This report describes the geotechnical engineering aspects of a seismic vulnerability analysis of the Alaskan Way Viaduct in Seattle, Washington. The Alaskan Way Viaduct is an approximately 40-year- old, two-level, reinforced concrete structure that carries some 86,000 vehicles per day along the Seattle waterfront.
Subsurface data from previous investigations for construction of the Alaskan Way Viaduct and other structures in the general area were collected and interpreted. The existing subsurface data was supplemented by additional borings, cone penetration test soundings, shear wave velocity tests, and laboratory tests. The existing and supplemental subsurface data were then combined to form a model of the subsurface conditions along the length of the Viaduct.
A probabilistic seismic hazard analysis was performed to develop design-level ground motions for use in the seismic vulnerability analysis. These ground motions were used as input for site-specific ground response analyses that predicted the nature of ground shaking along the length of the Viaduct. The ground response analyses also predicted cyclic loading levels that were used in subsequent liquefaction analyses.
Foundation stiffness and damping characteristics of the existing foundations were analyzed for use in analyses of the dynamic response of the Alaskan Way Viaduct superstructure.
The subsurface investigations revealed evidence of potentially liquefiable soils along the majority of the length of the Alaskan Way Viaduct. Review of historical documents verified that the soils were deposited in a manner that has produced liquefiable soils in other parts of the U.S. and the world, and that liquefaction has been observed near the Alaskan Way Viaduct in past earthquakes. A detailed liquefaction hazard evaluation that considered issues of susceptibility, initiation, and effects was conducted. The evaluation indicated that widespread liquefaction would be caused by the design-level ground motions. This liquefaction could cause bearing failure of the pile foundations that support the Viaduct. It could also produce significant post-earthquake settlement and lateral movement that could cause severe damage or collapse of the Viaduct.