Six decades of service
The Alaskan Way Viaduct has seen its fair share of critics. Before the double-deck highway took its place on Seattle’s downtown waterfront in the 1950s, transportation planners and city leaders spent nearly two decades debating the best way to fix the city’s growing traffic problems. Even Ray Finke, city engineer at the time the viaduct was built, acknowledged the structure’s imperfections. “It is not beautiful,” Finke said, as designs for the viaduct neared completion.
Beautiful or not, the viaduct quickly became a fixture of Seattle’s skyline and a vital north-south route through downtown. The two-mile-long structure gave drivers an alternative to the busy city streets, and even offered impressive views of Elliott Bay and the Olympic Mountains. By the end of the 20th century, it was among the state’s busiest and most important sections of highway, carrying 110,000 cars each day.
The path to replacement
In February 2001, a 6.8 magnitude earthquake struck Seattle. When the shaking stopped, sections of the viaduct had sunk several inches. Crews stabilized the structure, but engineers agreed that had the quake lasted a few moments longer the viaduct would have collapsed. Mother Nature had sent a clear message: the viaduct needed to be replaced.
Thus began one of the most scrutinized public processes in state history. During the next decade, state and local agencies studied more than 90 alternatives. Ideas included tunnels, surface streets and bridges. But while everyone agreed something needed to be done, it seemed no one could agree on the best approach.
In 2007, with much of the public’s attention focused on the waterfront section of the viaduct, leaders decided to take a fresh approach. Crews would begin a separate project to replace the viaduct’s south end. Doing so would allow them to remove almost half of the viaduct while discussion about the waterfront section continued.
Time passed. Environmental studies were published. The debate intensified. Cars, trucks and buses continued to rumble over the viaduct each day.
But the passage of time, while frustrating to politicians and the public alike, brought with it an unforeseen development. Tunneling technology was advancing at a remarkable rate. So fast, in fact, that by the end of a yearlong stakeholder effort in 2008 an alternative that had previously been considered too expensive reemerged: a bored tunnel.
In January 2009, leaders from the state, county, city and port recommended a bored tunnel – along with a host of other improvements – to replace the waterfront section of the viaduct. It was the only alternative that would allow SR 99 to remain open during construction, maintaining a vital stretch of state highway.
A detailed environmental study of the tunnel concluded in summer 2011, followed by approval from the Federal Highway Administration making it official: crews would dig the world’s largest-diameter bored tunnel beneath downtown Seattle. Afterward the viaduct would be demolished, ushering in a new era for the downtown waterfront.
Replacing the viaduct’s south end
Meanwhile, crews had already begun replacing the southern mile of the viaduct. This new section is a side-by-side roadway, instead of a stacked viaduct. It has wider lanes and shoulders, and a more earthquake-resistant design. Foundations for the roadway are buried up to 260 feet deep – nearly half as tall as the Space Needle – to reach beyond unstable soils and into sturdy ground.
In February 2011, crews demolished a portion of the on-ramp connecting SR 99 to First Avenue South, marking the first time any part of the viaduct had been torn down. Then, during a nine-day closure in October 2011, an army of machines moved in to demolish the viaduct’s south end, chunk by seismically vulnerable chunk.
A temporary bypass structure now connects the south end replacement to the viaduct along the waterfront, allowing SR 99 to remain open during tunnel construction. The two projects will be permanently connected just before the tunnel opens to traffic.