Large Drilled Shaft Foundations and Bi-Directional Test Program for the West Seattle Link Extension Project-Video

Video Transcription

Video Summary

Sound Transit is developing the $4 billion West Seattle Link Extension, adding 4.1 miles to the regional light-rail network. The project includes 2.1 miles of elevated guideway, a half-mile tunnel, four stations, and a cable-stayed bridge over the Duwamish Waterway. Elevated structures will use precast-girder or segmental box-girder spans, with drilled-shaft foundations reaching up to 295 feet deep.<br /><br />Subsurface conditions include fill, liquefiable granular deposits, and deep glacial till. Because of significant seismic hazards—including potential lateral spreading—the design uses large drilled shafts and caps to provide lateral resistance. Sound Transit is evaluating refinements to seismic load combinations and whether controlled plastic hinging could reduce foundation costs.<br /><br />Two sacrificial, full-scale, 10-foot-diameter drilled-shaft load tests, approximately 200 feet long and bearing in glacial till, are planned near the bridge. The tests will assess shaft resistance, settlement behavior, and construction challenges, potentially enabling shorter, more economical foundations.

Keywords

West Seattle Link Extension

Sound Transit

drilled-shaft foundations

seismic design

glacial till