Building over a highly sensitive urban subway station ‐ Achieving the near‐impossible through collaboration

Abstract The previous buildings around the existing underground U‐Bahn U3 Westbahnhof Station in Vienna, were demolished to make way for the new IKEA building. This was constructed with a deep excavation pit in the Miocene clay silt under the effect of groundwater. The deepest part of the excavation was only about four metres above the tunnel roofs of the U‐Bahn tubes. The main technical challenge was to develop a construction method that would cause as little uplift and deformation of the underground tunnels, including tracks and switches (especially curvature), as possible. A structural engineering design was developed based on comprehensive geotechnical investigations, existing documentation, condition assessments and mathematical forecasts. This design envisaged sectional excavation at depth with simultaneous construction of the above‐ground floors. The aim was to keep the load caused by the excavation directly above the U‐Bahn tunnels as low as possible. A comprehensive monitoring system was implemented to monitor structural deformations. During construction, the actual system behaviour largely corresponded to the forecasts. There were isolated but manageable leaks in the underground tunnels, which were successfully sealed after completion of the IKEA shell construction.