Credit: Original article published here.
Seattle’s new Elliott Bay Seawall promises to be a big win for the city for years to come.
By Shari Held
If you’re like most people visiting Seattle, you’ll head to the waterfront. After all, what’s a trip to the Emerald City without experiencing Pike Place Market, the Great Wheel and the other tourist attractions?
What all those people walking along the waterfront probably don’t realize is they are on top of one of the nation’s most sophisticated and innovative, as well as award-winning, infrastructure systems: Seattle’s new Elliott Bay Seawall. And precast concrete played a big part in its construction.
The original seawall
The original, two-mile seawall, built between 1916 and 1934, consisted of 20,000 old-growth timber piles. The seawall’s construction and subsequent thriving waterfront helped put Seattle on the map, enabling the city to become the major supply depot for Alaska and also setting its deep-water seaport ahead of its competition. Unfortunately, it nearly obliterated the marine life habitat along the shore.
By the 2000s, the seawall had exceeded its expected service life and was deteriorating from old age, the appetite of a marine creature called a gribble and the harsh marine environment. Still, the seawall protected the city’s infrastructure and utilities. But when the 6.8 magnitude Nisqually earthquake hit Seattle in 2001, it caused devastating damage to the city’s waterfront.
Seattle needed a new seawall to protect its waterfront infrastructure and utilities from earthquakes. And while they were building it, the city wanted to encourage the return of marine life and its once-flourishing salmon corridor.
The scope of the project, which began in 2013, was large. The main part involved replacing 3,500 feet of the original two-mile structure. Besides the seawall, the plan included ecological features, a habitat beach, a restored roadway and a promenade for a total project cost of nearly $400 million.
An ideal project for precast
Logistically, using precast concrete materials helped improve working conditions in the cramped environment of downtown Seattle’s waterfront. Environmentally, precast eliminated the sensitive issue of pumping fresh concrete so close to the water. Structurally, precast worked hand-in-hand with the city’s goal of simultaneously improving the soil around the waterfront. Using precast also minimized disruption to adjacent businesses and popular tourist attractions.
“It was a lot of dollars,” said Jessica Murphy, with the City of Seattle Department of Transportation, who served as project manager for the seawall project. “Precast was a very early value-engineering proposal that we latched on to quickly.”
Precast also exceeded the city’s 75-year design-life requirement.
“You get very good quality and control and lifespan from precast, and that was one of the things the city was very concerned about,” said Steve Hiester, P.E., Oldcastle Infrastructure project manager for the Elliott Bay Seawall. “They don’t want to redo this again anytime soon.”
Fabricating for performance
The joint venture team of Mortenson/Manson, the main contractor, selected Oldcastle Infrastructure in Auburn, Wash., to produce the seawall’s structural precast components – 385 Z-shaped superstructure elements, 391 face panels and 434 habitat shelves. Oldcastle used a concrete mix appropriate for marine conditions with a minimum compressive strength of 6,000 psi to manufacture the precast components. The precast concrete was fortified with fly ash and slag and reinforced with epoxy-coated rebar to increase its durability and lifespan.
“It was a complicated mix design,” Hiester said. “The performance requirements were pretty stringent. The city was concerned about chloride content, shrinkage and permeability.”
The massive, 20-ton precast Z-segments are designed to support the seawall’s cantilevered precast sidewalk, serving as the backbone of the seawall structure. Workers poured two Z-segments per day.
Each segment is reinforced with 4,000 pounds of rebar and checks in at 8-feet wide by 9-feet tall. Oldcastle mainly used custom steel forms, although particleboard, which was changed out every day, was used in areas where a rough texture was desired.
The face panels were 8-feet wide and 15 inches thick and ranged from 8 to 20-feet tall. Height variances were dictated by the different ground elevations at the bottom of the bay. Oldcastle poured three face panels per day.
The large amount of variation within each element was also challenging. Among the Z-segments there were 39 unique configurations, 165 for the face panels and 15 panels for the habitat shelves.
In addition, the face panels came in three surface finish patterns – a cobblestone, marine-life texture featuring barnacles and starfish and panels with the tide levels (high, mid, low) marked on them.
“It wasn’t just producing the same thing over again,” Hiester said. “There were a lot of form changes, and a lot of formliner changes. And since they were for downtown Seattle, there were a lot of utilities that had to go through the face panels, and, to a certain extent, the Z-segments.
“Keeping track of all that was one of the bigger challenges.”
It took Oldcastle just over 18 months to produce all the elements. Mortenson/Manson installed the precast in phases so Oldcastle kept the structures in the plant yard and shipped them to the job site as needed using flatbed trucks and lowboys.
Laying the groundwork
In 2013, prior to Oldcastle fabricating the structural precast elements, the Seattle waterfront was being readied to accommodate the new seawall.
“A big part of the project was ISM – improved soil mass,” said the project manager with Mortenson/Manson. “This is technology that’s been used before, but you don’t see it a lot. It wasn’t contemplated originally going into the project.”
Jet grouting, a process where grout is injected into holes drilled into the ground, was used to stabilize the ground and create a solid barrier that would withstand seismic events and keep water away from the project. As an added bonus, using this process meant workers could solidify the existing, deteriorating timbers so there was no need to remove and dispose of them.
It was one of the largest jet-grouting projects ever completed globally, requiring 5,750 jet-grout columns, measuring 3-to-6 feet in diameter, and placed at depths of 40-to-90 feet.
Putting it together
To ensure there weren’t any unwanted surprises or delays during installation, Mortenson/Manson adopted a “practice before you play” approach, simulating the installation process to better understand the concrete mix properties and how to handle the material. This learning experience prior to installation at the job site cut down on installation time.
“We did a ton of planning and mock-ups,” the Mortenson project manager said. “We were fortunate, and it all worked out as we had hoped.”
First, the cast-in-place support slab was poured. Then, the Z-segments were placed using a large crawler crane or a rough terrain crane, depending on the location of the segments. Concrete was then poured around them to form a closure wall to connect them. This innovative installation technique also helped the installation proceed quickly.
Prior to installation, stainless steel threaded rods were inserted into the face panels to support the habitat shelves, which weighed up to 2,685 pounds each. The shelves were then lowered into place using a crane, bolted to the panels and grouted in place. Last of all, the face panels were installed.
It took Mortenson/Manson about two years to complete the installation of the precast structural elements.
“Once we got going we could set each piece in essentially a matter of minutes,” the Mortenson project manager said. “But it wasn’t continuous.”
It wasn’t as easy as it sounds. For one thing, the precast concrete elements had tight, 1/8-inch tolerances.
“We had to set 37,000-pound precast elements that close for essentially three-quarters of a mile,” the Mortenson project manager said. “That was certainly a challenge.”
Designing for sustainability
Constructing a new seawall gave Seattle an opportunity to improve its habitat for migrating salmon. Migrating young salmon prefer a sunny path within a few meters of land. But seawalls, piers and docks create shady, murky areas that newly hatched salmon try to avoid, putting them at risk. Additionally, the old seawall did not provide an ideal habitat for the salmons’ food source.
The design of the new seawall solved both problems. The patterned precast face panels create an attractive environment for the marine life that young salmon like to eat, and the cantilevered precast sidewalk panels feature light-penetrating panels that help guide the salmon along the waterfront as they journey to the Salish Sea and beyond.
The Elliott Bay Seawall project was challenging in many ways. It had an aggressive schedule, exacting production and installation tolerances and tough working conditions. Despite that, it garnered awards for categories from design to sustainability, including the 2017 Project of the Year (Disaster/Emergency Construction/Repair Category) from the American Public Works Association Washington Chapter.
“There was very good collaboration among all parties involved – the owners, the architects and engineers – to make this project go smoothly,” Hiester said. “And that’s what it really takes. Collaboration makes all the difference in the world.”
The high-profile project, which wrapped up in 2017, also earned high merits with the City of Seattle. Murphy is currently serving as construction project manager for Waterfront Seattle, the next phase of the city’s waterfront development. This phase involves removing a double-decker freeway and adding a bike path and other amenities to the waterfront. It also includes revamping Pike Place Market and the aquarium.
“We’re excited in Seattle,” Murphy said. “The new seawall was much needed. It is literally and figuratively the foundation for our future waterfront.”
Shari Held is an Indianapolis-based freelance writer who has covered the construction industry for more than 10 years.