Variations on the strong buoyancy of floating elements can make anchoring stays act as rigid supports. That decides a minimum buoyancy in structures submerged by tension stays. Extra buoyancy may also be needed against oscillations and therefore the tension stays require very strong anchoring. Inside compressed gas can prestress submerged stays, pontoons and tube shells. Submerged tunnels can be made unsink-ingable by hollow, prestressed shells. Unsinkable tunnels can rationalize their installation and they may also be demanded for secure traffic. There are only economic limits for the length of unsinkable tunnels. Ferries are unfavorable for transport of gods in containers on trains. A submerged railway tunnel between Norway and Denmark may therefore be profitable. However, some crossings of the Baltic may give higher profit. In fjords screened against heavy weather, a bridge on submerged pontoons may cost less than tunnels. Fairway for seatraffic can be provided under a bridge of tubearches prestressed by compressed gas.
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