Alaskan Way Viaduct Replacement Transforming Seattle

Removal of viaduct is opening up downtown waterfront to the public

The Seattle underground scene has a new star: The Alaskan Way Tunnel, a nearly two-mile-long, double-decked road tunnel that is improving travel through the city’s downtown and paving the way for a revitalized public waterfront.

The tunnel, which carries traffic along Washington State Route 99, replaced the Alaskan Way Viaduct (AWV) – a fixture of downtown Seattle’s Elliott Bay waterfront for more than 65 years. Between 1953 and 2019, the AWV was one of the main north-south highway corridors through Seattle, until recently carrying approximately 100,000 vehicles per day.

Efforts to replace the viaduct accelerated shortly after the Nisqually Earthquake, which struck the Puget Sound region in February 2001. The powerful earthquake damaged the double-decked viaduct. Repairs and strengthening kept the roadway functioning, but the 2001 earthquake highlighted the structure’s seismic vulnerability.

Later that year, the Alaskan Way Viaduct Replacement Program was established, led by the Washington State Department of Transportation (WSDOT) in partnership with the Federal Highway Administration, King County, the City of Seattle and the Port of Seattle.

“WSDOT was faced with the need to replace a vulnerable and critical transportation corridor within Seattle’s downtown and waterfront core,” said Paula Hammond, WSP USA transportation market leader, who prior to joining the firm was involved with the planning of the tunnel project as the Washington State transportation secretary from 2007-2013. “We had never developed a project of this magnitude—of such economic and quality of life importance to the Seattle community—and knew we had to bring our A-game through strong partnerships and community engagement to succeed.”

WSP has served as general engineering consultant (GEC) to the WSDOT for the $3.3 billion program and been involved in every aspect of the work since 2001, from planning through construction. WSP provided the conceptual design and developed the technical requirements for the design-build tunnel project, prepared environmental documents and final design of some major project elements and coordinated and managed a consultant team consisting of 62 subconsultants.

Tunnel Vision

Using highly sophisticated virtual design and construction (VDC) and building information modeling (BIM) techniques prior to construction, WSP created dynamic, three-dimensional animations to illustrate conditions at the site and help WSDOT officials visualize the impacts of proposed solutions.

“Design visualizations were an integral part of the program, used extensively to educate and inform the public, and as a primary design tool to effectively determine the viability of proposed concepts and eliminate conflicts between design elements for the replacement,” said Ginette Lalonde, current project manager for WSP.

WSP created a video simulation to show, from a “worm’s eye view,” the proposed route for the tunnel and its proximity to other underground structures, including utilities and buildings. Another lifelike video prepared by WSP and released by WSDOT in 2009 showed a simulation of the viaduct’s collapse from a hypothetical seismic event similar to the Nisqually Earthquake.

“The viaduct posed an immense public safety risk, and we all knew we were racing against Mother Nature,” said Mike Rigsby, who served as WSP’s project manager during the early stages of the program. “The video simulation attracted huge media attention and heightened public awareness about the life-safety risk of the seismically vulnerable structure.”

WSP drew upon computer-aided design (CAD) and geospatial data to develop digital 3D and BIM for all major parts, pieces and processes of the project. These models enabled the project team to understand the project, design alternatives, and view traffic impacts and construction phasing from any point of view, allowing for more informed decision making by multiple stakeholders, while supporting public outreach efforts.

An example of the team’s extraordinary effort was with an early design-bid-build enabling project—the Holgate-to-King Street Viaduct Replacement. While the project was out for bid, a late change in the adjoining bored tunnel project required revising and resubmitting 75 percent of the design documents, which included changing and reprinting 1,400 documents over three months.

“One of the most challenging aspects was dealing with and still meeting project milestones,” Lalonde said. “Even though final alignment decisions were delayed, the project had to remain on schedule.”

Digging Bertha

After years of planning and careful exploration of more than 75 possible options to replace the Alaskan Way Viaduct, the decision was made to bore a double-decked tunnel through downtown Seattle to replace the viaduct.

The tunnel design met stringent seismic standards capable of withstanding a 2,500-year earthquake. It also would feature smart transit technology, including more than 300 cameras that monitor traffic, safety conditions and security.

Digging began in July 2013 as “Bertha” began to make its way underneath Seattle. The tunneling boring machine (TBM) was named for Bertha Knight Landes, Seattle’s first female mayor. Standing five-stories tall and weighing some 7,000 tons, Bertha was the world’s largest TBM at the time digging began.

Bertha’s enormous size provided the capacity needed to use one machine to drill a tunnel large enough for four traffic lanes – two lanes stacked upon two others – thus eliminating the need for a second bore. The upper deck carries southbound traffic, while the lower deck carries the northbound traffic.

Tunneling moved forward until December 2013, when the TBM failed. At that time, only 10 percent of the tunnel had been built. The contractor, Seattle Tunnel Partners, and the machine manufacturer, Hitachi Zosen developed a bold plan to raise the machine from below ground for extensive repairs above. Bertha was fully repaired and resumed digging in February 2016, and on April 4, 2017 broke through into the disassembly pit—building the tunnel’s final ring shortly after.

Throughout the dig, more than 4,000 monitoring devices kept watch for ground movement along the two-mile tunnel alignment. Using a variety of above-ground devices, 158 buildings – including historic buildings – were monitored in real-time. Ground and vibration monitors operated 24 hours a day for the length of the project.

The extensive monitoring along the tunnel alignment would allow an immediate response if movement exceeded contract specifications. In the end, all the monitoring systems confirmed an amazing feat – there was no surface ground movement under Seattle buildings or streets caused by tunneling.

When the TBM’s tunneling and ring-building work ended, the massive machine had to be taken apart to allow road building to continue.

Bertha’s signature 57-and-a-half-foot steel cutterhead was cut into pieces and hauled to a local steel recycler. Hitachi Zosen salvaged what it could for reuse. Hitachi also donated pieces of the cutterhead to the Port of Seattle and gave Seattle’s Museum of History and Industry several cutting tools and the TBM’s control panel for future display.

Waterfront Seattle

The completed tunnel opened to traffic on Feb. 4, 2019 following a weekend grand opening celebration where the public had an opportunity to walk through the tunnel and learn more about the city’s plans for the waterfront—a section of the city that had long been cut off from the rest of the city by the unwieldy and noisy viaduct.

In addition to making travel safer, WSDOT officials also recognized that replacing the viaduct provided the city with an opportunity to restore access between downtown and a revitalized waterfront area.

“By its nature, moving transportation corridors underground frees up a lot of valuable surface land,” said Mike Colyn, WSP’s project manager for Waterfront Seattle. “The AWV Program’s completion provides nine acres of new open space adjacent to the downtown Seattle waterfront and gives the waterfront back to the public.”

With demolition of the viaduct completed in November 2019, attention has now turned to waterfront improvements.

Waterfront Seattle is a $688 million program designed to reinvigorate the waterfront as an inviting, lively and family-friendly public space; connecting residents and visitors with cultural, commercial and civic destinations, and fostering growth and development opportunities for area businesses and nearby neighborhoods.

WSP is part of the team helping deliver this new amenity. As a subconsultant to the City of Seattle, WSP is leading the engineering component of the project and has provided 3D design visualization, cost risk assessment, project management, civil engineering, traffic engineering, stormwater management, storm drainage, electrical duct banks, risk management, and structural design support.

Although waterfront construction should be complete in 2024, segments of the waterfront will open this year, including the newly rebuilt Pier 62, which will provide views of Elliott Bay and the city skyline, as well as a floating dock for boating. Eventually the waterfront will feature parks, bike paths, playgrounds, a seawall, and improved road access to the area.

ACEC Grand Award

The Alaskan Way Viaduct Replacement Program has been recognized with multiple honors, including the 2019 Grand Conceptor Award from the American Council of Engineering Companies (ACEC) as the best overall engineering project in the U.S.

That award followed wins of the ACEC national 2019 Engineering Excellence Grand Award for Transportation and the ACEC-Washington Gold Award for Transportation.

“The success of the project is not just measured by the tunnel’s completion or the awards, but by the success and the careers of the team of people whose hard work completed it,” Lalonde said. “It’s truly amazing that the tunnel is completed and to watch how the program is transforming the City of Seattle.”

Rigsby added that the program’s complexity “attracted the best people from around the company and around the country,” who rose to meet every challenge along the way.

“I grew and learned so much about environmental, engineering, urban design, public involvement and so much more,” he said. “The technical complexity of planning, getting environmental clearance, and helping to contract for the largest bored tunnel in the history of the world was amazing. We were literally pushing the state of the art and involving every discipline.”