Hydrogeologic Exploration during Excavation of the Lötschberg Base Tunnel (AlpTransit Switzerland)

Abstract.

Many tunnels that are currently being built through the Alps have to cross fractured and karstified formations at great depth and many hundreds of meters below the ground water table. The detection and characterization of permeable structures ahead of the advancing tunnel face is vital for these projects. We present new prediction and analysis methods and actual results from a deep Alpine tunnel in Switzerland, which has recently crossed 3.5 kilometers of karstified and fractured Limestones under an overburden of 1000 meters. The exploration programme in this part of the tunnel was unique and includes about 10’000 m of cored boreholes tested systematically with open hole and packer tests. From the high resolution transient pressure records of these experiments we derived not only standard formation properties but also diagnostic information including type, orientation and dimension of permeable structures. The subsequent tunnel excavation at these locations allowed to directly map the permeable structures and to evaluate the diagnostic model results.