The 57-km long (37 miles) Gotthard Base Tunnel, under construction since 1997, will connect the North and South sides of the Swiss Alps, and thus will be a main transportation axis for Europe, being the longest twin railway tunnel in the world. Its construction has been split into five sections, with three intermediate accesses. With overburden up to 2,000m the rock mass is affected by high stress. The southern 17 km section was recently completed in October 2006, by means of two TBMs, each 9m in diameter, and was excavated for the most part in massive gneisses, where not affected by tectonically disturbed zones. These were expected to be more or less perpendicular to the tunnel axis. At the excavation level, close to 1 km beneath the surface, most of brittle tectonic section was crossed obliquely, and was found to have a more cataclastic character than expected. The paper discusses the reasons for such unforeseeable conditions and the way they affected TBM progress.
The Gotthard Base Tunnel is the main element of a general railway concept aiming to connect North and South Europe with high-speed trains -- the AlpTransit project. Currently, the crossing of the Alps is made at an elevation of about one thousand meters. Normal trains are not designed to climb more than about one percent of gradient. To reach the crossing altitude the lines are elongated inside spiral tunnels, excavated at the end of the 19th century, with curve radii too small to allow high speeds. The time needed to travel from Milan to Zürich is about four hours. The purpose of a base tunnel is to cross the Alps at a lower elevation and along a more or less straight line, allowing high speeds. The travel time will be more than halved.