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Construction Close-Up: Nine Miles of Pipe Piledriving Now Underway
To the general public, the driving of the Skyway foundation piles is the most visible - and audible - portion of the East Span project to date. On Wednesday, Feb. 12, the first permanent pile was driven, its top standing high above the deck of the existing bridge as it was hammered into the Bay floor.
The construction of the Skyway's 28 piers will require the driving of 160 tubular steel piles, each composed of two welded sections, over 21 months. In all, 9.5 miles of pile will be driven, requiring 163 pile splices, 40 miles of welding bead and 23,598 pounds of weld metal.
Here's how they're doing it:
Most foundations will require six piles each; four foundations will require four. Each pile is an eight-foot diameter steel tube barged to the site from a production facilities in Southern California and Mare Island in the north Bay. The piles vary in thickness from three inches at the bottom to two inches at the top. The longest single section is 244 feet. The longest spliced section is 380 feet long. Each section weighs 200 to 239 tons.
From the barge, the pile is lofted - picked up by a derrick crane. It is then stabbed - put into position in a pile-driving template equipped with hydraulic gates and arms that guide the pile at a precisely calculated angle, or batter, as it is being driven into the Bay.
Each pile is then driven by a giant hydraulic device called a hammer, suspended on a derrick crane. The largest hammer at work on the Bay has a 90-ton head that delivers up to 1.2 million foot-pounds of force onto the pile.
The pile is driven until it is 33 feet above the top of the foundation box that has been set inside the cofferdam.
The top half of the pile is then lofted and positioned on the first. This bottom edge of this pile is beveled at 30 degrees and fitted with spacers to create a small gap between the two halves where the welding will occur.
The welding of each pile is a precise process that relies on specially designed welding machines that run on tracks placed along the circumference of the pile. It takes four hours alone to set up the welding operation for each pile, not including the placement of temporary welding shed over the pile joint. Prior to the start of welding, ceramic heating pads are placed on the pile to preheat the weld area to 250 degrees, promoting better welding conditions.
Each welds requires a four-man crew working with two welding machines, one for each half of the pile. Each completed weld requires 70-100 passes around the pile at 26 feet per pass.
Once the welding is completed and finished to specifications, the spliced pile is then driven into the Bay floor through foundation box. The top 14 feet of the pile, 12 feet of which is a temporaryextension, is then removed . The extension is stored for use on another pile.
The piledriving template is then picked up by the derrick crane and repositioned for another drive. The driven pile is cleaned out, its bottom is plugged with concrete, and the water is pumped out. Eight plates are then welded around the exterior of the pile to tie it to the foundation box. A cage of reinforcing steel is inserted into the pile and grout is inserted into the cavity between the pile and the 10-foot diameter hole it was driven through.
Ultimately, all the piles at each location and the top of the foundation box will be filled with concrete to form a pier that will support a column and the roadway.