Design and construction of the new railway bridge over the Neckar river - challenges of the foundations

As of the beginning of 2016, as part of the Stuttgart 21 infrastructure project, the Deutsche Bahn is building a four-line railway bridge across the Neckar River in Stuttgart. The Neckar railway bridge has an overall length of 345 m and comprises of two abutments and six support axes. The structure crosses rail lines from the city rail network (Stadtbahn), the main trunk-road, the B 10, inner city roads, the Neckar River, pedestrian and cycle ways as well as diverse public and private utilities. The bridge structure is located within the core mineral water protection zone, with its strict restrictions regarding permissible depth of penetration into the subsoil and groundwater reservoirs. The mineral water springs lie within the construction site 26 m below the surface as artesian aquafers with an artesian pressure of circa 10 m above ground level. From a groundwater management perspective, and following a program of five ground investigations and their evaluation, a raft foundation at the mid-level of the "Unterkeuper", built using cut-andcover technique with a pneumatic caisson was chosen as the most economical foundation type for the three primary supports. A program of further hydrochemical and geomechanical ground investigations were undertaken during the detailed design phases. Following an evaluation of these additional ground parameters it was possible to substitute the pneumatic caisson for a more economical deep foundation type using large diameter piles. A management concept for possible scenarios was needed for the large diameter pile construction to achieve the required consent for derogations from the banned ground penetration in the area of the mineral springs.