Seattle — The Seattle Department of Transportation made public the conclusions of an independent review of the Alaskan Way Viaduct. The report presents the findings of an engineering consulting firm, CH2M HILL, retained by SDOT to review existing studies in order to evaluate the potential effects of ground settlement on the structural capacity of the viaduct.

The report’s conclusions support the Washington State Department of Transportation’s (WSDOT’s) decision to keep the viaduct open to traffic; however, refinement of some of WSDOT’s analysis of certain portions of the structure is recommended, especially because additional settlement is expected from continuing tunnel construction activities. Based on this finding, SDOT is recommending that WSDOT complete additional analysis before resuming underground drilling for the new tunnel.

SDOT’s consultant reviewed more than 30 structural studies, reports, design calculations, and settlement analyses prepared by WSDOT and WSDOT’s design-build contractor for the tunnel project. The consultant considered the extent to which previous analyses accounted for significant settlement of the viaduct, most of which occurred following the 2001 Nisqually earthquake and before 2011, as well as much smaller amounts of settlement since tunnel work began in 2012.

The conclusion that the viaduct remains safe is good news to SDOT engineers who are counting on the viaduct continuing to carry traffic for two years longer than previously planned, until the tunnel project is completed.

The full report is available on SDOT’s web page,