3-D NUMERICAL SIMULATION OF STRUCTURALLY-CONTROLLED, PROGRESSIVE FAILURE OF A LARGE-DIAMETER EXPERIMENT BOREHOLE THROUGH FAULTED OPALINUS CLAY SHALE (PF EXPERIMENT, MONT TERRI URL, SWITZERLAND)

In this paper, we present a numerical study of an in-situ experiment conducted at the Mont Terri Underground Research Laboratory, which aims at investigating the spatio-temporal evolution of structurally-controlled overbreaks during underground excavation in faulted Opalinus Clay shale (OPA). In the so-called Progressive Failure (PF) experiment, a research borehole with a diameter of 0.6 m and length of 12.9 m was drilled in 2020 through a major fault zone and numerous tectonic fault planes in OPA at an angle of 30–45° with respect to the strike of the structures and progressive failure phenomena are being monitored. To understand the physical mechanisms that drive rock mass damage and borehole overbreaks in faulted OPA, we developed a 3-D numerical model using the discrete element method to simulate the deformational and failure evolution during and shortly after the excavation (short-term) and open-drift (long-term) phases. Our model includes a realistic representation of the in-situ fracture network including the major fault zone and two sets of tectonic faults. We discretized the geomechanical model using Voronoi cells. New fractures can propagate by yielding of “virtual” joint elements embedded in between adjacent Voronoi cells. Our simulation captures essential excavation-induced geomechanical responses, including rock deformation, fault slip and new fracture growth, some of which form borehole breakouts, and, thus, allow a comparison with in-situ tomographic observations of the experiment site (see abstract by Ziegler et al.). In this presentation, we focus on short-term progressive failure, while the simulation of long-term damage evolution will be presented in subsequent work.