Main breakthrough of Gotthard Base Tunnel

On Friday, October 15, 2010, the tunnel boring machine Sissi completed the final meters of rock separating Faido and Sedrun, marking the main breakthrough of the eastern tube of the Gotthard Base Tunnel. This event signified a major milestone in the construction of one of the world's most ambitious engineering projects.

Precision of the breakthrough

The two halves of the tunnel tube joined with an 8 cm horizontal deviation and 1 cm vertical deviation. This was well within the allowed tolerance of 25 cm horizontally and 12 cm vertically, demonstrating the high level of precision achieved during the construction.

Length and structure of the tunnel

The Gotthard Base Tunnel consists of two tubes, each 57 km long, separated by 40 meters. This makes it the longest railway tunnel in the world, with the combined length of both tubes reaching 114 km, slightly less than the road distance between Paris and Orléans.

Engineering challenges

The tunnel is constructed beneath a mountain that weighs up to 2,300 meters of rock above the tracks, nearly seven times the height of the Eiffel Tower. The construction required precise alignment of excavation fronts from intermediate sites rather than the tunnel portals, to shorten the overall construction time.

Tunnel boring machine details

Sissi, the tunnel boring machine that completed the breakthrough, measures 9.43 meters in diameter, roughly the height of a three-story building. Its cross-section covers nearly 70 square meters, equivalent to a large apartment, and it crushed approximately 125 cubic meters of rock in the final 1.8 meters of excavation.

Precision navigation methods

In the Gotthard massif, where GPS signals are unreliable, surveyors used a combination of methods to ensure accuracy. A GPS network on the surface provided reference points, while an 800-meter vertical shaft in Sedrun allowed measurements to be taken underground. Surveyor gyroscopes, which detect Earth's rotation to find geographic north, were crucial in maintaining alignment.

Importance of the reference system

The surface GPS network and the underground shaft and gyroscopes created a unified reference system for both construction teams. This system allowed the two excavation fronts to work within the same reference frame, ensuring they met at the correct point under the Alps.

Achieving the precision

The horizontal deviation of 8 cm was about a third of the allowed 25 cm margin, and the vertical deviation of 1 cm was roughly one twelfth of the 12 cm allowed. These results were remarkable, especially considering the scale of the tunnel and the distance from the portals to the meeting point.

Completion and inauguration

The tunnel was officially inaugurated on June 1, 2016, nearly 5 years and 7 months after the main breakthrough. This period allowed for the completion of the tunnel's structure, installation of infrastructure, and final preparations for its opening to traffic.