Project details

SR 99 Bored Tunnel Alternative Design Build Project

Project Description

State Route 99 is one of only two north-south highways that pass through Seattle.

In 2001, a 6.8 magnitude earthquake shook the Puget Sound region. The Alaskan Way Viaduct survived, but concrete cracked and some columns shifted as much as 5 inches. Repairs strengthened the road but removing the viaduct became a critical public safety issue.

During years of discussion over replacement options that followed the earthquake, tunneling technology vastly improved. It became feasible to build a large, single-bore tunnel underneath Seattle, keep the viaduct open and keep traffic moving above. In 2009, the Washington State Legislature voted to fund the plan to replace the viaduct with a tunnel large enough to build a double-deck highway inside.

The Washington State Department of Transportation hired Seattle Tunnel Partners for the job, who then hired Hitachi Zosen to build what was, at the time, the world’s largest tunnel boring machine, nicknamed Bertha.

OMA’s Role in the Project

The task was unprecedented and the journey anything but easy. Bertha broke down, requiring a repair effort that itself was a remarkable engineering feat.

The new tunnel opened in February 2019 and has set the gold standard for tunnel safety. Its fire suppression systems, ventilation systems and intelligent traffic systems are state of the art – all built inside a tunnel designed to withstand an earthquake as powerful as a 9 magnitude (a 2500 year recurrence interval).

The SR 99 tunnel allows drivers to bypass downtown Seattle traffic, and, with the demolition of the seismically-vulnerable Alaskan Way Viaduct, opened up the Seattle waterfront for a worldclass revitalization that includes multi-modal travel and more than 20 acres of new public space.




Seattle Tunnel Partners; Tutor Perini / Dragados, JV


State Route 99, Washington