At the same time, the tunnel is considered to be the variant that is likely to provoke the fewest conflicts with nature, residential areas and commercial areas. Autobahn GmbH estimates a planning and construction period of 13 years - so it is still relatively unclear what the tunnel will look like. The first tenders for planning and construction firms are not expected for another four to six years. But some scenarios can already be played out now.

Rheinspange: First autobahn tunnel under the German Rhine
In any case, a highway tunnel under the Rhine would be a first, at least on the German side, says Sabrina Kieback, spokeswoman for Autobahn GmbH. Although there are already district heating and sewage tunnels under the Rhine, there is no tunnel for cars. A similar project on the Rhine is planned in Basel, but the realization of the underwater tunnel there is also a long way off. In Germany, the only two car tunnels under rivers are the Emstunnel near Leer and the Elbtunnel in Hamburg.

In principle, however, a road tunnel under water is nothing unusual, says Markus Thewes, holder of the Chair of Tunneling, Pipeline Construction and Construction Operations at the Ruhr University in Bochum. Around the turn of the millennium, he was involved in the construction of a road tunnel in the Netherlands: the 6.6-kilometer Westerschelde Tunnel. "Road tunnels under rivers are technically feasible and there are many successful examples of them, also in Germany." A tunnel under water is not necessarily more complex to build than a tunnel under pure ground, he said. Nevertheless, "A tunnel is naturally filled with air and therefore has buoyancy," says Thewes. "So the load above it, called the overburden, has to be heavy enough to keep the tunnel in place.”

There are several ways to approach the construction. The traditional option would be a so-called immersed tunnel, in which the river channel is opened, the necessary parts are lowered into the depths and installed there, and then the opening is closed again. However, this method is very costly, so it is questionable whether it is suitable for a project under the Rhine - after all, there is heavy shipping traffic here.

The more modern alternative, which is currently preferred for the Rhine bypass, would be to bore the tunnel into the depths from the mainland. Here, so-called tunnel boring machines are used. These are also used to compensate for the pressure on the tunnel. The groundwater pressure above the tunnel is therefore crucial for construction. "In general, the deeper the tunnel is below the water table, the higher the pressure. However, there are already tunnels under deeper waters than the Rhine," says Thewes. As a rule of thumb, there needs to be space of about one tunnel diameter between the riverbed - the bottom of the body of water - and the tunnel. Experts speak here of around nine to 16 meters in diameter.

The nature of the ground is also a decisive factor in how such a tunnel is approached. The challenges here vary: "Rock is more difficult to bore. Sand and gravel, on the other hand, require more support," says Thewes. If Autobahn GmbH actually decides on the drilling option, the tunnel will of course need an entrance and exit. How far these are created from the respective shore depends on how deep the tunnel runs under water and how large the angle of inclination is that it may have coming from the mainland.

According to the current planning, the Rheinspange will consist of two tunnel tubes. Anything else would be unusual, says Andreas Zenz, who heads the construction maintenance division at the Cologne branch of Autobahn GmbH. You have to think of it like this: The bigger the hole under the Rhine, the more difficult it is to stabilize. For that reason, it is better to have two tubes next to each other, each of which is stable in itself. If an accident happens in one tube, people can escape to the other. At present, two lanes are planned in each direction, with each direction having its own tube. Sometimes there is also a third tube, for example for additional escape routes. In the case of the Elbe Tunnel, there are even four tubes - the first three were built using the lowering technique, the fourth using a tunnel boring machine.

In principle, tunnels under water have advantages over bridges when it comes to navigating, the landscape and the weather, says expert Thewes. For example, motorists are not exposed to strong crosswinds. There are also appropriate precautions against flooding, he says, referencing the Dutch Westerschelde Tunnel: "There, dikes are built around the tunnel portal that can collect water.”
(Orig. text: Nina Bärschneider / Translation: Carol Kloeppel)