A lot of bridges have been built in the state of Washington, but few, if any, pose the construction challenges of the new SR 520 Floating Bridge and Landings over Lake Washington.
At 7,710 ft. (2,350 m) long, the $750 million project will be the longest floating bridge in the world when it’s finished and it’s every inch the formidable task it sounds.
“If you go out on the lake you see a flotilla of equipment,” said Dave Becher, Washington Department of Transportation project director. “Barges, tugboats... The floating bridge has to be assembled on the lake. We barge concrete from another site. We load mixer trucks and bring them down.”
But to have so much equipment on the lake presents lots of challenges.
“There are very restrictive environmental requirements. There are very complicated orchestrations of construction equipment. It’s a lot of construction, a lot of potential for problems. The scale is so massive. You have to do almost everything from the water. You just don’t see it very often.”
The original floating bridge was built in 1953 and is still in use while the new bridge is under construction. The old bridge has only two lanes in each direction, no shoulders and no sidewalk. The pontoons are vulnerable to windstorms and its columns vulnerable to earthquakes.
“The state spent a lot of money over the years to maintain it,” Becher said. “At a certain point there are diminishing returns. At some point we needed to replace it. The state decided now is the time to do that.”
The contract to build the new bridge was awarded to contractor Kiewit/General/Manson (KGM). It will feature two general purpose lanes, one HOV/transit lane and wide shoulders in each direction. There will be a 14-ft. (4.3 m) pedestrian/bicycle path on the north side, and the bridge is being built to accommodate light rail in the future.
The replacement project got underway in 2012 with an original finish date of 2015, but several of the early pontoons required repairs due to design errors and that caused delays. The new completion date is now set for 2016.
Working on the water with heavy equipment is not only challenging, at times it’s not even possible.
“The other challenge with that much equipment is getting it to the lake,” said Greg Meadows, deputy construction manager. “You’re talking the longest floating bridge in the world. There are certain pieces of equipment they would like to have, but it cannot fit through the locks to get it there. One of the challenges: How to build what needs to be built with the equipment that can get to the bridge itself? We solve a little with a tower crane we built that will be removed at the end of the project. And we’ll use two derrick barge cranes to pick a single unit. Just the size and complexity of the project as a whole presents challenges.”
The project begins with the pontoons — 77 of them. Thirty-three are being built off site in Aberdeen, the other 44 are being built in Tacoma.
The longitudinal pontoons run along the highway and are 365 ft. (111 m) long and 75 ft. (22.9 m) wide. Currently, there are 21 of the pontoons in place, running adjacent to the old bridge.
The pontoons are built in a dry, gated casting basin. Once it’s complete, the gate is removed and the basin flooded.
“We tow it out into the Pacific, up and around the horn through Neah Bay, through the straights, up into the Ballard Locks and up onto the lake,” said Meadows. “The reason they are the size they are is to get them through the locks, which are 80 feet wide. They fill it up. It’s pretty impressive.
“Once we get them on the lake, we add supplemental pontoons, smaller ones, 60 feet wide. We attach them to the side of the longitudinal pontoons for additional buoyancy. We bring them into position with the tug boat, pull them together and once we have enough pressure on it, we will pump the water out from between the pontoons.”
The pontoons are joined at each joint by bolts measuring 22 ft. (6.7 m) long by 3.5 in. (8.9 cm) in diameter.
“All the joining operation happens on the lake which is one of the big challenges,” Meadows said. “We’re doing on it all on the pristine waters of Lake Washington. We haven’t had any incidents yet. To get it to its final configuration, at each joint there are a total of 80 bolts. That’s what is holding the bridge together. Each bolt is pretensioned to 750,000 pounds.”
When it is completed the bridge will float on the surface of the lake with seven feet exposed above the water and 21 ft. (6.4 m) submerged. Each end of the bridge will feature a transition span, which will reach 70 ft. (21.3 m) tall on the east and slightly less on the west end, allowing for the navigation of ships. One of the reasons the state opted for a floating bridge as opposed to the traditional construction is because the earth below Lake Washington consists of diatomaceous — or super soft — soil.
“Lake Washington is a really deep lake,” said Becher. “When you get to the center it drops off really quick. It gets down to about 200 feet in depth. It gets so cloudy and murky, it becomes cost ineffective to build a traditional bridge. The floating bridge becomes more cost effective. Otherwise, if you wanted to sink traditional anchors, you’d have to create some sort of super span or suspension which would be difficult to do. You’d have to have elevation, you’d have to have a certain clearance. People in this area really value their view.”
The project also involves three different types of anchors.
In the shallower area along the edges, they use pile anchors, a concrete drilled shaft with a connection point on top for the anchor. In the areas where the lake starts to slope off, they use gravity anchors, a large concrete box benched into the slope and filled with rocks. The majority of the anchors are fluke anchors, which Becher described as looking like an open pyramid, jetted into the bottom of lake, buried under rocks and attached to an anchor cable. Floating bridges are unique, more like ships than bridges, Becher said.
“If you took a cross section of a pontoon and took off the top deck, it would look like a hollow box with all these compartments, all these internal water tight doors. Like a ship, if something hit it, it is designed to handle that. It can handle any kind of event, or type of failure where cells get flooded with water.” The bridge is designed to last 75 years.
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