Seismic Design of the New Elliott Bay Seawall Using the Conditional Mean Spectrum

The new Elliott Bay Seawall will replace an existing 75 year old seismically vulnerable structure that stretches along downtown Seattle’s waterfront. A gravity wall concept consisting of a matrix of interlocked jet grout soil cement columns was selected to replace the aging sheet pile and timber platform structure. This paper presents seismic analysis of the new gravity wall system which included the development of earthquake ground motions using the conditional mean spectrum (CMS) rather than the more conventional uniform hazard spectrum (UHS). For locations similar to Seattle, Washington, USA, where the seismic hazard is governed by crustal and subduction zone events which have distinctly different spectral shapes and durations, applying ground motions compatible with the UHS results in a significantly conservative soil improvement design versus applying ground motions compatible with the CMS. A comparative assessment of the design using UHS based motions and CMS based motions is presented.