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SR 520 - West Approach Bridge North Project Construction Progress Tracker

Last updated: Dec. 20, 2017

In August 2017, we opened the West Approach Bridge North (WABN) to traffic, just under three years after construction began. During that time, work crews built (and later removed) a temporary work bridge, demolished the R.H. Thomson “Ramps to Nowhere” and constructed the 1.2-mile-long bridge, among many other local improvements. With a few final items left to finish, the contractor plans to complete the project by Milestone Checklist - with all three boxes checked for Montlake off-ramps, Westbound lanes, and the SR 520 Trailearly 2018.  

Ongoing and upcoming work 

The SR 520 trail – the cross-lake bicycle and pedestrian path – is now open! This trail is the last major piece of the WABN project. Last month, we restriped the old SR 520 west approach bridge’s eastbound lanes to add an HOV lane east of Montlake. Now, crews will spend time completing some final tasks on the project, including restoring the Foster Island trail below and near SR 520. To complete this work, we may still need intermittent road, lane and/or trail closures. If we do, we’ll be sure to put information about the closures on our Orange Page and send it out through our email updates.

The bridge-building process

In fall 2014, prior to starting construction of the permanent WABN structure, the contractor built a temporary work bridge. This structure, nearly a mile long, served as a platform for crews to store materials and deploy equipment as they constructed the permanent bridge over shallow water. Crews used barges to access the eastern most bridge sections that stand in deeper water.

In February 2015, crews began installing the first casings for the permanent WABN structure. After they secured the casings into the lakebed, crews removed soils, placed metal rebar inside for added strength and filled the casings with concrete. Once the concrete cured, crews removed the top portion of the casings, creating the shafts. These shafts form the foundation for the new, solid bridge columns. The solid bridge columns increase the seismic stability of the new bridge, compared to the hollow columns that support the old bridge.

After crews finished constructing the solid columns, they installed 353 girders, 108 isolation bearings, and 41 sections of bridge deck. We poured the last section of bridge deck in April 2017, and three months later, in July 2017, we opened the first part of the bridge – the Montlake-area off-ramps. Then, in August 2017, crews completed and opened the new westbound lanes that connect drivers from the floating bridge to Montlake.

The pieces that make up the West Approach Bridge North (WABN)

 Diagram of the various parts of WABN  Casing

Structure legend
The graphic above shows how the different pieces fit together to form WABN.

A bridge casing is a large steel pile that provides the permanent mold for the bridge's concrete foundation. After fixing each of the 99 casings into the lakebed, crews drilled out soils, placed metal rebar inside for strength and then filled the casings with concrete to create the shaft.

 Shaft Installation  Rebar Cage

Shaft installation
This large machine drilled soils out of the casings before crews placed metal rebar inside for added strength. Once the rebar was placed inside, crews filled the casings with concrete to create the 99 shafts.

Rebar cages
Rebar cages strengthen the foundation and columns. Crews tied all 194 rebar cages for shafts and columns by hand.


The west approach bridge built in the 1950s has hollow columns that are vulnerable to earthquakes. All 95 new columns are made of solid, rebar-reinforced concrete.

Isolation Bearings
The old west approach bridge does not have isolation bearings, which makes it more vulnerable to earthquakes. The new WABN structure has 108 isolation bearings, which allow the columns to move freely from the roadway deck during an earthquake.

Girder Deck

Girders are the backbone of the WABN structure. Before pouring concrete for the bridge deck, crews installed 353 girders to create a base for the new roadway.

Roadway deck
Crews poured 41 sections of roadway deck to complete the 1.2-mile-long WABN structure. After that, crews ground, paved, and striped the deck before opening the bridge to traffic.

Ramp Removal 

In order to build the WABN project, crews removed the old westbound SR 520 off-ramp and the never-completed R.H. Thomson “Ramps to Nowhere” as part of our effort to restore the Arboretum to a more park-like setting. In August 2017, crews removed the final pier cap and columns of the old westbound SR 520 off-ramp.
Image shows a pictures from January 2017 and one from June 2017 showing how the ramps are no longer there.