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This Tunnel-Bridge Combo Will Float

Thursday, July 28, 2016

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How do you solve the problem of building a crossing where traditional engineering methods won't work? Transportation officials in Norway are considering the world’s first floating submerged tunnel as their solution.

As part of its “E39 ferry-free project,” the Norwegian Public Roads Administration is weighing the possibility of constructing a Submerged Floating Tube Bridge (SFTB) as part of its update to the national highway, according to the Norwegian University of Science and Technology.

SFTB rendering
The Norwegian Public Roads Administration / Vianova

If a a Submerged Floating Tube Bridge (SFTB) is constructed as a fjord crossing in Norway, it will be the first such structure of its kind in the world.

If implemented, it would be the first such crossing structure of its kind.

A Unique Solution

The E39 is part of Norwegian national road system connecting Nørresundby, Denmark to Trøndelag, Norway. Along its path, the coastal highway crosses several fjords, which can reach widths of up to 5 kilometers (more than 3 miles) and depths of up to 1 kilometer (more than 3,200 feet).

Traditionally, the 1,100-kilometer (683.5-mile) route includes seven ferry crossings to bridge the fjords, Forbes reported Friday (July 22). Whereas the current requirements can take 21 hours of travel time, by making the route ferry-free, travel could be reduced to 11 hours.

However, these fjord settings call for unconventional solutions for a crossing, the school’s Department of Engineering said. The SFTB is one of the proposed answers.

How It Works

NUST describes the proposed structure as a buoyant tunnel, also known as an Archimedes Bridge, that floats at certain depth in the water. It allows crossings to be built in areas with “extreme geometric constraints like those found in a fjord,” NUST researchers said.

The Norwegian Public Roads Administration first revealed the proposed concept for the SFTB with a promotion video in 2013.

The Archimedes Bridge concept comes from the Greek mathematician’s studies of buoyancy. Because buoyancy is the result of the upward force exerted by a fluid that opposes the weight of an immersed object, as long as the immersed object weighs less than the water it disperses, it will float.

“And this is the key concept behind a submerged floating tube-bridge—you want it to float above most of the water while still remaining submerged,” Forbes explained.

The SFTB is slated to be set at 30 meters (about 100 feet) below the water’s surface so that water traffic at the surface will not be disrupted. Moreover, the depth will not interfere with submarines that navigate the fjords.

Floating pontoons connected to the shore are one option to support the SFTB in its placement. Alternately, it can be anchored to bedrock via cables or tethers, which act as a mooring system to hold the structure in a fixed position.

A Bridge and a Tunnel

“For the user, the structure will be indistinguishable from a tube-shaped tunnel,” NPRA senior engineer Arianna Minoretti told Forbes, “but in terms of its behavior, it has much more in common with a bridge.”

Minoretti said the SFTB would consist of two concrete tubes connected to each other by trusses. The tubes would be constructed in sections in dry dock before being sealed and floated to the installation site.

The finished tubes would be filled with water for ballast until they reached the necessary mass to float at the desired level.

The pontoons or cables would keep the SFTB in a “fixed, highly stable position,” according to Minoretti, so drivers won’t be aware of outside water movements.

SFTB rendering cutaway
The Norwegian Public Roads Administration / Vianova

“For the user, the structure will be indistinguishable from a tube-shaped tunnel,” NPRA senior engineer Arianna Minoretti told Forbes, “but in terms of its behavior, it has much more in common with a bridge.”

The depth of the SFTB would also protect it from weather and most of the environmental loads, the team added, but there are still a number of areas for consideration. NPRA is currently undertaking a feasibility study for the proposed project.

“Knowledge of the environment is very important,” Minoretti noted, “so in addition to [a] detailed simulation, we have started a campaign of measurement of wind speed, current, bedrock geology, etc., so that we can accurately compare our model to the real-world environmental loads.”

Still Not Set

The SFTB is not a sure thing at this point. It is one of three proposals the NPRA is considering. The others include a suspension bridge and floating bridge, not unlike Seattle’s SR 520 Bridge.

However, a traditional bridge would be a challenge for the area, the business magazine said. Beyond requiring a span reaching 3 miles (which would become one of the world’s longest), the above-surface structure would be exposed to corrosive sea air and strong winds.

Minoretti herself favors the SFTB, Forbes noted.

“I really hope we get to build the SFTB,” she said. “For an engineer like me it’s a dream project—it would be the first in the world, and it answers the question of how best to pass through incredible landscapes that need to be preserved, without damaging them.”

She is also optimistic that the innovative structure would spur a new trend in construction across the globe.


Tagged categories: Asia Pacific; Bridges; concrete; EMEA (Europe, Middle East and Africa); Infrastructure; Latin America; Marine; North America; Offshore; Program/Project Management; Transportation; Tunnel

Comment from Gary Burke, (7/28/2016, 8:06 AM)

This is unreal! Would be crazy to drive underwater!

Comment from Antonio Leal, (7/28/2016, 12:33 PM)

Amazing and beautiful and innovative design. It will be a big challenge, but based on road construction, mixed with off-shore technologies, will be a beautiful future school for all, keep us informed please the progress of this gorgeous design!

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