Assessing karst-hydraulic hazards in tunneling-the Brunnmuhle spring system-Bernese Jura, Switzerland

Drilling a tunnel in a karst region requires the assessment of disturbances in the hydrogeological functioning of aquifers and flow-systems as well as disturbances for the tunnel itself. A complete characterization of the karst hydrogeological functioning and the establishment of prevention measures are essential in order to insure a suitable construction. The paper relates investigations which were conducted in the frame of the construction of a safety gallery (Sisto) in the bernese Jura (Switzerland). This gallery is located close to the outlet of a regional karst system and parallel to an existing highway tunnel, which met numerous disorders in the past due to the intersection of active karst conduits with a discharge reate exceeding 1 m(3)/s. The KarstALEA method was applied along the Sisto trace in order to identify sections with a high probability of karst occurrences and to characterize the type of karst related problems. The main issue turned out to be related to water head and discharge, which could potentially be cut by the tunnel construction. The system discharge rate may frequently exceed 10 m(3)/s, and most of this water could potentially flow into the tunnel. Therefore, a hydraulic model was set up in order to assess expected flow rates and heads if conduits are intersected during the construction. Based on this model and on meteorological and hydrological real-time measurements, a predictive model was established in order to anticipate high-flows events.