The aging Alaskan Way Viaduct has sunk another quarter-inch along Seattle's central waterfront, a routine state inspection found. At its weakest point...

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The aging Alaskan Way Viaduct has sunk another quarter-inch along Seattle’s central waterfront, a routine state inspection found.

At its weakest point, near the state ferry terminal, the 54-year-old elevated highway now rides 5 inches lower than it did before the Nisqually Earthquake of Feb. 28, 2001. Many of its underground supports were built in weak fill soil.

“The viaduct is still safe for drivers. We are very concerned about the new settlement, but it does not come as a huge surprise,” Harvey Coffman, bridge preservation engineer for the Washington State Department of Transportation, said this morning.

The department noticed new sinking during a quarterly inspection last week.

In late September or early October, the state will begin a $5 million strengthening project for that part of the viaduct. Steel rods will be installed to surround and reinforce the foundations of four vulnerable columns. The vertical rods will extend deep into the ground and reach stable soil.

Traffic will continue as normal during the six-month project, spokeswoman Emily Fishkin said.

In recent years, the DOT has said 6 inches of sinking is the threshold to require emergency reinforcing.

Politicians have not yet decided whether to replace the waterfront highway with a bigger elevated structure, or tear it down in favor of a nonhighway, “surface-transit” strategy. In the meantime, the state plans to replace the viaduct’s southern stretch, from Qwest Field to South Holgate Street in Sodo.

For more information: www.wsdot.wa.gov/projects/Viaduct/