The Program includes replacing a 2.2-mile long elevated highway structure which has taken motorists along downtown Seattle’s waterfront since 1950. Our participation with WSDOT, the City of Seattle, and a large consultant team has involved:

• Contribution to the Environmental Impact Statement
• Evaluation of over 70 alternatives
• Division of the project into design and construction of five separate projects
• Multiple alignment changes
• Evaluation of field and laboratory test data from over 500 explorations
• Design-Build process (supporting the Owner) to construct a bored tunnel
• Demolition of the existing structure planned for 2018

Project Phases Include

SOUTH HOLGATE TO SOUTH KING STREET VIADUCT REPLACEMENT (H2K) PROJECT
The southern mile of the viaduct was in an industrial area that supports the Port of Seattle, railroads, two sports stadiums and other industries.  In 2007, while debate about how to replace the northern portion of the viaduct continued, sate leaders decided to replace the southern mile with new side-by-side bridges.  The primary geotechnical challenge in this area was the presence of over 200 feet of sand and silt deposits that are susceptible to strength loss during an earthquake.  The presence of these soft and loose soils required that the bridges be supported on very deep and large foundations (drilled shafts and pipe piles).  Ground improvement was required to protect the foundations from large lateral loads generated by earthquake effects.  The approach embankments to the bridges required that lightweight expanded polystyrene blocks be used to mitigate stability and settlement.  Shannon & Wilson completed the geotechnical design to meet WSDOT’s requirements and provided support during construction to review Contractor approaches and ideas.

SR 99 BORED TUNNEL PROJECT
In 2008, the State of Washington decided to replace the viaduct through Downtown Seattle with a bored tunnel, extending more than 250 feet below the city.  WSDOT chose to use the design-build method of project delivery.  Shannon & Wilson compiled all existing subsurface data, performed additional subsurface explorations (some extending more than 300 feet deep), performed conceptual evaluations, and prepared a large geotechnical data report and a geotechnical baseline report.  After the design-builder, Seattle Tunnel Partners (STP), was selected in 2010, Shannon & Wilson supported WSDOT in reviewing design and construction submittals related to geotechnical issues, reviewing settlement monitoring data, evaluating STP’s differing site condition claims, and providing geotechnical and environmental support as needed.  In late 2013, Bertha, the tunnel boring machine, malfunctioned and over the next two years STP worked to repair the machine, which included creating a large rescue shaft.  Tunneling resumed in January 2016 and was completed in April 2017.   The completed highway and tunnel opened in early 2019.