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The Skyway Substructure: Footings, Piles and Piers
Construction of the Skyway begins below the Bay. Working amid the tides and dealing with deep layers of sediment, crews will use unique techniques and mammoth equipment to build the underpinnings of the new bridge. Here's how they'll do it:
The substructure of the Skyway consists of three major components:
In mid-October (will update this later) crews took the first step in constructing the Skyway substructures when they started driving ribbed, interlocking sheets of steel some 30 feet deep into the Bay at each pier location. These sheets will form a cofferdam encircling each pier location.
Once the coffer dam walls are completely assembled, crews will use a clamshell bucket to dig a seating bed for a pre-cast footing. These 1,000-ton steel structures, fabricated in Texas and barged to the site, have openings for the piles to be driven through. The footings will be partially filled with concrete at their final location at the Bay floor.
After the bed is graveled and the footing set in place, additional gravel will be poured to fill between the footing and the coffer dam. At that point, the cofferdam will be dewatered - pumped free of water - leaving in essence a hole in the Bay above the footing.
Once the footings are in place, a 150-foot tall steel work platform called a pile-driving template will be installed around the footing. Manufactured in Washington and resembling an oil rig, these structures will rise higher than the future Skyway deck and serve as giant guides for driving the piles through the footings for the Skyway's 24 piers.
The 8.5-foot-diameter steel tubular pilings, six each for most piers, will be battered, or driven at an angle, some 300 feet deep into the Bay substrata by a one of the world's largest pile-driving hammers.
In all, 160 piles will be set, their sections welded in two long segments as they are driven in sequence. Crews will then clean out the piles, insert a rebar cage into each and fill them with concrete. When the pilings are set, the template will be dismantled and moved to the next location.
A 3-foot thick slab of concrete will then be poured on top of the footing to make a solid conglomerate upon which the piers supporting the roadway can be built. In addition, a concrete sleeve will be poured atop the footings. Although the footing will be submerged in the Bay shallows, the sleeve will be visible at the waterline.
Within the sleeve, the footings will have two open sockets into which reinforcing steel will be set for the construction of the piers, which will be poured in stages, called lifts. Concrete will be barged to the piers and pumped into the forms. Three to four lifts will be required for each pier.
Each finished pier will actually consist of four connected columns with an opening in the middle where an access stairway will allow engineers and maintenance crews to inspect the pier interiors and footing caps.
Go to the next step - Skyway: The Roadway.