Since 1988, Nippon Steel Corporation has developed a new submerged tunnel tube production method consisting of prefabrication method which does not required graving docks, composite structure, and reliable flexible joint. The Kobe Minatojima Submerged Tunnel, the first steel-concrete sandwich tunnel tube, was produced in the graving dock. This development has been completed with the first tube completion of The Naha Submerged Tunnel, which the double-skin steel tube shell body including a bellows joint was towed to Okinawa from Kitakyusyu and was placed high fluidity concrete while afloat early spring in 2001.
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