With the first tower crane erected on the Danish coast and dredgers working offshore, the massive challenge of building the world’s longest immersed tube tunnel over the next eight years is underway. The 17.6km-long combined road and rail tunnel will link Denmark and Germany across the open waters of the Fehmarnbelt, and the client, Femern A/S, aims for the tunnel to be open before the end of the decade.
“Work is progressing very quickly on our primary site in Rødbyhavn, and the future fixed link is already clearly visible in the landscape,” Jens Ole Kaslund, technical director with Femern tells Tunnels and Tunnelling International.
The immersed tube tunnel will be a key hub in Europe’s network of strategic transport corridors and has EU funding support. Most of the financing to build the toll-link crossing, though, comes from Denmark which has been driving planning and procurement of the huge bi-national project over the last decade, despite extended challenges. It is on Danish shores that the mega construction yard for the project is taking form.
The huge construction yard has two parts – a precasting factory with six parallel production lines to make the massive concrete elements, most weighing at least 73,500t; and a specially-built harbour holding drydocks, each serving two production lines, that will float the enormous elements to a common staging area to be readied for passage out to sea.
To begin the colossal enterprise, the civil engineering works have focused for more than a year mainly on dredging and creating the harbour. Onshore, meanwhile, there has been little to see above ground so far, except earthworks and roads in advance of the vast precasting factory. All that is about to change.
TUNNEL ELEMENTS
Fehmarn’s immersed tube tunnel will be a world-beating assembly of 79 standard concrete elements, interspersed with 10 special elements, at approximately 1.8km intervals.
Each standard element will be a gigantic hollow concrete structure in its own right. They are to be 217m long by 8.9m high and 42.2m wide, and formed of an assembly of nine slices, or segments, of the same cross-section and each 24m long, T&TI was told. The base of each segment will be cast, then walls, and finally a roof deck to complete the structure.
While Fehmarn’s standard concrete elements are longer and more massive than those on any other immersed tube tunnel, its special elements are slightly bigger again. They are taller, to accommodate a deeper basement to house operational support equipment, and they are a little wider, too. Otherwise, they have mostly the same cross section to align pairs of road and rail cells throughout the entire length of the concrete tunnel.
The special elements are of a size and number, in fact, that they would, by themselves, be longer and bigger than many existing immersed tube tunnels around the world.
Denmark is no stranger to immersed tube tunnels, and not small ones either. At the other side of the country, connecting the capital, Copenhagen, to Malmø, in Sweden, is the Øresund fixed link which has concrete elements of 55,000t – also huge in global terms. However, while massive, they are still smaller than Fehmarn’s standard elements, and Øresund was not built with any special elements.
ROAD TO CONSTRUCTION
The Fehmarn fixed link is forecast to be a major boost to economic growth in northern Europe, enabling faster transport between Germany and Scandinavia, via Denmark. But despite the anticipated benefits, and the use of an established tunnel technology, albeit on a gargantuan scale, getting this far with project development has taken much longer than had been expected for Femern A/S.
Femern A/S is one of the fixed link companies owned by Denmark’s Sund & Baelt Holding group, which is partly owned by the government. The group built and now operates the Storebelt tunnel and bridge crossing, and owns half the Øresund link.
Fehmarn development saw an immersed tube design picked in 2011, procurement started the following year, while much planning work was underway for applications made in 2013. But while Denmark gave approval in 2015, the process was unexpectedly, and extensively, drawn-out in Germany, severely delaying the start of main construction on most works.
The client picked the design-build joint ventures (JVs) for the civils packages in 2016, but the contracts were signed on a conditional basis only, and patience was sought all round.
Dredging and reclamation contractor for harbour works on both coasts is Fehmarn Belt Contractors (FBC), a Dutch JV of Boskalis, Van Oord and Sweco. The contractor building the tunnel, portals, ramps, and seabed trench is Fehmarn Link Contractors (FLC), a JV of Vinci, Per Aarsleff, Wayss & Freytag, Max Bogl, CFE, Soletanche Bachy and BAM; Cowi is consultant, and the subcontractor is Dredging International. The JV won various lots covering all the range of main works.
By late 2017, the client had introduced a contingency plan with a revised schedule that saw, conservatively, main works start by late 2020. That has mostly come to fruition; the exception was an early kick-off on harbour dredging at Rødbyhavn, thanks to the green light granted by the Danish Government.
FBC JV formally commenced the dredging and harbour works in late 2019. Only in recent months was FLC JV’s contract activated, and work is now gearing up to build the precasting factory and associated drydocks in the harbour.
“We’re very pleased with the progress we’ve made in just 12 months of construction,” Kaslund tells T&TI. “Many years of construction still lie ahead, but we’re off to a great start.” The client is supported on the project by a consultancy consortium of Rambøll, Arup and TEC.
CONSTRUCTION YARD
On the Danish coastline, the construction yard is a close neighbour to both the future site of the tunnel portal and the existing ferry terminal at Rødbyhavn, from where sailings reach Puttgarden on the German coast.
The scale of the construction operation for Fehmarn is huge by global civil engineering standards but also for an immersed-tube tunnel project. With the schedule looking at production and immersion of all 89 concrete elements over approximately four years, Fehmarn’s strategy has been to take a highly industrialised approach to the process, compared to many other immersed tube projects.
Even while those other projects have called for significant construction works in themselves, they were much smaller in comparison to Fehmarn, especially in the numbers of their elements, but also their sizes. Often, all the concrete elements could be cast beside each other in a large drydock, then it would be flooded and the group floated out. Otherwise, smaller drydocks would be used and only a few elements cast and floated out at a time, in a cyclical process.
Fehmarn will also have a cyclical process operating on the production lines and drydocks that point out to sea together like long fingers. The factory will hold five production lines for standard concrete elements, and one line is to be dedicated to production of the specials. This arrangement was jointly decided by the client and contractor as the optimum solution, T&TI was told.
Femern A/S also said the sheltered, climate-controlled factory will help ensure quality and consistency in concrete production – which is key to the performance of a structure designed to have a 120-year design life. A concrete testbed at Rødbyhavn has been working on concrete mix design for the project for more than 10 years.
The floors of the factory’s production lines and the drydocks will be on the same level, and connected by skidding beams. Concrete elements will pass along the beams from the factory to the drydock, each ‘receiving area’ serving two feeder lines.
Gates will seal-off the drydocks from the factory. Inside, with the end of the elements sealed off by bulkheads, the drydocks will be flooded. The floated concrete elements will pass through another gate, on the opposite side, to gradually emerge into the staging area of the harbour.
T&TI was told that the precasting factory is expected to be ready for partial operation in late 2022, and be fully operational the following year. Each standard segment will take about one week to produce, resulting in production times for standard elements of around nine weeks. Overall, the entire factory is expected to produce an element every 10-14 days.
The schedule anticipates the first concrete elements to be ready from late 2023 or early 2024, and floated out of the drydocks shortly afterwards. They will then be towed out, slowly immersed and positioned in a trench that has been prepared by dredging the seabed. After placement, the concrete tunnel will be covered for protection from shipping.
“There are many challenges in a project of this scope and size,” Kaslund tells T&TI. “Managing the logistics alone in a mega-project like Fehmarn, with the enormous manpower, machinery and raw materials required, is a significant challenge. However, we are well prepared, and we feel confident in our ability to handle any problems that may arise during the construction phase.”
The sailings and placement of the concrete elements are to take place in the middle of this decade, finishing by late 2027 or early 2028. Afterward, the project will shift gear to the fit-out phase for basement equipment chambers and each of the pairs of long road and rail cells which will run from coast-to-coast. The fit-out will include ventilation, lighting, signalling and signage, road surfacing and rail tracks to ensure everything is in place for future operations.
T&TI was told that Femern A/S anticipates the Fehmarnbelt tunnel to be completed in mid-2029.
