Computing the Water Inflows Discharge and Assessing the Impacts of Tunnels Drilled in Hard Rocks: The A89 (France) Motorway Case Study

Abstract

Most Hard Rocks were exposed to deep weathering processes, from which their hydraulic conductivity is mostly inherited, within their stratiform fissured layer located below the low hydraulic conductivity unconsolidated saprolite, and within the permeable vertical fissured layer at the periphery of preexisting geological discontinuities. From this conceptual model, first the water inflows discharge into the tunnels and second the surface hydrogeological and hydrological impacts due to shallow motorway tunnels have been forecasted. These three tunnels, up to 4 km long and whose depth ranges between 0 and 300 m below ground level (bgl), are located on the A89 motorway in France. The method is based on: 1. the location of the tunnel within or below the various layers constituting the weathering profile. Three different weathering profiles with ages from Triassic to post Miocene were identified, mapped, and characterized (fissuration) in the various lithologies of the area; 2. steady state groundwater discharge measurements in existing tunnels (railway) of the area where weathering profiles were similarly mapped; 3. application of the Goodman et al. (1965) analytical solution to: (i) inverse steady state railway tunnels groundwater discharge into the hydraulic conductivity of the various layers of the weathering profiles, and (ii) compute the discharge of the future motorway tunnels, on the basis of the hydraulic conductivity of these weathering profiles. The actual discharge of the now completed motorway tunnels validates the accuracy of the methodology. This method proves to be very efficient for forecasting the water inflows into shallow tunnels (a few 100 m deep) in hard rock areas.