Before the excavation of the new gallery Ga08 in the Mont Terri Underground Rock Laboratory (Switzerland), which joined the existing gallery Ga04 in 2008, the end-face of gallery Ga04 was instrumented in 2007 to characterize the evolution of the rock mass with geochemical and geophysical methods. The noble gas content of a 12-m long horizontal borehole evidenced that desaturation processes occurred in the first 2 m, where pre-existing fractures accommodated the stress change during excavation of gallery Ga04 four years before. These first 2 m are associated with the so-called Excavation Damaged Zone (EDZ). As an inflow of pore water was observed in this borehole few weeks after its drilling, continuous self-potential (SP) measurements were performed to characterize its dynamics. After the drilling of new sub-horizontal boreholes in the end-face, strong localized variations of SP occurred. The comparison with the geological features of the rock mass suggests that these dynamic anomalies have to be associated with fluid circulation in pre-existing tectonic fractures that were primarily reactivated by the excavation of the gallery Ga04 and subsequently by the drilling operations.
Times Cited: 0 0