Fehmarn Belt Contractors (FBC) consortium took three years to dig the trench and removed around 15 million m3 of spoil from the seabed. At the peak of the work up to 70 vessels were operating.

Pedro da Silva Jørgensen, technical deputy director at Femern A/S, said it was the largest dredging operation in Denmark’s history, and it had been challenging.

“The subsoil between Denmark and Germany is a complex mixture of different soil types from soft clay to hard limestone. This has given rise to some significant challenges along the way, which we have successfully solved in collaboration with our contractors. Therefore, we are happy and proud that we are now at the finish line,” he said.

Several times during the operation the dredgers encountered huge blocks of granite from the Ice Age which were particularly difficult to remove. One of the largest weighed 70 tons. It is now on display next to the construction site at Rødbyhavn.

FBC project director Bart Pröpper said the operation required a range of dredging equipment, including trailing suction hopper dredgers, backhoe dredgers, specially developed and built grab dredge pontoons and boulder clearance vessels.

“On all these vessels, we made the necessary improvements to deal with the difficult soil types at great depths and get the job done in time. During the execution of this technically complex project, the safety of our employees and subcontractors was paramount, and we are therefore not only proud and excited about the completion of the tunnel trench, but also that we recently achieved a major milestone in safety performance,” he said.

Most of the excavated soil was placed behind dykes off the coast at Rødbyhavn where it has created 300ha of land for wildlife and recreation.

In the coming months FBC will remove the temporary dyke in front of the tunnel portal, preparing the area for the immersion of the first tunnel element later this year.

The 18km-long immersed tunnel beneath the Baltic Sea will connect Rødbyhavn on Lolland in Denmark and Puttgarden on the German island of Femern. It will consist of 79 standard elements, each 217m long, and weigh 73,500 tonnes. In addition, 10 special elements will house the tunnel’s technical installations in a special sub-basement.

The elements are being produced on six production lines at the world’s largest concrete factory. The elements will be sailed out, lowered into the tunnel trench, and connected individually.

Femern A/S expects to immerse the first tunnel element later this year and the link will be completed by 2029.