Stone Columns and Earthquake Drain Liquefaction Mitigation for Federal Center South in Seattle, Washington

Abstract

This paper presents site conditions, design, and construction of a liquefaction and lateral spreading mitigation program for the project of the Federal Center South building located in Seattle, Washington. The area is dominated by the Cascadia Subduction Zone, and the site is located within the 4- to 6-km-wide zone for the Seattle Fault zone. It is also adjacent to the Duwamish River and on top of potentially liquefiable soils, thus creating a potential for slope instability or lateral spreading. Soil properties were determined by SPT, CPT, a Surface Shear Wave Velocity Survey, and a Seismic Cone Survey. The initial evaluation of slope stability indicated that mitigation was required. Stone columns were installed to mitigate liquefaction. Earthquake drains were also installed to aid in the drainage to avoid lateral spreading / liquefaction, as well as to increase the effectiveness of the stone column densification. The stone column and earthquake drain design was analyzed using the computer program FEQDRAIN (Pestana, et al., 1997). Confirmed by the post-treatment CPTs, the combined stone column and earthquake drain treatment program effectively densified the liquefiable sands and mitigated the site lateral spreading hazards.