Underground planners are faced with many different challenges in built-up areas.
It will be particularly challenging in the area around the future Pilgramgasse station: this must be turned into an underground intersection while the U4 continues to operate.
The listed Otto Wagner station building must be subject to special protection. The new U2 tunnels need to pass under the Vienna river (Wienfluss). It cannot be re-routed, even for a construction project, due to its current-related propertied, and can swell to just below the top of the protective walls in the case of flooding.
This means that particular care must be taken around the riverbed during the construction work.
