The ground must be surveyed and the subsoil explored so as to determine how the underground line should run and which construction techniques are required.
This includes conducting exploratory drilling down to a depth of around 50 metres, which provides information about the rock stratification and groundwater levels. Another aim is to locate any war relics, particularly unexploded bombs.
Geologists and geotechnicians also drill into the ground around 75 times along the planned U2-route.
Deciding on the drilling points is not an easy task: Streets should be blocked as little as possible and care must be taken to avoid pipes and cables already in the ground.