Vienna is a densely populated city that has grown overtime, and it is suitable on geologically complicated land. Various soil formations overlap each other. The Danube and its tributaries are responsible for many water-bearing layers. Consequently, building an underground line in Vienna represents a major challenge for everyone involved in its planning and construction.
Three different underground construction methods are used depending on the subsoil properties and the presence of buildings on the surface: the cut-and-cover method, the New Austrian Tunnelling Method and tunnel-driving using a tunnel boring machine.