Demolishing the outdated and seismically vulnerable viaduct cleared the way for ambitious new phases of development along the Seattle waterfront.
Demolition teams from Kiewit worked northward up the viaduct, removing sections of deck, cutting the concrete into sections, and using a crane to lower the material down the slope for removal.
Our team created a slope protection system plan to manage stormwater runoff above the slope during demolition and reduce the risk of instabilities due to erosion.
GeoEngineers also provided subgrade recommendations for surface streets and features near the exit of Battery Street Tunnel north of Denny Way.
The hillside beneath SR 99 was partially protected from rainfall by the viaduct, and GeoEngineers staff watched carefully for signs of instability as it was exposed to more rain and stormwater than it had for decades.
GeoEngineers evaluated subsurface soil conditions by reviewing existing geotechnical information from the client, the Department of Natural Resources and GeoEngineers’ own in-house records from extensive geotechnical investigations in the Seattle downtown corridor.
Alaskan Way Viaduct and Battery St. Tunnel Demolition
Safely removing a 70-year-old elevated roadway to make way for a renovated Seattle waterfront.
Since the 1950s, the Alaskan Way Viaduct and Battery Street Tunnel have carried traffic through downtown Seattle, Washington. The viaduct is an elevated roadway running directly along the Seattle waterfront, and is connected to Aurora Avenue and points north via the Battery Street Tunnel. Together, the two structures carried State Route 99 through Seattle and formed a critical transportation artery.
These structures performed well 70 years ago, but over time the viaduct became an eyesore and barrier to further development along the waterfront, and both structures became outdated and vulnerable to earthquake damage. The Alaskan Way Viaduct was damaged during the 2001 Nisqually Earthquake. Although it was reinforced and made safe for daily use, engineers knew that neither the viaduct or tunnel could meet modern seismic standards. To solve these challenges, officials rerouted viaduct traffic beneath Seattle’s downtown core through a new, longer, SR 99 Tunnel and planned to demolish the Alaskan Way Viaduct and Battery Street Tunnel.
Safely demolishing an elevated roadway in a dense urban area is not easy. In this challenging setting, GeoEngineers provided geotechnical services and construction observation as a subconsultant to Kiewit Infrastructure West for various components of the viaduct demolition, as well as for the decommissioning of Battery Street Tunnel and the reconfiguration of related surface streets.
The GeoEngineers team was also responsible for protecting the steep slope immediately below and beneath the viaduct. This hillside was partially protected from rainfall by the viaduct, and GeoEngineers staff watched carefully for signs of instability as it was exposed to more rain and stormwater than it had for decades. Our team created a slope protection system plan to manage the slope during demolition.
Throughout demolition, officials needed to maintain pedestrian access to the waterfront and specifically the Colman Ferry Dock, a hub for the Washington State Ferry network. The project team built the Temporary Marion Street Pedestrian Bridge to solve this challenge and provide access during construction. GeoEngineers made geotechnical recommendations for the bridge. To save money and time, the team used existing foundations from the viaduct and nearby Columbia Street Ramp.
Approach
- GeoEngineers evaluated subsurface soil conditions by reviewing existing geotechnical information from the client, the Department of Natural Resources and GeoEngineers’ own in-house records from extensive geotechnical investigations in the Seattle downtown corridor. By relying on existing data, GeoEngineers saved the client from the additional costs of new geotechnical investigations.
- The geotechnical team used their characterization of subsurface materials to provide design recommendations for demolishing the viaduct, decommissioning the Battery Street Tunnel, building the Temporary Marion Street Pedestrian Bridge, protecting surface slopes near the southern portal of the Battery Street Tunnel and for surface streets and features near the exit of Battery Street Tunnel north of Denny Way.
- Our team created a slope protection system plan to manage the slope beneath the viaduct during demolition. The plan laid out drainage and surface water control measures, made recommendations for erosion control, analyzed the staging and placement of temporary equipment and included a monitoring plan for inspections during and after demolition.
- The team also made geotechnical recommendations for surface streets and features near the exit of Battery Street Tunnel near Denny Way.
Results
GeoEngineers leaned on its expansive experience with subsurface conditions in downtown Seattle to provide careful geotechnical analysis and recommendations for the demolition and decommissioning of the Alaskan Way Viaduct and Battery Street Tunnel. The project cleared the way for a new SR 99 Tunnel with additional transportation capacity, built to modern seismic standards, and in the process prepared the waterfront for ambitious new phases of development.
