Transbay Tube
The Transbay Tube is an underwater rail tunnel that 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, and attaches to twin bored tunnels. The section of rail between the nearest stations totals in length. The tube has a maximum depth of below sea level.
Built using the immersed tube technique, the Transbay Tube was constructed on land in 57 sections, transported to the site, and then submerged and fastened to the bottom – primarily by packing its sides with sand and gravel.
Opened in 1974, the tunnel was the final segment of the original BART system to open. All BART lines except the Orange Line operate through the Transbay Tube, making it one of the busiest sections of the system in terms of passenger and train traffic. During peak commute times, over 28,000 passengers per hour travel through the tunnel with headways as short as 2.5 minutes. BART trains can reach their highest speeds in the tube, up to, although trains typically operate at unless recovering from a delay.
Conception and construction
Early concepts
The idea of an underwater rail tunnel traversing San Francisco Bay from Oakland Point was suggested by the San Francisco eccentric Emperor Norton in a proclamation that he issued on May 12, 1872. Emperor Norton issued another proclamation on September 17, 1872, threatening to arrest the city leaders of Oakland and San Francisco for neglecting his earlier proclamation.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, from the foot of Market St. to the Oakland mole, was almost exactly the same as today's Transbay Tube. His proposal called for building on bay mud, which anticipated some of the seismic design aspects of the finished Transbay Tube, and was estimated to cost up to. A competing bridge-and-tunnel proposal was advanced in July 1921 by J. Vipond Davies and Ralph Modjeski, closer to the alignment of a proposed Southern Crossing, between Mission Rock and Potrero Point in San Francisco due east to Alameda. Davies and Modjeski were critical of the ventilation issues that would arise from a long combined automobile and rail tunnel, indirectly endorsing the idea of a dedicated tunnel for electric rail traffic. The Davies and Modjeski proposal was joined by twelve other proposed projects to cross the Bay in October 1921, several of which featured rail service through long tunnels.
In 1947, a joint Army-Navy Commission recommended an underwater tube as a means of relieving automobile congestion on the then-ten-year-old Bay Bridge. The recommendation was issued in a report undertaken to determine the feasibility of the Reber Plan.
Construction
Seismic studies commenced in 1959, including boring and testing programs in 1960 and 1964, and the installation of an earthquake recording system on the Bay floor. The tube's route was modified after preliminary surveys were unable to identify a continuous bedrock profile, requiring more precise boring and probing of the Bay floor. The route was deliberately chosen to avoid bedrock as much as possible so the tube was free to flex, avoiding concentrated bending stresses.Design concepts and route alignment were completed by July 1960. A 1961 report estimated the cost of the Transbay Tube at. Construction was started on the tube in 1965, and the structure was completed after the final section was lowered on April 3, 1969. BART sold commemorative bronzed aluminum coins to mark the placement of the final section. Prior to being fitted out, the tube was opened for visitors to walk through a small portion on November 9, 1969. The tracks and electrification needed for the trains were finished in 1973, and the tube was opened to service on September 16, 1974, five years after the originally-projected completion date, after clearing California Public Utilities Commission concerns regarding the automated dispatch system. The first test run was performed by a train under automatic control on August 10, 1973. Train No. 222 ran from to in seven minutes at and returned in six minutes at the full speed of, carrying approximately 100 passengers including BART officials, dignitaries and reporters.
The tunnel is set in a trench wide with a gravel foundation deep. Lasers were used to guide the dredging of the trench and the laying of the gravel foundation, maintaining route accuracy of within for the trench and for the foundation. Construction of the trench required dredging of material from the Bay.
The structure is made of 57 individual sections that were built on land at the Bethlehem Steel shipyard on Pier 70 and towed out into the bay by a large catamaran barge. After the steel shell was completed, water-tight bulkheads were fitted and concrete was poured to form the -thick interior walls and track bed. They were then floated into place, and the barge was tethered to the Bay floor, acting as a temporary tension leg platform. The section was ballasted with of gravel before being lowered into a trench packed with soft soil, mud, and gravel for leveling along the Bay's bottom. Once the section was in place, divers connected the section with the sections that had already been placed underwater, the bulkheads between placed sections were removed and a protective layer of sand and gravel was packed against the sides. Cathodic protection was provided to resist corrosive action from the Bay's salt water.
The project cost approximately $180 million in 1970, with $90 million of that cost being spent on construction, the remainder going towards laying rails, electrification, ventilation and train control systems.
Configuration
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 western span of the Bay Bridge between the San Francisco Peninsula and Yerba Buena Island, and emerges in Oakland along 7th Street, west of Interstate 880.The tube has 57 sections; each section ranges from long. The average length of each section is, measured along the tunnels' bore; sections are wide, high, and weigh approximately each. To conform with the route, 15 tube sections were curved horizontally, 4 were curved vertically, 2 had horizontal and vertical curves, and the remaining 36 sections were straight. Each section of the Tube cost approximately, based on the construction contract. The steel shell is thick, and has just enough strength to support its own weight and resist hoop stresses; an external consultant, Professor Ralph Brazelton Peck, convinced project engineer Tom Kuesel that thin shells were adequate because the soil loads would naturally form an arch.
The tube consists of two tunnels and a central maintenance/pedestrian gallery. Each tunnel has a bore approximately in diameter, with the track centerline offset towards the outside from the bore centerline. The tunnels flank a gallery which contains maintenance and control equipment in the upper gallery, including a pressurized water line for firefighting. Each tunnel has 56 doors opening into the lower gallery, spaced approximately apart, numbered consecutively from the San Francisco side of the tube. The doors are locked from the gallery side and can be opened inwards from the tunnel through emergency hardware. Between doors, the tunnel has narrow wide walkways adjacent to the gallery space.
The upper section of the gallery space is also used as a duct, moving of air under forced circulation. The tunnels are vented to the atmosphere at the San Francisco and Oakland ends and are vented to each other with remotely-operated dampers long by high over every third door.
Each end of the tube is secured to the vent structures with a patented sliding seismic joint which allows six degrees of freedom. As designed, the joints allow movement of up to along the tube's axis and up to vertically or laterally. A restaurant was constructed atop the San Francisco transition structure on a pier behind the Ferry Building. The Oakland vent structure is located in the middle of a Port of Oakland container yard.
Seismic retrofitting
The Transbay Tube has required earthquake retrofitting, both on its exterior and in the interior. The total cost of seismic retrofits was estimated at in 2004.A 1991 study, commissioned at the recommendation of the Governor's Board of Inquiry in the wake of the 1989 Loma Prieta earthquake, found the seismic joints would "likely remain intact and functional after the next earthquake." However, settling of the tube within its trench and the Loma Prieta quake had reduced the allowable movement of the seismic joints to as little as.
The 1991 study was followed by a more detailed BART Seismic Vulnerability Study, published in 2002, which concluded 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 or cause movement that would exceed the capacity of the sliding seismic joints. Retrofitting work required the fill to be compacted, to make it denser and less prone to liquefaction. Compaction started in Summer 2006 at the east end of the tube, on property belonging to the Port of Oakland. A 2010 paper concluded the distance the tube would rise due to liquefaction was limited based on model testing of potential liquefaction mechanisms, and questioned the justification for the compaction effort.
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. A vehicle was custom-built to handle the, thick plates; once hoisted in place, the plates were bolted to the existing concrete walls and welded together, end-to-end. The contract for was awarded to California Engineering Contractors for installation. In order to complete this work during 2013, BART closed one of the two bores of the tube early midweek, resulting in delays of 15–20 minutes. The work, originally estimated to last approximately 14 months, was completed by December 2013, after only 8 months of construction.
In December 2016, BART awarded a contract to perform further seismic retrofitting. In this phase, a new steel liner and higher-capacity pumps would be installed to reduce the possibility of flooding the tube, as the existing pumps would not be adequate in the worst-case seismic event. Work was projected to start in the summer of 2018 and is scheduled to take more than two years to complete. Service through the tube would be reduced or eliminated during the first hour and the last three hours of the service day.