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Construction Close-up: Building the Cofferdams Holes in the Bay.
In mid-November, KFM construction crews began building the offshore cofferdams that will allow construction of the new East Span piers. A cofferdam is essentially a steel box in the Bay that provides a secure, predominantly watertight area below the waterline for construction of the bridge's foundation.
Cofferdams will be required for 20 of the Skyway contract's 28 piers. Each cofferdam will require the deployment of one of the largest derrick barges in the industry, thousands of tons of steel, and the skills of dozens of highly skilled craft workers and engineers.
Here's how they'll do it:
For each cofferdam, the heavy lifting begins with an enormous General Construction Company derrick barge named the DB General. Capable of lifting more than 700 tons, the General is used to transport and install in the water the giant steel frames upon which the cofferdam walls are erected. Using a 400-ton-capacity spreader beam to help support the derrick's lift cables, the General lifts the 300-plus ton steel frame from the construction yard in Oakland and takes it on board.
The frame, with cross-bars and main beams forming a box the size of a 6,400 square-foot warehouse, is then taken by the DB General to a location on the Bay just north of the existing bridge where one of the new bridge piers will be constructed in coming months.
The frame is lowered slowly into the Bay waters until just the top portion is visible above the surface. Four 80-foot "spud piles" are driven at the four corners of the frame to hold the enormous cage in place and secure it to the Bay bottom. A vibratory hammer attached to the General's derrick cables is clamped onto the top of each spud pile and activated to drive them.
The next step is to turn the inside of the submerged box-shaped structure into a construction site by building cofferdam around it.
Back at the Oakland construction yard, 2,720 tons of steel divided into more than 1,700 pairs of interlocking sheets four feet wide and between 60 feet and 80 feet long lay stacked and ready to be loaded aboard the DB Vancouver, a smaller General Construction Company derrick barge with a 135-ton capacity crane. The sheets, which work like interlocking building blocks, are barged to the frame location.
The sheets are raised from the barge deck and lowered vertically until they penetrate the Bay bottom, locking into each other with exact precision until they form a border surrounding the submerged cage on all four sides.
Each four-sided cofferdam consists of 80 paired sheets, four corner units, and two single sheets needed to close an odd-shaped gap on two sides. Once enclosed, the completed cofferdam resembles a warehouse with corrugated siding. A vibratory hammer is lowered onto each pair of sheets, applying enough vibration to "shake down" the sheets deeper into the Bay. Each will be driven 40 to 50 feet beneath the Bay floor.
While the cofferdam does not provide a water-tight environment, an elaborate pumping system can be used to de-water the interior of the structure as activity within commences. Special care is taken to keep the water level inside the structure even with the water level outside as the Bay tide fluctuates. This minimizes pressure on the cofferdam walls.
Excavators are the next to arrive on the job, using a clamshell bucket to scoop tons of Bay mud out of the box. The mud is replaced with gravel fill, which is spread evenly to create a level surface. This surface becomes a seating bed for the 1,800-ton footing through which the permanent bridge piles will ultimately be driven.
When the water is pumped out after the footing is installed, a stable hole in the Bay results. Inside, a range of activities will take place as the beginnings of the East Span structure are built.
Ten cofferdams will be constructed for the eastbound bridge structure. Once these 10 footings and piers are in place, the frames and cofferdams will be disassembled and moved north for construction of the substructure for the westbound skyway.