The Øresund bridge is one of the most mesmerising structures ever built, thanks to the fact that it transforms into an underwater tunnel.

The Øresund bridge is a marvel of modern engineering. It consists of a combined road and rail line that runs for eight kilometres, at which point it then transitions into an underwater tunnel. In total, the bridge/tunnel stretches 12 kilometres, connecting Denmark and Sweden.

The bridge first opened in July of 2000 as a primary means of transit for the entire region. The structure carries a data cable through its infrastructure that serves as a connection hub for much of the internet access across Europe.

In terms of notable feats of achievement for the bridge, the Øresund bridge is the longest combined road-rail bridge in the entire European continent.

Why the Øresund bridge was built

Deciding to build a bridge that goes over a large waterway that then transitions into a submerged tunnel seems like a rather peculiar choice.

It's not often that we see tunnels transiting waterways, and it is even more uncommon that we see tunnels beginning in the middle of a waterway. However, the design constraints of the project justify the final construction.

The engineers behind the bridge had to design a transit structure that was tall and wide enough to let shipping traffic transit the channel – thus the bridge portion. However, the bridge also couldn't be so tall that it obstructed the nearby Copenhagen airport.

Engineers were concerned that the high shipping bridge would be too much of an obstacle to landing and taking off planes that they decided to transition the relevant portion of the bridge into a tunnel.

The biggest fear of engineers was that after construction, a plane might crash into a support tower of the bridge, creating large interruptions in international travel.

In the initial scoping phase of the project, a design competition was held to determine how the bridge and tunnel structures should be built. This was done both for publicity, but also to pool collective engineering minds to ensure optimal final design.

Another notable constraint to the bridge was that it needed to have a rail line under the bridge. The Øresund bridge needed to transport everything, cars, rail cars, and international internet cables.

The rail constraint kept the engineers from going with a traditional cable suspension bridge, which would've been too shaky for trains. Rather, they settled on a cable-stayed design, which transfers weight through multiple cables back to main towers.

The final failsafe in the design process was to leave each support tower completely unconnected from the others. This ensures that if a plane ever were to crash into one of the support towers, the other might be able to keep the structure from completely collapsing.

As you might be able to tell, the Øresund bridge was designed to last quite some time, with all possible failure modes in mind.

How the bridge and tunnel were built

Construction of the Øresund bridge was an engineering marvel all it's own. It was part road bridge, part rail bridge, part tunnel, part rail line. The number of design constraints and moving parts to the process was absolutely stunning.

The biggest issue in the design process was figuring out how to transition the structure from a bridge into a tunnel.

There wasn't any land or point where this could be done easily, so engineers had to dredge the seafloor to build a massive artificial island where the tunnel could begin. The artificial island that was built is named Peberholm.

video: https://youtu.be/-yLZYETYlmM

This island was the main transition point and focal point of the entire transit system, but it was made entirely of dredged seafloor material. This meant that the tunnel couldn't be drilled, rather the tunnel had to be pre-built into the island.

Engineers created 55 million total kilograms of tunnel segments that were sealed shut using specially designed ships underwater, where they were then dropped into place and backfilled.

In total, the Øresund bridge cost $4.5 billion at the time, which was paid for through government funding and loans initially, but is now being recollected using tolls from drivers that cross the bridge.

The toll system is expected to completely pay for the bridge within 30 years of its completion date, or 10 years from now, in 2030.

As for the construction of the bridge supports, there are four main towers. These massive towers were poured in sections that were placed using specially built rigs.

Another aspect of the structure that is quite unique and interesting is that a maintenance system and vehicle were built into the structure.

Since daily inspections were needed on the structure to ensure safety, there's a motorised gantry under the railway portion of the bridge. This vehicle is equipped with a variety of tools, even a large hydraulic arm that can reach the road's surface.

Video: https://youtu.be/JPR36tIak8U

While many major construction projects can be quite detrimental to the environment, the major care was taken during construction to not harm the surrounding underwater ecosystem. The artificial island is now home to over 500 species of plants and has become a haven for biologists.

In the greater picture, the Øresund bridge is one of eight bridge-to-tunnel projects in the world, and it continues to serve as a transit hub for road, rail, and internet travel across all of Europe.

This article was written by Trevor English and is reproduced with kind permission from InterestingEngineering.com. Find the link to the original article here.