Various special techniques are available for building foundations. The use of ductile cast iron piles has proven to be a competitive method for pile foundation. Here, a ductile cast iron pipe with a length of 5 m is driven into the soil by means of a hydraulic hammer. Once a specified ram criteria is reached, during which the soil cannot be penetrated further within a defined interval, the ram process is finished and the cast iron pile cut to target height. The advantage of this method is that the remaining part can be used for the next pile. Each piece of a pile has a sleeve so that the pile can be enlarged by inserting a new piece of pile. The piles can transfer their load into the ground either as grouted friction piles or end-bearing piles which transfer the load into the ground through their tip.
Below the protected monument, the „Kaufhof“ building at Kö in the heart of Dusseldorf, a tunnel with a connecting platform to the existing railway station „Heinrich-Heine-Allee” is built, which is exactly situated in the middle of the new “Wehrhahn” line.