DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE | ROCK TUNNELS


Backfill experiences plastic deformations under the


longitudinal compression exerted by the plug. This can be deduced from Figure 19, which shows an increase of the preconsolidation stress. The zone experiencing plastic deformation extends up to a distance of around


twice the excavation diameter (~ 20 m). To study the effect of plug-lining interface strength


on performance of the sealing structure, three different simulations were performed, considering different values for the interface friction angle. An additional complementary simulation was performed without the presence of the plugs, corresponding to the scenario in which these elements do not contribute to equilibrium (only backfill material confining the sealing core). Naturally, longitudinal displacements obtained are


higher for lower friction angles. Accordingly, the swelling pressure is reduced due to deconfinement. This effect is more evident at the end of the core where radial and longitudinal component of swelling pressure are reduced by 20% and 40%, respectively, with respect to the reference case. However, values of the radial and longitudinal stress


components in the central part of the core are only 2% and 12% lower, respectively, indicating that the central part of the core maintains the swelling pressure even if the shear strength of the plug-lining interface is reduced or if the plugs are removed. In Figure 20 the final swelling pressure (radial and


longitudinal components) obtained in the different simulations is plotted against final dry density at points C1a, C2a, and C3a. In this figure is also shown the fitting curve already presented in Figures. 7 and 16, derived from the experiments performed by Bernachy-Barbe et al. (2020). It can be observed that, in general, the reduction of the interface strength implies a reduction of the final dry density, and therefore of the final swelling pressure. However, in this case, the effect of decompression on core performance is limited as it is located only close to the ends of the core.


EDZ Recompression Creep strains lead to a significant increment of radial stress during the operational phase (first 100 years). The radial stress increases by nearly 1 MPa during this period. Afterwards, during the post-closure phase, the increment of stress due to host rock long-term deformation is minor and is limited by the presence of the compressible lining, which allows the rock to deform without increasing the stresses in the lining. After core saturation, recompression of the EDZ


only occurs in the part where the lining was removed allowing a direct contact between the rock and the core. At the sections where the lining is not removed, the presence of the compressible lining does not allow the transmission of the pressure exerted by the core towards the host rock.


CONCLUDING REMARKS Advanced simulations have been performed to assess the performance of large-diameter sealing structures in


the framework of a deep geological radioactive waste disposal in an argillaceous host rock, for the current sealing concept in the Cigéo facility, in France. Large 3D models have been developed including the main components of the sealing structure (expansive core, retaining plugs, and backfills), and key details at smaller scales (such as compressible linings and interfaces). The presented formulation tackles the strongly


hydro- mechanical coupled phenomena related to the excavation, exploitation, and post-closure phases of the disposal. Advanced constitutive models were employed, able to reproduce most of the important aspects of the behaviour of the involved materials. In particular, the BExM and a non-linear hardening/


softening and time-dependent Mohr–Coulomb based model were considered for the expansive core material and for the Callovo-Oxfordian claystone, respectively. Several complex phenomena underlying the response of the sealing structures were contemplated, such as the EDZ generation during the excavation procedure, the natural hydration of the core, the swelling pressure development, and the achievement of the final equilibrium of the whole system. These challenging simulations provided qualitative


and quantitative results on key aspects about the performance and long-term integrity of the, proving to be a useful tool to support the design and the optimisation of a safe radioactive waste disposal. Results show that, during the post-closure phase, the


EDZ is recompressed by the swelling of the core and part of the initial stresses are recovered. Due to the presence of the compressible lining, the recompression of the EDZ only occurs in the part where the lining was removed and there is direct contact between the rock and the core. On-going research is aimed at including additional


effects, such as concrete degradation, gas flow passing through the sealing system, and self-sealing of induced fractures.


The financial and technical assistance of Andra.


The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


This paper was originally published online in Rock Mechanics and Rock Engineering Journal in March 2024, and is in 57(6), 4133-4158 (2024), from Springer, under a Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0/). Open access funding for original publication of the paper was enabled by CRUE-CSIC. As permitted under the particular open access facility, this version of the original paper has been abridged and edited for space, and images have been adapted to house-style. The original paper is available in full at: https://doi.org/10.1007/s00603-024-03813-w.


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