Geomechanical behaviour of Opalinus Clay at multiple scales: results from Mont Terri rock laboratory (Switzerland)
The paper represents a summary about our research projects conducted between 2003 and 2015 related to the mechanical behaviour of Opalinus Clay at Mont Terri. The research summarized covers a series of laboratory and field tests that address the brittle failure behaviour of Opalinus Clay, its undrained and effective strength, the dependency of petro-physical and mechanical properties on total suction, hydro-mechanically coupled phenomena and the development of a damage zone around excavations. On the laboratory scale, even simple laboratory tests are difficult to interpret and uncertainties remain regarding the representativeness of the results. We show that suction may develop rapidly after core extraction and substantially modifies the strength, stiffness, and petro-physical properties of Opalinus Clay. Consolidated undrained tests performed on fully saturated specimens revealed a relatively small true cohesion and confirmed the strong hydromechanically coupled behaviour of this material. Strong hydro-mechanically coupled processes may explain the stability of cores and tunnel excavations in the short term. Pore-pressure effects may cause effective stress states that favour stability in the short term but may cause longer-term deformations and damage as the pore-pressure dissipates. In-situ observations show that macroscopic fracturing is strongly influenced by bedding planes and faults planes. In tunnel sections where opening or shearing along bedding planes or faults planes is kinematically free, the induced fracture type is strongly dependent on the fault plane frequency and orientation. A transition from extensional macroscopic failure to shearing can be observed with increasing fault plane frequency. In zones around the excavation where bedding plane shearing/shearing along tectonic fault planes is kinematically restrained, primary extensional type fractures develop. In addition, heterogeneities such as single tectonic fault planes or fault zones substantially modify the stress redistribution and thus control zones around the excavation where new fractures may form.
KeywordsClay shale Excavation damaged zone Undrained shear strength Pore-pressure response Suction Tectonic structures Nuclear waste disposal
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Most of the funding of the projects described in this article was provided by the Swiss Federal Nuclear Safety Inspectorate (ENSI) with cost-sharing contributions from Swisstopo (Federal Office of Topography, Switzerland), BGR (Federal Institute for Geosciences and Natural Resources, Germany) and Chevron (USA). We also highly appreciate the scientific and technical contributions made by many partner organizations and scientists. Important partners of these projects have been ENSI (Martin Herfort, Meinert Rahn, Ernando Saraiva), BGR (Kristof Schuster, Torsten Tietz, Dieter Boeddener, Friedhelm Schulte, and Wilfried Stille), Swisstopo (Christophe Nussbaum, Nicolas Badertscher, Olivier Meier, David Ja¨ggi, Claude Giarardin, and Lukas Glur), University of Alberta at Edmonton (Derek Martin), CEMI (Peter Kaiser, Andrew Corkum), Queen’s University (Mark Diederichs), the Technical Universities of Torino (Marco Barla), TU Graz (Manfred Blümel), the Geodetic Metrology and Engineering Geodesy Group of ETH Zurich (Stephan Schütz, Florence Vaudan), and many colleagues and students from the Department of Earth Sciences at ETH (Corrado Fidelibus, Keith Evans, Frank Lemy, Valentin Gischig, Jonas von Rütte, Jürgen Hansmann, Freddy Xavier Yugsi Molina, Christian Haug, Sophie Gschwind, Sebastian Zimmer, Linda Wymann, Nicolas Kupferschmid, Patric Walter, Matthew Perras, Claudio Madonna, and Hansruedi Maurer). We are grateful two the two reviewers (Prof. Derek Martin and Dr. Bill Lanyon) for their valuable comments.
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