Making a New Path: State Route 99 Tunnel Replacement

The replacement of the SR 99 Tunnel is the largest project in the Alaskan Way Viaduct Replacement Program being spearheaded by the Washington State DOT

By Brianna Fries

The Alaskan Way Viaduct in Seattle, Washington, is an elevated highway that originally opened in 1953. This incredible structure carries the State Route (SR) 99 through Seattle and has safely allowed the transportation of hundreds of thousands of cars, and their passengers, for the past 50 years. However age and the forces of nature took their toll on the viaduct when, in 2001, the Nisqually earthquake shook things up and caused large amounts of damage. This led to a series of necessary repairs and replacements that spanned from 2011 to present day.

Acting as a vitally important highway for the Seattle, WA area, the Alaskan Way Viaduct is one of the only north/south highways that go through the city. While it was initially repaired after the 2001 earthquake to ensure that it would remain safe for everyday driving, the viaduct still needed additional work to make sure it met today’s standards rather than the standards that were in place when it was created in the ‘50s. Due to its advancing age and its seismic vulnerability, the viaduct needed to be fully replaced.

After the 2001 Nisqually earthquake, it became apparent that the Alaskan Way Viaduct needed to be replaced to address its age and seismic vulnerability.
The Project Plan

The Alaskan Way Viaduct Replacement Program, which is being spearheaded by the Washington State Department of Transportation (WSDOT), consists of 30 projects, with the largest part of the project being the replacement of the SR 99, a two-mile long, double-decked, underground tunnel. Construction on the tunnel began in 2009 after the program received $3.3 billion in funding authorized by the Washington State Legislature. That government funding was also paired with additional funds that will come from the required tolling of the SR 99 which is expected to cover up to $200 million in costs for building, maintenance, and operation.

In order to tackle the SR 99 Tunnel portion of the project, the WSDOT chose to utilize a design/build approach with Seattle Tunnel Partners (STP), a joint-venture of Dragados USA and Tutor Perinini Corp, acting as contractors. The replacement program required a great amount of collaboration and cooperation between the Seattle Tunnel Partners, who were responsible for the means and methods of construction, and the WSDOT, which took care of oversight and verification.

Replacement of the SR 99 tunnel was tackled through a design/build method proposed by the Washington State DOT.
A New Challenge: Rescue Bertha

While the project was already a challenge in itself, the project team had other hurdles they had to handle, the biggest of which was a breakdown in the tunneling machine. The breakdown of this piece of equipment ended up delaying the project by two whole years.

“At this time tunneling started, the tunnel-boring machine (TBM) nicknamed Bertha was the largest in the world – 57.5 feet in diameter,” stated the Spokesperson for the Washington State Department of Transportation, Laura Newborn. “About 1,000 feet into the tunnel drive, the machine overheated and shut down. After several months of investigation, the design-builder, Seattle Tunnel Partners, concluded the seals designed to protect the tunnel boring machine’s main bearings had failed.”

Bertha needed to be repaired for construction to move forward, so STP and Hitachi Zosen (the manufacturer of the TBM) designed a rescue project along Seattle’s waterfront. STP engineered a 120-foot deep, 80-foot in diameter shaft that was supported by interlocking concrete piles, moved the machine into the giant pit, and engineered a spectacular lift of the cutterhead and drive unit to the surface where Bertha could be repaired.

Once the tunneling machine was fixed, the cutterhead and drive unit were reassembled, lowered back into the pit, and reconnected to the rest of the tunnel-boring machine. Bertha resumed tunneling and made a breakthrough to the other side in April 2017. It was a good thing that the TBM was fixed, since it provided the ability to construct the double-decked highway in the first place.

The SR 99 tunnel was created using a tunnel-boring machine nicknamed Bertha, which was the largest tunneling machine in the world at 57.5 feet in diameter.
Unique New Features

The tunnel is quite the unique piece of infrastructure. Newborn detailed that, “There are many unique features of the SR 99 tunnel. The two-mile long tunnel is one of only two single-bored double deck tunnels in the world. Its sophisticated safety system includes more than 100 safety zones in the tunnel that can operate independently or as a whole.”

Alongside the enhanced safety system, the tunnel accounts for fire suppression by including eight miles of linear heat detectors, a deluge sprinkler system, and a ventilation system, which can both extract air from the tunnel and push fresh air inside. Additionally there is an exit corridor on the tunnel’s west side that has been built to include its own unique ventilation and fire suppression systems as well.

And that’s not all. The SR 99 tunnel was even built with an AM/FM override, a public address system, a cellphone system, and multiple cameras to monitor traffic and security.

The SR 99 tunnel should be completed and reopened to the public by fall of 2018.

Unique and proactive thinking can be seen both inside and outside the tunnel. The northbound off ramp that leads to downtown Seattle from the tunnel includes a first-of-its-kind bridge that is made to be flexible during an earthquake. Designed and tested in collaboration with the Earthquake Structures Lab at the University of Nevada-Reno, the bridge’s nickel and titanium steel support rods and bendable concrete allow the bridge to flex back into shape after a strong earthquake occurs.

While extensive progress has been made on the Alaskan Way Viaduct Replacement Program, it is still in progress. However, the new SR 99 tunnel is expected to finish and re-open as early as fall of the year 2019.

Brianna Fries is an Assistant Editor for American Infrastructure magazine. She may be reached at

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.