A microcracks-induced damage model for initially anisotropic rocks accounting for microcracks closure

We formulate a new micromechanical damage model for anisotropic rocks. This model accounts not only for the coupling between material initial anisotropy and the damage-induced one, but also for the opening/closure status (the so-called unilateral effects) of evolving microcracks. A closed-form expression of the overall free energy of the microcracked medium is implemented in an appropriate thermodynamics framework to derive a complete damage model for initially anisotropic rocks. The salient features of this model are fully illustrated. Then, its capabilities are demonstrated through an application to a Taiwan argillite subjected to direct tensile loading (including off-axis ones) for which the damage model well captures experimental data (mechanical response, growing damage rocks strength). Finally, the response of the studied rock along a tensile loading followed by an unloading and a reloading in compression is provided in order to illustrate the so-called unilateral damage effects due to microcracks closure

On micromechanical damage modeling in geomechanics: influence of numerical integration scheme

Tunnel excavations in deep rocks provide stress perturbations which initiate diffuse and/or localized damage propagation in the material. This damage phenomenon can lead to significant irreversible deformations and changes in rock properties. In this paper, we propose to model such behavior by considering a micromechanically-based damage approach. The resulting micromechanical model, which also accounts for initial stress, is described and assessed through the numerical analysis of a synthetic tunnel drilling in Opalinus Clay. A particular emphasis is put on the numerical integration of the model. In particular, an appropriate choice of the latter is required to ensure the numerical stability and a confident prediction of excavation damaged zone around tunnels

A two scale anisotropic damage model accounting for initial stresses in microcracked materials

In a recent study [15], we proposed a class of isotropic damage models which account for initial stresses. The present paper extends this approach to anisotropic damage due to growth of an arbitrarily penny-shaped microcracks system. The basic principle of the upscaling technique in the presence of initial stress is first recalled. Then, we derive a closed-form expression of the elastic energy potential corresponding to a system of arbitrarily oriented microcracks. It is shown that the coupling between initial stresses and damage is strongly dependent of the microcracks density and orientation. Predictions of the proposed model are illustrated through the investigation of the influence of initial stresses on the material response under non monotonous loading paths. Finally, by considering a particular distribution ofmicrocracks orientation, described by a second order damage tensor, it is shown that the model is a generalization of the macroscopic damage model of Halm and Dragon [9], for which a physically-based interpretation is then proposed

A benchmark of the large-scale in-situ PRACLAY Heater test

editorial reviewedDeep geological disposal is widely considered as one of the most sustainable solutions for isolating radioactive waste from the biosphere and ensuring its long-term management. Understanding the thermo-hydro-mechanical (THM) behavior of the host rock is important for the design of geological disposal. In Belgium, a poorly indurated clay named Boom Clay is studied as a potential host rock thanks to its low intrinsic permeability, its excellent self-sealing property and its capability of adsorption of radionuclides. Laboratory tests [1] and former in-situ small and intermediate scale heater tests [2] carried out in the HADES underground research facility (URF) in Mol (Belgium) already showed the strong hydro-mechanical coupled behavior of the host rock. However, the relatively limited size of these tests suffers from the inevitable mechanical disturbance induced by the installation of the heater and a lower accuracy in reproducing the thermal pressurization in the excavation damaged zone (EDZ). A large-scale in-situ heater test PRACLAY [3] (Fig. 1) is thus now conducted in HADES URF to reproduce the thermal impacts in the EDZ and in the near field and to verify at large scale the far field performance. A 2D benchmark, carried out in the framework of the European Joint programme EURAD HITEC [4], is proposed to model the PRACLAY heater test with fully coupled THM finite elements and to investigate the in-situ behavior of the host rock. The geometry of this model is a cross-section of a supported heating gallery and host rock perpendicular to the gallery axis. Only a quarter of the full gallery is modelled thanks to the symmetry of the problem and the boundary conditions. The numerical modelling comprises four primary phases: excavation, waiting, artificial injection, and heating phases, conducted by adjusting boundary conditions of gallery wall and the linner. An extensive monitoring system established around the PRACLAY gallery enables the observation of temperature and pore water pressure changes within the Boom Clay. The comparison between the numerical prediction and in-situ measurement are carried out. The computation is performed with the finite element code LAGAMINE, developed at the University of Liege. The thermal pressurization due to the discrepancy of thermal dilation between solid and fluid phases is well predicted in the EDZ. To well reproduce the evolution of pore water pressure, the dependency of the permeability on the deformation is introduced in the implemented modelling [5]. The small strain stiffness theory based on the HSsmall model is also taken into account [6]. Finally, a good agreement is obtained between the in-situ measurement and the numerical results (Fig. 2). The benchmark provides valuable insights into the THM impact on the host rock and reliable indications of the model capacity

A benchmark of the large-scale in-situ PRACLAY Heater test

editorial reviewedDeep geological disposal is widely considered as one of the most sustainable solutions for isolating radioactive waste from the biosphere and ensuring its long-term management. Understanding the thermo-hydro-mechanical (THM) behavior of the host rock is important for the design of geological disposal. In Belgium, a poorly indurated clay named Boom Clay is studied as a potential host rock thanks to its low intrinsic permeability, its excellent self-sealing property and its capability of adsorption of radionuclides. Laboratory tests [1] and former in-situ small and intermediate scale heater tests [2] carried out in the HADES underground research facility (URF) in Mol (Belgium) already showed the strong hydro-mechanical coupled behavior of the host rock. However, the relatively limited size of these tests suffers from the inevitable mechanical disturbance induced by the installation of the heater and a lower accuracy in reproducing the thermal pressurization in the excavation damaged zone (EDZ). A large-scale in-situ heater test PRACLAY [3] (Fig. 1) is thus now conducted in HADES URF to reproduce the thermal impacts in the EDZ and in the near field and to verify at large scale the far field performance. A 2D benchmark, carried out in the framework of the European Joint programme EURAD HITEC [4], is proposed to model the PRACLAY heater test with fully coupled THM finite elements and to investigate the in-situ behavior of the host rock. The geometry of this model is a cross-section of a supported heating gallery and host rock perpendicular to the gallery axis. Only a quarter of the full gallery is modelled thanks to the symmetry of the problem and the boundary conditions. The numerical modelling comprises four primary phases: excavation, waiting, artificial injection, and heating phases, conducted by adjusting boundary conditions of gallery wall and the linner. An extensive monitoring system established around the PRACLAY gallery enables the observation of temperature and pore water pressure changes within the Boom Clay. The comparison between the numerical prediction and in-situ measurement are carried out. The computation is performed with the finite element code LAGAMINE, developed at the University of Liege. The thermal pressurization due to the discrepancy of thermal dilation between solid and fluid phases is well predicted in the EDZ. To well reproduce the evolution of pore water pressure, the dependency of the permeability on the deformation is introduced in the implemented modelling [5]. The small strain stiffness theory based on the HSsmall model is also taken into account [6]. Finally, a good agreement is obtained between the in-situ measurement and the numerical results (Fig. 2). The benchmark provides valuable insights into the THM impact on the host rock and reliable indications of the model capacity

Axisymmetric transient modelling of a wind turbine foundation in cohesionless soil using the Prevost’s model

peer reviewedSuction caissons are more and more used for offshore foundations. This paper deals with the cyclic modelling of suction caissons using the Prevost's model. The case study is a 8m large diameter embedded in dense No. 0 Lund sand. Parameters for the model are calibrated using drained triaxial tests. A parametric study concerning the influence of the constitutive law, the skirt length and permeability is carried out

Using local second gradient model and shear strain localisation to model the excavation damaged zone in unsaturated claystone

The drilling of galleries induces damage propagation in the surrounding medium and creates, around them, the excavation damaged zone (EDZ). The prediction of the extension and fracture structure of this zone remains a major issue, especially in the context of underground nuclear waste storage. Experimental studies on geomaterials indicate that localised deformation in shear band mode usually appears prior to fractures. Thus, the excavation damaged zone can be modelled by considering the development of shear strain localisation bands. In the classical finite element framework, strain localisation suffers a mesh-dependency problem. Therefore, an enhanced model with a regularisation method is required to correctly model the strain localisation behaviour. Among the existing methods, we choose the coupled local second gradient model. We extend it to unsaturated conditions and we include the solid grain compressibility. Furthermore, air ventilation inside underground galleries engenders a rock– atmosphere interaction that could influence the damaged zone. This interaction has to be investigated in order to predict the damaged zone behaviour. Finally, a hydromechanical modelling of a gallery excavation in claystone is presented and leads to a fairly good representation of the EDZ. The main objectives of this study are to model the fractures by considering shear strain localisation bands, and to investigate if an isotropic model accurately reproduces the in situ measurements. The numerical results provide information about the damaged zone extension, structure and behaviour that are in very good agreement with in situ measurements and observations. For instance, the strain localisation bands that develop in chevron pattern during the excavation and rock desaturation, due to air ventilation, are observed close to the gallery