Copenhagen’s Cityringen
Specialist geotechnical consultancy
The Copenhagen Cityringen is a circular metro line in the centre of the Danish capital, inaugurated in 2019 with the aim of steering the city towards carbon neutrality. The line comprises two parallel tunnels spanning 17.4 km, 17 new stations at approximately 30 metres below street level, and 5 ventilation shafts.
Within this context, the geotechnical consultancy focused on the Frederiksberg Allé and Aksel Møllers Have stations and the Sønder Boulevard shaft, which were considered representative cases of the general behaviour of this type of structure.
Both the stations and the shaft were constructed using the bottom-up method. Specifically, the temporary external retaining structure for the two station boxes was built with secant pile walls socketed into the
Copenhagen Limestone and braced at multiple levels, enabling safe excavation to formation level—approximately 20–30 m below ground—while limiting settlements and lateral displacements in the surrounding urban environment.
The Sønder Boulevard shaft followed a similar scheme; however, continuous diaphragm walls were used instead of secant pile walls to resist earth pressures.
Given the construction methods adopted on site for the secant pile walls and diaphragm walls, together with the highly corrosive nature of Copenhagen’s groundwater, considerations were developed regarding the effective durability of the steel reinforcement in these temporary concrete structures.
Accordingly, the long-term behaviour of the temporary works (the secant pile walls at the stations and the diaphragm walls at the ventilation shaft)
was analysed with reference to its effects on the earth-pressure regime acting on the permanent station structure over time.
In particular, comparative finite-element numerical analyses were carried out to assess changes in the earth-pressure distribution acting on the final internal lining of the station boxes and the shaft, and to evaluate the implications for the design of the permanent station and shaft structures.
