Progressive failure of a circular excavation drilled through faulted clay shale — Experimental setup, rock mass structures and damage evolution at excavation boundary

The PF experiment at the Mont Terri Underground Rock Laboratory (MT URL) examines how structurally controlled rock mass damage evolves and spreads in faulted Opalinus Clay shale excavations. Installed in 2020 near the Main Fault of the MT URL, the PF (Progressive Failure of Structurally Controlled Overbreaks) experiment consists of a central 0.6 m wide experimental borehole representing a circular high-level waste repository drift at a scale of 1:6.5, and six parallel monitoring boreholes at different distances above the experiment borehole. Changes to the unsupported excavation boundary of the experimental borehole were surveyed for four years using a custom-built automated close-range photogrammetric survey system. Together with structural data from the six monitoring boreholes, a detailed model of the fracture network surrounding the Main Fault was established. The Main Fault at the test location consists of a 0.6 m-thick zone of scaly clay, sharp fault zone boundaries and a few mm-thick fault gouge layer. The hanging wall and footwall are transected by two sets of mm-thick tectonic faults.We explored the evolution of borehole convergence and visible damage inside the experiment borehole during ventilation, transient resaturation, and a fully resaturated phase between October 2020 and August 2024. We mapped the formation of new fractures, overbreaks, and slippage along pre-existing faults, and specifically assessed the time-dependent formation of borehole wall breakouts and induced cracks using a deep-learning model. In this study, we present the data acquisition and processing methods and discuss the progressive and saturation dependent damage evolution at the excavation boundary. The transient rock mass response above the experiment borehole, as derived from geophysical monitoring across monitoring boreholes, are described in a companion paper.