The Progressive Failure (PF) project explores experimentally and numerically structurally-controlled damage evolution in faulted Opalinus Clay shale at 1:5 scale in the Mont Terri URL, Switzerland. The in-situ experiment consists of a central, large-diameter experiment borehole (representing a repository drift) and six monitoring boreholes. The experiment borehole intersects tectonic fault planes and a major fault zone; its air humidity is controlled to simulate open and closed drift phases. The rock mass damage evolution is recorded by manifold techniques, including borehole photogrammetry, active seismic and electrical resistivity tomographies, and recordings of pico-seismicity, aiming at detecting and observing damage initiation and evolution around the borehole. A discontinuum numerical model is built to mimic the tectonic fractures explicitly and investigate the damage evolution around the experiment borehole. This paper provides an introduction to the experiment and highlights key datasets collected over the first two years of the project.
In the context of nuclear waste repositories, the integrity of the geological barrier is a major concern. In unfavourable situations, steep and acute angled fault zones may lead to deep rock mass damage and/or large caving/overbreak above repository drifts or caverns. However, the presence of tectonic fault zones with offset < 20 m at the repository depth of 800-900 m below ground surface cannot be predicted precisely nor completely from measurements (e.g., seismic investigations) carried out at the ground surface prior to repository excavation. Very critical are steeply dipping and persistent discontinuities or weak fault zones striking at angles smaller than about 30° to the tunnel axes (e.g., Bieniawski, 1989). Structurally-controlled failure may be initiated during excavation and damage the geological barrier progressively, i.e., rock damage may evolve over long periods of time, driven by hydromechanically coupled processes and delayed support reactions. Such zones can substantially reduce the effective thickness of the geological barrier and may lead to abandoning of repository drift sections for waste disposal, increasing the required subsurface space of a high-level waste (HLW) repository. Up to now, the extent, properties, and progressive formation of such damaged zones have not been investigated under in-situ conditions. It is of central importance to evaluate fault zone hazard scenarios related to repository construction and long-term safety (Ziegler and Loew 2020).
