Anchors ahoy! Final anchor set for the new 520 floating bridge

By Guest Bloggers Nathan Karres and David Gitlin

If you crossed the SR 520 floating bridge this morning, you may have caught a glimpse of our contractor crews completing a major project milestone: we placed the 58th of 58 total anchors for the new SR 520 floating bridge.

It’s March madness on Lake Washington as one of the final four
fluke anchors is lowered to the lakebed. Photo taken March 12, 2013

Construction crews pose for a group photo moments before submerging
the last of 58 anchors for the new SR 520 floating bridge.
 When you’re crossing Lake Washington on 520 or I-90, it’s easy to forget that you’re actually floating on the lake’s surface above 200 feet of water. The new SR 520 floating bridge will be no different. In order to hold the bridge in place and help withstand winds and waves, we built and installed three kinds of anchors:

  1. Forty-five fluke anchors will secure the bridge in the deepest parts of the lake by creating drag that resists pulling from bridge pontoons floating on the lake’s surface. Weighing 100 tons and with a surface area of 910 square feet (think 20 elephants packed into the floor space of a typical apartment), the immense weight, size and shape of fluke anchors ensure they stay firmly in place. Fluke anchors are covered with mounds of heavy rock to strengthen their hold. 
  2. Eight gravity anchors will secure the bridge where the lake bottom is sloped with hard soils. These massive concrete boxes are filled with rocks to hold the bridge in place by shear enormity. 
  3. Five shaft anchors have been drilled directly into the lake bottom where the lake is too shallow for gravity anchors. 

 Gravity and fluke anchors were built at the north end of Lake Washington in Kenmore, at a pair of specifically designed concrete casting areas. After the concrete cures and anchors are ready to go, they’re transported by barge and floated to the bridge site. This is no small feat – each anchor weighs up to 420 tons!

Crews set each anchor at a specifically mapped location in the lake, all the while carefully navigating around the numerous anchor cables for the existing floating bridge. Each anchor will eventually be attached to a new SR 520 bridge pontoon. While remaining pontoons are being constructed in Aberdeen and Tacoma, the anchors will be temporarily connected to mooring buoys bobbing atop the lake.

With one major component of the new SR 520 floating bridge complete, another has only just begun: crews in Medina are preparing to pour the first roadway section for the new bridge. Casting this roadway section is the first of thousands of feet of pavement that will be constructed for the project. 

Check back for more updates as we share our progress building the world’s longest floating bridge.