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?
The concern of the Norwegian people and government for the environment places constraints on possible resolutions to this transportation conundrum [1]. Perhaps the most pressing challenge for engineers on the NTP E39 plan is how to pass the Sognefjord—a 3,700 meter stretch at its narrowest point between Lavik and Oppedal [1]. Although Norway already has 35 tunnels that run underwater, Sognefjord’s maximum depth of 1,300 meters (about one mile), makes standard underwater tunneling impractical and detrimental to the sensitive coastal ecosystem [5]. If the Sognefjord can be crossed, however, the Norwegian Public Roads Association (NPRA) hypothesizes that bridges and tunnels can be easily built for the six remaining fjord crossings [1].

A Bridge to Nowhere?

Underwater tunnels that descend over one mile below sea level, such as the Holland Tunnel in New York City, are dangerous and expensive. In these cases, engineers are forced to consider whether or not the necessity of a tunnel outweighs the safety risks that would be encountered with modern transportation. The NPRA considered both suspension and floating bridge types in its technical analysis of the Sognefjord crossing. The proposed suspension bridge would consist of a double tower construction, with each tower reaching higher than 150 meters, which is double the height of the Eiffel Tower [1]. Its span of 3,700 meters would break the world record of the longest suspension bridge [1]. To put that in perspective, a suspension bridge across the Sognefjord would be three times the length of the Golden Gate Bridge in San Francisco [1].
A second proposal was a “triple span floating suspension bridge” in which two of the bridge’s four towers would be on land on either side of the fjord, and two would be floating on pontoon-like structures anchored to the seafloor using rigging lines and anchors [1]. This proposal built off inspiration from the grand number of offshore oil and wind industries that are common in Norway. Each tower would have a total height of 200 meters with the pontoons supporting concrete floating towers boasting a 75 meter diameter. They would dive nearly 200 meters into the water and be anchored on either side with mooring lines- a common technique in the engineering of offshore oil rigs [1].

Is a Submerged Tunnel Really So New?

Briefly put— no. In 1996 the Forum of European National Highway Research Laboratories (FEHRL) explored the concept of building submerged floating tunnels in several principal locations in Europe [6]. A 100+ page report titled “Analysis of the Submerged Floating Tunnel Concept” outlined the basic concepts of Submerged Floating Tunnels (SFTs), design and construction engineering, safety, and environmental concerns. In fact, the paper introduces a brief timeline of underwater tunnels that spans at least 100 years.
Although running underwater tunnels have become more common, they are a distinct cry from floating underwater structures. The most obvious difference is perhaps that an underwater tunnel is completely anchored to a surface, namely bedrock (Fig. 2). While a floating submerged tunnel is also fixed, it is done so with various methods. The range of anchoring techniques is necessary to stand against volatile movements caused by currents or watercraft movement [6].
Figure 2: Water-spanning structures: 1: Suspension bridge 2: Submerged floating tunnel 3: Immersed tube 4: Undersea tunnel.