Earthquake simulation highlights the vulnerabilities of the Alaskan Way Viaduct

By guest blogger Ron Paananen

The double-deck Alaskan Way Viaduct, a fixture on Seattle’s downtown waterfront for more than five decades, was already showing signs of wear and tear when the last major earthquake struck in 2001. That 6.8 magnitude seismic event further weakened the structure by damaging its joints and columns and causing sections to settle into the loose fill soil in which it was built.

In the years since, crews have kept a close eye on the viaduct through quarterly inspections and have strengthened several columns to prevent further damage to the structure, but the threat of another earthquake was always present. During this time new soil data and a better understanding of local and regional seismic behavior clarified exactly how vulnerable the viaduct is to another earthquake. In 2007 we released a report that concluded there is a higher chance – specifically, a one in 10 chance in the next 10 years – of an earthquake occurring that could cause portions of the viaduct and adjacent seawall to collapse. The vulnerability analysis is available on our Web site.

A simulation based on the 2007 report demonstrates how disastrous a strong earthquake could be for the Alaskan Way Viaduct. It shows what could happen if a seismic event more intense than the 2001 earthquake were to shake the Puget Sound region again. To say that the damage to the viaduct and the seawall would be severe would be an understatement.

We understand the risk, and we are making progress to replace this vulnerable structure. Early next year we will begin major construction to replace the southern mile of the viaduct, and the state, King County and the City of Seattle have agreed to a plan to replace the section along the waterfront (The plan, calling for a bored tunnel beneath downtown, is currently under environmental review.) State and city crews also continue to monitor the structure and ensure it remains safe for drivers.

We are also installing an automated closure system next year that will keep drivers from using the viaduct after an earthquake, fire, or other event compromises the structure. The new system will use the latest in monitoring technology, including GPS antennas and wireless equipment, to detect structure and ground movement. New signs and gates at the viaduct’s ramps and entrances will detour traffic away from the structure during an emergency, and advance warning signs will notify drivers in SODO, West Seattle, downtown and north of Seattle about any closures.

The specter of another major earthquake, however, is always present. That is why we are determined to replace the Alaskan Way Viaduct before Mother Nature makes the decision for us.

You can watch the video on our streaming server if you can't access it on YouTube.