Clay shales are currently being assessed as possible host rock formations for the deep geological disposal of radioactive waste. However, one main concern is that the favourable long-term isolation properties of the intact rock mass could be negatively affected by the formation of an excavation damaged zone (EDZ) around the underground openings. This thesis investigated the deformation and failure process of a clay shale, namely Opalinus Clay, with particular focus on the influence of anisotropy on the
short-term response of circular tunnels. To achieve this goal, a hybrid continuum-discontinuum numerical approach was used in combination with new field measurements from the Mont Terri underground research laboratory. The response of Opalinus Clay during the excavation of a full-scale emplacement (FE) test tunnel was characterized by geodetic monitoring of wall displacements, radial extensometers
and longitudinal inclinometers. The deformation measurements indicated strong directionality induced by the combined effect of in situ stress field and presence of bedding planes striking parallel to the
tunnel axis, with the most severe deformation occurring in the direction approximately perpendicular to the material layering. Computer simulations were conducted using a newly-extended combined
finite-discrete element method (FEM/DEM), a numerical technique which allows the explicit simulation of brittle fracturing and associated seismicity. The numerical experimentation firstly focused on the
laboratory-scale analysis of failure processes (e.g., acoustic activity) in brittle rocks, and on the role of
strength and modulus anisotropy in the failure behaviour of Opalinus Clay in tension and compression.
The fracturing behaviour of unsupported circular excavations in laminated rock masses was then analyzed under different in situ stress conditions. Lastly, the modelling methodology was applied to the
aforementioned FE tunnel to obtain original insights into the possible EDZ formation process around emplacement tunnels for nuclear waste. The calibrated numerical model suggested delamination along bedding planes and subsequent extensional fracturing as key mechanisms of the damage process potentially leading to buckling and spalling phenomena. Overall, the research findings may have a potential impact on the constructability and support design of an underground repository as well as implications for its long-term safety assessment procedure.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:OTU.1807/43649 |
| Date | 10 January 2014 |
| Creators | Lisjak Bradley, Andrea |
| Contributors | Grasselli, Giovanni |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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