A submerged floating tunnel (SFT), also known as submerged floating tube bridge (SFTB), suspended tunnel, or Archimedes bridge, is a proposed design for a tunnel that floats in water, supported by its buoyancy (specifically, by employing the hydrostatic thrust, or Archimedes' principle). The tube would be placed underwater, deep enough to avoid water traffic and weather, but not so deep that high water pressure needs to be dealt with; usually a depth of 20 to 50 m (66 to 164 ft) is sufficient. Cables either anchored to the seabed or to pontoons on the surface would prevent it from floating to the surface or submerging, respectively.
Construction
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: After the tube is prefabricated in sections in a dry dock and the sections are moved to the site, one way is to first seal the sections; sink them into place, while sealed; and, when the sections are fixed to each other, break the seals. Another possibility is to leave the sections unsealed, and after welding them together at the site, pump the water out. The ballast 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 to achieve negative buoyancy so they tend to remain on 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, depending on the buoyancy of the submerged floating tunnel: slightly positive or negative, respectively.
Applications
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.
Proposals As of 2016, a submerged floating tunnel has never been built, but several proposals have been presented by different entities.
Europe In Norway, a first patent on this structure was presented in 1923 by Trygve Olsen ("Submerged pontoon bridge") and a new request was done in 1947 by the engineer Erik Ødegård. The interest has been revived during the last centuries with several studies in Norway, but it is just with the studies done by the Norwegian Public Road Administration (NPRA) that the feasibility of the structure is proven, with the recent developments of the offshore structures. The Norwegian Public Roads Administration (NPRA) has investigated the technical and economic potential for eliminating all ferries on fjord crossings along the western corridor (European route E39) between Kristiansand and Trondheim. This project also linked with FEHRL through the Forever Open Road programme. If the project were to proceed it estimated to cost $25 billion and be completed by 2050. 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 National Highway Research Laboratories) an International Association of over 30 National Road Centres. Furthermore, the Provincial Administrations of Como (Como Lake) and Lecco, in Italy, have officially shown great interest in the Archimedes' 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).
China 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 planned 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 through in the middle, with two railway tracks flanking them. It was later reported that the pilot project would now be a tourist observation tunnel to allow undisturbed viewing of the ruins of flooded Hecheng city, which are currently only viewable by scuba diving. 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. According to Elio Matacena, the President of Ponte di Archimede International, the only difficulty building such tunnels in deeper waters is the price of the structure. Namely, the cables, which are very expensive, would be very long. He also notes that the tunnel 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 environmental studies show that the tunnel would have a very low impact on aquatic life.
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