Complicated excavation altering an existing tunnel – the west portal of the Visp by-pass tunnel, Switzerland

Apart from the caverns necessary for the underground junction and the connecting tube over-passing the new motorway, an unusual design problem is posed by enlarging the existing Vispertal tunnel to become the south tube of the motorway. At the northern portal, called Schwarzer Graben (“black trench”), the alignments of the old and the new tunnel deviate by a small angle, necessitating the partial demolition of heavy steel supports of the old tunnel, close to the surface of a steep, weathered rock slope (Fig. 2).