Lots of questions have been pouring into the Doc's mailbox about the ongoing Pinellas Bayway bridge replacement project. The nearly 50-year-old, two-lane drawbridge will be replaced with a higher four-lane bridge, and work has been in progress for a few months. Here's a rundown of the most frequently asked questions and the inside skinny we got from Kris Carson of the state Department of Transportation:
How do the new bridge pilings get driven into the sand?
The pilings are driven using a diesel pile-driving hammer. Depending on the height that the hammer drops on a given setting, it transfers a large amount of energy into the pile to force it into the ground.
Is there bedrock under the sand and water that those pilings go into?
There is no bedrock in the area. The pilings are driven down to a hard sand layer or to a deeper hard clay layer to achieve the required strength to support the bridge.
What about the motion of the water? How do the pilings not tip over?
The design plans include a minimum depth that the pilings must be driven in order to make sure the bridge supports are not affected by the current and wave action (referred to as "scouring''). The design calculates the anticipated worst scour that would occur during a 100-year storm event and the minimum depth is much deeper than that worst-case scenario. So say we are hit by a storm on the scale of the epic 1921 hurricane that pummeled Pinellas County with 100-mile winds. The new bridge should still be standing.
How far down under the surface do they go?
Depending on the location, the piles are driven into the ground to depths varying between 75 to 115 feet below the water elevation. Once all of the pilings are driven for each pier location, they are cut off at the required top elevation and a reinforced concrete footing is built on top of the 16 piles. Each footing measures is 22 feet by 22 feet and 6 feet deep. The columns and pier caps on top of each footing are being constructed to eventually support the new concrete beams and concrete bridge surface.
Speaking of concrete, how much is in one of the 16 piers?
Many components come together to make up one of the support piers for the new bridge. First, there are 16 pilings for each separate column being constructed on the project (two columns per pier). Once a piling is driven to the specified depths, a steel-reinforced concrete footer is constructed on top of the piling. The footers contain approximately 100 cubic yards of concrete. On top of the footers are the columns and pier cap. Since these vary in height between 17 to 65 feet for the new bridge, the overall concrete varies between 110 cubic yards to 230 cubic yards. So, for each pier in the water, the overall concrete quantities vary between 210 and 330 cubic yards.
In total, more than 23,500 cubic yards of concrete and 2,300 tons of reinforcing steel will be placed on site to build the new bridge. This total doesn't include the concrete and steel used to construct the pre-cast pilings and beams. To put that in perspective, we're talking about enough concrete to fill 280 residential swimming pools.
How much asphalt will be used?
Nearly 6,700 tons will be used to build the new lanes and resurface the existing travel lanes.
What about landscaping?
More than 100 palm trees and other landscaping will be planted along the bridge and its new wider approaches.
How many people are working on the project?
Approximately 60 people are working on the project on a given day, including the prime contractor, Orion Marine, and various sub-contractors and DOT staff.
How much does the whole thing cost?
Orion Marine Group was awarded a $40 million contract.
When will the new bridge be done?
The completion date is sometime near the end of 2014 or early 2015, depending on weather and other factors.
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