Seattle’s State Route 99 tunnel completion

Seattle’s State Route 99 tunnel completion

by Jon M. Casey

As I begin writing this article (on Jan. 11), at 10 p.m. local time, Seattle’s Alaskan Way Viaduct between South Spokane Street and the south end of the Battery Street Tunnel will close forever. Once that takes place, the scheduled three-week process of intensive construction work by WSDOT to realign State Route 99 from the viaduct to the tunnel entrances in both directions begins. Since both the tunnel and viaduct will be closed to traffic during this construction, daily commuters will face considerable delay and confusion on the short term. In the end, however, most agree that the new tunnel beneath downtown Seattle will be a blessing to the area when it is finally completed. Even though the tunnel is scheduled to open the first week of February, it will take another two weeks to complete the new northbound off-ramp into downtown Seattle. In all, the disruption will go on for approximately six weeks.

Nevertheless, most commuters welcome the tunnel opening for several reasons. First, it will make way for the removal of the double-deck Alaskan Way Viaduct, a structure that currently carries approximately 90,000 vehicles per day. This structure has been in use since its construction in the 1950s.

Second, the added safety of traveling underground for two miles in what is known to be an earthquake zone, versus above ground on a viaduct that could easily collapse,  gives commuters the added assurance that they will be able to drive safely under most conditions. Planners took notice three decades ago when the 1989 Loma Prieta Earthquake in Oakland, CA, took the lives of 42 people who were killed while traveling on an elevated roadway, much like the Alaskan Way Viaduct.

Finally, with the removal of the closed viaduct system, the improvements planned for the 20-acre waterfront area will provide visitors easy access from the downtown area and the historic Pike Place Market to a waterfront park that will provide a public space for thousands of visitors each year.

Planning begins

According to Brian Russell, HNTB Corp. project manager, the idea of an alternative way to travel in downtown Seattle began to take shape following a local earthquake in 2001. While sections of the SR 99 viaduct received slight to moderate structural damage, a nearby railway tunnel, built in the very early 20th century, received none. This highlighted the added safety that can be found within tunnel structures during earthquake events. Russell summarized the progress of the tunnel project during a webinar presentation given during the Roads & Bridges Virtual Expo on Oct. 10, 2018.

Following the repair and reactivation of the viaduct roadway, the Alaskan Way Viaduct Replacement Program was established. Planners began looking at how they might tunnel beneath Seattle’s downtown area as a way to replace the aging viaduct roadway system. Ultimately, a single-bore, 57-foot diameter, two-mile long tunnel containing a double-deck roadway and ample evacuation safety measures became the plan.

Getting the job done

Once the contract was awarded, Seattle Tunnel Partners began construction in 2010 using design plans created by the architect firm HNTB Corp. Seattle Tunnel Partners is a joint venture of New York-based Dragados USA, a wholly owned subsidiary of Dragados, S.A., the construction division of ACS Group of Spain, and Tutor Perini Corporation, based in Sylmar, CA. During this massive project, design-build contractor Tutor Perini Corp. worked with Fisk Electric Company, Desert Mechanical Inc., Frontier-Kemper Constructors Inc. and Superior Gunite, among others, to accomplish this historic task.

Three years later, tunneling began in July 2013 on the $1.4 billion project after the Hitachi-Zosen Corporation’s $80 million tunnel boring machine (TBM), named “Bertha,” arrived at the jobsite. The machine was given the name Bertha in honor of Seattle’s first mayor, Bertha Knight Landes. It was built in Osaka, Japan. There, it was disassembled into 40 pieces for shipping and sent to the Port of Seattle.

Upon arrival in Seattle, Bertha was reassembled in the launch pit on the south end of the future tunnel — 326 feet long and 57.5 feet in diameter, weighing in at 6,700 tons. During its time of operation, the borer moved in 6.5-foot increments toward the north end of the project.

Along the way, tunneling met with a number of delays caused by maintenance scheduling and a major breakdown after hitting an unexpected, buried steel pipe. According to a overview, “Work was halted on Dec. 6, 2013 after the machine overheated and shut down approximately 1,083 feet (330 m) into the planned 9,270-foot-long (2,830 m) route. Investigations later revealed the seal system that protects the machine’s main bearing had been damaged. Three days prior to stopping, the machine mined through an 8-inch steel well casing used to help measure groundwater in 2002 around the Alaskan Way, drilled as part of the planning phases of the project. Whether this pipe had anything to do with the machine’s failure is at the center of legal dispute between WSDOT and the contractor, Seattle Tunnel Partners. This delay lasted for more than two years as the workers had to dig a 120-foot (37 m) vertical shaft down to Bertha’s cutting head to repair it” (   

By January 2016, the tunnel had only reached 1,560 feet of its 9,270-foot journey. After the resumption of boring in January 2016, the crew made steady progress. During its journey, the boring machine tunneled as deep as 200 feet beneath ground level. During the time the machine did its boring work directly beneath the existing viaduct, traffic was rerouted for the two weeks it took to bore the 292 feet. In all, once the initial repairs had been made, it took another 14 months until Bertha made its appearance at the north end of the project on April 4, 2017. It took four more months to dismantle and remove the TBM from the site.

Roadway preparation and construction was ongoing. While the tunnel itself was made up of pre-cast concrete sections, the roadways were cast in place. Operations buildings were constructed at either end of the tunnel, designed to provide ventilation, administration and other needed functions. Inside the tunnel, the two-mile-long roadway is two lanes in either direction with an 8-foot safety shoulder in each direction. The tunnel contains state-of-the-art safety systems that include fire protection, a deluge system and single point extraction capabilities. Continuous visual and sensory monitoring is in place for motorist safety.

What’s next?

Beginning Saturday, Jan. 12, Scarsella Bros. Contracting are working to complete the ramp and road connections between the new tunnel and SR 99. Most of the work will take place in the existing construction areas for the SR 99 tunnel. There will also be some bridge sign work on SR 99 and surface street paving. A ribbon cutting ceremony and community celebration is scheduled for Feb. 2 – 3. The tunnel will be open to traffic in one direction until work can be finished on the northbound off-ramp into downtown Seattle. The completed tunnel and roadway project is expected to be completed by mid-February.

Following the reopening of SR 99, work to demolish the Alaskan Highway Viaduct will begin. Once the viaduct is removed, the “Waterfront Seattle” Project will continue in earnest, having begun in 2017. According to the project’s website,, “The program spans the waterfront from Pioneer Square to Belltown and includes a rebuilt Elliott Bay Seawall, a new surface street providing access to and from downtown and new parks, paths and access to Elliott Bay. Waterfront Seattle is led by the City of Seattle’s Office of the Waterfront, working closely with civic leaders, stakeholders and the broader Seattle public to create a ‘Waterfront for all.’” When completed, the project will include a pedestrian walkway, a waterfront park and other visitor amenities. In total, the project is expected to be ongoing until 2024.

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