Data from the depths of the seabed will ensure stable ground under the tunnel

10 to 35 m below the seabed at a position immediately north of Puttgarden, advanced equipment is currently transmitting information about all movements in the subsurface.

This data retrieval is part of the extensive geotechnical investigations carried out by Femern A/S since 2008. All key data from the investigations will be included in the tender material for the fixed link.

Project Manager, Jens Kammer, Femern A/S, who has been in charge of the investigations from the outset, is confident about the outcome.

”In reality, although we’ll continue testing the ground conditions until the end of 2013, we have already produced so much information that we can provide a description of the ground conditions as a ”client deliverable”, which is a key element in the future tender. Should the ongoing measurements result in further information, this will naturally be shared with all relevant parties,” says Jens Kammer.

In Jens Kammer’s view, the Fehmarnbelt is as geotechnically challenging as were Storebælt and Øresund when the two links were being planned. Indeed, during the course of their investigations, the experts have subjected the subsurface to all possible impacts.

An important discovery

The first investigations of the subsurface took place as far back as 1963. Subsequently in 1995 and 1996, seismic investigations proved that the subsurface consisted of chalk similar to that found in the cliffs on the island of Møn. On the surface is a layer of fat clay, some of the finest in the world. What wasn’t immediately obvious, however, was that just north of Fehmarn, the clay layer has been disturbed down to depths of approximately 50 m because it was folded and tilted during the Ice Age, which is an important discovery.

”This type of clay contains water and when the clay is exposed to loading, it gradually presses the water away and settles,” explains Jens Kammer. “Similarly, through unloading, it will gradually absorb water and swell.  This type of movement, of course, can affect the construction of the Fehmarnbelt link, which is why we have designed an underwater test field where we can conduct our tests on a large scale.”

The tests are conducted in partnership with Rambøll Arup JV, Per Aarsleff A/S, GEO, and Norwegian Geotechnical Institute. The latter has experience from Norwegian North Sea oil explorations and is regarded as the world leader in its field.

”I can categorically state that we’re testing the limits of technology and that we’ve developed new methods, which have been necessary. This is the first time for a tunnel to be built with such dimensions and in such ground conditions,” Jens Kammer explains.

Largely speaking, the investigations have comprised all conceivable tests. As the soil to be removed for the tunnel is heavier than the tunnel itself, experts working in the test field have excavated 50,000 m3 of soil to a depth of 10 m to simulate the construction phase, measured movements and partially replaced it with sand to simulate the eventual weight of the tunnel.  

As a result, during the seven month period of excavations, the sea bed rose by around 2 cm. An expected outcome, believes Jens Kammer.

A number of other elements are covered by the investigations. What happens, for instance, if piles with a half metre diameter are installed 25 m into the seabed? How will their bearing capacity develop over time? Or what will happen if a large, deep trench is dug in the seabed and the edges of the trench are pressed away from each other by two massive plates subject to hydraulic pressure? Finally, around 100 borings to a depth of 50-100 m under the seabed have been undertaken. During the borings, continuous strength measurements were carried out and undisturbed samples for laboratory analysis were recovered.

All results from the geotechnical investigations will be made available to interested parties. Some of the information has already been published, e.g. ”Ground Investigation Report”, which summarises previously published reports, and is available at Femern A/S’ website under ”Publications 2011”.