Analysis of Seawall Concepts Using Yielding Soil Anchors

Stretching 8,016 feet (2,443 meters) along downtown Seattle's waterfront, the 75-year-old Alaskan Way Seawall provides the interface between the city's downtown core and Elliott Bay. Damage to the seawall during the 2001 Nisqually earthquake led to investigations that confirmed the seawall is deteriorating and seismically vulnerable to potential liquefaction of the loose soils that underlie the structure. The City of Seattle is planning to replace the seawall. To further advance the proposed replacement program, two replacement concepts were developed. One used secant pile technology while the other used yielding soil anchors in combination with soil cement (jet grouting). A soil-structure interaction program was used successfully to model the complex dynamic interaction between the soil, the existing seawall and timber relieving platform, and the structures proposed to replace the seawall. The structural and geotechnical engineers collaborated on the development of an innovative hybrid system in which the soil improvement or secant pile wall resists service loads and liquefied soil pressures while the yielding soil anchors resist the inertial effects of a seismic event. This paper will review the development of the replacement alternatives analysis, the lessons learned, and the importance of successful structural-geotechnical collaboration in order to provide a solution to a complex soil-structure interaction problem.