The ‘Cityringen’ project in Copenhagen was completed with the opening of the new M3 Cityringen line in September of 2019.
The urban mass transport system in Copenhagen
The Metro is an important part of the urban mass transport system in Copenhagen. At an estimated cost of approximately 25 billion kroner (3,3 billion euros), connecting 17 new stations with 15 km running underground, the Cityringen connects Vesterbro, Frederiksberg, Nørrebro, Østerbro with the inner city. As the biggest infrastructure project in Copenhagen since the 17th century, this massive construction project is helping Copenhagen reach its target of being the world’s first carbon-neutral capital by 2025.
Cityringen is an underground railway connecting the centre of Copenhagen to the surrounding inner-city areas. The project will continue with further extensions to the urban quarters Nordhavn and Sydhavn, totalling 24 new metro stations by 2024.
The Metro Company (Metroselskabet) will run two new lines on Cityringen – one circle route (M3) running around the entire track and one pendulum line (M4), which will run between Sydhavn and Nordhavn via the city centre. This new line provides extra services for the additional passengers on the busiest part of the route.
As with the existing Copenhagen Metro, the M3 Cityringen includes driverless train units and operates within gaps of less than two minutes during peak hours.
Plans for the Cityringen construction were approved by the Danish Parliament on 1 June 2007, and the initial contract was awarded in January 2011, with the main construction work commencing the following summer. The contract, which covers 15 km twin bored tunnels, 17 stations and 5 shaft structures, was awarded to Copenhagen Metro Team.
The construction consortium consists of three Italian companies: Salini-Impregilo, Tecnimont and SELI, and in addition the building consortium has some 250 subcontractors and suppliers.
Historic construction site
If P&C Insurance has been part of the group of insurers providing cover for the construction project (CAR Insurance), which includes public liability insurance and insuring physical damage due to the construction work and cover for the Tunnel Boring Machines (TBMs) used in the tunnelling project. The sheer number of contractors involved in the construction consortium highlights the complexity associated with large building projects like this.
Large infrastructure projects in dense urban areas create major challenges, such as a considerable number of changes to Copenhagen city’s utility grid, which had to be implemented before the actual construction work could be started. Around the same time, archaeologists from the Museum of Copenhagen worked on what the museum terms “Northern Europe’s largest archaeological excavation to date”. The archaeologists discovered some remains of the ancient city gate and wall, as well as other finds that resulted in the revision of Copenhagen’s early history.
The city is noted for its historic buildings, which had to be addressed in the planning and construction phase. Even though it is a challenge to build a metro in a dense city, with the narrow streets of Copenhagen, only two existing buildings have had to be demolished to make room for the new metro line and its stations.
According to Russell Saltmarsh (Seconded to Metroselskabet from engineering firm Arup, via COWI Arup Systra JV), responsible for the risk management of the civil works at Metroselskabet, the project has several risks that need to be mitigated. One of the biggest risks associated with any tunnel work in a dense urban environment is the impact on the existing buildings due to settlement caused by the tunnelling, especially considering that nearly half the tunnelling is in mixed soil conditions which can be very demanding and challenging for the contractor.
This was a focus area early on in the project with an assessment being carried out on all buildings within 200m of the alignment to identify sensitive or historic buildings that might be at risk of damage. Once the contractor was appointed it carried out a more detailed assessment of the buildings within a 50m corridor of the tunnel alignment.
An internationally accepted three-stage process
The assessment, based on predicted settlement contours from the tunnelling, follows an internationally accepted three-stage process. For each stage of assessment, a progressively more detailed analysis is carried out until it can be demonstrated that the work will not cause damage to existing buildings, as defined in the contract; or the assessment is used to determine the mitigation measures required. In several locations, the tunnels are only a few meters below the foundations of existing buildings, or operational metro stations.
To mitigate the possibility of excessive settlement of the ground causing damage to existing buildings, specialist techniques, such as compensation grouting, have to be applied under some buildings. A sleeved pipe is grouted into a predrilled hole beneath a foundation. Cement is injected at strategic locations, which results in a controlled heave of the overlying soil and structures, mitigating the effects of settlements when the tunnel machines passes through.
For sensitive buildings, 3D analysis of the ground and building structures has been carried out, and the results of this analysis are constantly compared with the results obtained from automatic monitoring systems installed on buildings to ensure that unexpected movements can be identified immediately.
Natural hazard risks
Natural hazard events are other risks to consider when engaging in infrastructure construction, and “in the Copenhagen Metro construction the flood risk is the dominant natural hazard risk,” says Fredrik Holmqvist, Head of Property RM Services Denmark at If P&C Insurance. Flooding events have been thoroughly analysed both for the construction phase and the operational phase. Consideration of the flood risk started during the preliminary design phase with hydraulic modelling of the entire city to model flood events.
This allowed the engineers to set flood threshold levels for both the temporary and permanent conditions, which were included in the contract requirements. This, together with the contractors’ own risk assessment and mitigation measures, has mitigated the flood risk as far as possible.
Article by: Kristian Orispää /If
Contributors: Fredrik Holmqvist /If & Metroselskabet