This paper is devoted to study the elastic–plastic damage behavior of heterogeneous shale rocks. The representative microstructure of this kind of rocks is first studied in order to define the representative elementary volume for the implementation of homogenization procedure. Three relevant material scales are considered. Inter-particle pores are distributed at the nanoscopic scale. Fine grains of calcite and kerogen are immersed at the microscopic scale. Large grains of minerals are embedded at the mesoscopic scale. Effective elastic properties of shale rocks are first determined by using a three-step linear homogenization procedure. The plastic damage behavior is estimated by developing a three-step nonlinear homogenization method. The effective plastic behavior of porous clay matrix with nanoscopic pores is described by an analytical model. The effects of small and large grains of various mineral inclusions are investigated by using a two-step incremental model. The damage due to progressive debonding of mineral inclusions is taken into account. After the implementation of the proposed model, comparisons between numerical results and experimental data are presented.

The development of a coupled elastoplastic damage constitutive model for argillites is presented. Emphasis is put on the description of post‐failure and coupled hydromechanical behavior of argillites during the desaturation/resaturation processes. A short summary of experimental investigations is given in the first part, which shows an important plastic deformation coupled with damage and a significant influence of water content on the mechanical behavior of argillites. Based on the framework of poroplasticity and continuum damage mechanics, a general constitutive model is proposed for the poromechanical behavior of argillites in both saturated and unsaturated conditions. Main features observed in experiments are taken into account, in particular the elastic degradation due to microcracks, residual strength, coupling between plastic deformation and induced damage, influence of water content on plastic flow, variation of permeability induced by microcracks as well as the deformation generated in the desaturation/resaturation processes. The performance of the model is examined by comparing numerical simulations with test data in representative load paths. Finally, the model is applied to hydromechanical coupling study of four time‐dependent tests subjected to a step change in relative humidity. Copyright © 2009 John Wiley & Sons, Ltd.

To enhance the understanding of thermal impact on the in situ behaviour of the Callovo–Oxfordian argillite, this paper presents an interpretation of an in situ heating experiment carried out in the Meuse/Haute Marne Underground Research Laboratory (M/HM URL). The argillite was heated successively by two constant heating powers: 277W and 925W. When subjected to thermal loading, the argillite exhibits an important volume change and a strong pore pressure response that significantly affect its hydraulic and mechanical behaviour. Numerical analysis has been performed by using a coupled theoretical formulation that incorporates a constitutive model especially developed for this material. Based on Biot's theory, this model includes the influence of interstitial pressure on the mechanical behaviour. The simulation obtained reproduces satisfactorily the results of the in situ experiment and the main observed patterns of behaviour. The interpretation and discussion of numerical results provide additional data that can help us to understand the thermo-hydromechanical behaviour concepts of saturated argillite formation.

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.

This paper presents a numerical modeling of hydromechanical response of a vertical shaft subjected to excavation in hard clay called argillite. The excavation procedure of the shaft is first presented. Based on experimental data obtained in conventional laboratory tests, a unified plastic and viscoplastic model is used to describe poromechanical behavior of the argillite in saturated and unsaturated conditions. This model takes into account the main features of poromechanical behavior of the material such as non-linear yield surface, non-associated plastic flow, sensitivity to water saturation, instantaneous and time-dependent plastic deformations. The model’s parameters are determined from experimental data obtained in triaxial compression tests and triaxial creep tests. The proposed model is implemented in a fully coupled finite element computer code in order to perform numerical modeling of boundary values problems. The numerical simulation of the shaft is then presented in the second part. Evolutions of displacement and pore pressure are evaluated during the excavation and compared with in situ measurement. It is shown that the hydromechanical responses of rock are affected by drying process inside the shaft. However, the influence of creep deformation seems to be negligible due to the relatively short duration of excavation.

In the context of a feasibility study for geological disposal of nuclear waste, an in situ heating experiment was performed in order to study the thermo-hydro-mechanical responses of a geological barrier to variation of temperature. The experiment was carried out at the Mont Terri underground research laboratory in Switzerland. This paper presents the numerical modelling of the experiment. An elasto-plastic damage model has been used to describe the mechanical behaviour of the geological formation (a mudstone). The constitutive model is extended to the description of the thermo-poro-mechanical behaviour of saturated porous media. A finite element method with fully coupled algorithm has been employed for the numerical solution of boundary value problems. Variations of temperature, pore pressure and deformation during heating phases have been evaluated and compared with in situ measurements. The numerical modelling allows the interpretation of coupled phenomena observed in the experiment.

Excavation of underground tunnels can be conducted by tunnel boring machines (TBM) or drill-and-blast. TBMs cause minimum damage to excavation walls. Blasting effects on excavation walls depend on the care with which the blasting is executed. For blast-induced damage in excavation walls, two issues have to be addressed: rate of loss of confinement (rate of excavation) and dynamic loading from wave propagation that causes both intended and unintended damage. To address these two aspects, laboratory dynamic tests were conducted for the determination of the dynamic properties of the Meuse/Haute-Marne argillite.In the present study, 17 tensile (Brazilian) and 15 compression split Hopkinson pressure bar (SHPB) tests were conducted. The test revealed that the dynamic strengths of the argillite are strain rate dependent. The average dynamic increase factors (ratio of dynamic strength to static strength) for tensile and compressive strength are about 3.3 and 2.4, respectively. A high-speed video camera was used to visualize the initiation of failure and subsequent deformation of the specimens. The direct compression specimens were found to deform and fail uniformly around the circumference of the specimen, by a spalling process. The SHPB Brazilian tests indicated that failure occurred in tension along the line of load application. Radial fractures were also observed. The test results can be used for the development of a dynamic constitutive model for the argillite for the prediction of damage in underground excavation utilizing the drill-and blast method.

This paper presents an elastoplastic damage model for argillites in unsaturated and saturated conditions. A short resume of experimental investigations is presented in the first part. Based on experimental data and micromechanical considerations, a general constitutive model is proposed for the poromechanical behavior of argillite in both saturated and unsaturated conditions. The proposed model is formulated within the framework of poroplasticity and continuum damage mechanics. Main features observed in experimental data are taken into account, in particular the elastic degradation due to microcracks, coupling between plastic deformation and induced damage, influence of water saturation on plastic flow and damage evolution, as well as variation of permeability with induced damage. The performance of the model is examined by comparing numerical simulation with test data in representative load paths. Finally, the model is applied to a hydromechanical coupling analysis of a cavity subjected to excavation and ventilation.

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 476 m 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.

This paper presents thermo-hydromechanical modeling in partially saturated hard clay in the frame work of feasibility study for nuclear waste storage. In the first part, based on the relevant experimental data, an elastoplastic damage model is developed for hard clay in saturated and unsaturated conditions. The model is applied to typical hard clay subjected to various loading paths. Comparisons between numerical predictions and experimental data are presented. In the second part, numerical modeling of coupled thermo-hydromechanical problems in saturated and unsaturated porous media is proposed. The finite element method is used with fully coupled algorithm. The proposed model is applied to the modeling of an underground storage of category C wastes. In this structure, the rock formation is subjected to cavity excavation, heating and desaturation. Coupled thermo-hydro-mechanics responses of underground storage, in both short and long term, are investigated using the proposed model.

New laboratory results on Callovo-Oxfordian argillite are presented and modelling of poromechanic behaviour of this rock in saturated and partially-saturated conditions is proposed. This rock is extensively being studied in the context of the underground research laboratory in Bure (France). The laboratory results show a clear dependency of the strength of this rock on the saturation state. The modelling of the behaviour of this rock is proposed using an effective stress approach. For the saturated rock the effective stress coincides with Biot's effective stress. The extension of the model in unsaturated field is made assuming an evolution of Biot's coefficient with the capillary pressure.

A new strategy for initial stress identification by stress relaxation methods, coupled with finite element calculation, is presented. It aims at assessing virgin stress state on the basis of most in situ tests, accounting for most constitutive laws. It uses an inversion method which consists in the minimisation, with a gradient-based algorithm, of a cost functional of least-squares type, which quantizes the difference between measured and computed data. The inversion methodology is firstly applied to an in situ overcoring test performed at Mont Terri laboratory, Switzerland. The inversion gave good results and allows us to validate the inversion methodology. The identification of in situ stresses at the Andra M/HM laboratory as also been attempted on the basis of dilatometer tests.

A numerical modeling is constructed to represent hydromechanical response of a deep hard clay shaft subjected to excavation, which belongs to an underground research laboratory operated by Andra. This study is carried out in the framework of 5th EC Euratom project Modex-Rep. Based on experimental data on core samples, a constitutive model for the poromechanical behavior of saturated and unsaturated rocks is presented. The proposed model is formulated within the thermodynamic framework of porous media and aimed to describe the main features observed in experimental data, especially plastic deformation and influence of water saturation on plastic flow. A series of numerical studies in axisymetric configuration have been performed by using a fully hydromechanical coupling FEM method. Qualitative and quantitative comparisons between numerical simulations and in situ experimental results are presented and discussed. The model predictions were found to be in good agreement with experimental data.

Linear and nonlinear THM analyses of the HE-D test in MontTerri Underground Rock Laboratory (Switzerland) are presented. The experiment consists to heat the Opalinus Clay formation for a long time in the undisturbed zone and monitoring the evolution of temperature, strains and pore pressure in the massif. By comparing different modeling cases with measured in situ data we show the role of the nonlinear behaviors on the overall response of the massif. The simplified analyses fail to correctly predict the evolution temperature filed, unless some modifications were made on the thermal flux. Concerning hydromechanical response, the plastic dilatancy of the rock around the heating borehole and the evolution of the water viscosity as a function of the temperature are the major factors affecting the distribution of pore pressure and its evolution in adjacent rock mass.

In the underground waste isolation projects such as the ANDRA'one in the site of Bure, the transport properties of the surrounding rock mass is of fundamental importance. To measure very low permeability, we use the modified version of the pulse test proposed by Hsieh et al. [Hsieh, P.A., Tracy, J.V., Neuzil, C.E., Bredehoeft, J.D., Silliman, S.E., 1981. A transient laboratory method for determining the hydraulic properties of ‘tight’ rocks — I. Theory. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. Vol. 18, pp. 245–252] which enables the intrinsic permeability, k, and the specific storage coefficient, Ss, of rocks such as mudstone to be characterized. In this paper, the special effort performed on the laboratory apparatus design, to ensure a good sensitivity of the rock response with respect to both parameters, k and Ss, is presented. In addition, two parameters identification procedures are proposed: the graphical method given by Hsieh et al. [Hsieh, P.A., Tracy, J.V., Neuzil, C.E., Bredehoeft, J.D., Silliman, S.E., 1981. A transient laboratory method for determining the hydraulic properties of ‘tight’ rocks — I. Theory. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. Vol. 18, pp. 245–252] and a parameter identification based on the solution of an inverse problem. The efficiency of the apparatus design and the parameters identification procedures is then demonstrated though some pulse tests performed on the Meuse/Haute-Marne mudstone.