About this Article
Written by: Sabrina Winarko
Written on: December 7th, 2016
Tags: civil engineering, mechanical engineering, transportation, building & architecture, water
Thumbnail by: Will Heilpern/Weforum
About the Author
Sabrina Winarko is a 21-year-old wanderlust that one-day hopes to vacation in Norway to see the Northern Lights.
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Volume XVIII Issue II > To Float or Not to Float?

How Does It Float?

Submerged floating tunnels (SFTs) generally float by the Archimedes Principle, which describes the upward force motion of an object that is submersed in a liquid. This principle can be applied to an SFT, which is lighter than water and is forced to the top surface of water- a phenomena described as positive buoyancy [6]. Negative buoyancy can be ascribed to SFTs that are heavier than water and describes the downward force as it sinks to the bottom.
Depending on the buoyancy design and environmental factors impacting the submerged floating tunnel, novel engineering methods may be required to anchor the SFT in place, such as column or tension leg supports [6]. Pontoon supports, on the other hand, may be utilized to support a SFT that would otherwise sink [6]. As submerged floating tunnels are subject to external forces such as water current, earthquakes, sub-zero temperatures, and watercraft movement, more support is necessary to maintain a constant, stable environment for the tunnel [7]. This maintains a sense of safety and structural integrity of the tunnel.

The Sognefjord Submerged Floating Bridge

The submerged floating tunnel proposed by the NPRA to span the Sognefjord is designed as a curved concrete structure consisting of two tubes, one for each direction of traffic [8]. It will be anchored to bedrock at the entrance and exit points of the tunnel. Designed to have negative buoyancy, the tunnel will be anchored by placing approximately 15 floating pontoons on the water’s surface [1].
The FEHRL study indicated that submerged floating tunnels should be situated deep enough underwater to mitigate the impact of surface wind, weather and watercraft disturbance [6]. However, SFTs should not be placed so deep that the increasing water pressure becomes too great of an obstacle [7]. Depending on the type of SFT and its proposed location, tunnels can be as placed as deep as 100 meters below the sea level. This, however, is a maximum depth: most proposed submerged floating tunnels, including the Songnefjord SFT, are designed to be placed at a maximum of 30 meters below sea level [6, 7].
The Sognefjord SFT in particular is designed to be placed 20 meters (about 100 feet) below sea level [2]. Driving a car at this depth may sound frightening- however, the SFT buoyancy experienced at this depth should be virtually imperceptible [6]. Furthermore, the supportive pontoons above are designed and placed in a way that causes minimal impact to important military and cruise ship activity in the fjord above.
The next big question to tackle is this: How much will this cost? The answer doesn’t come cheap. The Sognefjord SFT is part of a 25 billion dollar E39 coastal highway improvement plan expected to be completed by 2035 [2, 4].

Safety First

Safety is a primary concern for all engineering studies concerning the feasibility of submerged floating tunnels [7]. The biggest concern is of the potential breach of water into the body of the tunnel. To combat this, safety routes (similar to modern tunnels) are integrated into the blueprints, ensuring a reliable exit in the event of an emergency [6]. It doesn’t stop there- a second concern is seismic activity [7]. These, and other natural disasters, are well planned for, and the impact of accidents (such as boat crashes) have been analyzed and considered in the Sognefjord tunnel [1], which would withstand the loss of several surface pontoons without sustaining damage to the tunnel below. Consequently, although a submerged floating tunnel has never before been built, the safety and experience for potential travellers is, by all calculations, projected to be equivalent to standard tunnel travel [6] [7].

Crossing New Expanses

According to experts, floating submerged tunnels are not only the future of transportation engineering, but may be the only promising way to cross great expanses [7]. The environmental impact of SFTs is also beneficial— as complex air treatment processes must be present to mitigate exhaust from underground vehicles, particulate matter that would otherwise be released untreated are processed and scrubbed before being released into the atmosphere [7]. Additionally, the sporadic ferry crossings currently used in Norway would obsolete and the associated carbon emissions consequently reduced [1]. The travel time between northern and southern Norway would be significantly reduced to just under 10 hours, lowering carbon emissions by 50%, and providing the economic benefits of saved time [1].
Although this method of engineering has yet to be successfully implemented, the Norwegian government remains optimistic for the promising environmental impact and traffic alleviation. Beyond that, the science and public demand make an underwater submerged tunnel an anticipated and a necessary advance in transportation technology [7].


    • [1] “Statens vegvesen - Coastal Highway Route E39 animation (English voice),” YouTube,​ 15-Apr-2013. [Online]. Available:​com/watch?v=l7en6etg​2mk&​&t=263. [Accessed: 04-Sep-2016].
    • [2] N State, “Ferjefri E39 | Statens vegvesen,” Staten​s vegvesen. [Online]. Available: http://www.vegvesen.​no/vegprosjekter/fer​jefrie39. [Accessed: 04-Sep-2016]
    • [3] A. Marshall, “What It Takes to Keep a 7,700-Foot Floating Bridge From Doom,”,​ 19-Apr-2016. [Online]. Available:​m/2016/04/takes-keep​-7700-foot-floating-​bridge-doom/. [Accessed: 03-Sep-2016].
    • [4] A. Marshall, “Yes, a ‘Submerged Floating Bridge’ Is a Reasonable Way to Cross a Fjord,”​, 07-Jul-2016. [Online]. Available:​m/2016/07/submerged-​floating-bridge-isnt​-worst-idea-norways-​ever/#slide-2. [Accessed: 03-Sep-2016].
    • [5] J. Ramey, “Norway's floating tunnel concept is both brilliant and terrifying,” Auto​week, 26-Jul-2016. [Online]. Available:​article/technology/n​orway-plans-worlds-f​irst-submerged-float​ing-bridges. [Accessed: 04-Sep-2016].
    • [6] “An Analysis of the Submerged Floating Tunnel Concept,” FEHRL:​1996. [Online]. Available:​/index.php?m=203. I​mage published by Transport Research Laboratory, after FEHRL, 1996. [Accessed: 03-Sep-2016].
    • [7] A.B. Kawade and S.P. Meghe, “Submerged Floating Tunnel,” Submerge​d Flaoting Tunnel. [Online] Available: http://www.engineeri​​d-floating-tunnel-ht​ml. [Accessed: 09-Sep-2016].  
    • [8] L. Winkless, “Could Norwegian Engineers Really Build A Floating Tunnel In A Fjord?,” Forbes, 22-Jul-2016. [Online]. Available:​m/sites/lauriewinkle​ss/2016/07/22/could-​norwegian-engineers-​really-build-a-float​ing-tunnel-in-a-fjor​d/#59d81fb56878. [Accessed: 04-Sep-2016].
    • [9] Will Heilpern, “Norway could build the world’s first floating tunnel,” Weforum, 29-Jul-2016. [Online]. Available: https://www.weforum.​org/agenda/2016/07/n​orway-could-build-th​e-worlds-first-float​ing-tunnel