The new seawall will be 15 ft inland of the original. A series of jet-grouted columns, approximately 4 to 5 ft in diameter, will stabilize the soil without requiring the removal of the old timber columns. The opaque cantilevered walkway will allow light to the new shallow-water habitat below. Courtesy of City of Seattle
The Seattle Department of Transportation will employ jet grouting in the replacement of a critical part of its transportation infrastructure.
December 17, 2013—Work began in mid-November on a project to replace the critical Elliott Bay Seawall in downtown Seattle. Portions of the seawall are nearly 100 years old and its vulnerability became increasingly apparent following the Nisqually earthquake. The 2001 temblor damaged portions of the structure and the Alaskan Way Viaduct—a two-tier, elevated portion of State Route 99 adjacent to the wall. A subsequent structural analysis determined that the seawall was deteriorating internally as well as externally and presented a seismic risk as well.
The existing seawall comprises a concrete face founded on a master pile wall, as well as a cantilevered concrete sidewalk and an anchoring system of large timber piles, says Jessica Murphy, P.E., a project manager for the Seattle Department of Transportation (SDOT). “What anchors that wall in place is a timber relieving platform and 20,000 vertical and battered timber piles driven into the soils behind the wall,” she says.
Over the years, these buried timber piles have significantly deteriorated, especially in key locations at which they anchor the wall to the support structure. In addition to age, the timbers have been attacked by invasive isopods—called gribbles. These small marine crustaceans bore into wood and digest the cellulose. This creates small holes in the wood, leaving the timbers spongy and structurally unsound.
“It’s also a problem that the timber piles that anchor [the seawall] don’t extend to solid soils,” Murphy notes. “They are all in a liquefiable soil area. The whole area is subject to liquefaction in a seismic event. This creates a combination of failure risks between seismic activity, tidal forces, and coastal storms along the waterfront.”
The existing seawall is supported by 20,000 old growth timber
piles that are deteriorating because of age and attacks from
small isopods known as gribbles. Courtesy of City of Seattle
The seawall replacement takes on added importance because of extensive underground utility infrastructure, an important surface street at the location of the wall, and the elevated roadway above. “This is a critical infrastructure project,” Murphy says. “If the seawall fails, the roadway fails, the utility infrastructure fails, the transportation network fails, and the elevated roadway could fail. That could be a pretty catastrophic event and [presents] a significant risk.”
To address the risk, the SDOT examined several options, including drilled shafts and secant piles to replace the wall. Because solid soils are as much as 60 ft below the surface, and the wall is subjected to strong lateral forces, the project would have required large diameter shafts, however.
“That’s an impactful construction technique in front of what we are trying to maintain as an active business area,” Murphy notes. The SDOT instead selected jet grouting for soil stabilization.
The construction team will first drill approximately 5,000 jet grouting holes, approximately 8 in. in diameter. A mixture of cement, air, and water will then be injected deep underground at high pressure through radial nozzles. This will create a series of 4 to 5 ft diameter columns underground. The solidified mass will serve as the foundation for a new gravity-system wall face.
Large portions of the seawall were constructed in the mid-1930s.
This photo from 1936 shows the old growth timber piles and
relieving platform in place. © Seattle Municipal Archives/Wikimedia
“By solidifying that liquefiable soil, we have solved one of our main vulnerability issues,” Murphy says. “Then building the new seawall solves the problem of the deterioration of the existing wall. But one of the benefits of the jet grouting is that we don’t have to remove any of the 20,000 piles.”
In fact, she says, jet grouting “can surround and entomb the existing timber structure down there. The wood can continue to deteriorate. The design works if the wood is there or not. It’s a pretty creative solution and ends up being less disruptive than some other options.”
The seawall was built in segments, some constructed as early as 1916. Large portions of the seawall, however, date to the mid-1930s. Seattle was already a city of more than 365,000, having grown dramatically in the previous decades during timber and gold booms. The seawall provided robust access to the ports, cementing the city’s status as a busy industrial hub.
The seawall displaced natural tidal flats, bridging the gap between the shore and deep-water ports that were created to accommodate the largest ships of the era. Due to current property lines and construction staging requirements, the new seawall will be located inland approximately 15 ft. SDOT will use this move to recreate some of the shallow-water areas that were lost with the construction of the original wall.
“It [provided] the opportunity to focus on some habitat improvement in what we call the pullback area,” Murphy says. “We created some shallow water for juvenile salmon migration [and for] small invertebrates and plant life to grow in that area.” To foster this, the new cantilevered sidewalk that will extend from the wall will have translucent glass panels to allow sunlight to pass through.
The project will cost $350 million, $290 million of which was provided by a November 2012 bond measure approved resoundingly by Seattle voters. The project is slated to be substantially complete in 2016.