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The building of the foundation for the Suspension section tower and the construction of the eastern most support structure for the Suspension roadway have been combined by Caltrans into one marine foundation contract.
The foundation for the 530-foot steel tower is called the T1 footing. It entails the construction of a concrete and steel base structure supported by 13 piles set deep into underlying rock.
The eastern support is referred to as E2. It features two pile-supported footings linked by a reinforced concrete box and surmounted by pier columns to support the bridge's twin road decks.
The contract for this project has not yet been awarded. Still, here's how they'll probably build T1 and E2:
In building the T1 foundation for the Suspension Section tower, crews will bore holes for 13 supporting piles 196 feet through the thin mud layer into the bedrock that forms the Bay bottom east of Yerba Buena Island. The Bay at this point has an average depth of 60 feet. Before any boring begins, a cofferdam may be built to provide a dry, safe working environment.
The T1 piles will consist of concrete cast in place around reinforcing steel cages. The upper portion of the piles, roughly the top 107 feet, will be encased in a permanent steel shell, 8 feet in diameter. The lower portion of the piles, roughly the lower 98 feet, is a "rock socket", with the steel-cage-reinforced concrete extending down into the bedrock without a steel casing.
Giant steel plates with holes in them to accommodate the piles will form the top and bottom of the footing frame, the walls of which will be made of vertical steel plates of varying thickness called web stiffeners.
The entire footing frame will be encased in concrete and will form a final footing structure 85 feet long, 73 feet wide and 21 feet thick.
Each of the two foundations will require eight piles driven vertically down to bedrock. Each pile will probably need to be driven in multiple sections totaling 347 feet. As one section is nearly completely driven, another section will be lifted and securely welded to the first section before being driven further into the Bay bottom.
A cofferdam - perhaps several - may be built around the piles to provide a working environment in the 40-foot-deep water. The two footings will be built of steel frames encased in concrete in similar fashion to T1.
Crews will then construct two reinforced-concrete pier-columns, roughly 120 feet tall.
The cap beams that will connect the piers to the Suspension Span road deck will be built under a different contract.
The T1/E2 project will be allowed 765 days to complete. Bids were scheduled to be opened on July 15, 2003 for this contract.