Experimental and numerical characterisation of the mechanical anisotropic properties of Boom Clay

The underground geological disposal of radioactive waste is today the most widely adopted solution in many countries to ensure long-term safety. In Belgium, Boom Clay represents a stable and potential host formation for the disposal of radioactive waste.

The research on Boom Clay was initiated in 1974 by the Belgian Nuclear Research Centre (SCK CEN). So far, many studies have been undertaken to understand its behaviour. Both laboratory test campaigns and in situ experiments in HADES URL have allowed a good understanding of the hydro-mechanical behaviour of the Boom Clay. However, open questions remain on the short- and long-term behaviour of clay around galleries. Dizier et al. (2022) have recently demonstrated that the clay mechanical anisotropy and the anisotropic in situ stress states have a major impact on the deformation modes of galleries. The gallery deformation increases with time due to the visco-plastic behaviour of the clay, which causes the load on the lining to increase with time, even after decades. Understanding the Boom Clay anisotropies and its long-term behaviour is critical to optimize the support design of a future deep geological repository. In order to address these questions, a first scientific research project entitled “Investigation of the long-term behaviour of Boom Clay”, was established from 2018 to 2022. The main objective was to improve our understanding of the long-term behaviour of Boom Clay thanks to a new experimental program and the development of a relevant constitutive law.

So far, the role of the anisotropy of the strength properties was neglected in Boom Clay. Except very few triaxial tests that have been performed with different orientations of the bedding (Dao, 2015), only in situ experiments offered the opportunity to explore the anisotropy in Boom Clay. In that way, Chen et al. (2011) have shown that by considering the anisotropic elastic properties of the Boom Clay in the analysis of the small-scale ATLAS heater test, it is possible to well reproduce the main thermo-hydro-mechanical behaviour when the clay is subjected to an increase of temperature. However, in this study, parameters were deduced by back analysis of in situ measurements. While the approach proposed by Chen et al. (2011) was efficient for reproducing Boom Clay behaviour around this small-scale test, it fails in reproducing accurately the Excavation Damage Zone (EDZ) and the observed convergence of the galleries of the underground research laboratory (URL).

This project aims to complement the current knowledge by focusing on the anisotropic behaviour of the Boom Clay and more particularly on the characterisation of the mechanical plastic anisotropy of the Boom Clay. The objective is to improve our understanding of the deformation mechanisms in the clay during the different phases of a geological disposal facility from the excavation to the thermal phase. It will be an added value in the optimisation of the disposal galleries by evaluating more precisely the loads acting on the galleries.

To reach this goal, this research project will be divided into three main phases:

1. An experimental programme focusing on triaxial tests on Boom Clay with different orientations to the bedding and at different confining pressure values.
2. The development and the implementation of a new constitutive law taking into account the anisotropic plasticity. This model will be inspired by Pardoen (2015) who developed an anisotropic elasto-visco-plastic model for Callovo-Oxfordian claystone.
3. Modelling of a long-term in situ experiment such as the Connecting gallery, for which more than 20 years of experimental data are available.

References

Chen GJ, Sillen X, Verstricht J, and Li XL, 2011: ATLAS III in situ heating test in boom clay: Field data, observation and interpretation. Computers and Geotechnics 38(5), 683-696, ISSN 0266-352X.

Dao LQ, 2015. Etude du comportement anisotrope de l’Argile de Boom. PhD thesis, Ecole des Ponts ParisTech, France.

Dizier A, Scibetta M, Armand G, Georgieva T, Chen GJ, Verstricht J, Li X.L., Léonard D, and Levasseur S, 2022. Stability analysis and long term behaviour of deep tunnels in clay. Geological Society, London, Special Publications: Geological Disposal of Radioactive Waste in Deep Clay Formations: 40 Years of RD&D in the Belgian URL HADES.

Pardoen B, 2015. Hydro-mechanical analysis of the fracturing induced by the excavation of nuclear waste repository galleries using shear banding. PhD Thesis, ULiège

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