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The Transbay Tube is an underwater rail tunnel which carries Bay Area Rapid Transit's four transbay lines under San Francisco Bay between the cities of San Francisco and Oakland in California. The tube is long; including the approaches from the nearest stations (one of which is underground), it totals in length. It has a maximum depth of below sea level. The tube was constructed on land, transported to the site, then submerged and fastened to the bottom (mostly by packing the sides with sand and gravel). This immersed tube technique is in contrast to bored tunneling, where rock is removed to leave a passage. All BART lines except the Richmond–Fremont line operate through the Transbay Tube, making it one of the busiest sections of the system in terms of passenger and train traffic. ==Conception and construction== The idea of an underwater rail tunnel traversing San Francisco Bay was suggested by the San Francisco eccentric, Emperor Norton, in two proclamations that he issued in 1872.〔 〕 Official consideration to the idea was first given in October 1920 by Major General George Washington Goethals, the builder of the Panama Canal. The alignment of Goethals's proposed tube is almost exactly the same as today's Transbay Tube. In 1947, a joint Army-Navy Commission recommended an underwater tube as a means of relieving automobile congestion on the then-ten-year-old San Francisco–Oakland Bay Bridge. Seismic studies commenced in 1959, construction was started in 1965, and the tube was completed in 1969. The tracks and electrification needed for the trains were finished in 1973, and the tube was opened to service in 1974. The structure is made of 57 individual sections that were built on land and towed out into the bay by a large barge. They were then positioned above where they were to sit, and lowered into a trench packed with soft soil, mud, and gravel for leveling along the bay's bottom. Once the sections were in place, bulkheads at each end of each of the sections were removed and a protective layer of sand and gravel was packed against the sides. The project cost approximately $180 million in 1970 (equivalent to $ in ). During construction, the Transbay Tube was also used briefly as a shooting location for the ending of George Lucas's film ''THX 1138''. The final vertical climb out to daylight was actually filmed, with the camera rotated 90°, in the incomplete (and decidedly horizontal) Transbay Tube. The scene was filmed before installation of the track supports, with Robert Duvall's character using exposed reinforcing bars as a ladder. The western terminus of the tube directly connects to the downtown Market Street Subway near the Ferry Building, north of the Bay Bridge. The tube crosses under the bridge between the San Francisco Peninsula and Yerba Buena Island, and emerges in Oakland along 7th Street, west of Interstate 880. The Transbay Tube has required earthquake retrofitting, both on its exterior and interior. Research done years after construction showed that the fill packed around the tube might be prone to soil liquefaction during an intense earthquake, which could allow the buoyant hollow tube to break loose from its anchorages. Retrofitting work required the fill to be compacted, to make it denser and less prone to liquefaction. On the interior of the tube, BART began a major retrofitting initiative in March 2013, which involved installing heavy steel plates at various locations inside the tube that most needed strengthening, to protect them from sideways movement in an earthquake. In order to complete this work during 2013, BART closed one of the two bores of the tube early on weeknights. The work, estimated to last approximately 14 months, was completed after only 8 months of construction. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Transbay Tube」の詳細全文を読む スポンサード リンク
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