Seattle Department of Transportation – Elliott Bay Seawall Project

On even the most visible projects, it’s not common that the owner invites the public to drop by the construction site. But the reconstruction of Seattle’s protective Elliot Bay Seawall is a once-in-a-generation project. Which is why the city has set up outreach initiatives and tours to help its citizens learn about the $410 million undertaking. “It’s part of Seattle’s history, not to be shied away from but to embrace it,” says Jessica Murphy, project manager for the Seattle Department of Transportation (SDOT).

The project will replace the existing seawall that support’s much of the city’s downtown infrastructure, including roads, freight routes, local and regional utilities, high-pressure gas mains, electrical and telecommunications wires and sewers. The seawall also abuts the Seattle Ferry Terminal, an important transportation point used by 8.5 million people each year.

The existing seawall was built between 1916 and 1934 before engineers had an understanding of the area’s seismic risk, Murphy says. As a result, the seawall’s foundation is not grounded in solid soil and is susceptible to failure in seismic events, putting the entire downtown area at risk. “Failure of the seawall is really catastrophic for us,” she notes of the potential danger.

The new seawall will be built to the same seismic standards required of bridges and is designed to withstand a 1,000-year seismic event, Murphy explains. The gravity-based structure’s design life is 75 years, but Murphy says it should last longer thanks to maintenance and adaptability. “Our wall holds up not on the strength of the wall face but on its mass,” she adds.

The section being replaced is 3,700 feet long – only about half the length of the entire seawall. “We’re tackling the section that’s the most deteriorated and protects the most sensitive upland infrastructure,” Murphy says. The zone includes the Alaskan Way Viaduct, a double-decker highway that runs along the Elliott Bay coast and carries 110,000 cars daily. Construction began in November 2013 and is scheduled for completion in fall 2017.

The other half of the existing seawall not included in the current project will be rebuilt at a later date, Murphy says.

Managing Budget

Critical infrastructure projects require careful selection of the builders. Most government projects follow a straight bidding process, where the qualified contractor with the lowest bid wins the job. But on mega projects, such as the seawall reconstruction, the contractor is chosen primarily for qualifications and experience. Seattle found that capable contractor in the Mortenson-Manson Joint Venture team, which is acting as the contractor/construction manager for the project. “This model has proven valuable for such a unique and one-of-a-kind project,” Murphy says. “We are clearly doing things that have not been done in this kind of combination before.”

Seattle residents endorsed the seawall project in November 2012 when 77 percent of voters approved a $290 million bond to fund construction. “We are a city that generally supports our infrastructure levies, but to that extent it was unheard of,” Murphy says of the public’s support. The original project budget was $300 million but has grown to $410 million as additional challenges arose. Additional funding will come from real estate excise tax receipts and a bond against the city’s commercial parking tax.

In addition to the budget increase, the project’s end date was moved from 2016 to 2017. “We chose to defer the last section of it so we could get a better handle on the costs,” Murphy explains.

The city delayed the seawall’s completion, not because construction was running behind, but because of opportunity. The seawall replacement is coinciding with the Alaskan Way Viaduct Replacement Program, a Washington State Department of Transportation project that will remove the double decker viaduct and bury it underground. The two-mile tunnel project has been delayed numerous times due to ongoing problems with the tunnel boring machine known as “Bertha.” The state now expects the tunnel to open to drivers in spring 2018.

The seawall reconstruction must be completed before the tunnel commences so that traffic can be rerouted from the old Alaskan Way to the new seawall road. Delaying the tunnel allowed SDOT to move back its own dates and relieve pressure on the seawall project’s timeline, Murphy says.

Making Progress

Although the seawall project’s schedule has been stretched out, SDOT and Mortenson-Manson continue to reach milestones. The nature of the project site has forced SDOT and its contractor to be innovative in the construction process. The seawall area sits entirely on fill soil over old tideflats. During a seismic event, the mushy soil behaves like water, making for a poor foundation. 

To reinforce that soil, the project is utilizing jet grouting. The process drills 8-inch-diameter holes into the soil as deep as 60 feet. Grout is then injected through the holes to stabilize the existing soil. “It gives us the foundation to actually build the seawall structure,” Murphy says. Jet grouting was completed in about 70 percent of the project area as of November. 

In addition to an effective way to strengthen the existing soil, jet grouting allowed SDOT to leave the 20,000 timber piles that supported the old structure in place. Alternative methods would have required the removal of timber through 8-foot-diameter shifts, adding cost and time to the project. But jet grouting simply fills and reinforces the soil around the existing support structure. “The timber structure is able to stay in place and it doesn’t compromise the new structure,” Murphy says.

Another aspect that has proven a challenge for construction is how to manage the water on site. The Elliott Bay tides can rise and fall 12 feet each day, while groundwater flows upland to the project area. Keeping the work area free from both water sources – not to mention rain – is no easy task.

Here, too, SDOT and Mortenson-Manson opted for an innovative approach. To manage the groundwater, contractors created a underground ice dam that protects the work site. The dam runs north and south parallel to the seawall along the base of a nearby hill. Holes as deep as 38 feet were drilled into the ground, allowing builders to install pipes carrying salt water. Salt water, which has a lower freezing point than fresh water, is pumped through the pipes at 29 F, freezing the groundwater and creating the ice dam over the course of about three weeks.

Pumping the groundwater out would have required SDOT to treat the liquid and could have created seismic issues and put stress on the viaduct structure. Going with the ice dam avoids both of those pitfalls. “It’s a higher initial cost solution but it came with fewer risks,” Murphy says.

Ice damming isn’t a new technology, but SDOT had never used it before. “Anytime you’re going to try something that’s not standard, there’s a little bit of nervousness around it,” Murphy says. “You want to do your due diligence. But it has been effective.”

With solutions in place for the major obstacles, the seawall project is expected to be a crucial part of downtown Seattle’s development for the next several decades. Protecting vital utilities and allowing the Alaskan Way tunnel to be built will pave the way for new public spaces. Murphy likens it to Boston’s Big Dig, which united the city by burying infrastructure and roads. 

The city envisions its reclaimed shoreline as a place for all people and a spark for surrounding development. “It’s really going to transform Seattle’s waterfront in a way [the city] has never seen before,” Murphy says. 

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