and concrete was poured to form the -thick interior walls and track bed. They were then floated into place (positioned above where they were to sit), 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 (equivalent to $ in ), 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