions. A weak sorption of I− and SO 4 2 - on argillite samples was always observed whatever the technique used. In the case of I−, this sorption appears to be partly reversible and kinetically controlled. Very low values of diffusive parameters were obtained for 75 SeO 3 2 - and are still not well explained. Last, diffusive parameters of each radioactive tracer tested can be plotted according to Archie’s law, which therefore allows an estimation of De(non-reactive species) from the knowledge of both the sample total porosity and the corresponding exponent m = 2 for argillite samples.

Comparison of the bulk geochemical features and thermal reactivity of kerogens from Mol (Boom Clay), Bure (Callovo–Oxfordian argillite) and Tournemire (Toarcian shales) underground research laboratories

Deep argillaceous formations are potential repositories for the long-term disposal of nuclear waste because of their low permeability and high sorption capacity with respect to radioelements and heavy metals. Such sedimentary rocks contain organic matter, mostly macromolecular and insoluble (kerogen). Upon temperature elevation related to high-level long-lived radioactive waste disposal, the kerogen may release significant quantities of gaseous and liquid effluents, especially oxygen-containing ones, which may influence the ability of the clay to retain radionuclides. The aim of the present study is to assess the global geochemical features and the thermal reactivity of the kerogens isolated from samples collected in the Bure and Tournemire sites, France (Callovo–Oxfordian Clay and Toarcian Shales, respectively) and to draw comparisons with data previously obtained for the Mol site, Belgium (Boom Clay). The study is based on a combination of elemental, spectroscopic (FTIR, solid state 13C NMR) and pyrolytic (Rock-Eval pyrolysis, Curie point pyrolysis–gas chromatography/mass spectrometry) analyses. Different levels of maturity and resulting differences in the relative abundance of oxygen-containing groups were thus observed for the three kerogens. This is linked with differences in their ability to generate CO 2 and various oxygen-containing, low molecular weight, water-soluble compounds under thermal stress, decreasing from Mol to Bure and to Tournemire.

A micromechanical model of elastoplastic and damage behavior of a cohesive geomaterial

The present study is devoted to the development and validation of a nonlinear homogenization approach of the mechanical behavior of Callovo-Oxfordian argillites. The material is modeled as an heterogeneous composite composed of an elastoplastic clay matrix and of linear elastic or elastic damage inclusions. The macroscopic constitutive law is obtained by adapting the incremental method proposed by Hill [Hill, R., 1965. Continuum micro-mechanics of elastoplastic polycrystals. J. Mech. Phys. Solids 13, 89–101]. The approach consists in formulating the macroscopic tangent operator of the material by considering the nonlinear local behavior of each phase. Due to the matrix/inclusion morphology of the microstructure of the argillite, a Mori–Tanaka scheme is considered for the localization step. The developed model is first compared to Finite Element calculations and then validated and applied for the prediction of the macroscopic stress–strain responses of argillites.

Micromechanics based modeling of the Callovo-Oxfordian argillite mechanical behavior

The present study is devoted to the development and validation of a non-linear homogenization approach of the mechanical behavior of Callovo-Oxfordian argillites. The material is modelled as an heterogeneous one composed of an elastoplastic clay matrix and of linear elastic or elastic damage inclusions. The macroscopic constitutive law is obtained by adapting the Hill-type incremental method [1]. The approach consists in formulating the macroscopic tangent operator of the material from the non-linear local behavior of its phases. Due to the matrix/inclusion morphology of the microstructure of the argillites, a Mori-Tanaka scheme is considered for the localization step. The developed model is first compared to Finite-Elements calculations and then validated and applied for the prediction of the macroscopic stress-strain responses of argillites.

Anisotropy of elastic, magnetic, and microstructural properties of the Callovo-Oxfordian argillite

Anisotropy of elastic, magnetic, and microstructural properties of the Callovo-Oxfordian argillite

Temperature-induced evolution of the elastic and magnetic anisotropy in argillite samples from Bure underground research laboratory, eastern France

Abstract The influence of temperature on magnetic and elastic properties of the Callovo-Oxfordian argillite formation, host of the French underground research laboratory for studying radioactive waste repository, has been investigated following two types of experiments. In the first experiment, rock samples were heated at 95 °C in an oven for 41 days, and in the second, heating–cooling cycles at increasing maximum temperatures were applied. Magnetic susceptibility and P-wave velocity measurements were conducted to investigate the effect of heating not only on the absolute values but also on the anisotropy. Whereas little variation was observed in the first experiment, the results of the heating–cooling cycles showed that different stages can be identified in the evolution of P-wave velocity and magnetic susceptibility with maximum temperature applied. At temperatures below 200 °C, the velocity increases sharply whereas the magnetic susceptibility remains unchanged: this is interpreted as being the result of mineralogical transformations, probably associated with dewatering of the clays. Above 200 °C, a large increase in susceptibility associated with a levelling off (or even a decreasing trend) of the velocity results from the competition between thermal cracking processes and another kind of mineralogical transformation affecting both properties. A magnetic mineralogical study showed that the main ferromagnetic mineral present is magnetite. An interesting result of the study is that in the temperature range investigated, the elastic properties are enhanced and the anisotropy is increased by raising the temperature, which has important consequences for the seismic monitoring of the storage site.

Determination of hydraulic properties of the Callovo-Oxfordian argillite at the bure site: Synthesis of the results obtained in deep boreholes using several in situ investigation techniques

Since 1991, ANDRA ( Agence Nationale pour la gestion des Déchets Radioactifs – National Radioactive Waste Management Agency) has been performing research on the possibility of geologic disposal of high level radioactive waste. In 1999, Andra began constructing an Underground Research Laboratory at Bure, a site located on the border of the Meuse-Haute-Marne departments, 300 km East of Paris. The laboratory is investigating the Callovo-Oxfordian argillite, a 130 m thick middle Jurassic stratum, at a depth of about 420 m. Argillite is a clay-rich sedimentary rock with low-permeability. Between 1994 and 2004, Andra collected from deep boreholes an impressive wealth of data covering a wide range of geosciences. This paper focuses on the hydraulic data related to argillite, including the results from short-term hydraulic packer tests and long-term monitoring of the formation pressures. Three types of tools are used on the site for investigations in deep boreholes. The first one is a conventional packer test tool used in the petroleum industry and adapted for hydrogeological purposes. The main objective is to determinate the permeability of the formation through short-term tests (24–72 h) at about 10 regular intervals. The two other types of tool are permanent monitoring devices. The electromagnetic pressure gauge (EPG) is totally isolated from the surface perturbations. There are no electric or hydraulic lines to the surface and the borehole is cemented. The advantage of this tool is that the formation almost recovers its initial pressure, avoiding disturbances from surface. Although the multi-packer equipment, installed in an open borehole can be affected by surface perturbations, it is used to measure pressure at different isolated levels in the same borehole ( i. e., 11 chambers in one borehole). Evaluations of the formation pressure (freshwater head) and hydraulic conductivity have been performed for all intervals investigated (19 short-term packer tests and 15 long-term monitoring sections). Borehole simulators were used to define a suitable flow model taking into account the complete pressure history of the borehole, and to derive best-guess estimates and uncertainty ranges for the hydraulic parameters. The sources of perturbations and the consistency of results are discussed in this paper. For instance, for a same interval tested through different techniques, an overestimation by one order of magnitude of the hydraulic conductivity due to a large overestimation of pore pressure during packer test was observed. In situ permeability estimations are also compared with those obtained from laboratory tests on core samples. Both short-term and long-term measurements provide values for the hydraulic conductivity at different scales with high consistency. This parameter is shown to be less than 2 × 10 −12 m/s. Pressures measurements from long-term monitoring are sufficiently accurate for determining formation hydraulic heads. A pressure profile in the argillite, derived from the extensive set of data currently available, shows an overpressure in the argillite 20–60 m above its surrounding formations. As a whole, the pressure data and derived hydraulic properties acquired from deep boreholes, offer a high degree of reliability and constitute a major contribution to the hydraulic characterisation of the low-permeable argillite formation. In 2006, this data will be complemented with measurements carried out in the Laboratory at 490 m depth, with the aim to characterize in greater depth the pressure profile of the argillite.

Influence of creep on water pressure measured from borehole tests in the Meuse/Haute-Marne Callovo-Oxfordian argillites

Porewater pressure is an important parameter for use in safety assessment of underground waste disposal. In order to measure porewater pressure in the Callovo-Oxfordian argillites, ANDRA has performed several in-situ tests, which consist in measuring time evolution of water pressure in an almost closed chamber. This was accomplished using an in-place pressure sensor coupled with an electromagnetic transmission device (called an EPG probe). The measured values show a small hydraulic overpressure (0.1 MPa) compared with the estimated value at the corresponding depth. In the framework of a scientific cooperation agreement between Andra and Ineris, a study was undertaken to examine whether all or a part of this overpressure could be attributed to the hydro-mechanical coupled processes linked with the creep of argillites and the stress relaxation in the experimental measuring chamber-nearby. In the whole, the numerical results were in good agreement with the measured results. Poroviscoplasticity can explain the increasing pressure in the borehole. The measured overpressure can be reached with adequate viscoplastic model parameters.

Pore network connectivity anisotropy in Jurassic argillite specimens from eastern Paris Basin (France)

In order to test the feasibility of nuclear waste storage, Andra, the French radioactive waste management agency, gave us the opportunity to study preserved specimens of Jurassic clay-rich rocks from eastern Paris Basin. These rocks, deposited during the Callovian and beginning of the Oxfordian, are dark- to light-grey marls that consist mainly in a mixture of clay, calcite and silt. Magnetic susceptibility and remanence vary according to the clay/calcite/silt ratios and the mineral preferred orientations are characterized by the anisotropy of the magnetic susceptibility. A few test specimens, sampled from borehole-core #HTM 102, and coming from the base and top levels of the Callovo-Oxfordian argillite formation, were subjected to connected porosity measurements using the mercury injection technique. By imposing mercury to flow parallel to a given direction, we were able to determine the anisotropy of connectivity along the three principal magnetic susceptibility axes. We find that the clay-richest specimens have a large and sub-isotropic connected porosity which is mostly accessible through the smallest pore threshold diameters (<0.02 μm). By contrast, carbonate-enriched specimens have anisotropic and smaller connected porosities accessible through larger pore thresholds (∼0.08 μm). Except in a carbonate-enriched specimen where the largest connectivity axis is vertical, attributed to tension cracks normal to bedding, the pore connectivity anisotropy positively correlates with the magnetic anisotropy, hence with the mineral arrangement.

Permeability of macro-cracked argillite under confinement: Gas and water testing

Argillite is considered a privileged candidate for long term nuclear waste storage. Yet argillite rock drilling often induces surface cracks that locally modify its permeability. This phenomenon located in a so-called Excavation Damaged Zone (EDZ) is of importance since permeability increase means lesser confinement capacity of the argillite rock. Potentially influencial phenomena occur when argillite is subjected simultaneously to normal stress variations and fluid seepage. Therefore, this extensive experimental study (6 month duration) on macro-cracked Callovo-Oxfordian argillite is aimed at distinguishing the contribution to rock permeability of mechanical loading (crack opening and closing) on one part and of chemically active fluid seepage (water) on the other. Steady state gas flow tests show that permeability K mainly depends upon crack closure cc, with values on the order of 10 −14 m 2. Permeability from transient water flow tests varies with test duration from 10 −18 to 10 −21 m 2. In both test types, K also depends upon confining pressure P c, mainly during the first three loading–unloading phases. A difference between water injection tests and gas injection tests is that the water-saturated rock sample swells. Swelling does not contribute to unload the crack zone but rather creates additional closure and pressure in the crack area. Indeed, water permeability is shown to depend upon cumulated crack closure a c, which sums up swelling and confinement-induced crack closure. Finally, this study outlines the strong effect of water upon crack closure amplitude and permeability. After a relatively short time (on the order of ten days), water flow within the crack drives the permeability back to very low values close to sound rock permeability (10 −21 m 2). This reflects a complete self-sealing of the macro-crack, which is an important factor for nuclear waste repository safety.

Characterization of sulphate sorption on Callovo-Oxfordian argillites by batch, column and through-diffusion experiments

Callovo-Oxfordian argillites are under investigation as a potential host rock for a high-level radioactive waste repository. The purpose of this study was to investigate sulphate sorption on two borehole samples (EST205 K119c and K100c), using three different and complementary laboratory techniques. Batch experiments revealed a fast, weak and reversible sorption of sulphate ions on Callovo-Oxfordian argillites. Quantification was difficult with this method due to uncertainties on low K d values obtained. Column and through-diffusion experiments confirmed this retention and allowed a better quantification. A clear dependence of sulphate sorption on both mineralogical composition and sulphate content was shown. For the K119c argillite, the sulphate isotherm was fitted with a Langmuir isotherm. K d values are ranging from 0.15 to 0.37 mL g −1, depending on total sulphate content.

Scientific investigation in deep wells for nuclear waste disposal studies at the Meuse/Haute Marne underground research laboratory, Northeastern France

Andra, the French National Radioactive Waste Management Agency, is constructing an underground test facility to study the feasibility of a radioactive waste disposal in the Jurassic-age Callovo-Oxfordian argillites. This paper describes the processes, the methods and results of a scientific characterization program carried out from the surface via deep boreholes with the aim to build a research facility for radioactive waste disposal. In particular this paper shows the evolution of the drilling programs and the borehole set up due to the refinement of the scientific objectives from 1994 to 2004. The pre-investigation phase on the Meuse/Haute-Marne site started in 1994. It consisted in drilling seven scientific boreholes. This phase, completed in 1996, led to the first regional geological cross-section showing the main geometrical characteristics of the host rock. Investigations on the laboratory site prior to the sinking of two shafts started in November 1999. The sinking of the shafts started in September 2000 with the auxiliary shaft completed in October 2004. The experimental gallery, at a depth of 445 m in the main shaft, was in operation by end 2004. During the construction of the laboratory, two major scientific programs were initiated to improve the existing knowledge of the regional hydrogeological characteristics and to accelerate the process of data acquisition on the shales. The aim of the 2003 hydrogeological drilling program was to determine, at regional scale, the properties of groundwater transport and to sample the water in the Oxfordian and Dogger limestones. The 2003–2004 programs consisted in drilling nine deep boreholes, four of which were slanted, to achieve an accurate definition of the structural features.

Experimental study of the thermo-hydro-mechanical behaviour of indurated clays

Indurated clays are being considered in a number of counties as a host rock for disposal of radioactive waste. This paper describes laboratory experiments carried out at the GRS geotechnical laboratory in Braunschweig on core samples taken from the Callovo-Oxfordian argillite at Bure in France and the Opalinus clay at Mont Terri in Switzerland. The experiments focused on investigating responses of the clay rocks to thermo-hydro-mechanical (THM) loading similar to that expected in repositories. Very complex coupled THM phenomena were observed in the tests, such as significant swelling and shrinking induced by wetting and drying, stress reaction to hydration and dehydration, pore-water pressure increase, thermal expansion and contraction, thermal effects on deformation and strength, and self-sealing. All the laboratory observations suggest that adsorbed pore-water is a key factor dominating the THM properties and processes in indurated clays.

Anisotropy of elastic, magnetic and microstructural properties of the Callovo-Oxfordian argillite

We investigated the anisotropic behaviour of the Callovo-Oxfordian argillite in laboratory experiments, and analysed the rock microstructure using different imaging techniques. The starting material was a core retrieved at 460 m depth in well EST204 at the site of the French Underground Research Laboratory in the Meuse/Haute-Marne region, from which 17 small subcores were prepared in three orthogonal directions. The anisotropy of magnetic susceptibility (AMS) was studied using a KLY3S kappabridge. The technique allows one to determine the susceptibility tensor which can be represented by an ellipsoid with principal axes K max, K int and K min in decreasing order. We found that the AMS ellipsoid is consistent with an oblate fabric. We have also investigated the elastic anisotropy through the measurement of P-wave velocity in different directions, following the procedure described in our previous works [Louis, L., David, C., Robion, P., 2003. Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions. Tectonophysics 370, 193–212; Louis, L., Robion, P., David, C., 2004. A single method for the inversion of anisotropic data sets with application to structural studies. Journal of Structural Geology 26, 2065–2072]. The anisotropy of P-wave velocity is significantly larger than the anisotropy of magnetic susceptibility. Interestingly, we found an excellent correlation between AMS and the anisotropy of P-wave velocity, with a minimum velocity V min and susceptibility K min fairly well constrained in the direction perpendicular to bedding. Both from the elastic and magnetic viewpoint, the argillite behave as a transverse isotropic medium, which is typical of a sedimentary fabric with no clear sign of tectonic fabric. Our data set on P- and S-wave velocity measurements allowed for the full determination of the transverse isotropy stiffness tensor as well as the Thomsen’s parameters within the framework of weak anisotropy. We also present some results of a study of the rock microstructure and its structural anisotropy using stereological techniques and X-ray microtomography. The microtomography technique revealed the existence of tubular structures made of dense material (pyrite), the origin of which is not clearly defined but has probably to do with bioturbation during the formation of the sediment.

Evaluation of damage-induced permeability using a three-dimensional Adaptive Continuum/Discontinuum Code (AC/DC)

Digging a shaft or drift inside a rock mass is a common practice in civil engineering when a transportation way, such as a motorway, railway tunnel or storage shaft is to be built. In most cases, the consequences of the disturbance on the medium must be known in order to estimate the behaviour of the disturbed rock mass. Indeed, excavating part of the rock causes a new distribution of the stress field around the excavation that can lead to micro-cracking and even to the failure of some rock volume in the vicinity of the shaft. Consequently, the formed micro-cracks modify the mechanical and hydraulic properties of the rock. In this paper, we present an original method for the evaluation of damage-induced permeability. ITASCA has developed and used discontinuum models to study rock damage by building particle assemblies and checking the breakage of bonds under stress. However, such models are limited in size by the very large number of particles needed to model even a comparatively small volume of rock. In fact, a large part of most models never experiences large strains and does not require the accurate description of large-strain/damage/post-peak behaviour afforded by a discontinuum model. Thus, a large model frequently can be separated into a strongly strained “core” area to be represented by a Discontinuum and a peripheral area for which continuum zones would be adequate. Based on this observation, Itasca has developed a coupled, three-dimensional, continuum/discontinuum modelling approach. The new approach, termed Adaptive Continuum/Discontinuum Code (AC/DC), is based on the use of a periodic discontinuum “base brick” for which more or less simplified continuum equivalents are derived. Depending on the level of deformation in each part of the model, the AC/DC code can dynamically select the appropriate brick type to be used. In this paper, we apply the new approach to an excavation performed in the Bure site, at which the French nuclear waste agency, ANDRA, is building an underground experimental laboratory. The modelling aims at predicting the damage-induced variations of permeability at the periphery of a shaft or drift during the first two years of its opening. In the first part of this paper, we present the concepts that have been developed to build a numerically efficient mixed model while minimizing the effect of continuum/discontinuum boundaries. We also describe the particle model developed to reproduce the short- and long-term behaviours of Callovo-Oxfordian argillite. In the second part of the paper, we present the numerical model built to simulate shaft excavation. We summarize the calibration process used to define the short- and long-term parameters, as well as the assumptions considered. Finally, we present an interpretation in terms of crack-induced permeability at the periphery of the shaft. First, a (crack transmissivity vs. opening) relation is calibrated using laboratory results and then applied to the crack network generated during the mechanical simulation. As the underground laboratory currently is being excavated, we will be able to compare our results with experience in the future. The predictions are plausible and show expected orders of magnitude.

Saturated and unsaturated behaviour modelling of Meuse–Haute/Marne argillite

Poromechanical behaviour modelling of the Callovo–Oxfordian argillite under saturated and partially saturated conditions is proposed using the equivalent stress concept. In comparison with the previous works on this rock, the particular form of the yield criterion and the plastic flow potential proposed here help to better describe the rock behaviour in tension–stress paths. The evolution of the poroelastic parameters due to the induced cracks is also considered in a simple way. Due to its physical nature, different from classical soils, the partially saturated behaviour of this rock could not be correctly described by any of the pre-existing theories usually used for the partially saturated porous media (soils). Based upon experimental results on this rock and developing some ideas proposed by other authors, an extension of the saturated elastic–plastic model in unsaturated field is proposed. The key hypothesis of this extension is the evolution of the Biot’s coefficient as a function of the suction, justified by laboratory results and micromechanical analyses. The predictions of the model in saturated and partially saturated conditions are compared with laboratory results and a good general agreement is found.

Paramètres contrôlant la distribution de la fracturation : exemple dans une série marno-calcaire du Jurassique supérieur (Est du bassin de Paris)

Study of fractures along a 490-m vertical section of marl/limestone alternations in the Upper Jurassic (Meuse/Haute-Marne underground research laboratory–eastern Paris Basin) reveals their organization and the different states of palaeostress. Type and extension of tectonic structures seem to be controlled principally by lithology and secondary by depth. Also, it appears deviations of Alpine palaeostresses between Kimmeridgian and Oxfordian formations. These deviations are related to the presence of marl/limestone contacts. The vertical evolution of current horizontal maximum stress shows a similar behaviour, with deviations at the walls of Callovo-Oxfordian argillites. These results allow us to point out and to discuss the impact of lithology, rheology and depth on fracturing occurrence and distribution. Furthermore, this study suggests the role of Callovo-Oxfordian as a barrier for fracture development between the limestones of Dogger and Oxfordian formations. To cite this article: G. André et al., C. R. Geoscience 338 (2006).