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

PPT Presentation

Pdf Summary

The West Seattle Link Extension (WSLE) will add 4.1 miles of light rail from SODO to Alaska Junction, including 2.1 miles of elevated guideway and four stations. Its structures include typical prestressed concrete girders, long-span segmental box girders, and a 1,690-foot cable-stayed bridge over the Duwamish River. Preliminary engineering is complete, final design is planned for 2025–2027, and service is targeted for 2032.<br /><br />The project faces deep, variable soils, liquefiable deposits, and significant seismic hazards associated with the Cascadia Subduction Zone. Design considers a 150-year operating-level earthquake and a 2,500-year maximum design event focused on preventing collapse. Because structures are sensitive to settlement and must resist substantial seismic and lateral-spreading loads, large-diameter drilled shafts were selected as the preferred foundation type. Shafts may reach 295 feet in length; many require ground improvement or multiple shafts with caps.<br /><br />Foundation design follows AASHTO and WSDOT guidance and uses subsurface data to assess axial and lateral resistance, group effects, and downdrag. Potential enhancements include reducing combined inertial and lateral-spreading loads and allowing in-ground hinging during the maximum design event.<br /><br />Two sacrificial test shafts are planned under separate contracts to reflect project shaft dimensions, geology, and construction conditions. Bi-directional load testing will measure side and base resistance, calibrate load–deformation behavior, and support use of higher LRFD resistance factors. The tests are also intended to validate design assumptions and inform construction methods, including excavation, shaft cleanout, concrete placement, and possible permanent casing. Embedded hydraulic jacks and redundant instrumentation along the reinforcement cage will help distinguish side resistance from base resistance.

Keywords

West Seattle Link Extension

light rail

drilled shafts

seismic hazards

Cascadia Subduction Zone

cable-stayed bridge

foundation design

bi-directional load testing

prestressed concrete girders

ground improvement