Electrical tomography monitoring of the excavation damaged zone of the Gallery 04 in the Mont Terri rock laboratory: Field experiments, modelling, and relationship with structural geology

The excavation of the new G04 gallery in the Opalinus clay of the underground rock laboratory at Mont Terri offered a unique opportunity to perform an electrical monitoring experiment of the evolution of the excavation damaged zone (EDZ) during the progression of the excavation. An electrode array was installed near the end of the first section of the gallery in order to monitor the evolution of the EDZ at the restart of the excavation works. Data sets acquired at different time intervals show conspicuous changes of the electrical resistivity. Forward modelling of a data subset indicates resistivity variations from 6 Ω m for undamaged Opalinus clay to 45 Ω m in the most damaged zones. Comparison with geological observations shows that the resistivity changes are strongly controlled by the local tectonics and by the bedding in the rock formation.