The largest railway project in Norway

The project consists of the longest twin rail tunnels in Scandinavia, connecting Oslo and Ski in Norway.

The twin tunnels, both 19.5 km long and with an internal diameter of 8.75 metres, connect the cities of Oslo and Ski, forming the central hub of inter-urban development to the south of the Norwegian capital. The consortium, made up of ACCIONA and Ghella, will also deliver all the railway facilities except the signalling systems. The contract type is EPC Design and Construction.


The aim of the work is to boost economic development in the area and to facilitate communication between the inhabitants of Oslo and Ski. Improving people's quality of life in the Oslo area will not be the only benefit - saving travel time will result in energy saving and a reduction in CO2 and greenhouse gas emissions.


The project is already open to rail traffic.


The tunnel boring machines began drilling the rock of the Ekeberg hill halfway along the route, at Åsland, from where two double shield tunnellers have dug north towards Oslo and two others have drilled south towards Ski. Åsland was also where the facilities that served the keystone and concrete plants were located.

The TBMs, with an excavation diameter of 9.96 metres, were designed to tackle the conditions they would face during excavation, such as high water pressures and strong rock resistance.

The tunnel drilling project has also included the drilling and blasting of two caverns for the assembly of the tunnellers, 370 metre rescue tunnels connecting the caverns, an escape tunnel of 2.7 km in length, two transport tunnels of 427 and 394 metres in length, connecting the caverns and rescue tunnels with the outside; and two auxiliary tunnels, 267 and 294 metres in length, available for various needs of the tunnelling operations.

The Follo Line is marked by the latest innovation and technology. Thanks to an advanced monitoring and control system for the four tunnellers, it has been possible to know at all times, and precisely, the progress of the construction, in order to propose complementary measures and solutions. In addition, the project has involved an application for controlling cutter consumption, developed by the ACCIONA innovation team.


  • Location: Oslo, Norway.
  • Contract type: Design and construction in EPC mode.
  • Infrastructure: Two twin tunnels, each measuring 19,5 km long, with an internal diameter of 8.75 m, dug into the rock using four double shield tunnelling machines.
  • Year of project completion: 2022


One of the priorities of the AGJV consortium whilst the project is being carried out has been to minimise the environmental impact.

  • AGJV used conveyor belts to move rock debris to the workplace in Åsland and eliminated the need for thousands of vehicle trips to complete the same task, avoiding the emission of more than 27,000 tonnes of CO2 during tunnel drilling.
  • AGJV has reused 75 % of the excavation material for the preparation of the so-called deposit area where Oslo City Council will subsequently undertake urban development.
  • In collaboration with ACCIONA, a complex tunnel water treatment system was designed that includes the addition of ferrous sulphate to the circuit, allowing the management of incoming water from the simultaneous excavation of 4 TBMs, with a discharge of approximately 200-250 m3/h, high pH and TSS values as well as high hexavalent chromium (CrVI) content. The chromium action plan implemented in the WTP has allowed the reduction of CrVI by 75 %. The treatment of water in the project has also allowed the reuse of water (more than 80 %) in work activities, minimising the consumption of natural resources.
  • Monitoring is carried out in order to meet both ACCIONA and the client's sustainability requirements during the execution of the work. This monitors, among other aspects, the consumption of energy and materials, prioritising the use of materials with EPD (developing it where necessary, in collaboration with ACCIONA's R&D department), certified timbers and using machinery with high environmental standards.

Project implementation complies with all of Norway's strict environmental laws and regulations, as well as with the corporate objectives of the National Railway Administration of Norway.


To observe how the largest railway tunnel in Scandinavia is being built, with work due to be finalised in 2021, since 2017 Oslo has had a visitor centre that provides detailed information on the particularities of the design and construction process. Citizens can see the difficulties of this ambitious project and how its various phases are being carried out, with audio-visual material, illustrations, documentation and 360-degree projections of the tunnel under construction.

ACCIONA's Engineering Business Line led the execution project as the main designer.

Technology and innovation

To design the concrete used for the tunnel reinforcement keystones, on the one hand a comparison of short fibres, both polymeric and steel, was carried out to optimally meet the requirements of residual resistance to flexotractions.

On the other hand, it was proposed, through a detailed study, to replace the tunnel of thermal curing of the keystones with the use of latest generation accelerating additives based on CSH gels.


At the same time, the productivity of the tunnelling machine has a significant financial impact on the project budget. This is why we worked with the Data Science team at ACCIONA's Digital Innovation Hub to develop a model that would predict critical stops with impact on the project, by analysing the machines' sensors.


At ACCIONA we face the challenge of making a predictive model that would improve machine productivity by studying engine failures to increase the time it is operational. Therefore, all the engine faults during different parts of the production were sought.


The model has been able to predict faults, with great precision, even up to 15 days in advance. The predictive technology developed will be used in the next tunnels to prevent critical engine failures.


Furthermore, the design and construction of a new contact line allows, among many advantages, to adapt to the space limitations due to the small diameter of the tunnels without having to reduce the maximum speed of the trains, increasing efficiency and security with respect to the current systems.

    Awards and recognitions

    The project has won and been a finalist in several international awards:

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