Led by the Office of the Waterfront and the Seattle Department of Transportation (SDOT), the reconstruction of the Elliott Bay Seawall is a $220 million project that will provide protection of critical infrastructure and utilities while improving the salmon migration corridor in the region. It will also serve as a foundation for current projects that are transforming Seattle’s waterfront.
The rebuild began in November 2013 and is anticipated to be complete by mid 2017, along a similar timeline to the Pike Place Market expansion and before the anticipated 2020 completion of the new Seattle Central Waterfront.
The city hired Parsons Transportation Group as the design firm on the project and Mortenson-Manson Joint Venture as the general contractor. “Each team was selected from a group of competing firms based on written proposals and interviews,” said Norm Mah, a senior public relations specialist for SDOT. “The city select team reviewed the proposals and conducted the interviews, taking into account each team’s qualifications and experience.”
Considering the state of the original wooden infrastructure, which is dated between 100 to 75 years, the decision made by the city to rebuild the seawall was one out necessity. Built between 1916 and 1934, the seawall runs from Broad Street, near Olympic Sculpture Park, down to South Washington Street and holds up the entire length of the downtown waterfront. Seattle’s natural shoreline stretches a few blocks deep underneath the city’s infrastructure and is supported by the seawall, which allows the actual shoreline to be out into deeper water. This was done in order to entice big ships to dock in Seattle instead of going further south to Tacoma.
According to representatives of SDOT during a guided tour earlier this week, the current state of Seattle’s seawall has become an issue of public safety because it is long past its design life. It stretches up to 60 feet deep in some areas underneath the city’s shoreline and consists of piles of timber, soil and sawdust with a relieving platform over the top. The only concrete aspect of the seawall is the wall face along the shoreline, which has begun to crack causing it to rot from salt water and marine termite damage. Holding up Seattle’s waterfront roadways, a viaduct and several utilities, it would cause severe damage if the seawall were to collapse, according to the same sources.
According to the SDOT representatives the new seawall is being built using a construction technique called jet grouting, which is a process using a high kinetic energy jet of fluid to break up and loosen the ground and mixing it with a thin slurry. It is not truly grouting but rather a hydrodynamic mix-in-place technique producing a soil-cement material. This process allows the existing timber piles to remain in place without having to extract and dispose of them. It also provides a slight give in the seawall, which allows it to withstand an earthquake.
Once this phase is completed, the next step would be to create a cast-in-place slab. This refers to the installation of a layer of concrete directly onto compacted or natural soil and is widely used in the construction of foundations for single industrial, commercial and residential buildings. They are usually reinforced with steel bars or welded wire fabric. The final step is to install 44,000-pound reinforcements, called z-panels, before installing the face panels of the seawall. The new face panels have a texture that will enhance the growth of sea plant life, in order to reclaim some of the nearshore salmon habitat.
“With the Seawall Project we’re already starting to see the new waterfront come together,” said Office of the Waterfront Director Marshall Foster in a prepared statement. “Today you can stand on the completed light-penetrating sidewalks and see temporary landscaping that starts to show how great the space will be when it’s finished.”