Résumé
Most Hard Rocks (HR) are or were exposed to deep weathering processes. It turns out that the hydraulic conductivity of HR is mostly inherited from these weathering processes (Lachassagne et al., 2011): (i) within their permeable Stratiform Fissured Layer (SFL) located below the low hydraulic conductivity unconsolidated weathered layer (saprolite). The thickness of both layers often reaches more than 100 m (Dewandel et al., 2006), (ii) and within the permeable vertical fissured layer developed at the periphery of or within preexisting geological discontinuities (joints, dykes, veins, lithological discontinuities, etc.) (Dewandel et al., 2011, Roques et al., 2012). From this conceptual model, the drainage discharge and the surface hydrogeological (piezometry in wells) and hydrological (discharge of streams) impacts of shallow highway tunnels drilled in a metamorphic series (metasedimentary and metavolcanic rocks) intruded by granitic bodies have been forecasted. These tunnels belong to the A89 highway recently opened (2012) in France between Balbigny and La Tour de Salvagny (Monts du Lyonnais, 50 km West of Lyon city). They are up to 4 km long, and their depth below ground level ranges between 0 and 300 m. The method is based on: