Anchoring the World’s Longest Floating Bridge

SR520 Bridge

Photo: WSDOT

You’re at the bottom of Lake Washington, 200 feet underwater. It’s flat as a pancake here, but the first 50 feet of soil is diatomaceous silt and clay, which is unspeakably unstable. Think microscopic glass Christmas tree ornaments with the consistency of chocolate mousse. Below that is 50 feet of very-soft clay (zero blowcount, to those in-the-know).

Try, just try, to anchor the new SR 520 Bridge in this chocolate mousse (remember, it’s a floating bridge that can’t be left to drift off to Renton or points unknown). And just for good measure, make each of the 58 anchors able to resist a horizontal load of 600 tons—four times what was needed for the old bridge.

Figure out that you’ll need three types of anchors. In areas along the side slopes, where the water is shallower and has competent soil, use a gravity anchor, but call it a box of rocks amongst your workmates.  Build it like a heavily reinforced concrete egg carton with only four compartments. Joke about the kind of eggs that would fit into a 40 foot by 40 foot by 23 foot carton.  Build them on a barge at the concrete plant in Kenmore at the north end of the lake.  Make them so heavy that that the only derrick large enough to lift one is too big to fit through the Ballard Locks. Tow the gravity anchors through the Ballard locks, though they barely fit, while the public looks on in astonishment.

Gravity anchor

Gravity Anchor on its way to the SR 520 Bridge site. Photo: Kiewit

Flood the 440-ton floating boxes with water to make them sink. Lower them to the lake-bottom and place them on a leveled-out gravel pad. Fill each of them with 1,700 tons of rock to make them heavy enough for lateral frictional resistance, or so they won’t budge.

Don’t stop there. Use a second type of anchor, a drilled shaft, along the shoreline where the lake is shallow enough that the box of rocks would have caused havoc as a navigational hazard. Make them ten feet in diameter and 100 feet tall, not as tall as the original Godzilla, but close enough.

Drilled Shaft

Ten-story-deep drilled shaft anchor. Image: KPFF Consulting Engineers

Then, use fluke anchors, the most technically challenging anchor, for the majority of the project. Make these fluke anchors from reinforced concrete plates three feet by 35 feet wide by 26 feet tall. Cast a steel tetrapod into the side so that the anchor cables can be attached to the I-bar at the end of the tetrapod. Explain that a “tetrapod” is a four-sided shape with triangular faces (not to be confused with a four-limbed vertebrate).

Fluke Anchor

Fluke anchor being jetted into the bottom of Lake Washington. Image: KPFF Consulting Engineers

Place the fluke anchors in a steel frame equipped with water jet tubes to drive them into the mud. Because the mud is chocolate mousse, place mounds of rock above and beside the fluke anchors. And then more rock. And then more rock. Good, that’s enough.

Now, celebrate. The Washington State Department of Transportation’s grand opening of the longest floating bridge in the world will be April 2 and 3, 2016. You can run, bike, or possibly meander across the bridge. Hopefully there will be food. You’re hungry after all that work.

Hart Crowser was the geotechnical engineer-of-record for the anchors for the new SR 520 Bridge. The design-build contractor was a joint venture of Kiewit/General/Manson. The structural engineer was KPFF Consulting Engineers.

Need more detail? Read the technical paper Geotechnical Design: Deep Water Pontoon Mooring Anchors or contact Garry Horvitz, PE, LEG, at garry.horvitz@hartcrowser.com

Fluke anchors on barge

Fluke anchors on barge.