A9 Bosruck Tunnel - design approaches for swelling rock / A9 Bosrucktunnel - Dimensionierungsansätze im quellenden Gebirge

Construction works were started for the 2nd tube of the Bosruck road tunnel on the A9 Pyhrn motorway in December 2009, as part of the ongoing upgrade to full motorway standard by ASFINAG. In the course of the more than 100-year history of tunnelling under the Grosse Bosruck, this tunnel is the fourth tunnel to be built in this part of the central mountain range of the Eastern Alps. The 4,766 m long single-track railway tunnel was constructed between 1901 and 1906, the ventilation and drainage tunnel for the road tunnel between 1978 and 1980, and the approximately 5,500 m long road tunnel for two-way traffic was constructed between 1980 and 1983.
Soon after being opened for traffic, substantial damage became apparent in the road tunnel. It was only possible to maintain operation by means of continuous and costly rehabilitation measures. In 2005, the extent of damage finally led to the decision to add a second tube to the road tunnel and to carry out a general rehabilitation of the existing tube of the road tunnel. The main conclusions regarding the swelling and squeezing behaviour drawn from the damage that had occurred in the first tube of the road tunnel were exploited for the design and construction of the second tube and are the subject of this paper.