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

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The West Seattle Link Extension (WSLE) will add 4.7 miles of light rail, including 2.8 miles of elevated guideway. Its structures include prestressed girders for typical spans, segmental concrete box girders for longer spans, and a cable-stayed bridge over the Duwamish Waterway.<br /><br />Foundation design is challenging because soft, compressible and potentially liquefiable sediments extend to depths of up to 270 feet. During major earthquakes, these soils may settle and spread laterally, imposing substantial forces on bridge piers. The preliminary design therefore favors drilled shafts: single shafts with ground improvement for typical piers, and groups of 10-foot-diameter shafts connected by caps for long-span and cable-stayed bridge piers. Designs account for axial and lateral loads, liquefaction, downdrag, and seismic inertial and lateral-spreading effects. The cable-stayed crossing is designed for a 2,500-year seismic event.<br /><br />To validate foundation assumptions and improve constructability, two full-scale bi-directional load tests are planned near the cable-stayed bridge’s main pylons. Sacrificial test shafts, approximately 225 and 125 feet long, will assess shaft side resistance and base resistance in the site’s layered soils. Instrumentation will measure loads, movement, compression, and strain. Results will be used to verify or refine design parameters and may justify higher resistance factors, potentially reducing shaft dimensions or quantities. The tests will also inform excavation, casing, base-cleaning, and concrete-placement methods.<br /><br />The paper presents the work at preliminary-design stage and notes possible future refinements to seismic analysis and in-ground plastic-hinge assumptions as additional data and research become available.

Keywords

West Seattle Link Extension

light rail

elevated guideway

cable-stayed bridge

Duwamish Waterway

drilled shafts

liquefaction

seismic design

load tests

foundation engineering