Alaskan Way Viaduct and Seawall Replacement

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GHD has provided utility coordination and relocation design services throughout the two-mile long Alaskan Way Viaduct corridor in Seattle, Washington.

GHD is responsible for data quality review for all utility mapping throughout the corridor, performing research of the City of Seattle’s GIS and other utility records dating back to 1900, resolving discrepancies, performing field checks, and integrating survey data.

GHD has completed conceptual utility relocation designs for the entire corridor, interfacing a full spectrum of public and private infrastructure in tightly constrained areas.  It is preparing detailed designs of water utility and drainage improvements for phased construction packages along the Alaskan Way corridor.

The south end of the project involves relocating existing water mains on East Marginal Way and Alaskan Way between South Atlantic Street and Pike Street.  These mains are a vital to the city’s water network, both as transmission lines and local supplies to adjacent customers.  Over the last 100 years, water mains of eight, 12, 20 and 24 inches were installed — the majority of them prior to 1920 – using cast iron pipes with lead joints. 

The existing single water main will be replaced with two 12-inch mains running South and East plus nine short connection mains.  The proposed system will include approximately 14,400 linear feet of 12-inch restrained joint ductile iron pipe. In addition, there will be 12 connection points between the existing and proposed systems. 

GHD also designed water main modifications to accommodate construction near the Battery Street Tunnel. A challenging element of this work involved the intersection of 20-inch and 24-inch high pressure mains. To maintain consistent service to the existing water system during this work, GHD’s engineers temporarily re-routed the 20-inch and 24-inch mains, ensuring that the system was not interrupted. Utility designs are being prepared in Civil 3D and applying City of Seattle design standards.

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