SDOT looks at seawall alternatives

22 06 2010

 

By KATIE ZEMTSEFF
DJC Journal Staff Reporter

Photos Courtesy SDOT and the U.S. Army Corps of Engineers: The city is considering two structural solutions for the problem. One is a secant pile wall approach. The other is a jet grouting approach.

The Seattle Department of Transportation and the U.S. Army Corps of Engineers are beginning the environmental review process necessary to replace the Elliott Bay seawall. Last week, a public meeting was held to present the project and solicit comments.

Comments will be accepted until July 19. Built between 1916 and 1934, the seawall runs along Elliott Bay from South Washington Street to Broad Street. SDOT and the Corps are doing environmental and feasibility analysis to replace the entire seawall, and will be studying alternatives. They are developing a draft environmental impact statement and a feasibility study.

The city wants to replace the central section of the seawall first, which runs from South Washington Street to Pine Street. It recently chose a multi-disciplinary team led by Tetra Tech to provide consultant services for the seawall replacement project. The contract is valued at about $18 million. The replacement is anticipated to cost $285 million.

The seawall helps protect downtown, trans-continental rail lines, major utilities, waterfront businesses and the Alaskan Way Viaduct. It was constructed on loose soils with a design that did not account for earthquakes. Marine organisms are eating away at its timber platforms, and the wall also has sections of un-reinforced concrete.

Bob Chandler, city program manager for the viaduct and seawall replacement, said the city is looking for a seawall that supports the waterfront while providing habitat value and creating space for people to interact with the water. “We have an opportunity,” he said. “We need to come up with an approach here that provides protection in a seismic event but that doesn’t necessarily mean this is a 35-foot high straight wall. It doesn’t mean that at all.”

The city is considering two structural solutions for the problem. One is a secant pile wall approach. This would involve drilling five to eight foot overlapping shafts behind the seawall face. The other is a jet grouting approach that would inject grout 13 feet below the relieving platform. Under pressure, the grout will fill the gaps between existing wood pilings to become a solid underground wall. The city would put a new facing on the sheet pile wall.

SDOT is designing the seawall fix for a 1,000-year earthquake. It wants comments from the public to provide citizen guidance.

Over the next six months, the team will be developing design alternatives for the seawall. SDOT plans to choose a preferred alternative next spring. Construction could potentially begin in 2013 and would likely last for two and a half years. The rest of the seawall replacement will be done following completion of the central seawall.

Chandler said the seawall work is tied to the replacement of the Alaska Way Viaduct. If the state chooses to pursue the bored tunnel option, tunnel work should be complete by the end of 2016. After which, the viaduct will be torn down and new roadway and open space connections will be constructed.

Chandler said the city wants the first phase of the seawall to be done before that construction begins, as it supports the viaduct area and would support new structures.

Peter Hahn of SDOT said a lot of attention will be paid to how the team does its work, in addition to what it designs and builds. Timing will be very important. “Our goal is not to bring this part of the city to a standstill for five or six years. That’s just not going to be acceptable.”

Heather Trim of People for Puget Sound commented at the meeting. She said this discussion is much better than a similar one that occurred in 2002 because the city wants to provide connections to the water. She said her organization would like the design team to come up with a range of options for the edge.

The scoping period will run from June 1 through July 19, 2010. For more information, visit www.seattle.gov/Transportation/seawall.htm.

 
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