The recently completed geotechnical investigations in the Fehmarnbelt have thrown up new and important knowledge about a particular clay layer, which has been a great challenge for other major construction projects until now. The engineers can now present specific proposals for the foundations of the Fehmarnbelt fixed link in the problematic clay layer, between the portal on the German coast and the tunnel itself.

Femern A/S geotechnical investigations have led to a better understanding of how Palaeogene clay reacts when it is subjected to different loads. In natural deposits the highly plastic and fat clay is in balance with its surroundings, and it has taken a long time to reach this state. When pressure is relieved during dredging work, the clay draws in water like a sponge, and expands. When it becomes subject to stress again, it settles together once more and creates folds and settlements, which contractors can only take into account when they have full knowledge of the soil and how it reacts.

Backfilling around the portal can give rise to differential settling which can be countered by various known methods, and it is ultimately up to the contractors to come up with specific solutions in their design.

Femern A/S has carried out particularly thorough investigations in a test field a kilometre off the German coast at Puttgarden, and has been analysing the results since 2009. Click here and and dk/14 for tunnelbuilder archive. Visit www.femern.com. 05/14.