CH2M Hill was retained by Seattle Department of Transportation (SDOT) to review existing studies in order to evaluate the potential effects of ground settlement on the structural capacity of the viaduct. The viaduct now needs to remain in operation for two years longer than originally intended as tunnelling operations are at a standstill following a breakdown of the tunnel boring machine (TBM).
SDOT has published the conclusions of the independent review of the viaduct carried out by CH2M Hill. The report’s conclusions support the decision by Washington State Department of Transportation (WSDOT) to keep the viaduct open to traffic during the project to replace it with a tunnel. SDOT is recommending that WSDOT completes additional analysis before resuming underground drilling for the new tunnel, especially because additional settlement is expected from the continuing construction activities.
CH2M Hill 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.
Tunnelling operations have been at a standstill for more than a year following the breakdown in the TBM. The machine, Bertha, has now been lifted to the surface in order for repairs to be carried out.
