A benchmark exercise to investigate the thermal effects on the Excavated Damage Zone

editorial reviewedDeep geological disposal with multi-barriers confinement is considered as one of the most reliable solutions for long-term management of radioactive wastes. Thermal-Hydro-Mechanical (THM) effects are likely to alter the confining function of clay host rock during the construction and lifetime of repository [1]. Specifically, the heat generated by the waste must not affect the favourable properties of the clay host rock for containment, especially its transport properties. In the vicinity of the drift, the excess pore pressure generated by the thermal expansion of pore water could induce fracture re-opening or propagation. In the far-field, the zone subjected to thermal loading from two neighbor galleries could induce tensile or even shear failure and/or reactivate old fractures. The above processes could potentially alter the permeability of host rock. The present work aims to reproduce the development of strain localization bands induced by multi-physical couplings associated with thermal effects. A 2D plane strain generic model is built from the benchmark exercise within the European joint programme EURAD HITEC [2]. The geometry of this model is a cross-section of a heating drift and host rock. Only a quarter of the full drift is modelled thanks to the symmetry of the problem and the boundary conditions. The full computation is characterised by three phases: excavation (0 ~ 24 h), waiting (24 h ~ 6 months) and heating (6 months ~ 10 years), conducted by adjusting boundary conditions of drift wall. The elasto-plastic mechanical law with hardening and softening is used, and a local THM second gradient model including microstructure effects allows a robust modelling of the post peak regime. The Callovo-Oxfordian claystone (COx) is selected as a candidate host formation due to its low permeability and good plasticity [3]. All the numerical modelling is performed with the finite element code Lagamine developed at University of Liège. During the tunnel excavation, the strain localisation was not triggered at the end with a radial stress reduction to 5% of initial confining pressure. At the beginning of the waiting phase, Figure 1 shows that the initiation of shear band is located at around 45°. Due to the progressive drainage (elastic unloading) during the waiting phase, the development of shear bands gradually weakens. Once the heating phase is activated, the plasticity develops rapidly. The THM couplings at the drift wall are evidenced by excess pore pressure, that induces distinct growth of shear bands. Thermal effect has a leading role in the development of strain localisation, and the shear bands occur preferably along with the minor principal stress direction

Thermal impact on the excavation damage zone around a supported drift using the 2nd gradient model

peer reviewedThe temperature increase induced by radioactive waste decay generates the thermal pressurisation around the excavation damage zone (EDZ), and the excess pore pressure could induce fracture re-opening and propagation. Shear strain localisation in band mode leading to the onset of micro/macro cracks can be always evidenced before the fracturing process from the lab experiments using advanced experimental devices, hence the thermal effects on the rock behaviour around the EDZ could be modelled with the consideration of development of shear bands. A coupled local 2nd gradient model with regularisation technique is implemented, considering the thermo-hydro-mechanical (THM) couplings in order to well reproduce the shear bands. Furthermore, the thermo-poroelasticity framework is summarized to validate the implemented model. The discrepancy of thermal dilation coefficient between solid and fluid phases is proved to be the significant parameter leading to the excess pore pressure. Finally, an application of a heating test based on Eurad Hitec benchmark exercise with a drift supported by a liner is studied. The strain localisation induced by thermal effects is properly reproduced. The plasticity and shear bands evolutions are highlighted during the heating, and the shear bands are preferential to develop in the minor horizontal principal stress direction. Different shear band patterns are obtained with changing gap values between the drift wall and the liner. A smaller gap between the wall and the liner can limit the development of shear bands

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