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

Fluke anchors on barge

Fluke anchors on barge.

Why an Earthquake Warning System Should Not Be a Priority In The Pacific Northwest

Earthquake_damage_Cadillac_Hotel,_2001_SmallerThe newest and hottest topics when it comes to disaster discussions in Oregon and Washington, as well as on the national level, are an earthquake warning system and earthquake prediction possibilities. They are the new obsession that has come on the heels of the New Yorker articles this summer. While we don’t object to advancing both of these methods to better warn of impending quakes and hopefully save lives, we do think that the discussion is premature, especially here in the northwest.

The first reason is that an earthquake warning system like that in Japan has to be implemented only with a comprehensive, aggressive, and continuous public education program. Without a full understanding of what you should do when your phone emits an ear piercing shriek warning of impending shaking, we risk even greater panic and possibly more casualties. Running out of buildings with unreinforced masonry or weak facades just before the shaking could put people at more risk of falling hazards outside of the buildings. It could also cause major traffic hazards as drivers try desperately to get across or get off bridges and overpasses. Unless we develop a much better awareness of what the public should do when they receive the warning, it may cause more problems than it solves.

But the real issue is that these technologies are acting as the bright shiny objects that are distracting all of us, from the public to the president, from the real issue: our infrastructure is in dire need of upgrades not only to prevent casualties, but also to encourage long term recovery.  We doubt 30 seconds of warning will seem as beneficial when the public doesn’t have wastewater for one to three years.  Further, a warning system that stops surgery or an elevator is not as important as making sure that the hospital or building itself is designed to withstand shaking. Especially in Oregon and Washington, all of our energy and funds need to be focused first on comprehensive and intelligent infrastructure improvements that increase our community resilience. And that needs to happen as quickly as possible. We implore you not to follow the flashing light! Urge our government to focus on the real issues, and encourage your colleagues and neighbors to personally prepare.

For more information contact Allison Pyrch at (360) 816-7398 or