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SR 99 - Spokane Street Overcrossing

Under construction

SR 99 timber bridge project Map

Construction to replace the timber-supported section of Highway 99 in Seattle is underway. The east half is complete (above, far right) and the west half is being built from the ground up (as seen above).

Southern approach at E. Marginal Way S. undercross
The timber-supported approach is the final section of the overcrossing to be upgraded to current earthquake safety standards.


November 2014

  • This project was declared operationally complete on Saturday, March 22. Operationally complete means the roadway is open to traffic and the majority of work is complete.
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The 470-foot-long bridge was built in 1959 as a temporary structure. Crews will replace the SR 99 timber bridge with a permanent structure made with Geofoam blocks, lightweight, Styrofoam-like material capped with concrete. 

The SR 99 timber bridge, also known as the Spokane Street Overcrossing (overpass), is located just south of the West Seattle Bridge and the Spokane Street Viaduct. During construction, crews will shift traffic and reduce the number of travel lanes on this section of SR 99 around-the-clock as they rebuild one half of the southern approach at a time. This bridge is also known as the SR 99 timber bridge.

When the work is done, the 470-foot span will be a new, stronger structure with a concrete and Geofoam embankment and an access opening for the Port of Seattle. Underground concrete columns and other upgrades will improve the footing of the approach and preserve this well-traveled section of Seattle's SR 99 for years to come.

Why is WSDOT replacing the SR 99 timber bridge located at the southern approach to the SR 99 Spokane Street Overcrossing?        
This section of the SR 99 Spokane Street overpass was built in the late 1950s. The timber supports are splitting and rotting, and are now considered obsolete.  

Replacing the timber section with a concrete and Geofoam embankment and adding underground columns to strengthen the footing will help the approach better withstand earthquakes.

What the heck is Geofoam?
Geofoam, aka expanded polystyrene, is a lightweight fill that will not decompose or decay. It looks pretty much like Styrofoam packing material, but it’s stronger and denser to withstand the rigors of construction.

On average, Geofoam is 100 times lighter than most soils and 20-30 times lighter than other fill materials. We will cap the Geofoam embankment with concrete.

The End Result
When the project is complete, the Geofoam and concrete embankment will better withstand earthquakes and resist damage, making for a stronger, safer bridge approach.

WSDOT also will build a new underpass opening just north of the approach for a future Port of Seattle freight access road to East Marginal Way S.

Bicycle and pedestrian access
Nearby sidewalks and pedestrian crosswalks will not be affected during construction. For information on any future bicycle or pedestrian access improvements on East Marginal Way South and the surrounding area, please contact the City of Seattle at (206) 684-7583 or

Project Benefits

Replacing the timber-supported approach with a concrete embankment helps the structure better withstand earthquakes. Additional project benefits include:

  • Installing new barrier - Enhances safety for motorists.
  • Installing energy efficient lighting - Provides maximum visibility with minimum energy consumption.
  • Removing chemically treated timber - The timber supports were treated with a chemical wood preservative called creosote. Removing the creosote-treated timbers improves the environment and eliminates the threat of the wooden beams catching fire.
  • Improving drainage - Upgrading the drainage system helps prevent drains from clogging, reducing the need for future maintenance.

These benefits protect drivers and keep traffic flowing on this key route into downtown Seattle.

What is the project timeline?

  • Nov. 29: Awarded to Midmountain Contractors, Inc.
  • February 2013: Construction begins.
  • Saturday, March 22, 2014: All lanes reopened to traffic.
  • Fall 2014: Expected completion.

Financial Information

Financial Data for PIN 109935A
Funding Source Amount ($ in thousands)
2003 Gas Tax (Nickel Funding) $0
2005 Gas Tax (TPA) $691
Pre-Existing Funds (PEF) $10,581
Total $11,272

Project signage will reflect the cost of construction engineering, project bid award and sales tax.

How can I get more information?
Kris Olsen
WSDOT Communications

Hien Trinh
WSDOT Project Engineer - Construction

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