Bridges, Roads Likely to Hold Up During Cascadia Earthquake, But May Take Time to Fully Reopen

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OLYMPIA — The big one. The 9.0 magnitude Cascadia earthquake that experts say will change the West Coast forever.

No one knows when, but scientists are pretty certain it will hit. Since the 2001 Nisqually earthquake, Washington has been working to better prepare its infrastructure for a large seismic event like a Cascadia earthquake.

But how well will our roads and bridges hold up?

"If you look at the preparedness side, we're in really good shape," state bridge engineer Mark Gaines said. "But on how well our structures perform, we have a lot of work to do there."

WSDOT currently retrofitting bridges

The Department of Transportation has been retrofitting its bridges on the West Side since 1990, but the effort took on more urgency after the Nisqually quake.

As of February, the department has retrofitted 323 bridges and partially retrofitted another 114. That's about a $144 million investment.

Bridges don't just mean those that span a body of water. Large sections of Interstate 90 are considered bridges because they go over other roads, according to the department.

All bridges have a superstructure — the part you drive on — that rests on a column pier that holds the bridge up. To retrofit means making sure columns under the bridges don't collapse during an earthquake. Part of retrofitting could be better anchoring the columns to the superstructure or filling hollow columns with cement grout.

Since 2001, all retrofitting of bridge superstructures and single column piers has been completed.

The department has focused mostly on bridges that were built in the 1960s and 1970s because those were designed with "no seismic consideration," Gaines said.

Bridges that have already been retrofitted include the new Tacoma Narrows Bridge, the S.R. 520 floating bridge and the new Alaska Way Tunnel in Seattle.

"But we have some pretty important bridges that have not," Gaines said.

Those still left to retrofit includes bridges with multicolumn piers and those that require further analysis because of their large or unusual structures.

The Interstate 5 Ship Canal Bridge and the State Road 20 Deception/Canoe Pass bridge are two that have yet to be retrofitted because of how complex their structures are .

WSDOT is focusing its initial retrofitting on what it calls lifeline routes. The 2017 Legislature allocated $171 million to WSDOT to finish retrofitting its lifeline bridges over 10 years.

The Interstate 5 Ship Canal Bridge and the State Road 20 Deception/Canoe Pass bridge are not necessarily part of the lifeline route, Gaines said, but they are important given their significance in the community and how many people rely on them.

Lifeline bridges are those that would be considered critical infrastructure and would allow emergency vehicles, goods and supplies to be delivered after the disaster.

The lifeline routes are chosen based on a combination of how well the bridges can withstand an earthquake, how quickly they can reopen and how needed they are to access support bases, said Taylor Hennessee, private sector and critical infrastructure program manager at the state Emergency Management Division.

The Emergency Management Division prioritized routes from Western Washington to Eastern Washington, where most of the federal staging areas will be after a large earthquake.

Prioritized routes include I-90 to Grant County International Airport, I-5 between Joint Base Lewis-McChord and the Bellingham Airport, and Highway 101 between William Fairchild Airport in Port Angeles to Tacoma.

Bridges that are not considered lifeline will be retrofitted as funding allows. The department estimates it will cost about $1.5 billion to properly retrofit all bridges in the state. That includes 114 bridges that are partially retrofitted but need additional work, and 471 bridges that have had no previous retrofit work.

It will take decades to finish all of that work, even with proper funding.

Reopening bridges will take time

Gaines said Washington retrofits their bridges for "life safety," meaning they'll hold up enough to save lives but not necessarily enough to avoid structural damage. The higher standard for retrofitting focuses on recovery, meaning the bridge is mostly undamaged and usable immediately after the event.

This standard is great for protecting people, Gaines said, but it doesn't do a lot for recovery.

A 2019 study by the Department of Homeland Security looked at the ability of the state's transportation systems to recover after a large earthquake, using the worst-case scenario. The study looked at the structure of bridges, when they were built and the ground movement and type of soil surrounding it, Hennessee said.

"We didn't look at restoration," she said. "We looked at reopening, how quickly can we reopen so we can move supplies."

According to the study, 621 bridges in the state would have no damage due to an earthquake and be able to reopen immediately.

Most bridges, however, would take longer. More than 300 bridges could reopen within two weeks of the earthquake. More than 600 bridges could take up to three months to reopen, and more than 350 could take up to 21/2 years to reopen.

Other preparations

There are other things the state can do to prepare. While bridges are important, WSDOT has other assets that have seismic needs, such as the roads themselves, Gaines said.

Earthquakes can cause liquefaction of the ground, meaning shaking is so strong it loosens up the soil enough so the ground essentially turns into liquid.

Liquefaction can also play a role in how quickly bridges can reopen after the earthquake, Hennessee said.

According to the Department of Homeland Security study, most roads located on liquefiable soil will be able to reopen within one day. About 13% will take up to two days to reopen, about 6% will take up to four days, and about 3% will take more than two weeks.

WSDOT can look at its roads to ensure they withstand some liquefaction, but Gaines said there is no current funding to do so.

To prepare for a large earthquake, both WSDOT, EMD and other state and national agencies take part in exercises to practice their response. The second Cascadia Rising exercise will take place next summer. It allows those who would be responding to the disaster, such as bridge engineers who can reopen roads and EMD staff who will be moving supplies across the state, a chance to practice.

While state agencies practice, the biggest advice for the public: be prepared. Officials suggest keeping two weeks of supplies on hand at all times.

"It may take some time to reopen bridges," Hennessee said. "Making sure people have supplies on hand is really important."

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