Gotthard Base Tunnel: breakthrough in west tunnel between Sedrun and Amsteg

Oct 24, 2007

Squeezing rock conditions successfully overcome

After one final blast, at 11 am today tunnellers from Sedrun and Amsteg could shake hands. Nine months earlier than planned, breakthrough took place in the west tunnel of the Gotthard Base Tunnel between the Sedrun and Amsteg sections with a horizontal deviation of just under 15 cm and a vertical deviation of only 1 cm. The concept of deformable steel inserts that was developed specially for the squeezing rock conditions of the Tavetsch Intermediate Massif proved a total success.

In the squeezing rock conditions of the Tavetsch Intermediate Massif North near Sedrun, a constructionally difficult zone with a length of approximately 1,200 metres had to be crossed. To counter the immense rock pressure, new methods of tunnel construction were used at Sedrun. In the squeezing rock conditions of this section, excavated cavities tend to close again unless countermeasures are taken. To provide support for keeping the excavated cross-section open, deformable steel rings were inserted which gradually close under the pressure of the squeezing rock until their maximum supporting force is attained. Although this technology was already known from German coal mines, it had never before been applied in these dimensions in tunnel construction.

The selected concept with deformable steel inserts more than fulfilled expectations. Despite sometimes very large deformations, driving and securing work in the difficult geological conditions of the north drive progressed so well that breakthrough took place nine months ahead of schedule. Driving in the Amsteg section was completed a long time ago. After each cutting a length of 10.7 km at outstanding advance rates, both tunnel boring machines reached their destination several months ahead of schedule: nine months in the east tunnel, six months in the west tunnel.

Of the total length of 153.5 km of the tunnel system being constructed under the Gotthard, on October 1, 2007, 105.1 km or 68.6 percent had been excavated.

Further information:
AlpTransit
Ambros Zgraggen
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Mobile: 079 655 96 65
Email: ambros.Zgraggen@alptransit.ch
Web: www.alptransit.ch

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