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

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Abstract: As part of the Final Environmental Phase, Sound Transit has completed the preliminary engineering for 2.8 miles of elevated guideway structures for the West Seattle Link Extension Project. Following an extensive bridge type study and evaluation, Sound Transit advanced the preliminary engineering for the guideways that primarily consists of two structural systems: a cable-stayed bridge over the West Duwamish Waterway, segmentally constructed concrete box girders for long-span guideways. The cable-stayed Duwamish Crossing will be the longest cable-stayed bridge carrying light rail in the United States and the first designed to withstand a 2, 500-year return period seismic event in a highly seismic zone.

Given the complex geotechnical conditions— characterized by liquefiable soils and glacial till as deep as 250 feet— the piers on the east and west shores of the Duwamish Waterway face significant lateral spreading forces in addition to inertial loads during seismic events. Consequently, the foundation systems were carefully selected and analyzed to optimize structural efficiency and cost-effectiveness. The adopted foundation systems at each of the long-span piers consist of multiple drilled shafts with a shaft cap. These foundations are designed to accommodate substantial vertical service loads and lateral seismic loads, making them well-suited for the heavy segmental box girder spans and the cable-stayed bridge. Other critical loading conditions, such as downdrag and the combined effects of seismic inertial and lateral spreading loads, were rigorously considered during the preliminary design phase.

To enhance design reliability, validate end bearing and side friction capacities, and refine installation methods, two full-scale bi-directional load tests are planned near the cable-stayed bridge foundation sites. These tests will be conducted prior to the start of final design of the foundation to ensure optimized design and constructability.

Mr. YANG JIANG; Mr. Dirk Bakker

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