Gotthard Base Tunnel - Tunnelling in the influence zone of two concrete arch dams / Gotthard Basistunnel - Tunnelbau im Einflussbereich von zwei Bogenmauern

The determination of the alignment of the Gotthard Base Tunnel made sure not to tunnel directly under the dams of the Vorderrhein AG power stations. The Gotthard Base Tunnel will, however, have to be driven through the zone, which could affect the three arch dams at Nalps, Santa Maria and Curnera.
The Zeuzier dam in Wallis (Valais) suffered severe damage in 1978 as the result of drainage of the rock mass for an investigation tunnel being driven in the vicinity. The cracks in the dam led to years of operational limitations and required extensive repair works.
A whole range of measures was undertaken for the Gotthard Base Tunnel in order to avoid a repetition. The construction of the tunnel near the dam was the subject of years of preparatory work. Since the end of 2005, the southward drive from Sedrun has been near the Nalps dam and since the middle of 2009, that from Faido has been within the zone influencing the Santa Maria dam. Long-term safe operation of both dams has to be ensured despite the near approach of the tunnelling works. Although the task is the same for both dams, completely different solutions were chosen. The following paper explains the reasons for this