Mega-Dredging ‘Shovels’ to Trench the Fehmarnbelt Tunnel

The Fehmarnbelt road and rail tunnel between Denmark and Germany will be the largest infrastructure construction ever built in Denmark. To achieve this monumental work, the first step will be to excavate the seabed for the tunnel. This will be the largest excavation at sea on Danish territory ever undertaken, and the dredging operations have been awarded to the Dutch dredging contractors Royal Boskalis Westminster, headquartered in Papendrecht, and Van Oord of Rotterdam.

Geographically, the Fehmarnbelt region extends from Hamburg across Schleswig-Holstein and the western part of Mecklenburg-Vorpommern, the Danish islands of Lolland, Falster and Zealand to Skåne in southern Sweden. Two straits have historically separated the region, the Øresund between Copenhagen, Denmark and Malmö, Sweden and the Fehmarnbelt between Fehmarn, Germany, just east of Puttgarden, and Lolland, Denmark. The Øresund Bridge has linked Denmark and Sweden since 2000. Under the stretch of water known as Fehmarnbelt, a tunnel will be trenched and immersed, going from Rødbyhavn on the Danish island of Lolland across to the German island Fehmarn. Presently only a ferry from Puttgarden to Rødbyhavn connects the two countries, creating a serious traffic bottleneck for passengers and freight.

While the Fehmarn connection will in many ways employ methods similar to those used during construction of the Øresund connection between Denmark and Sweden, the Fehmarnbelt project’s dimensions are far more remarkable. The Øresund crossing is comprised of both an immersed tunnel and a cable-stayed bridge. Although it was a ground-breaking project at the time, and provided a game-changing economic boost for both countries, the tunnel trench at the Øresund connection was only 4 kilometers long (2.5 miles). The Fehmarn connection will be 18 kilometers (more than 11 miles) long, 12 meters (39 feet) deep and more than 40 meters (131 feet) wide.

Large Capacity Mechanical Dredges

Technology and the capacity of dredging equipment has grown enormously in the last 20 years. Boskalis and Van Oord will be deploying some of their largest digging machines at Lolland on the Danish side, which are actually some of the largest excavators in the world. While the exact machinery has not yet been determined, the operations are based on using mechanical dredges like, for instance, the mega-backhoe Magnor, launched in 2015. To appreciate the size of this massive shovel: imagine that its bucket capacity is big enough to pick up an entire shipping container with room to spare. Specifically, the Magnor’s bucket has a capacity of 40 cubic meters (52 cubic yards). With a total capacity of 4,100 kW, the Magnor’s bucket can lift up to 67 tons of dredged material, and can dig to depths of 40 meters (131 feet). It is 72 meters (240 feet) long, with a breadth of nearly 20.5 meters (67 feet) and 40-meter-long (131-foot) spuds. Backhoes are increasingly being used for environmental reasons because they minimize turbidity, which is a key requirement for the Fehmarnbelt project.

The design of the Magnor is similar to Boskalis’s Baldur, which is now the second-largest backhoe in its fleet with a capacity of 24 cubic meters (31 cubic yards). It too was developed in-house. The Magnor’s BK 12700 DD Greenline excavator is based on the largest mining machine available today, the Caterpillar 6090. The name ‘Greenline’ reflects the fact that the machine has low fuel consumption and reduced emissions of harmful substances. A Boskalis team modified and assembled the entire excavator and the complete lifting section including the boom, stick, hydraulic cylinders and different sizes of buckets were developed in house.

Van Oord’s mega-backhoe Goliath, built in 2009, is likewise of extraordinary large proportions. The Goliath is a Backacter 1100 – an excavator with three spud poles fitted to a pontoon. It can be equipped with a bucket of 25 to 40 cubic meters (32 to 52 cubic yards) and can work at a dredging depth of up to 26 meters (82 feet). It has a total installed power of 5,000 hp, comparable to the horsepower of 50 large automobiles. The Simson, a sister backhoe with slightly smaller but similar capacity to the Goliath, was also launched in 2009.

These mechanical dredges are heavy duty workhorses suited to dredging a variety of soils. In the case of Fehmarnbelt, these soils are characterized on the German side by Paleogene clay and some clay-till with boulders. In the middle or central basin area, soils are comprised of gyttja, sands, silts and clays, and on the Danish side, the soils are primarily thick deposits of clay-till.

“The advantages of such mega-mechanical dredging machines are that they are cost efficient and simultaneously less harmful to the environment because they spill less sediment back into the water,” said Jørgen Andersen, head of the project for Femern A/S.

Even considering the enormous size of these dredges, the excavation is expected to take about 18 months before the trench is ready. Approximately 19,000,000 cubic meters (24,850,000 cubic yards) of sand, rock and clay will be removed, some of which will be used to build more than 3 square kilometers (more than 1 square mile) of new land on the south coast of Lolland.

Once the tunnel is finished and the elements are connected, the dredging contractors will cover the tunnel with layers of sand and gravel that will protect the tunnel from ship traffic and make it invisible to marine animals.

The Fehmarnbelt tunnel will bring the region together and provide new opportunities for consolidating its respective strengths. Work is well underway in a large number of cross-border organizations, including the Fehmarnbelt Committee, Fehmarnbelt Development, Fehmarnbelt Business Council (FBBC), STRING and the Baltic Development Forum.

The Fehmarnbelt tunnel connection has been a multi-year project and requires bilateral approval from both the governments of Denmark and Germany. “The current focus is on the German Plan Approval decision, and the German authorities are making good progress on that front. A plan decision is expected to be finished by summer this year 2018. We expect construction to start in 2020 with the dredging operations as the first order of business,” said Jens Villemoes, head of media relations at Femern A/S.