Paper contribution

Plastic swelling and hardening mechanisms of in-situ heated Callovo-Oxfordian claystone

Junfeng Ren; Philipp Braun; Carlos Pula; Minh-Ngoc Vu; Siavash Ghabezloo

Proceedings
EUROCK 2026
Pages
617-622
Submitted
28 January 2026
Accepted
15 June 2026
Published
12 August 2026

Summary

Abstract

Callovo-Oxfordian (COx) claystone is the candidate host rock for the future French deep geological repository for radioactive waste, designed to experience thermal loads of up to 90 °C during operation. In this context, this study characterizes cores extracted after an in-situ heating test where the maximum temperature reached 95°C. A multistep isotropic test was performed on a sample resaturated close to the in-situ stress state. Drained and repeated mechanical unloading-reloading cycles were carried out. Reversible elastic strain and irreversible plastic strain were decomposed from specially designed loading steps to characterize the evolution of plastic yield mechanisms. Results indicate that the sample exhibited plastic swelling during the unloading paths, when the effective mean stress decreased below 8 MPa. The unloading also seemed to have reduced the compression yield limit, since during subsequent reloading, we observed plastic compression at stress levels below the initial state. Furthermore, unloading after loading to high confining pressure revealed that the swelling yield limit increased, exceeding the initial stress level, thereby demonstrating a hardening effect. These observations could be represented by a swelling yield limit at a low effective mean stress, which is strongly coupled with the compression yield limit during the hardening process. The observed swelling during mechanical unloading could be relevant for in-situ stress paths involving decreasing mean effective stress, namely tunnel excavation, fracture self-sealing, thermal pressurization and retrieval of laboratory specimens.

Contributors

Authors

  1. Junfeng Ren École nationale des ponts et chaussées, France
  2. Philipp Braun École nationale des ponts et chaussées, France ORCID 0000-0003-3207-5018
  3. Carlos Pula Andra, Châtenay-Malabry, France
  4. Minh-Ngoc Vu Andra, Châtenay-Malabry, France
  5. Siavash Ghabezloo École nationale des ponts et chaussées, France

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References

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