Subsurface exploration and transient pressure testing from a deep tunnel in fractured and karstified limestones (Lötschberg Base Tunnel, Switzerland)

This paper summarizes the results from a subsurface exploration campaign carried out in fractured and karstified limestones and marls of the Lötschberg Base Tunnel in the Swiss Alps. The exploration program in the 3.2 km long Doldenhorn Section consisted of 50 cored and 13 destructive horizontal predrillings of 250–300 m length each (in total 14,853 m of predrillings). These boreholes were used to locate possible tunnel inflows and characterize their hydraulic properties. A new efficient hydrogeological test protocol has been developed and was systematically and successfully applied. The test protocol consists of short constant rate outflow and recovery tests in open boreholes. The on-site test analysis allowed reliable predictions of the hydrogeological conditions ahead of the advancing tunnel face in brief period of time, leading to decisions regarding further reconnaissance drillings, tunnel driving or sealing measures. Detailed analysis of the transient pressure responses of the hydrogeological tests carried out after tunnel completion led to important new findings regarding flow model identification (type and orientation of conductive fractures), aquifer parameters and boundary effects. Even though the hydrogeological tests had to be carried out within a short time frame (a few hours) they allowed characterization of large scale aquifer properties relevant for long term tunnel inflow behavior.