Through the history of base tunnels crossing Karavanke mountain range the development of NATM is studied. Both, railway tunnel constructed in 1906 and road tunnel constructed in 1991 faced the most difficult conditions in extremely weak permo-carboniferous rock. Together with high overburden such rock represents extreme conditions for tunneling, resulting in large convergences and over breaks. Based on the conclusions made after the construction of first two tunnels through Karavanks some key technical solutions, for construction of second road base tunnel, are pointed out.
Karavanke mountain range represents a natural border between Western and Southern Europe. The three mountain passes (Podkoren, Jezersko and Ljubelj) crossing the Karavanke mountain range represented the shortest route between the two parts of Europe i.e. between Austria and Slovenia. During winter, the three passes were often impassable due to immense amount of snow, and a transport was redirected to diversionary routes. To achieve a constant and efficient transport route a railway base tunnel was constructed in 1906, and several decades later followed by road base tunnel constructed in 1991. In 2014 project preparations for construction of second tube of a road base tunnel have begun.
During the construction of a railway base tunnel extremely weak permo-carboniferous rock layers together with high overburden (max. 962 m) caused extreme conditins for tunneling. Old Austrian method used at that time, which based on a stiff temporary timber support, was unable to bear the extreme pressures acting on the tunnel lining. Almost a century later construction of a road base tunnel was performed according to NATM allowing large deformations by leaving longitudinal slots in the primary lining. Dense rock bolting that increased the shear strength of a rock was also incorporated. The technique justified its reputation but pointed out several possibilites for improvement.
Challenges faced during the construction of railway tunnel and first tube of a road base tunnel, such as high overburden, extremely weak permo-carboniferous rock, methane gas inflows, water and liquid rock mass inflow, etc., resulted in extreme conditions for tunneling. Additionally, during the construction of a second tunnel tube the effect on the first tunnel tube needs to be minimized.