Assessment of field measurements of three deep excavations from Copenhagen’s Sydhavn metro line

Although several authors have investigated the performance of deep excavations over the past decades, most of the case studies analysed flexible retaining systems and/or soft ground conditions. Nevertheless, strict deformation requirements in constructions in urban environments frequently impose stiff design solutions for excavations. This study expands the existing data in the literature by evaluating the performance of three deep metro station excavations, located in the vicinity of buildings, and constructed by the bottom-up method in stiff over-consolidated deposits as well as soft rocks in Copenhagen. The data from the long-term comprehensive instrumentation program was employed for this purpose. Lateral wall deflections, axial forces of prestressed anchors and struts and ground water levels were analysed and discussed in accordance with the construction plan. The measured lateral wall deflections at the three case studies indicate a typical cantilever deflection shape until the installation of the first anchor, while the wall starts bending towards the excavation after the second excavation stage. The ratio between the maximum wall deflection and the excavation depth is at most equal to 0.25%, which is close to the lower boundary of reported ranges for available case studies. Corner effects were observed at one case study (Sluseholmen) where the width over length ratio of the underground excavation was relatively small. A linear correlation between the maximum wall deflection and the excavation depth was observed for wall deflections measured after the installation of the first support.