Anomalies of noble gases and self-potential associated with fractures and fluid dynamics in a horizontal borehole, Mont Terri Underground Rock Laboratory

• Published in: Engineering geology 2013-04, Vol.156, p.46-57
• Main Authors: Maineult, Alexis, Thomas, Bertrand, Nussbaum, Christophe, Wieczorek, Klaus, Gibert, Dominique, Lavielle, Bernard, Kergosien, Bruno, Nicollin, Florence, Mahiouz, Karim, Lesparre, Nolwenn
• Format: Article
• Language: English
• Subjects: Applied sciences; Boreholes; Building structure; Buildings. Public works; Construction (buildings and works); Drilling; Earth Sciences; Environmental Sciences; Exact sciences and technology; Excavation; Excavation Damaged Zone; Fracture mechanics; Galleries; Geochemistry; Geotechnics; Global Changes; Horizontal; Mont Terri Underground Rock Laboratory; Noble gases; Opalinus clay; Rock; Sciences of the Universe; Self-potential; Soil investigations. Testing; Soil mechanics. Rocks mechanics; Underground; Underground structure
• ISSN: 0013-7952; 1872-6917
• DOI: 10.1016/j.enggeo.2013.01.010

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 2m, where pre-existing fractures accommodated the stress change during excavation of gallery Ga04 four years before. These first 2m 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. ► Noble gas content and self-potential were measured in a borehole in Mont-Terri URL. ► Noble gas profiles evidenced the existence of a 2-m thick Excavation Damaged Zone. ► Localized self-potential anomalies occurred after drilling operations. ► Pre-existing fractures have been reactivated and served as flowpaths for pore water.