Callovo-Oxfordian Claystone Research Articles

Multi-scale modelling of gas flow in nanoscale pore space with fractures

Abstract In this work, a multi-scale pore network with fractures is developed against experimental data in a wide range of degrees of water saturation. The pore network is constructed based on the measured microstructure information at several length scales. The gas transport is predicted by different gas transport equations (e.g. Javadpour, dusty gas model (DGM), Civan and Klinkenberg), which can consider the fundamental physics mechanisms in tight porous media, such as Knudsen diffusion and viscous flow. Then, the model is applied to simulating the gas permeability of the Callovo-Oxfordian (COx) claystone. The predicted gas permeability is basically in good agreement with the experimental data under different degrees of water saturation. Then the effects of micro-fissures are studied. The results suggest that this model can predict the gas flow in other tight porous media as well and can be applied to other fields such as carbon capture and storage.

Statistical representative elementary area of shale inferred by micromechanics

This paper provides new estimates for the Representative Elementary Area (REA) sizes of the following two shales that are actively being studied in the framework of deep disposal of radioactive waste: Callovo-Oxfordian (COx) claystone from the Meuse/Haute-Marne underground research laboratory (Eastern France) and Toarcian argillite from the experimental station of Tournemire (Southern France). The REA sizes, named LREA, were obtained from two mineral maps following the classic “counting box” (CB) method and a statistical approach that introduces the concept of a “statistical” REA. Following this approach, a “statistical” REA is related not only to the microstructure and the properties of each of the components but, above all, to a given precision in the estimation of the effective property depending on the number of realizations that one is ready to generate. The probabilistic concept of realization is here, from a practical viewpoint, a subdomain of a mineral map in which the apparent morphological or mechanical properties have to be calculated. In this study, the apparent elastic moduli of the subdomain have been estimated using two micromechanical models. The first micromechanical model consisted of an inclusion-based model for which spherical nonclay grain is embedded in a clay matrix in which the values of its transverse isotropic stiffness tensor have been taken from literature. The second micromechanical model was an isotropic inclusion-based model for which a spherical nonclay grain is embedded in a clay matrix; the elastic moduli values have been inverted by a Monte-Carlo approach from the engineering moduli of both shales under study. Our calculations have shown the following results: (i) the statistical morphological LREA considering the surface clay fraction are of the same order of magnitude as those measured in other shales and those obtained by the simple CB method, with relative error values between 5 and 10%, (ii) the mechanical LREA values associated with the bulk modulus and shear modulus are significantly greater than that of the morphological LREA, and (iii) the mechanical LREA estimates of the shear modulus are greater than that of the bulk modulus. Moreover, our study highlights that these qualitative results do not depend on the chosen micromechanical models and, thus, would be independent of the underlying anisotropic nature of shale.

Investigation on anisotropy of mechanical properties of Callovo-Oxfordian claystone

Anisotropic mechanical properties of the sedimentary Callovo-Oxfordian clay rock were investigated with triaxial compression and extension, cyclic compression, and creep tests. The tested cylindrical samples differ in bedding orientation. The compression was accomplished by increasing axial stress at constant radial stress. The extension was carried out vice versa: radial stress was increased while axial stress was kept constant. Moreover, extension was also achieved by simultaneously increasing radial stress and decreasing axial stress under constant mean stress. Five independent elastic parameters were obtained at different deviatoric stresses during cyclic loading-unloading compressions. The deformation and strength were determined in relation to the load direction inclined to bedding and in relation to the load paths. The claystone exhibits elastoplastic stress-strain behaviour with volume compaction until failure. The strength is dependent on both load path and load direction to the bedding. For a given major load direction, the compressive strength is higher than the extensional one and the lowest strength occurs by extension at constant mean stress. In each load path, the maximum strength values are reached parallel and normal to the bedding, while the minimum occurs at the load orientation of about 45° during compression and of about 30° during extension. The elastic stiffness increases with deviatoric loading due to hardening. The deformability is time dependent.

Determination of Multiple Thermo-Hydro-Mechanical Rock Properties in a Single Transient Experiment: Application to Shales

The experimental determination of the thermo-poro-elastic parameters of low-permeability geomaterials is difficult because particular care has to be taken with respect to the homogeneity of the pore pressure field during the tests. A new transient step loading procedure is presented for isotropic rock tests in drained and undrained conditions, and under thermal and mechanical loading. Two different proposed protocols allow to measure various drained and undrained parameters in one single test. The observed specimen deformations are analysed based on analytical solutions for the pore pressure diffusion (Braun et al. in Rock Mech Rock Eng 51:1361–1378, 2018), which enables to back calculate the material permeability. This provides more information in less time, compared to conventional monotonic loading experiments. A demonstration is given on the Callovo–Oxfordian claystone.

Microbial and Geochemical Investigation down to 2000 m Deep Triassic Rock (Meuse/Haute Marne, France)

In 2008, as part of a feasibility study for radioactive waste disposal in deep geological formations, the French National Radioactive Waste Management Agency (ANDRA) drilled several boreholes in the transposition zone in order to define the potential variations in the properties of the Callovo–Oxfordian claystone formation. This consisted of a rare opportunity to investigate the deep continental biosphere that is still poorly known. Four rock cores, from 1709, 1804, 1865, and 1935 m below land surface, were collected from Lower and Middle Triassic formations in the Paris Basin (France) to investigate their microbial and geochemical composition. Rock leachates showed high salinities ranging from 100 to 365 g·L−1 NaCl, current temperatures averaging 65 °C, no detectable organic matter, and very fine porosity. Microbial composition was studied using a dual cultural and molecular approach. While the broad-spectrum cultural media that was used to activate microbial communities was unsuccessful, the genetic investigation of the dominant 16S rRNA gene sequences revealed eight bacterial genera considered as truly indigenous to the Triassic cores. Retrieved taxa were affiliated to aerobic and facultative anaerobic taxon, mostly unknown to grow in very saline media, except for one taxon related to Halomonas. They included Firmicutes and α-, β-, and γ-Proteobacteria members that are known from many subsurface environments and deep terrestrial and marine ecosystems. As suggested by geochemical analyses of rocks and rock leachates, part of the indigenous bacterial community may originate from a cold paleo-recharge of the Trias aquifer with water originating from ice melting. Thus, retrieved DNA would be fossil DNA. As previously put forward to explain the lack of evidence of microbial life in deep sandstone, another hypothesis is a possible paleo-sterilisation that is based on the poly-extremophilic character of the confined Triassic sandstones, which present high salinity and temperature.

Some aspects of the hydro-mechanical behaviour of Callovo-Oxfordian (COx) claystone around a gallery parallel to the principal horizontal minor stress

The Meuse/Haute-Marne Underground Research Laboratory (MHM URL) is an outstanding scientific tool that provides reliable data on the behaviour of Callovo-Oxfordian (COx) claystone. More than 10,000 sensors have been installed in the argillaceous formation to measure rock properties and behaviour under several configurations in order to assess coupled processes. Since 2004, data have been acquired continuously through several measurement campaigns –mainly in galleries at the main level, −490 m (clay unit level –, i.e. UA level), but also in a gallery at −445 m [silty carbonate unit level (USC)]. This experiment has increased understanding of the hydro-mechanical behaviour of claystone and highlight various phenomena. The research program carried out in the MHM URL galleries also is dedicated to technology improvement and demonstration issues of several disposal systems.1 At MHM URL, the principal horizontal major stress (σH) is oriented at NE150° , and the principal vertical stress (σv) is nearly equal to the principal horizontal minor stress (σh) that is oriented NE065° . The ratio σH∕σh is close to 1.3 and varies with depth and the rheological characteristics of the respective layers. This paper presents the hydro-mechanical coupling (i.e. interaction between the interstitial water and the solid skeleton) during and after excavation work. The influence of the in-situ stress fields on several types of support was studied for a gallery oriented according to the principal minor horizontal stress (σh). This paper presents an analysis of measurements recorded during and after the construction of an experimental gallery, referred to as the GER, from 2013 to 2015. The GER experimental gallery is an 83-m-long tunnel, in which four different supports were tested: sliding arches; shotcrete; compressible wedges; and poured concrete. Sliding arches, shotcrete and compressible wedges constitute the initial supports that were installed during excavation. The poured concrete was set up about 9 months after the section excavation as a final support, first at the bottom and then at the vault of the gallery. Hydrogeological and geotechnical measurements help to describe the hydro-mechanical response of the surrounding rock mass. Experimental results highlight the anisotropy of the loading from the surrounding rock mass and the influence of the stiffness/flexibility of the support.

Active porosity in swelling shales: insight from the Callovo-Oxfordian claystone

In swelling shales, significant clay–water interactions take place along the faces of the smectite minerals contained in the clay fraction, giving rise to the distinction between free water and adsorbed water. Further insight was recently gained by means of microstructure investigations, showing that the hydration mechanisms of pure or compacted smectites also explain the hydration and swelling behaviour of the Callovo-Oxfordian claystone, a possible host rock for deep radioactive waste disposal in France. In this rock, the proportion of water molecules adsorbed in intra-platelet pores was estimated to be around 25% of the total porosity, with 75% of the porosity containing free water. On the basis of these findings, the data of high-precision poroelastic measurements conducted in an isotropic compression cell showed that the porosity to account for a proper calculation of Skempton's B coefficient is not the total porosity, but the 75% proportion of the porosity corresponding to free water. This conclusion is believed to be of some interest with respect to the prediction of the rock response around the disposals at great depth, given that hydro-mechanical numerical simulations are often carried out within the framework of poroelasticity.

Mechanical Behavior of Claystone in Lateral Decompression Test and Thermal Effect

Based on a theoretical analysis of stress evolution in surrounding rocks of an underground cavity during excavation, a series of lateral decompression (LD) tests are designed and performed in this study to investigate the mechanical behavior of the Callovo–Oxfordian (COx) claystone. The mean stress is kept constant in each LD test. A number of unloading–reloading cycles are included in each LD test. Furthermore, to evaluate the effect of temperature on mechanical strength and elastic properties of the COx claystone, the LD tests are performed under five different values of temperature from 20 to 90 °C. The shear strength as well as failure pattern in the LD tests are analyzed. The variation of elastic moduli is determined and analyzed from the unloading–reloading cycles. The effect of temperature on both the shear strength and elastic moduli is identified and characterized. The COx claystone showed a shear failure in the LD tests, and the failure was characterized by the creation of a localized macro-crack and of a damaged zone with many micro-cracks around the macro-crack. The temperature increase induced a degradation of the shear strength of the COx claystone. An obvious mechanical damage of elastic properties was observed in the COx claystone during the shear stress loading in the LD tests. This damage was more important in the lateral direction than the axial one, leading to an induced anisotropy of the material.

Size Effect on the Time-Dependent Closure of Drifts in Callovo-Oxfordian Claystone

In this study, we analyzed the influence of the excavation size and the sequence of excavation on the convergence evolution in drifts excavated in Callovo-Oxfordian claystone. The drifts, which are part of the Underground Research Laboratory of the French National Radioactive Waste Management Agency (ANDRA), had diameters ranging between 0.7 m and 6.3 m. The measurements showed an anisotropic convergence with the maximum and minimum values in horizontal and vertical directions. Using a semi-empirical law, convergence evolution was fitted independently for each drift. The results show that the values for the parameters describing the time-dependent properties of the system rock-support depended upon the time elapsed for the support installation and the rate of excavation. For drifts following the same sequence of excavation, these parameters did not depend upon the size of the excavation. Finally, the analysis of a drift cross section enlargement showed that the convergence evolution was more isotropic and that the values for the parameters describing the time-dependent properties were lower, suggesting that overexcavation could limit the delayed response of the ground.

Experimental Modelling of the Caprock/Cement Interface Behaviour under CO2 Storage Conditions: Effect of Water and Supercritical CO2 from a Cathodoluminescence Study

In the framework of CO2 geological storage, one of the critical points leading to possible important CO2 leakage is the behaviour of the different interfaces between the rocks and the injection wells. This paper discussed the results from an experimental modelling of the evolution of a caprock/cement interface under high pressure and temperature conditions. Batch experiments were performed with a caprock (Callovo-Oxfordian claystone of the Paris Basin) in contact with a cement (Portland class G) in the presence of supercritical CO2 under dry or wet conditions. The mineralogical and mechanical evolution of the caprock, the Portland cement, and their interface submitted to the attack of carbonic acid either supercritical or dissolved in a saline water under geological conditions of pressure and temperature. This model should help to better understand the behaviour of interfaces in the proximal zone at the injection site and to prevent risks of leakage from this critical part of injection wells. After one month of ageing at 80 °C under 100 bar of CO2 pressure, the caprock, the cement, and the interface between the caprock and cement are investigated with Scanning Electron Microscopy (SEM) and cathodoluminescence (CL). The main results reveal (i) the influence of the alteration conditions: with dry CO2, the carbonation of the cement is more extended than under wet conditions; (ii) successive phases of carbonate precipitation (calcite and aragonite) responsible for the loss of mechanical cohesion of the interfaces; (iii) the mineralogical and chemical evolution of the cement which undergoes successive phases of carbonation and leaching; (iv) the limited reactivity of the clayey caprock despite the acidic attack of CO2; and (v) the influence of water on the transport mechanisms of dissolved species and thus on the location of mineral precipitations.

Modelling of Short-Term Interactions Between Concrete Support and the Excavated Damage Zone Around Galleries Drilled in Callovo–Oxfordian Claystone

Production of energy from nuclear power plants generates high-level radioactive nuclear waste, harmful during dozens of 1000 years. Deep geological disposal of nuclear waste represents a reliable solutions for its safe isolation. Confinement of radioactive wastes relies on the multi-barrier concept in which isolation is provided by a series of engineered (canister, backfill) and natural (host rock) barriers. Few underground research laboratories have been built all over the world to test and validate storage solutions. The drilling of disposal drifts may generate cracks, fractures/strain localisation in shear bands within the rock surrounding the gallery especially in argillaceous rocks. These degradations affect the hydro-mechanical properties of the material, such as permeability, e.g., creating a preferential flow path for radionuclide migration. Hydraulic conductivity increase within this zone must remain limited to preserve the natural barrier. In addition, galleries are currently reinforced by different types of concrete supports such as shotcrete and/or prefab elements. Their purpose is twofold: avoiding partial collapse of the tunnel during drilling operations and limiting convergence of the surrounding rock. Properties of both concrete and rock mass are time dependent, due to shotcrete hydration and hydro-mechanical couplings within the host rock. By the use of a hydro-mechanical coupled finite-element code with a second-gradient regularization, this paper aims at investigating and predicting support and rock interactions (convergence and stress field). The effect of shotcrete hydration evolution, spraying time, and use of compressible wedges is studied to determine their relative influence.

How rock samples can be representative of in situ condition: A case study of Callovo-Oxfordian claystones

Within the framework of feasibility studies for a reversible, deep geological repository of high- and intermediate-level long-lived radioactive waste (HLW, IL-LLW), the French National Radioactive Waste Management Agency (Andra) is investigating the Callovo-Oxfordian (COx) formation near Bure (northeast part of France) as a potential host rock for the repository. The hydro-mechanical (HM) behaviour is an important issue to design and optimise different components of the disposal such as shaft, ramp, drift, and waste package disposal facilities. Over the past 20 years, a large number of laboratory experiments have been carried out to characterise and understand the HM behaviours of COx claystones. At the beginning, samples came from deep boreholes drilled at the ground surface with oil base mud. From 2000 onwards, with the launch of the construction of the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), most samples have been extracted from a large number of air drilled boreholes in the URL. In parallel, various constitutive models have been developed for modelling. The thermo-hydro-mechanical (THM) behaviours of the COx claystones were investigated under different repository conditions. Core samples are subjected to a complex HM loading path before testing, due to drilling, conditioning and preparation. Various kinds of effects on the characteristics of the claystones are highlighted and discussed, and the procedures for core extraction and packaging as well as a systematic sample preparation protocol are proposed in order to minimise the uncertainties on test results. The representativeness of the test results is also addressed with regard to the in situ rock mass.

Drained Triaxial Tests in Low-Permeability Shales: Application to the Callovo-Oxfordian Claystone

Drained triaxial testing is challenging in low-permeability claystones (10−20 m2). This paper presents a method of testing low-permeability clay rocks in a standard triaxial cell. In this system, the resaturation of the specimen and the drainage conditions were enhanced by reducing the drainage length to 19 mm, the specimen radius. To do so, two geotextiles were placed around the top and bottom ends of the specimen, with no connection between them. Resaturation was hence performed by forcing water infiltration into the specimen from the upper and lower geotextiles, with a maximum infiltration length of around 19 mm, resulting in reasonable saturation durations. High-precision local measurements of radial strains were also achieved by ensuring direct contact between the LVDT rod and the specimen through the membrane. A poroelastic numerical calculation was carried out, and it was shown that, with these drainage conditions, a strain rate of 6.6 × 10−8 s−1 was satisfactory to ensure good drainage when shearing claystone specimens. After a check test made on a low-permeability sandstone with well-known mechanical characteristics, two tests were carried out to investigate specimens of the Callovo-Oxfordian claystone, a possible host rock for deep geological disposal in France. The results compare well with other published data from drained triaxial tests.

Theoretical Analysis of Pore Pressure Diffusion in Some Basic Rock Mechanics Experiments

Non-homogeneity of the pore pressure field in a specimen is an issue for characterization of the thermo-poromechanical behaviour of low-permeability geomaterials, as in the case of the Callovo-Oxfordian claystone (k < 10−20 m2), a possible host rock for deep radioactive waste disposal in France. In tests with drained boundary conditions, excess pore pressure can result in significant errors in the measurement of material parameters. Analytical solutions are presented for the change in time of the pore pressure field in a specimen submitted to various loading paths and different rates. The pore pressure field in mechanical and thermal undrained tests is simulated with a 1D finite difference model taking into account the dead volume of the drainage system of the triaxial cell connected to the specimen. These solutions provide a simple and efficient tool for the estimation of the conditions that must hold for reliable determination of material parameters and for optimization of various test conditions to minimize the experimental duration, while keeping the measurement errors at an acceptable level.

Discrete Analysis of Damage and Shear Banding in Argillaceous Rocks

A discrete approach is proposed to study damage and failure processes taking place in argillaceous rocks which present a transversely isotropic behavior. More precisely, a dedicated discrete element method is utilized to provide a micromechanical description of the mechanisms involved. The purpose of the study is twofold: (1) presenting a three-dimensional discrete element model able to simulate the anisotropic macro-mechanical behavior of the Callovo-Oxfordian claystone as a particular case of argillaceous rocks; (2) studying how progressive failure develops in such material. Material anisotropy is explicitly taken into account in the numerical model through the introduction of weakness planes distributed at the interparticle scale following predefined orientation and intensity. Simulations of compression tests under plane-strain and triaxial conditions are performed to clarify the development of damage and the appearance of shear bands through micromechanical analyses. The overall mechanical behavior and shear banding patterns predicted by the numerical model are in good agreement with respect to experimental observations. Both tensile and shear microcracks emerging from the modeling also present characteristics compatible with microstructural observations. The numerical results confirm that the global failure of argillaceous rocks is well correlated with the mechanisms taking place at the local scale. Specifically, strain localization is shown to directly result from shear microcracking developing with a preferential orientation distribution related to the orientation of the shear band. In addition, localization events presenting characteristics similar to shear bands are observed from the early stages of the loading and might thus be considered as precursors of strain localization.

Migration of Gas in Water Saturated Clays by Coupled Hydraulic-Mechanical Model

Understanding the gas migration in highly water saturated sedimentary rock formations is of great importance for safety of radioactive waste repositories which may use these host rocks as barrier. Recent experiments on drainage in argillite samples have demonstrated that they cannot be represented in terms of standard two-phase flow Darcy model. It has been suggested that gas flows along highly localized dilatant pathways. Due to very small pore size and the opacity of the material, it is not possible to observe this two-phase flow directly. In order to better understand the gas transport, a numerical coupled hydraulic-mechanical model at the pore scale is proposed. The model is formulated in terms of Smoothed Particle Hydrodynamics (SPH) and is applied to simulate drainage within a sample reconstructed from the Focused Ion Beam (FIB) images of Callovo-Oxfordian claystone. A damage model is incorporated to take into account the degradation of elastic solid properties due to local conditions, which may lead to formation of new pathways and thus to modifications of fluid transport. The influence of the damage model as well as the possible importance of rigid inclusions is demonstrated and discussed.

Multi-step triaxial compressive creep behaviour and induced gas permeability change of clay-rich rock

Clay-rich rocks are potential geological barriers for geological repositories of radioactive waste. The long-term safety of a repository requires a full characterisation of the time-dependent thermal hydro-mechanical behaviour of the clay-rich rocks. This paper investigates the creep deformation and induced permeability change of the Callovo-Oxfordian (COx) claystone by a series of multi-step triaxial compressive creep tests. The effects of confining pressure and deviatoric stress are both studied. Three values of effective confining pressure are 2 MPa, 6 MPa and 12 MPa, according to the in situ stress estimation at the Bure underground research laboratory in France. In each test, the deviatoric stress is progressively loaded in five steps so that the primary, secondary and tertiary stages of creep can be evaluated. In the last loading step, the tested sample may exhibit a creep failure process. During the creep tests, the gas permeability of the partially saturated COx claystone is measured by a transient pulse decay method in concurrence with the material creep deformation. The results obtained show that both the confining pressure and deviatoric stress have important effects on the time-dependent deformation and permeability evolution of the claystone. Analysis of creep strain rates during the last creep step indicates that there exists a threshold for the creep-induced failure of the COx claystone. The permeability evolution in the triaxial creep process is closely related to the microstructural evolution of rock, including the closure of pre-existing micro-cracks, the compaction voids and the creation, propagation and coalescence of micro-cracks.

A new view on iron-claystone interactions under hydrothermal conditions (90 °C) by monitoring in situ pH evolution and H2 generation

Many countries are considering the geological disposal of high-level radioactive waste combining natural clayey and engineered barriers (e.g. steel canister, bentonite, clay host rock, concrete). This configuration can lead to metallic iron-claystone interactions. Although a lot of experimental data on iron/clay interactions already exists, none of it allows the understanding and quantifying of the close interactions between the mineralogical transformations and the iron corrosion process. To fill this gap, a parametric kinetic investigation of iron/claystone interactions at 90°C in a water saturated-anoxic medium was carried out. A new approach is adopted to monitor a series of original experiments with various Fe0/claystone mass ratios. The continuous in situ monitoring of H2 production and of pH time-evolution allows to precisely follow the reaction rate and the chemical changes induced by the mineralogical transformations. In addition, gas composition and solid run products were characterized. We highlight that the Callovo-Oxfordian claystone of the Paris Basin (France) investigated here, in contact with metallic iron, drives the iron corrosion process, by controlling some medium parameters (like pH) and by acting as a Fe-consumer incorporating the released iron into neoformed Fe-rich clays minerals.

Thermal Volume Changes and Creep in the Callovo-Oxfordian Claystone

The Callovo-Oxfordian (COx) claystone is considered as a potential host rock for high-level radioactive waste disposal at great depth in France. Given the exothermic nature of radioactive wastes, a temperature elevation planned to be smaller than 100 °C will affect the host rock around the disposal cells. To gain better understanding of the thermal volumetric response of the COx claystone, a new thermal isotropic compression cell was developed with particular attention devoted to monitoring axial and radial strains. To do so, a high-precision LVDTs system ensuring direct contact between the LVDT stem and the claystone sample through the membrane was developed. A short drainage length (10 mm) was also ensured so as to allow full saturation of the sample under stress conditions close to in situ, and fully drained conditions during compression. High-precision strain monitoring allowed to observe a volumetric creep under stress conditions close to in situ. A drained heating test under constant stress carried out afterwards up to 80 °C exhibited a thermoelastic expansion up to a temperature of 48 °C, followed by thermoplastic contraction at higher temperature. Creep volume changes, that appeared to be enhanced by temperature, were modelled by using a simple Kelvin–Voigt model, so as to estimate the instantaneous response of the COx claystone and to determine its thermal expansion coefficient. The temperature at which the transition between thermal expansion and contraction appeared is close to the maximum burial temperature of the Callovo-Oxfordian claystone, estimated at 50 °C. This is in agreement with what has been already observed on the Opalinus Clay by Monfared et al. (2012) that was interpreted as a thermal hardening phenomenon, showing that the material kept the memory of the highest temperature supported during its geological history.

Effect of anisotropy and hydro-mechanical couplings on pore pressure evolution during tunnel excavation in low-permeability ground

Continuous monitoring around drifts excavated in the Underground Research Laboratory (URL) of The French National Radioactive Waste Management Agency (Andra) in Callovo Oxfordian claystone revealed the development of a fractured zone (extensional and shear fractures) induced by the excavation. Moreover, the field monitoring during the excavation showed an anisotropic pore pressure field around the drifts with zones of marked overpressure. For drifts parallel to the horizontal major principal stress, even though the initial stress state in the drift section is quasi-isotropic, the pore pressure evolution and the mechanical response are anisotropic. These observations suggest that the intrinsic anisotropy of the material plays a key role in the response of rock formation. A poroelastic analysis of the pore pressure change induced by drift excavation is performed. The principal objective is to simulate the main trends of the pore pressure evolution with a simple model taking into account the elastic anisotropy of the material. It is shown that depending upon the permeability and the degree of anisotropy of the rock, strong overpressures can be induced by the excavation. Furthermore, it is observed that the distribution of overpressures around drifts depends also on the drifts orientation. Finally, an analysis of the onset of failure shows the key role of the hydro-mechanical coupling on the extension of the failed zone around the drifts.