Experience gained from the restoration of the water table at the Brixlegg tunnel / Erkenntnisse aus der Bergwasseraufspiegelung im Tunnel Brixlegg

For the construction of the 4.2 km long Brixlegg tunnel, it was necessary to lower the water table by 50 m inside the mountain. This proved to have a bad effect on the artesian mineral and healing water springs at Bad Mehrn. To protect these, the responsible water authority prescribed a lining of the tunnel waterproof to 6 bar, in order to ensure that the groundwater table could rise again after the completion of tunnelling.
After the pressure tight tunnel lining had been successfully completed with the intended grout canopy at the transition from the pressure-tight to the drained sections of the tunnel, the groundwater initially rose quickly, but then stagnated after a few months. Continuous monitoring in the tunnel showed increasing high water pressures of up to 3 bar at the invert of the drained tunnel, despite functioning drainage at the sides.
Extensive investigations led to the supposition that this was not large-area seepage of water in the rock mass but rather localised lateral seepage along the old temporary construction drainage, which had been grouted. Targeted additional grouting was therefore carried out in the area of the old drainage and its filter bodies, which was very successful. The ingress of water into the drained section was completely stopped with the result that the water table started to rise again. The artesian flow of mineral water in Bad Mehrn started again on the 18th November 2008 and also has its original mineral content.
The additional grouting work led to hydrogeological and geotechnical knowledge, which could also be useful for future projects. In particular, the grouting of a construction drainage with its filter body at water pressures of 5 bar represents a special measure, which needs to be prepared in detail. Later leaks in this area lead to horizontal hydraulic short-circuits in the rock mass, which under some circumstances could considerably impair the intended purpose of a pressure-resistant tunnel lining to protect a water reservoir in the rock mass.