Precise knowledge of mountain structure and the qualities of rock are key to tunnel building. Geological findings influence the route the line takes, the driving operations, and ultimately the success of a project. At the Gotthard the steeply angled rock formations, mostly gneiss and granite, make things particularly difficult. Exploratory boring is carried out to investigate the more unstable Clavaniev zone and, especially, the Piora basin. The rock also needs to be continually analysed as the tunnel is being driven forwards. The results feed into geological models that are constantly updated.
« In general, the rocks are moderately favourable for tunnelling, the Triassic zones much less so. »Federal Office of Transport, Preliminary geological report on the Splügen railway, p. 32, 28.02.1975