EUROPE: The first concrete element for the Fehmarn Belt tunnel has departed its Danish work harbour for immersion to the seabed. The operation marks a critical step in better connecting the Danish and German railway networks, as well as continental Europe and Scandinavia.
The 217 m long element left the Danish harbour in Rødbyhavn on May 4, and arrived at its destination on May 6, where it is ready to be submerged. The tunnel element weighs 73 500 tonnes, and contractors added 4 500 tonnes of ballast concrete inside to ensures the element is heavy enough to sink.
The Fehmarn Belt tunnel is one of Europe’s largest infrastructure projects. Ultimately, the finished infrastructure will reduce the train journey between Hamburg and København to 2½h. The tunnel will be the world’s longest immersed tunnel once it is finished.
Balancing rail and road
Each tunnel element contains four main tubes, of which two are for the railway and two are motorway tunnels. A separate service tube will house technical installations. The structure is sealed at both ends and completely filled with air.
However, the road traffic tubes are heavier than the railway sections. Therefore, the element is not naturally balanced. To counter this, the outer railway tube features temporary water chambers. These chambers ensure the structure remains entirely horizontal during the immersion process.
Lasse Vester, Deputy Contract Director at Sund & Bælt, highlighted the difficulty of the immersion task. ‘This is a very complex manoeuvre, which has never been carried out on this scale before’, he said. ‘The elements of the Fehmarn Belt tunnel are both wider, heavier and longer than those used to build the Øresund link, and we will be working in significantly deeper water.’
Specialised vessels directing the descent
Five tugboats are assisting with the transport of the tunnel segment. A purpose-built vessel named IVY handles the actual immersion. This vessel consists of two pontoons called IVY 1 and IVY 2. Each pontoon measures approximately 105 m long and 45 m wide.
A Femern A/S factsheet explains that IVY grips the tunnel element at both ends. Subsequently, the vessel lowers the concrete section using a controlled system. Operators utilise 66 winches and approximately 23 km of steel cable. Consequently, this equipment can immerse tunnel elements to depths of around 40 m.
The element will rest in an 18 km excavated trench. This trench stretches across the seabed from Rødbyhavn to Puttgarden. Prior to immersion, workers laid a bedding layer of gravel. This preparation ensures the concrete element sits in the correct position.
Vester said ‘it is a major task with very little margin for error. We have to immerse an element that is as long as two football pitches, within just a few millimetres. That places great demands on the equipment and on our contractor, which is why they have been preparing for this task for a long time.’
The entire transport and immersion process will take several days. Afterwards, contractors will place rocks and gravel along the sides to secure the element.