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Seismic Vulnerability

Image of a WSDOT visually inspecting the I-5 bridge
DOT employees visually inspect the I-5 bridge.

Existing bridges do not meet current earthquake standards

The two existing I-5 bridges, built in 1917 and 1958, are supported by timber pilings driven approximately 70 feet below the river bottom. Recent geotechnical studies have shown that the sandy soil under the bridges will likely liquefy during a significant earthquake and could cause severe damage to the bridges and possibly collapse.

Replacement bridges will be built to withstand earthquakes

Supports for the replacement bridge would have been extended through the sandy river bed to solid ground or rock. This would have improved structural safety in an earthquake to avoid collapse.

The replacement bridge would have been designed so that it remains functional, with minor repairs after a seismic event that occurs once in 500 years. Additionally, the bridge would have been designed so that it would not have collapsed when subjected to an event that occurs once in 2,500 years. The intent of this design is to prevent loss of life.

Additional information on the geologic and soils assessment of the project area can be found in the Geology and Soils Technical Report from the Final Environmental Impact Statement.

Related Links

Panel Assessment of Interstate Bridges Seismic Vulnerabilities