Shallow Tunnelling in Soft Ground

The Brenner corridor is part of the new North-South European rail link for high-speed trains and combined transport services connecting Munich in Germany and Verona in Italy. As the Lower Inn Valley in Austria is narrow and densely populated with towns lined up along the Inn river, the characteristics of the alignment of the new route is that 83 % of the 41 km long Unterinntal route will run underground. The new route includes the tunnel Vomp-Terfens which is about 8 km long and is constructed through different types of ground - competent rock and soft ground (quaternary sediments such as ground moraine and gravel). The applied support measures varied from 150 mm steelfibre reinforced shotcrete in the hard rock sections (dolomite) to 300 mm reinforced shotcrete lining with advance support consisting of double layered pipe roof umbrella in the sections with low overburden and layers of potentially unstable gravel.

Two advance support systems were used in similar ground conditions - forepoling with spiles and a pipe roof umbrella support. This paper deals with conclusions drawn from the monitoring results in the tunnel, on the surface and from the evaluation of a horizontal inclinometer which was installed in one of the pipe roofs. The main objective pursued by the authors is the comparison of the mode of loading and its effect on the overall system behaviour, not an evaluation of the "advantages" or "disadvantages" of the two different types of advance support systems. In the authors' opinion there are always "two plus" possibilities to solve a tunnelling task - the chosen solution depends on a variety of factors such as the given geotechnical boundary conditions, the adopted safety level and economic considerations.