Sealing measures at Sedrun almost complete

The southward drive of the Gotthard Base Tunnel in the Sedrun section is currently approximately 1300 metres below the eastern valley flank of the Nalps reservoir. In mid-September 2006 in the west tunnel, water ingress of around 12 litres per second occurred which then stabilised at around 8 litres per second. To investigate the water-bearing zone, AlpTransit Gotthard Ltd ordered four exploratory bores to be made. At the end of November 2006, injection work was started. A total of more than 3,000 metres of injection bores were drilled, and more than 70 tonnes of cement pressed into the rock. By this means, by the Christmas break, the water inflow was reduced to 5 litres per second. With a second injection campaign, by February 2007 the water ingress should be reduced so much further that driving to the south can be resumed.

Challenging tunnel construction
Whenever a tunnel is driven, water seepage from ground level must always be expected. Even in deeply lying drives such as the Gotthard Base Tunnel, water ingresses are unavoidable. This drainage of water causes fissures in the rock to close to a certain extent, which can lead to deformations on the surface. Such phenomena were detected in Switzerland during construction of the exploratory drive for the Rawil tunnel near the Zeuzier reservoir, and in the area affected by the Gotthard road tunnel.
The overriding goal of AlpTransit Gotthard Ltd is therefore to secure the tunnel drive in such a way that the reservoirs lying above it are not damaged. So that this goal can be accomplished, AlpTransit Gotthard Ltd has initiated various measures: Advance explorations and injections at the tunnel drive as well as a unique automatic monitoring system for the Curnera, Nalps and Santa Maria reservoirs.

Monitoring system has proved itself
Experience to date in the Sedrun drive to the south shows that construction of the tunnel has caused deformations in the range of millimetres in the surface terrain. In view of the limited settlement, no damage has been detected so far.
More surprising to technical specialists is the fact that the observed deformations have already occurred with very limited volumes of water. The ingresses to the Gotthard Base Tunnel encountered so far are much less than those formerly observed in the Rawil exploratory drive and the Gotthard road tunnel. After several years experience, it can now be said that the monitoring system installed in 2000, as well as the close and cooperative collaboration with the power station operators and the controlling bodies of the Swiss government, have proved themselves very well.

Work status
On January 1, 2007, of the total of 153.3 km of tunnels, shafts and passages making up the Gotthard Base Tunnel, 102.07 km, or approximately two thirds, had been excavated. In 2006 alone, more than 17 km of the tunnel system under the Gotthard were excavated.
With the foundation stone laying ceremony in Camorino, construction of the Ceneri Base Tunnel officially started on June 2, 2006. Besides other work, preparations for the installation sites at Camorino and Sigirino for subsequent construction of the tunnel are proceeding at full speed.


Further information:
AlpTransit Gotthard AG
Zentralstrasse 5
CH-6003 Luzern
Tel: +41 (0)41 226 06 06
Fax: +41 (0)41 226 06 00
Email: info@alptransit.ch
Web: www.alptransit.ch