Gypsum karst problems along an alpine motorwaytunnel

The 1271 m long cross-border Fuessen tunnel is the missing link of the German Freeway A7 and the Austrian Fernpass route B 314. In March 1996, during running excavation works, an unexpected ground water inburst of 400 l/sec occurred. Voluminous grouting measures were then required to stop the water inflow. To explain this unexpected inflow, the model of a collapse dolina has been forwarded. The dolina is believed to have developed due to heavy leaching of the gypsum bearing strata of the Raibl formation during glaciation, covering an area of app. 100 x 80 m at surface. The burried dolina is surrounded by collapse breccias, which provide a reference for the extremely steep walls of the sink hole. As a consequence, and in order to avoid further lowering of the ground water level in the Faulenbach valley, the tunnel had to be excavated within a protective shield of pre-grouted rock over a total length of 240 m.. This paper gives a brief description of the geological conditions and the material in the dolina itself and the dolina margin.