PROGRESSIVE FAILURE OF A REPOSITORY DRIFT IN CLAY SHALE (PF EXPERIMENT, MONT TERRI PROJECT)

Tectonic fault zones with vertical offsets < 20 m at depth levels of a nuclear waste repository (400–800 mbgs) cannot be predicted precisely nor completely from seismic measurements carried out at the ground surface prior to repository excavation. Excavating repository drifts or access tunnels into such faults at acute angles may lead to large overbreaks and deep rock mass damage. Such zones could substantially reduce the effective thickness of the geological barrier and affect long-term safety. In low-permeability clay shales structurally-controlled failure is initiated during excavation and damage of the geological barrier is expected to develop progressively over decades to thousands of years driven by hydromechanically and thermo-mechanically coupled processes and deterioration of tunnel support systems.

The PF experiment explores structurally-controlled damage evolution in faulted Opalinus Clay shale in a 1:5 scale in-situ experiment at the Mont Terri URL. The experiment consists of a central, large-diameter experiment borehole (representing a repository drift) and six monitoring boreholes. The unsupported experiment borehole dissects a rock mass with different degree of tectonic fracturing and a major fault zone. We control the borehole’s air humidity and temperature to simulate open and closed drift phases. The rock mass damage evolution is recorded using manifold techniques, including photogrammetric surveys of the experiment borehole, active seismic and electrical resistivity tomographies, as well as recordings of pico-seismicity, aiming at exploring and localizing macro- and microscopic damage propagation evolving in the crown of the experiment borehole. We will present the evolution of damage propagation recorded over the first about 1.5 years.