Assessment of 100-Year-Old Foundation System to Meet Current Retrofit Load Demands

Results are presented of a geotechnical investigation and foundation analysis as a component of the seismic retrofit to the Amtrak King Street Station in Seattle, Washington. The purpose of this effort was to assess the condition of the existing pile foundation, to quantify the foundation response to revised seismic loads, and to provide recommendations for retrofitting of the foundation. King Street Station is a one- to three-story brick masonry structure with a 12-story clock tower. The building was constructed in 1906 in the area of a reclaimed tide flat. The foundation for the structure consists of timber piles; however, no information was available on the length of the piles or whether they were treated with a preservative. The approximate length of the piles was established at 9.7 m using ground-penetrating radar. A core sample obtained from one of the timber piles showed them to be untreated timber and in good condition. The subsurface investigation revealed a liquefiable granular fill soil in the upper 6 m underlain by medium-dense marine sands overlying dense glacial soils. An engineering analysis of the expected performance of the piles during the design seismic event showed that the piles could settle approximately 100 to 355 mm because of liquefaction of the marine sand layer and that the deflection of the piles under lateral loading was substantially in excess of the static loads. Steel pipe minipiles were recommended as a mitigation measure.