The undersea tunnel goes back 200 years in time


The Thames Tunnel in London was a world first, and the immersed tunnel under the Fehmarnbelt will, itself, be a world record based on technology with major Danish input.

Graphic showing the evolution of immersed tunnels

When the fixed link under the Fehmarnbelt is finished in late 2021, it will be the world's longest combined immersed tunnel for road and rail. The 18 kilometre tunnel will connect Denmark and Germany between Rødbyhavn and Puttgarden. The tunnel uses innovative ideas and also builds on proven technology and knowledge that has been gathered, developed and refined over 200 years.

The history behind modern tunnel construction dates back to the early 1800s, when the industrial revolution was in full swing to change Britain and the rest of the western world. The engineers’ dream was to build a big tunnel for horse-drawn carriages and pedestrians under the Thames in London. 

Prior to that, tunnel construction can be traced back to ancient times. The human race has always been attracted to shortcuts, whether cutting through mountains or crossing waters quickly and safely.

The oldest known tunnel actually documented, is a form of pedestrian tunnel under the Euphrates.  It was probably excavated about 4,200 years ago in Babylon in what is now Iraq. It was about one kilometre long, and it was possible to walk upright in there, apparently.

Remains of a bridge over the River Havos in Greece, believed to be 3,600 years old, have also been found. Neither bridge nor tunnel exists anymore, however.

This, in turn, leads to another famous tunnel, which was excavated across a mountain on the Greek island of Samos around 2,600 years ago.

Researchers have subsequently worked out how the mathematician and engineer, Eupalinos, made elegant use of ancient mathematics and measurement technology to solve a classic challenge: to ensure that the two excavations from each side of the mountain met inside it. Eupalinos achieved this by using geometry and common sense, having surveyed the mountain. He had his men digging in two straight horizontal lines from each side of the mountain right from the start.

Just before the middle, the digging direction changed a few degrees horizontally to the right and left respectively. The Greek engineer looked after the vertical part by keeping the bottom level and increasing the height of the last section from the one side. From the other side, the roof was kept level and the base was enlarged. If the tunnel section did not meet after all, both the ceiling and the floor could be enlarged so the effect was roughly the wide end of two funnels meeting.

It turned out that the precautions were unnecessary. The world's first properly-documented tunnel was constructed vertically to within a few centimetres of accuracy. This one kilometre plus tunnel for pedestrians and water can still be visited. Eupalinos’ achievement still sparks admiration among engineers and mathematicians today. See more here: http://en.wikipedia.org/wiki/Tunnel_of_Eupalinos

1843: The Thames Tunnel in London

There would be another 2,400 years before engineers really found a way to build tunnels under water.

Early in the 19th century, Great Britain benefited from the Scot, Thomas Cochrane, and Frenchman, Marc Isambard Brunel’s idea, which culminated in a patented technology for digging bigger underwater tunnels through soft ground and making them stable with an inner lining of bricks.

The inspiration came in part from a woodworm's ability to drill through timber using a shield. The wriggling back and forth would activate a set of teeth to scour material away. The two engineers constructed a huge shield of cast iron and wood and then excavated a shaft down from the riverbank, where the tunnel digging work itself could begin. Moving the tools that protruded through the shield would work the soil loose, which was then removed so that the shield could be moved forward. Led by Marc Brunel and his son Isambard, The Thames Tunnel Company began to dig under the river in 1825.

The technique worked and, in 1843, the tunnel for pedestrians under the Thames was ready for operation. It was 406 metres long and comprised two horseshoe-shaped tubes, 11 metres wide and almost seven metres high. It was the world's first fully functional underwater tunnel. The tunnel was a technological revolution at the time, and became a huge success – on the opening day alone 50,000 Londoners went through it, and there were actually user tolls with a one penny charge to walk through it. The plan was for the tunnel to also be used for horse-drawn carriages carrying goods. This never came to fruition, however, as the money had run out and plans for wide shafts with a spiral track at each end of the tunnel, which would have allowed for the horse-drawn carriages to drive into it, were abandoned. The tunnel is still used and is today part of the London Overground train system. See photos and read more about Thames tunnel here: http://www.ikbrunel.org.uk/thames-tunnel.

1910 and 1930: Michigan Central Railway Tunnel, USA and Canada

The technology then developed rapidly. In the US, engineer Howard A. Carson in the 1890s led the development of the innovative new concept of building immersed tunnels. They would be constructed of steel and concrete and consist of tunnel elements manufactured on land. The elements would be towed out onto the waters to be crossed, lowered to the bottom and assembled into a tunnel. In this situation it was considered to be simpler and cheaper than digging or drilling through the ground. The principle worked. But the idea was ahead of its time so, at first, it was only used for approximately 60 metre long sewer pipes of less than three feet in diameter.

But the concept worked and, in 1910, the world's first immersed tunnel for transporting goods and people opened. This was the Michigan Central Railway Tunnel under the Detroit River. The revolutionary railway tunnel was 2.6 kilometres long, built of steel and connected the cities of Detroit and Windsor in the US and Canada respectively. In 1930, it was complemented by another immersed tunnel for cars and both are still in operation today. It was also the world's first underwater link between two countries. Read more about the Michigan Central Railway Tunnel: http://www.dwtunnel.com/AboutUs.aspx.

1942: Maastunnel, Netherlands

In Europe, the idea of immersed tunnels had a new variant: they would be made of reinforced concrete and be cast as rectangular boxes, in contrast to the US where immersed tunnels were and are typically built as circular steel tubes and covered with concrete.

Concrete was more easily available than steel in Europe after the war and the rectangular shape minimised the cross-section. Thus, immersed tunnels in concrete could be built at competitive prices.

Danish engineers with M. Lassen Nielsen and A. E. Bretting led the development and introduction of the new tradition, which prevails in Europe, from the very beginning.

After five years’ work, Europe's first immersed tunnel cast in concrete opened in the Netherlands in 1942. It went under the River Maas in Rotterdam and had two tunnels for cars and two separate tubes for cyclists and pedestrians. The Danish engineering and consulting firm, Christiani & Nielsen, was one of the leading companies on the project. The design was a forerunner of today's immersed tunnels. It was efficient to produce the elements on land and then assemble them under the water. Read more about the Maas Tunnel here: http://www.architectuurinrotterdam.nl/building.php?buildingid=183&lang=en

Christiani & Nielsen were also advisers, when Denmark in 1969 – with the Limfjord Tunnel – got its first immersed tunnel.

Bored and immersed tunnels are becoming a widely used alternative to bridges, because the technology has evolved and enabled effective construction of highly secure links. The trend these days is toward tunnels instead of bridges in many infrastructure projects. This is partly because traffic through the tunnels is independent of the weather outside and also because they do not affect the surrounding countryside or urban area in the same way as a bridge does. Today, traffic is therefore moving underground increasingly to preserve the landscape as much as possible when planning and building new infrastructure. Boston and Stockholm are good examples. 

1974: Transbay Tube Tunnel, USA

In the United States the tunnels got longer. In 1974, the 5.8 km-long Transbay Tube tunnel in San Francisco opened for trains. It was built using the American method based on a steel frame and is still the world's longest immersed tunnel. In 1994, the 50 kilometre long bored rail tunnel between Britain and France opened and is the world's longest tunnel under water today. Read more about the Transbay Tube Tunnel, USA 1974: http://www.bart.gov/about/history/history2

Six years later, the world's longest immersed tunnel in concrete for both cars and trains was ready, as part of the Øresund link. It consisted of 20 composite elements and was just over four kilometres long, including the two tunnel portals at each end. Each element weighed 55,000 tons and was among the largest prefabricated tunnel elements in the world.

2000: Øresund tunnel, Denmark and Sweden

The greatest technological breakthrough on the Øresund tunnel was that the tunnel elements could be produced with consistent quality using industrial batch production close to the tunnel construction site. Read more about the Øresund tunnel: http://www.femern.dk/forside/anlagsfase/historien-om-oresundsforbindelsen/oresundstunnelen

The tunnel entrence at the Øresund Fixed Link

Since then, a number of immersed tunnels using similar technology in a number of countries have appeared. Among these is in South Korea, where the innovative approach of the Øresund link is further refined; another is in Turkey, where the deepest immersed tunnel constructed is under nearly 60 metres of water. See more about the Turkish tunnel between Asia and Europe: http://da.wikipedia.org/wiki/Marmaray

An immersed tunnel is currently being built in China, which will be the world's longest until the Fehmarnbelt link is completed. Danish engineering consultants are involved in many of these projects and this helps to further develop the technology.

This collected experience is involved in the development of the concept for the tunnel under the Fehmarnbelt, which takes the proven technology one step further. In particular by supplying a lower floor section in each of the ten special elements making up some sections of the tunnel, and by using longitudinal ventilation in the tunnel, rather than up through the cross-section.

The Fehmarnbelt link will not only be the world's longest and biggest immersed tunnel, but it will also be one of Europe's safest road sections. Among other things, this is because of the way it is set up so that oncoming traffic is avoided, the fact that the usual weather-related problems for traffic will not occur inside the tunnel and because of the innovative ventilation concept combined with a comprehensive monitoring and security system.

And so, tunnel construction is heading into a new era.

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