A Submerged Floating Tunnel, or SFT (also suspended tunnel or Archimedes bridge) is a tunnel that floats in water, supported by its buoyancy (specifically, by employing the hydrostatic thrust, or Archimedes' Principle).[1]
The tube is placed underwater, deep enough to avoid water traffic and weather, but not so deep that high water pressure needs to be dealt with—usually 20–50 m (60–150 ft) is sufficient. Cables either anchored to the Earth[1] or to pontoons at the surface[2] prevent it from floating to the surface or submerging, respectively.
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The concept of submerged floating tunnels is based on well-known technology applied to floating bridges and offshore structures, but the construction is mostly similar to that of immersed tunnels: One way is to build the tube in sections in a dry dock; then float these to the construction site and sink them into place, while sealed; and, when the sections are fixed to each other, the seals are broken. Another possibility is to build the sections unsealed, and after welding them together, pump the water out.
The ballast used is calculated so that the structure has approximate hydrostatic equilibrium (that is, the tunnel is roughly the same overall density as water), whereas immersed tube tunnels are ballasted more to weight them down to the sea bed. This, of course, means that a submerged floating tunnel must be anchored to the ground or to the water surface to keep it in place (which of these depends on which side of the equilibrium point the tunnel is).
Submerged floating tubes allow construction of a tunnel in extremely deep water, where conventional bridges or tunnels are technically difficult or prohibitively expensive. They would be able to deal with seismic disturbances and weather events easily (as they have some degree of freedom in regards to movement), and their structural performance is independent of length (that is, it can be very long without compromising its stability and resistance).
On the other hand, they may be vulnerable in regards to anchors or submarine traffic, which therefore has to be taken in consideration when building one.
Likely applications include fjords, deep, narrow sea channels, and deep lakes.[3]
A submerged floating tunnel has never been built, but several proposals have been presented by different entities.
| Date | Place | Country | Proposer | link |
|---|---|---|---|---|
| late 1800s | English Channel | United Kingdom | Sir Edward James Reed | [4] |
| 1969 | Strait of Messina | Italy | Alan Grant | [5] |
| 1998 | Høgsfjord | Norway | Norwegian Public Roads Administration | [6][7][8] |
| April 16, 2003 | Transatlantic tunnel | N/A | Discovery Channel's Extreme Engineering (Season 1, episode 3) | [9] |
| ? | Funka Bay, Hokkaido | Japan | Society of Submerged Floating Tunnel Technology | [10][11] |
| ? | Lake Washington, Seattle | United States | James Felch / Subterra | [11][5] |
| ? | Vancouver Island | Canada | Ministry of Transportation of British Columbia, Canada | [12] |
| ? | Lugano Lake | Switzerland | [5] |
Ponte di Archimede International, an Italian company, investigated the SFT in collaboration with the Norwegian Roads Research Laboratory, the Danish Road Institute and the Italian Shipping Register, with a financial grant from the European Union and the coordination of FEHRL (Forum European Highways Research Laboratories) an International Association of 22 Public Road Administrations. Furthermore the Provincial Administrations of Como (Como Lake) and Lecco, in Italy, have officially shown great interest in the Archimede's Bridge for crossing the Lario and the study of the submerged floating tunnel in the Strait of Messina has been promoted by Ponte di Archimede S.p.A. and verified with a feasibility analysis by the Italian Naval Register (RINA).[13]
The SIJLAB (Sino-Italian Joint Laboratory of Archimedes' Bridge), created in 1998, between Institute of Mechanics, Chinese Academy of Sciences, China and Ponte di Archimede S.p.A., is financed by the Italian Ministry of Foreign Affairs, the Chinese Ministry of Science and Technology and the Institute of Mechanics of the Chinese Academy of Sciences.
The consortium has started to build a 100m demonstration tunnel in Qiandao Lake in China eastern province of Zhejiang. Inside it, two layers of one-way motorways will run though in the middle, with two railway tracks flanking them.[14] The Qiandao Lake prototype will serve to help plan for the project of a 3,300-meter submerged floating tunnel in the Jintang Strait, in the Zhoushan archipelago, also situated in Zhejiang.[15][16][17]
According to Elio Matacena, the President of Ponte Archimede di International, the only constraint to build such tunnels in deeper waters is the price of the structure. Namely, the cables, which are very expensive, would be very long. He also refers that the bridge is capable of supporting more weight than a traditional bridge, which has very strict weight limits, while being up to two times cheaper. Matacena points out that the environmental studies carried on show that the bridge would have a very low impact on the aquatic life.[18]
Indonesia has also expressed interest in this technology. For the infrastructure, that would connect Bali to Thailand, there were two options, a conventional bridge or the undersea tunnel.
On 2004 the tunnel option was more widely discussed, specially when Kwik Kian Gie, then the Minister of National Development announced that a European consortium was interested in investing in the undersea tunnel between Java and Sumatra. The budget was told to be around 15 billion US dollars for the undersea tunnel in the Sunda Strait; in long term it would link up Bali, Java, Sumatra, Malaysia and Thailand in an uninterrupted chain. The project was planned to start construction in 2005 and be ready to use by 2018, and was a part of the Asian Highway.[19]
However, the bridge option was later favored.[20]
In 2007, Indonesian experts, led by Ir. Iskendar, Director for the Center of Assessment and Application of Technology for Transportation System and Industries, participated in a meeting with SIJLAB engineers, from the sino-Italian Archimedes Bridge project.[16][21] As an archipelagic country, consisting of more than 13 thousand islands, Indonesia could benefit of such tunnels. The conventional transportation service between islands is made mainly by ferries. Archimedes bridges (Terowongan Dasar Laut, in Indonesian) could thus be an alternative to connect adjacent islands, in addition to bridges.